Page 1 of 14 ENS4152 Project Development Proposal a.docx

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ENS4152 Project Development
Proposal and Risk Assessment Report
Baxter Research Robot: Solving a Rubik’s Cube
Chris Dawes
Student # 10282558
30 Mar 2015

Supervisor: Dr Alexander Rassau
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Abstract
Robotics is currently used to perform many tasks but many of
these are simple repetition of a
predefined method. By combining AI with robotics we can
greatly increase the applications of
robotics. An algorithm that combines the vision and servo
systems of a Baxter Research Robot
with a solving solution for a Rubik’s cube will demonstrate that
the use of even simple AI with
robotics allows complex tasks to be completed. Further
integration of object recognition will
allow the task to be completed in a dynamic environment, and
further increase the areas
robots are capable of working within.

1. Introduction
1.1. Motivation
The Baxter Research Robot by Rethink Robotics is a dual arm
robot, with seven degrees of
freedom per arm, released in 2012. Developed to be affordable,
flexible in its purpose, and
above all else safe, Baxter includes three cameras, one on each
wrist and the other on its head,
and a screen for displaying information relating to Baxter’s
current task. The robot is designed
to be a versatile research platform while containing the same
hardware as its industry
counterpart, allowing research to translate into industrial
applications (Rethink Robotics,
2015).
In general robotics artificial intelligence (AI) has been
developed separately to robotics, but is
now starting to become integrated. Unfortunately current AI is
fragmented as each application
focuses on one area, as opposed to making a true AI that thinks

like a human (Bogue, 2014).
Current usable AI is more akin to ‘smart’ robotics where
decisions are made and problems
solved by the robot in very specific applications. In industry,
robots are expanding into areas
that require more flexibility allowing robots to fill many more
positions in increasingly complex
areas (Hajduk, Jenčík, Jezný, & Vargovčík, 2013). Mobile
robots are even becoming more
common place, allowing for dynamic and spread out
workspaces. These are all due to adding
sensing and analysis to robots allowing them to react to
dynamic environments.
To further robotics in industry, multi robot work cells have been
designed that combine
several robots working on the same part while cooperatively
performing either one task, such
as welding and the required handling, or multiple tasks at the
same time (Hajduk, Jenčík, Jezný,
& Vargovčík, 2013). The number of activities these work cells
can perform increases
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dramatically, as the complexity of the task or tasks can be
higher while the robots don’t need
to be capable of performing the whole task individually.
For performing more human tasks, dual arm robots have begun
to emerge (Hajduk, Jenčík,
Jezný, & Vargovčík, 2013). By having two arms on one robot,
mobility can be enhanced while
providing some benefits of multi robot systems, further
extending the use of industrial robots.
Dual arm robots have a greater operational efficiency as they
have a centralized controller,
whereas multiple robots are individual standalone systems, thus
needing individual controllers
and additional centralized controllers for communication and
task assignment (Zhou, Ding, &
Yu, 2011). An example of dual arm robotics can be found in
space applications. Development
of dual arm space robotic systems is currently achieved by
relying on tele-operation from an
external location. This requires the same sensory information
that could be used by an AI to
automate tasks without the need of constant external control.
This could lead to adapting dual

arm robots with AI to industrial applications, especially where
the task or environment is
hazardous to humans. This could lead to much safer workplaces
along with decreasing running
costs and increasing efficiency while maintaining high quality.
Current research with Baxter has explored many different
research areas. For motion planning
an approach was developed that performed a learned task while
avoiding obstacles and
reacting to changing task related objects (Bowen & Alterovitz,
2014). This is executed in a
closed loop manner, where an automatic control system is
regulated by a feedback loop. This
is implemented by constantly updating information on task-
related objects.
The area of object grasping has been explored with Baxter by
research into using grippers that
can adapt to the shape and size of an object (Jentoft, Wan, &
Howe, 2014). By applying tactile
sensing to each of the three ‘fingers’ of the gripper, information
regarding the object shape
can be received, and grips can be adapted. Additionally by

analysing sensor data from
occurrences where the object is ejected from the hand, these
forces and grips can be avoided.
In addition to motion planning and object grasping, real-time
tracking of articulated objects
has been demonstrated with Baxter (Schmidt, Newcombe, &
Fox, 2014). Using Dense
Articulated Real-Time Tracking (DART), objects consisting of
rigid articulated bodies can be
tracked. These can include many object such as chairs, humans
and doorknobs. As position can
be determined, robots can more easily interact with objects.
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A demonstration of the Baxter Research Robot solving a
Rubik’s cube has been developed
previously (Coles-Abell & Pugh, 2014). This was undertaken to
demonstrate the ease of
integrating Baxter Research Robot into a research application.
They developed a workflow
consisting of detect, verify, solve, manipulate and complete.

Baxter was taught to use a flatbed
scanner to image each face of the cube. This information was
fed into OpenCV, an open source
computer vision library, and processed generating a mapping of
the cube. This mapping was
then verified using custom algorithms to ensure that the cube
was a valid cube. If this failed
the cube was rescanned. This mapping was sent to the Kociemba
algorithm which solves the
cube and returns a set of required manipulations. Using the
standard grippers the
manipulations are achieved by Inverse Kinematics moves.
AI and robotics have been developed separately and are only
recently being combined. By
combining these a higher level of automation for robotics can
be achieved. As explored
previously, robotics is currently going through a change where
AI has the ability to improve
automation and flexibility of robots greatly in many industries
and environments. This project
is aimed at developing an algorithm that combines a simple
form of AI with a Baxter Research
Robot to automate a task. By choosing the task as solving a

Rubik’s cube, vision recognition
and analysis systems are combined with motion planning and
manipulation of objects to
demonstrate the added flexibility by implementing simple AI.
1.2. Objectives
The overall objective of this project is to develop an algorithm
that controls the vision and
servos of the Baxter robot to allow it to pick up a Rubik’s cube,
visually analyse the cube, find a
solution, and allow the Baxter to manipulate the cube to solve
it. As research has already been
done on this, I will be aiming to use only the Baxter robot and
no external hardware. This
means that the project will require only the Baxter robot and a
computer setup as a
workstation to communicate to the robot.
The solving algorithm must allow any combination of a 3x3
Rubik’s cube to be solved. This
requires a set of rules for solving the Rubik’s cube to be
programmed, as opposed to using a

database of solutions. The vision must be able to recognise and
determine placement of a
Rubik’s cube placed in front of Baxter on a table, and to
recognise the different colours on a
Rubik’s cube with accuracy.
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Movements made by Baxter will be mostly programmed in, such
as the movements for
analysing the cube and for manipulating it. Motion planning
will be needed for retrieving the
cube and placing it back. Ideally the cube will be placed on any
spot in reach of Baxter.
As an extension the algorithm could also include the ability to
solve a 4x4 Rubik’s cube. This
will require additional research into a solving solution, and
possible modification of the
grippers. Additionally the vision software will also be required
to identify the type of Rubik’s
cube, and additional movements to manipulate the 4x4 cube will
need to be defined.

1.3. Significance
By demonstrating the use of an algorithm that solves a task
using vision recognition systems,
servo systems and motion planning system, along with a solving
solution, it shows robots can
be used in more dynamic and variable systems. This increases
the applications of robots
greatly and allows more complex tasks to be automated in
industry. In turn safety in
workplaces can be improved by replacing the worker, quality of
the products can be ensured,
and speed of tasks can be improved.
Currently robotics is used mainly in repeat tasks, where high
repetition is needed, whereas
developing intelligent automation will allow an increase in
available environments and tasks
that can be performed by robots. Applying this to dual arm
robotics extends task further,
allowing human like manipulations to be performed. This
combination of intelligent
automation and dual arm robotics will further the uses of
robotics greatly in all applications,

from space, to industry and manufacturing.
2. Proposed Approach
The project can be broken down into a number of task that need
to be completed sequentially
and successfully for the objective to be achieved. All the
programming will be done in Python
using the Integrated Development Environment (IDE) Visual
Studios. Python was selected as a
programming language as the Baxter robot has an Application
Programing Interface (API) that
allows direct python control of the Baxter robot. Additionally
the Python programming
language has been developed with emphasis on code readability
allowing creation of well laid
out code.
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The majority of the work will be done on two computers, one of
which I have set up for

programming, and one with simulation software Gazebo (Open
Source Robotics Foundation,
2014). This tool is designed for robot simulation capable of
simulating detailed environments
and multiple robots and allowing algorithm testing. The
computers will be available at all times
meaning work on the programming will not be limited by
resource availability. For movement
planning and manipulation MoveIt will be used (Sucan &
Chitta, n.d.). This allows movement
planning without having to interact with Baxter physically,
reducing the impact of the robots
unavailability.
When working with Baxter RViz will be used (Rethink
Robotics, 2014). This tool allows the
current configuration of Baxter to be displayed, along with
camera vision and individual sensor
values which will allow superior control and visualisation of
Baxter’s movements.
Only one Baxter robot is available but there are multiple
projects running on Baxter. A division
of time to be spent testing and working with Baxter will need to

be established to allow all the
projects ample use of Baxter. However, use of the simulation
software should allow some
work to be done without working with Baxter directly and thus
allowing work to continue at a
similar pace.
First vision recognition will be used to identify a Rubik’s cube
placed in front of Baxter. Then
inverse kinematics will be needed to obtain the cube, given the
relative positions of the
gripper and the cube. The initial placement of the cube will
need to be recorded for returning
the cube. A vision system is already provided with Baxter but
colour recognition will need to be
added. This will require research on relevant software and to
pick the optimal solution through
a trade-off analysis.
Vision recognition will also serve the purpose of identifying the
placement of the 6 different
colours. As this system could have errors a check to ensure that
there is 9 of each colour will
be put in place to determine if any errors have occurred. If an

error occurs, the colour will
either be inferred or the cube will undergo a rescan. All of the
colours will be saved either as
strings or values in tables, each representing a face. The colour
will be determined by
comparing the pixel value to a range of values that represent
each colour. By including a range
of values for each colour the vision recognition should be more
robust.
Before the cube can be scanned, a set of movements must be
defined that will scan all 6 sides
of the cube one at a time. This will require programming in a
set of movements for each arm to
perform, with the colour recognition being used between each
movement to scan the cube.
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MoveIt will be used to plan the motions for Baxter’s arms and
then these motions will be
converted into Python code.
The saved colour positions will serve as the inputs to a solving
solution that will solve the

Rubik’s cube and output a list of face rotations that will need to
be performed. These rotations
will be translated to sets of movements that are predefined
within the code and then
performed in order. As many programs have been written to
solve a Rubik’s cube, research will
be done into an efficient solution that requires the least time to
solve, and the smallest
number of movements. However the ability to integrate this
program into python, and allow it
to interact with the rest of the algorithm will be the most
important criteria that will ultimately
determine the solution used.
After the cube is solved, Baxter will then return the cube to its
initial position and the
algorithm will end. This will be achieved using reverse
kinematics based on the saved initial
position of the cube.
3. Timeline
A Gantt chart can be found as Attachment 2. This chart assumes
that by week 12 of semester

two, all project work should be completed for presentation in
week 13. The chart also omits
semester two assessments as dates cannot be determined at this
time, although the
assessments will be worked on in parallel to the project. The
final report will require
continuous work and as such will run for most of semester two.
The semester one report and
presentation have been given two weeks each, and are assigned
time parallel to task work.
As seen in the chart each task has been further broken into
subtasks. Each task is defined as a
section of the project that can be developed and tested by itself
which allows for more
targeted debugging. Each task for the project are completed
sequentially as they are designed
to be modules that will be combined after all project work
related tasks are completed. Even
though time for testing each module has been assigned, rigorous
testing has been allowed for
once all the tasks have been combined. This is for unseen issues
that may arise. Additionally
two weeks has been given no project work as this is exam time

for other units.
Given the generous time allowed, and the various testing that
occurs, this timeline is realistic
and should allow for any unseen setbacks.
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4. Risk Assessment
Majority of the project work is done on a computer, with testing
the algorithm done by using
the Baxter robot. This is reflected in the overall low risk rating
shown by the risk assessment
matrix as Attachment 1. The risks have all been assigned a three
letter reference that fits in the
following categories:
- Supervisor
- Personal
- Ergonomics
- Equipment

- Computer
- Software
Many of the risks are inherently low due to low probability of
occurrence or low impact on the
project if they were to occur. The two medium risks are eye
strain due to computer work and
loss of project data. These risks required minimisation as they
couldn’t be removed
completely. For risk ERG_03 the approach to reduce the risk is
to require regular breaks from
the computer, and to ensure ergonomic eye angles and screen
distance are abided by. Risk
CMP_01 is medium because of its impact to the project. Losing
project work can set back the
project by weeks and as such must be minimised. By using
cloud storage as an additional
backup to having the files stored on multiple computers, this
reduces the risk impact to low.
Cloud storage also has the added benefit of allowing multiple
computers to have access to the
same version of project files.

5. Progress to Date
Current progress has been made to get the Baxter research robot
ready to be used. A
workstation installed with Ubuntu 14.04 with Robot Operating
System (ROS) Indigo has been
set up to allow a permanent connection to Baxter. The software
on Baxter itself has also been
updated to the latest version, 1.1. The arm configuration
program has also been run to ensure
correct sensor measurements and precise movements by Baxter.
However the configuration
program will need to be run every month as recommended by
Rethink Robotics.
Page 9 of 14
A personal Laptop has also been installed with Ubuntu 14.04
and ROS. This is for the purpose
of simulating the Baxter robot when physical testing is
unavailable. This laptop can also be
setup to connect to Baxter, if the permanent workstation stops
working.

6. Conclusion
Automation through the use of robotics is currently at a stage
where many tasks are simple
repeating a predetermined method. By applying the growing
field of AI to automation, robots
role can expand greatly, increasing the environments they can
be used in alongside expansion
of the tasks that can be performed. By developing a set of
algorithms that harnesses a solution
algorithm to solve a Rubik’s cube, with vision recognition
systems, manipulation and
movement planning systems and a dual arm robot, a task that is
relatively complex can be
completed. This can change how robots are used in not only
industry, but in applications as
complex as space maintenance and exploration.
7. References
Adorno, B. V. (2011). Two-arm Manipulation: From
Manipulators to Enhanced Human-Robot
Collaboration. Université Montpellier II - Sciences et

Techniques du Languedoc.
Retrieved from https://tel.archives-ouvertes.fr/tel-00641678/
Bogue, R. (2014). The role of artificial intelligence in robotics.
The Industrial Robot, 41(2), 119-
123. doi:http://dx.doi.org.ezproxy.ecu.edu.au/10.1108/IR-01-
2014-0300
Bowen, C., & Alterovitz, R. (2014). Closed-Loop Global
Motion Planning for Reactive Execution
of Learned Tasks. In K. Lynch, & L. Parker (Ed.), IROS (pp. 1-
7). Chicago: IROS.
Chen, F., Sekiyama, K., Cannella, F., & Fukuda, T. (2014).
Optimal Subtask Allocation for Human
and Robot Collaboration Within Hybrid Assembly System.
Automation Science and
Engineering, IEEE Transactions on, 11(4), 1065-1075.
doi:10.1109/TASE.2013.2274099
Coles-Abell, H., & Pugh, V. (2014). Baxter Solves Rubik's
Cube. Retrieved from Baxter Research
Robot Wiki:
http://sdk.rethinkrobotics.com/wiki/Baxter_Solves_Rubiks_Cub
e
Hajduk, M., Jenčík, P., Jezný, J., & Vargovčík, L. (2013).
Trends in industrial robotics
development. Applied Mechanics and Materials : Robotics in

Theory and Practice, 282,
1-6. doi:10.4028/www.scientific.net/AMM.282.1
Page 10 of 14
Jentoft, L. P., Wan, Q., & Howe, R. D. (2014). Limits to
Compliance and the Role of Tactile
Sensing in Grasping. International Conference on Robotics and
Automation (pp. 1-6).
Hong Kong: International Conference on Robotics and
Automation.
Open Source Robotics Foundation. (2014). Gazebo. Retrieved
from Gazebo:
http://gazebosim.org/
Rethink Robotics. (2014). Baxter Research Robot Datasheet.
Retrieved from Rethink Robotics:
http://cdn-staging.rethinkrobotics.com/wp-
content/uploads/2014/08/BRR_009.09.13.pdf
Rethink Robotics. (2014). Baxter Research Robot Wiki.
Retrieved from Rethink Robotics:
http://sdk.rethinkrobotics.com/wiki/Main_Page
Rethink Robotics. (2014). Rviz. Retrieved from Baxter Research
Robot Wiki:

http://sdk.rethinkrobotics.com/wiki/Rviz
Rethink Robotics. (2015). Baxter Research Robot. Retrieved
from Rethink Robotics:
http://www.rethinkrobotics.com/baxter-research-robot/
Schmidt, T., Newcombe, R., & Fox, D. (2014). DART: Dense
Articulated Real-Time Tracking.
Robotics: Science and Systems (pp. 1-9). California: Robotics:
Science and Systems.
Sucan, I. A., & Chitta, S. (n.d.). MoveIt! Home. Retrieved
March 1, 2015, from MoveIt!:
http://moveit.ros.org/
Zhang, T., & Ouyang, F. (2012). Offline motion planning and
simulation of two-robot welding
coordination. Frontiers of Mechanical Engineering, 7(1), 81-92.
doi:10.1007/s11465-
012-0309-4
Zhou, J., Ding, X., & Yu, Y. Q. (2011). Automatic planning and
coordinated control for
redundant dual-arm space robot system. The Industrial Robot,
38(1), 27-37.
doi:http://dx.doi.org/10.1108/01439911111097823

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Attachment 1 – Risk Assessment Matrix
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Risks Consequences
Current
Risk Treatments
Current Level of
Risk
Additional

Risk Treatments
Residual Level of
Risk
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SUP_01
Supervisor is not contactable/unavailable

for an extended period of time
Advice and guidance cannot be given, possibly
allowing misinformation and a lack of help
when needed.
The unit co-ordinator can be
approached for general information.
1 2 2 L None Required 1 2 2 L
PER_01 Sickness or unrelated injury
Reduced work progress or complete stoppage.
Can set behind the whole project causing
stress.
Computers available at residence
has required software to work on
much of the project, allowing work
to continue, abet at a slower pace.
PER_02 Project work causes too much stress
Reduced ability to work on both project and
other units. Can fall behind further increasing

stress.
Maintain health level of
commitment to work, while allowing
for time spend outside of project
and unit work.
ERG_01 Bad posture or sitting position
Sore back, shoulders and neck. Will reduce
productivity and act as a distraction preventing
focused work.
Adjustable chairs with movable seat
angle and back position allowing an
ergonomic seating position.
ERG_02
Repetitive Strain Injuries in hands or
wrists
Computer work becomes painful impacting on

both project work and unit work.
Regular breaks from computer
work.
2 2 4 L
Implement regular wrist exercises.
Use correct hand position according
to ergonomic standards.
1 2 2 L
ERG_03 Eye strain/soreness Work must be stopped to rest eyes.
Have regular breaks from computer
work.
3 2 6 M
Correct ergonomic eye level and
distance from screen.
2 2 4 L
Page 12 of 14
EQU_01 Baxter robot breaks completely
Baxter cannot be used to test or demonstrate
project.

Simulation software Gazebo can be
used in place of Baxter for proof of
concept.
EQU_02 Collision of Baxter robot with person Affected area
can have slight soreness.
Baxter's limited force application
(lifting limit of 2kg) and slow motion
ensures all collisions are low
impact.
2 1 2 L
Area near Baxter to be clear of
persons before and during use.
1 1 1 L
EQU_03 Some systems of Baxter stop working
All the main functions are needed, and as such
this could completely eliminate using Baxter.
Use of Gazebo simulation software
in place of Baxter may be required
but will be sufficient to perform tests

and present the project.
CMP_01 Loss of data
Time and effort wasted by losing data due to
hard drive crashes or data corruption.
Copies of all project files are on
different computers and backed up
on USB's
2 3 6 M
Use of cloud storage to add another
backup of project data and allow
syncing between devices.
1 1 1 L
CMP_02
Workstation computer that connects to
Baxter fails
Communication through workstation to Baxter
and thus testing cannot be done.
Laptop is set up with required

software and can be used instead
of lab computer. Software can be
reinstalled on a different computer
in the lab. This would require a
couple hours of work.
CMP_03
Either personal computer used for
programming work fails
Limit personal computer work and therefore set
back or slow down project work.
Multiple computers set up to work
on.
SFT_01
Communication software becomes
unsupported
Cannot communicate and thus use the Baxter
robot.
Gazebo Simulation Software can

be used in place of Baxter robot.
SFT_02 MoveIT software becomes unsupported
Motion planning cannot be done using the
software. Can cause a large setback.
Motion planning will require test
with the Baxter robot, or use of
Gazebo simulation software.
Page 13 of 14
SFT_03 Rviz software becomes unsupported
Detailed information about Baxter cannot be
displayed while testing.
Although less information can be
obtained, tests can still be
performed. Debugging of Baxter
can be done by rqt_console

software.
SFT_04
Visual Studios IED becomes
unsupported
Programming cannot be done using Visual
Studios, stopping relevant work.
Another IDE can be used in place
to allow programming work to
recommence.
SFT_05
Gazebo Simulation software becomes
unsupported
Accurate simulation of Baxter robot in an
environment cannot be done. Can slow down
project development.
Physical testing with Baxter must
replace simulation tests.

Activity Overall Risk Rating 0.00 Low
Page 14 of 14
Attachment 2 – Timeline Gantt chart
30-Mar 13-Apr 27-Apr 11-May 25-May 8-Jun 22-Jun 6-Jul 20-
Jul 3-Aug 17-Aug 31-Aug 14-Sep 28-Sep
Total for Task
Research best vision system for detecting cube
Motion planning to grab Rubik's cube
Testing
Total for Task
Raise cube into head camera vision
Identify and save values of colours
Motion planning to allow all sides to be analysed
Testing
Total for Task

Research and determine best solution method
Implement into python and Testing
Total for Task
Motion planning for each face rotation required
Testing
Total for Task
Place cube in original position and Testing
Total for Task
Testing and Finalising
Total for Task
Poster Presentation
Project Report
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“Sister Aimee: Saint or Sinner”
Below are some questions to consider in our viewing of the
video. Please turn these into your TA.
These will be graded Pass/Fail. If you show us that you watched
the video, you will be fine.
1. How does the teaching of evolution figure into McPherson’s
spiritual struggles?
2. What spurred early Pentecostals? How did things go for her
in China?

3. Upon returning to the U.S did she find meaning in domestic
life?
4. Who did her message appeal to? What did they like about
Sister Aimee’s message? Do
you think this was a conservative or radical message?
5. Did everyone welcome Sister Aimee? How did she respond?
6. What changed by the time she went to Baltimore? How does
she justify her shift away
from classic Pentecostalism?
7. Why California? How did the first healing service (Balboa
Park) go for Sister Aimee?
8. Today we hear a lot about mega-churches. What role did
Sister Aimee play in this
movement? How did Sister Aimee use the radio?

9. Did everyone welcome Sister Aimee in California (come back
to this question)? On
what bases was she opposed? How did she respond?
10. What does she make of the Scopes trial? What does her
position suggest about the
division between church and state?
11. About the radio. . . . . did it ever cause her in trouble? Did
everyone buy that she had
drowned? How does she explain her disappearance and
reappearance?
12. So, what do you think? What happened to her at that day of
beach? What motivated the
District Attorney?
13. Where does she go from there? How do things end for her?
Two Wrap up questions
• Is Sister Aimee’s story important? Why?
• Do you think her message was conservative, radical, or
apolitical?

ENS4152 Project Development
Proposal and Risk Assessment Report
A MECHANICAL REDESIGN OF THE CAVALI
WELD INSPECTION PIPE CRAWLER
Matthew Foster
STUDENT NUMBER 10339728
26th of August, 2016
Supervisor: Dr Douglas Chai
Ethics Declaration Checklist (to be completed by student)

Does this project involve the use of: YES/NO
(a) Human participants, NO
(b) Previously collected confidential data, NO
(c) Animals for scientific purposes? NO
If ‘YES’ to any of the above, then the proposal will not be appr
oved and you will not be allowed
to proceed with this project.
By submitting this report through the unit website for assessmen
t, you certify that the
information provided above is true and correct.
Page 2 of 17
Abstract
This report details the progress report of an industry collaborati
on in conjunction with Applus+.
It covers the redesign of Applus+’s laser‐inspection
internal pipe crawler from a mechanical
perspective. The crawler redesign incorporates new functionaliti
es to meet client demand. This
includes the addition of vertical traction, chassis redesign to sup
port a line laser, 5D 8” bend

manoeuvrability, and subsea capability to within 50m. The proj
ect was overall found to be low
risk. The highest risk areas are financial costs incurred by dama
ge to equipment and downtime.
1. Introduction
1.1. Background
Non‐destructive testing (NDT) is the process of inspecting
asset integrity without causing
damage to the asset. The industry is relatively new but has show
n rapid expansion over the last
20 years. NDT alone has contributed to a 50% reduction in incid
ent failure rate in oil and gas [1].
Currently, $2.1 billion USD is spent per annum alone on
NDT methods to maintain liquid
pipelines [1]. The main area of focus within asset integrity is w
eld inspection for construction
and operational piping. Large hollow stainless steel pipes are w
elded together end‐to‐end to
create structural and functional elements of facilities.
These pipes can be used to support
extremely heavy loads or chamber fluid flows. Small defects in
the welding between the two

pipes can lead to disastrous outcomes such as ingress,
buckling, or leaks. Initially, asset
inspection was handled by trained personnel. They would ventur
e into small pipes which were
often dark, dirty, and or dangerous. This proved to be a costly,
high risk operation that often
meant it would get neglected and pushed into the
backlog. The advancement of robotic
technology has drastically increased the scope and safety of ND
T operations since then, allowing
for remote controlled machinery to perform the necessary tasks.
Applus+ RTD is a global industry‐leading asset inspection comp
any that focuses on NDT
methods. In Australia, they work closely with energy and resour
ce sectors to provide quality
assurance of asset integrity. The CAVALI (Camera Aided Visua
l And Laser Inspection) is a pipe
crawler robot designed provide feedback on the integrity of inte
rnal pipe welds. It was created
in April 2015 on a 4‐month lead time by a team of Applus+ RT
D engineers. It was created in April
2015 on a 4‐month lead time by the Applus+ engineering team.
The relatively short lead time

meant that the product specifications were made only to meet th
e needs of the client. This
resulted Applus+ compromising on the ideal range of
functionalities. This report details an
industry collaboration between final year Edith Cowan
University students and Applus+
engineers to redesign the CAVALI to meet those functionalities.
The perspective of this report
Page 3 of 17
will focus primarily on how to accomplish the mechanical criter
ia, but will detail the major
electrical and software components for a holistic understanding
of the process.
1.2. Motivation
Industry collaborations can often be difficult to arrange with dif
ferent goals and expectations
from all parties. However, when done successfully they
can be an extremely rewarding
experience. The motivation behind choosing an industry collabo
ration with Applus+ RTD is a
multifaceted approach that is already yielding great results. The
personal motivation extends to

wanting a project that is relevant to industry, and explores the c
urrent engineering technologies
and methods. This can act as a positive catalyst when it
comes to future discussions and
opportunities. Not only being familiar with how a certain
industry works, but actually
contributing to that industry provides a massive
competitive edge over peers relying on
academic results and theoretical knowledge. That competitive e
dge comes to fruition when
looking for graduate offers, or even entrepreneurial ambitions.
Working with industry leaders Applus+ RTD opens the
path to a wide range of
opportunities. The Applus+ group has established themselves as
a highly reputable company
built on 80 years of excellence. They set a high standard
on innovation, precision, and
knowledge; this aligns with the philosophy of the design team.
Within Applus+ RTD, product
design is handled by the Applications Centre. The applications c
entre has a strong track record
of producing cutting edge designs tailored to fit client needs. Th
e high quality outcomes have

resulted in substantial value added to the Applus+ group. As suc
h, they been given more lenient
budgetary constraints on future design projects including
the CAVALI redesign. This is an
important factor in final year projects as it one of the most com
mon reasons why projects can
fall short or fail.
There are currently 16 different non‐destructive testing
methods using a variety of
mediums [2]. The application centre team focus primarily on the
design of crawlers that use
laser, ultrasonics, visual, thermal, and radiography. This offered
multiple options for project
selection. Some potential projects included an ultrasonic nozzle
scanner, a pressure balanced oil
filled bladder, and a predictive programming calibration. Howev
er, the redesign of the CAVALI
was chosen because it best compromised between both parties’
desired outcomes. Applus+ RTD
prioritised the redesign of the CAVALI as one of the top items i
n the backlog. CAVALI’s inability
to function subsea or on vertical traction had recently resulted i
n a lost business deal. However,
the redesign is currently backlogged and estimated to take at lea

st a year. As such, it worked
well within the time constraints of both parties. The design team
agreed that the CAVALI was
the best choice because of its challenging yet achievable scope.
The design team has also had
previous experience with installation of a line laser. This allowe
d for a more accurate estimate
Page 4 of 17
on time required, and applicable knowledge to speed up
the design and implementation
process.
1.3. Objectives
The objective of the project is to upgrade Applus+ RTD’s signat
ure internal pipe crawler. The
CAVALI (Figure 1) is designed to drive
inside pipes and check for weld defects [3]. It uses a
combination of laser ranging and camera visuals to
provide imaging feedback on new
construction welds.
Those goals can be split up into the three engineering fields of s
oftware, electrical, and

mechanical. The redesign is to be built up from inception which
will removed constraints at the
cost of additional time investment.
Figure 1 The CAVALI Pipe Crawler
Software
The upgrade of the LIDAR scanning apparatus
is the top priority of the project. The original
CAVALI uses a dot laser which can take up to 30 minutes to sca
n a single weld. The laser will scan
a loop of the weld in 1mm iteration in welds that are estimated t
o be up to 20mm in longitudinal
length. Switching to a line laser can speed up this process to un
der 40 seconds. The line laser is
capable of scanning the entire length of the weld in a single rota
tion.
Electrical
The main electrical component upgrade will consist of changing
the communication protocol
from Ethernet to Controller Area Network. It will also be
pivotal in supporting the various
mechanical and hardware components, as well as establishing th

eir respective constraints. The
main focus areas include the power supply and transformation, p
ower supplied to the drive
mechanism, power supplied to rotate the laser, and space requir
ed to house components and
cables.
Mechanical
The mechanical has a large variety of potential functionality up
grades. These include vertical
traction, chassis redesign to support a line laser, capabilities to
pass through 5D 8” bends, leg
extension to 42” pipe diameter, and subsea sealing to within 50
m. Each of these functionalities
Page 5 of 17
can be achieved by adopting current market technologies. The c
hallenge lies in creating a design
that achieve all of those functionalities, or partially achieve the
m with compensation methods.
Discussion between both parties has been a critical factor
in establishing the potential
outcomes and expectations of the project. A top down
list was developed to set out those

outcomes into four possible categories.
1. Complete Build –
most desirable outcome using actual materials and passing the f
actory
acceptance test. Beyond Applus+ RTD’s expectations.
2. 3D Printed Build – highly desirable outcome that can
show problems not visible in
modelling software. Beyond Applus+ RTD’s expectations.
3. CAD/CAM Drawings –
desirable outcome that can realistically lay out how the design
will function. Applus+ RTD’s higher expectations.
4. Concept Drawings – acceptable outcome that illustrates
a proposed design. Applus
RTD’s expectations.
It is the goal of the design team to reach the complete build or 3
D printed build outcome.
The complex design of the project integrates the various
fields of study within
engineering. This can lead to vagueness and ambiguity when de
ciding the responsibility of a
task. As such, a holistic approach will be used to overcome tho
se issues. This means all members

of the design team will need to understand every system in the d
esign, not just those within
their field of study. This is a costly time investment, but should
lead to a better result.
1.4. Significance
The significance of the project is lies in its positive value to all
involved parties. The design team,
Applus+ RTD, and Edith Cowan University (ECU) all benefit gr
eatly so long as an acceptable
outcome is reached. Failure to reach an acceptable outcome is o
ne of the most notable risks
which is covered in the risk assessment section below.
Design Team
The design team gains first‐hand experience with a real enginee
ring project. Alongside this,
access to a wealth of resources at the Applus+ RTD workshop. S
ome of these resources include
knowledge from working professionals, technical
documents, buying power, and heavy
machinery. Real projects can give insight into future career path
s, operational knowledge, and
valuable industry contacts.

Applus+ RTD
Applus+ RTD benefit by gaining an improved design or product
without having to divert current
staff to backlogged items. Alongside this, a fresh perspective is
brought to the design process.
Page 6 of 17
The gain can be summarised as significantly adding value to the
company by upgrading their
product and increasing their market potential.
Edith Cowan University
ECU benefits by improved relations with industry. A successful
project reflects on the teaching
quality and facilitation provided by the university. This
can lead to potential future
collaborations, and better networking opportunities. It can
also attract potential students
looking to study at ECU.
Alongside this, the desired functionalities have not yet been suc
cessfully combined into a
single pipe crawler. Successfully achieving all design criteria w
ould be creating a state‐of‐the‐art

product.
2. Proposed Approach
The redesign of the CAVALI is a complex operation that will dr
aw from the mechanical, electrical,
and software engineering fields. Achieving the best results start
s with the team selection. The
design team was selected based on strong track records,
previous experience, and group
cohesion. The group is comprised of Matthew Foster
(Mechanical), Rick Hurlbatt
(Mechatronics), and Jarred Asquith (Instrumentation and Autom
ation). The group has opted for
a specialised approach aiming to complete the critical componen
ts to a higher degree. However,
there is a void of knowledge with electrical engineering knowle
dge. Applus+ RTD has agreed to
this approach and arranged to assist in this area if needed.
The engineering design process will be derived from the templat
e provided in Shigley’s
Mechanical Engineering Design [4]. The process follows the fol
lowing steps: Identification of

need; Definition of problem; Synthesis; Analysis and Optimizati
on; Evaluation; Presentation.
Each step has a feedback loop into every previous step to illustr
ate the iterative nature of design.
Another engineering design process template will be
supplemented from Engineering Your
Future [5]. This template expands on each step in more detail.
The work will be divided
into tasks based on the applicable field of study, and then
assigned to the respective member of the design team. An overv
iew of the major tasks and
assignment can be seen in Attachment 4.
The mechanical perspective poses a large challenge due to
combination of desired
functions. The individual functions can be achieved by
relying on the engineering body of
knowledge, and existing solutions already available in market.
There will be a large amount of
time invested to researching client demand, and communicating
with Applus+ RTD. Design ideas
will be graded against the Mechanical Design Functionality Crit
eria as seen in Attachment 3.
The aim is to create a priority order that can shape and give wei
ghting to design ideas.

It also allows for necessary compensation methods to be conside
red earlier during the design
Page 7 of 17
process rather than being an ad hoc thought. For example, a pot
ential design could have subsea,
5D bend manoeuvrability, and vertical traction capabilities.
However, it could be limited by
having a leg extension factor of 1.5. This means the maximum v
ertical pipe diameter is only 1.5
times that of the minimum. If the minimum diameter is set for a
n 8” pipe, then the maximum
would be 12”, and unable to take on the larger diameters. A com
pensation method could be to
use exchangeable legs with various lengths to reach the
larger sizes. As such, the earlier
discussion and research method will optimise the design process
. To facilitate this, the design
team will be working at the Applus+ RTD workshop one day a
week. The workshop has access
to manufacturing machinery such as 3D printers, CNCs, and lath
es. They also have a large supply
of engineering materials such as stainless steel and Delrin. Thes

e will be useful resources during
the prototyping stages.
Standardised software will be used for ease of collaboration. Me
chanical modelling will
be done on Solidworks 16, electrical circuitry on Altium
16, and software programming on
LabVIEW 15.
The three major mechanical functionalities can be categorised
into vertical traction,
subsea, and pipe manoeuvrability. Initial ideas and research are
as are listed below:
Vertical Traction
The principle of this relies of heavy friction between contact sur
faces to prevent the crawler
from sliding. Dreadnaught wheels can accomplish this by provid
ing a larger contact surface area
with more stable supports. This design also favours hub motors,
which could eliminate any
moving parts through the chassis and keeping the electrical com
ponents well protected.
Subsea
The main area of research with subsea capability is

understanding material properties and
tolerances. Ingress could result in disastrous damage to the elect
rical components so this will
require quality assurance by factory acceptance testing.
Pipe Manoeuvrability
The leg extension method could be handled in numerous ways s
uch as spring loaded, pneumatic
control, electric servo control, or scissor lift extension. The mos
t important consideration is to
guarantee there is enough tangential force relative to the pipe to
generate ample friction.
3. Timeline
…
Desalination 356 (2015) 226–254
Contents lists available at ScienceDirect
Desalination
journal homepage: www.elsevier.com/locate/desal
Nanofiltration membranes review: Recent advances and future
prospects
A.W. Mohammad a,b,⁎, Y.H. Teow a, W.L. Ang a, Y.T. Chung
a, D.L. Oatley-Radcliffe c, N. Hilal c,d

a Department of Chemical and Process Engineering, Faculty of
Engineering and Built Environment, Universiti Kebangsaan
Malaysia, 43600 Bangi, Selangor Darul Ehsan, Malaysia
b Centre for Sustainable Process Technology (CESPRO),
Faculty of Engineering and Built Environment, Universiti
Kebangsaan Malaysia, 43600 Bangi, Selangor Darul Ehsan,
Malaysia
c Centre for Water Advanced Technologies and Environmental
Research (CWATER), College of Engineering, Swansea
University, Singleton Park, Swansea SA2 8PP, UK
d Qatar Energy and Environment Research Institute (QEERI),
Doha, State of Qatar
H I G H L I G H T S
• State-of-the-art overview of NF membranes and its approach
on membrane fabrication and modification
• The trend and progress in the development and application of
NF membranes over the past 5 years
• Fouling and fouling mitigation in the application of NF
membrane
• Future direction in NF membrane development
⁎ Corresponding author at: Department of Chemical an
Darul Ehsan, Malaysia.
E-mail address: [email protected] (A.W. Moham
http://dx.doi.org/10.1016/j.desal.2014.10.043
0011-9164/© 2014 Elsevier B.V. All rights reserved.
a b s t r a c t
a r t i c l e i n f o
Article history:
Received 12 August 2014
Received in revised form 29 October 2014

Accepted 30 October 2014
Available online 11 November 2014
Keywords:
Nanofiltration
Modeling
Fabrication
Wastewater treatment
Desalination
Fouling
Nanofiltration (NF) membranes have come a long way since it
was first introduced during the late 80's. With
properties in between those of ultrafiltration (UF) and reverse
osmosis (RO), NF membranes have been used
in many interesting applications especially in water and
wastewater treatment and desalination. Other applica-
tions include those in pharmaceutical and biotechnology, food
and non-aqueous types of application. This review
will comprehensively look at the recent advances in NF
membranes research. Significant development has taken
place in terms of the fundamental understanding of the transport
mechanism in NF membranes. This has been
translated into predictive modeling based on the modified
extended Nernst–Planck equation. Similarly various
methods have been used to fabricate improved NF membranes
especially through interfacial polymerization
incorporating nanoparticles and other additives, UV
grafting/photografting, electron beam irradiation, plasma
treatment and layer-by-layer modification. New applications
were also explored in many industries. However
fouling is still a prevalent issue that may hinder successful
application of NF membranes. Efforts towards NF foul-
ing prevention and mitigation have also been reported. The
review ends with several recommendations on the
future prospect of NF membranes research and development.

© 2014 Elsevier B.V. All rights reserved.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 227
2. Fundamental of NF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . 228
2.1. Separation mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 228
2.2. Characterization of NF membranes . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228
2.3. Predictive modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 229
3. NF membrane fabrication and modification . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230
3.1. Interfacial polymerization (IP) . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231
3.1.1. Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . 231
3.2. Grafting polymerization . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 232
3.2.1. UV/photo-grafting . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . 232
3.2.2. Electron beam (EB) irradiation . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 232
3.2.3. Plasma treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . 233
d Process Engineering, Faculty of Engineering and Built
Environment, Universiti Kebangsaan Malaysia, 43600 Bangi,
Selangor
mad).

http://crossmark.crossref.org/dialog/?doi=10.1016/j.desal.2014.
10.043&domain=pdf
mailto:[email protected]
http://www.sciencedirect.com/science/journal/00119164
227A.W. Mohammad et al. / Desalination 356 (2015) 226–254
3.2.4. Layer-by layer (LbL) . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . 233
3.2.5. Other grafting methods . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . 233
4. NF membrane applications . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236
4.1. Environmental application . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . 236
4.1.1. Groundwater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . 236
4.1.2. Surface water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . 237
4.1.3. Wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . 239
4.2. Desalination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . 240
4.3. Pharmaceutical and biotechnological application . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
4.4. Non-water application . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 241
4.5. Food industry application . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 244
5. Fouling and fouling mitigation . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244
5.1. Fouling mechanism and type of fouling . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245
5.2. Fouling prevention and mitigation . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
6. Future prospects in NF membrane technology . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248
7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 249
Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 249
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 249
1. Introduction
Nanofiltration (NF) membranes have come a long way since it
was first recognized in the late 80's. With properties in between
ultrafil-
tration (UF) and reverse osmosis (RO), NF membranes possess
pore
size typically of 1 nm which corresponds to molecular weight
cut-off
(MWCO) of 300–500 Da. NF membranes in contact with
aqueous
solution are also slightly charged due to the dissociation of
surface func-
tional groups or adsorption of charge solute. For example,
polymeric NF
membranes contain ionizable groups such as carboxylic groups
and
sulfonic acid groups which result in charged surface in the
presence of
Table 1
Commercial nanofiltration membranes with specification of the
manufacturers.

Membrane Manufacturer MWCO (Da) Maximum temperature
(°C) pH r
NF270 Dow Filmteca 200–400 45 2–11
NF200 Dow Filmteca 200–400 45 3–10
NF90 Dow Filmteca 200–400 45 3–10
TS80 TriSepb 150 45 2–11
TS40 TriSepb 200 50 3–10
XN45 TriSepb 500 45 2–11
UTC20 Torayc 180 35 3–10
TR60 Torayc 400 35 3–8
CK GE Osmonicsd 2000 30 5–6.
DK GE Osmonicsd 200 50 3–9
DL GE Osmonicsd 150–300 90 1–11
HL GE Osmonicsd 150–300 50 3–9
NFX Syndere 150–300 50 3–10
NFW Syndere 300–500 50 3–10
NFG Syndere 600–800 50 4–10
TFC SR100 Kochf 200 50 4–10
SR3D Kochf 200 50 4–10
SPIRAPRO Kochf 200 50 3–10
ESNA1 Nitto-Denkog 100–300 45 2–10
NTR7450 Nitto–Denkog 600–800 40 2–14
a Midland, Michigan, USA.
b Goleta, CA, USA.
c Tokyo, Japan.
d Le Mee sur Seine.
e Vacaville, CA, USA.
f Wilmington, Massachusetts, USA.

g Somicon AG, Basel, Switzerland.
a feed solution. Similar to RO membranes, NF membranes are
potent
in the separation of inorganic salts and small organic molecules.
Key distinguishing characteristics of NF membranes are low
rejection
of monovalent ions, high rejection of divalent ions and higher
flux com-
pared to RO membranes. These properties have allowed NF to
be used in
niche applications in many areas especially for water and
wastewater
treatment, pharmaceutical and biotechnology, and food
engineering.
Some commercial membranes, together with their properties and
top layer composition as specified by the manufacturer are
given in
Table 1. Various aspects of NF membranes have been covered in
a few re-
view papers [1,2] while Schäfer et al. [3] have written a
comprehensive
ange Stabilized salt rejection (%) Composition on top layer
N97% Polyamide thin-film composite
50–65% CaCl2
3% MgSO4
5% Altrazine
Polyamide thin-film composite
85–95% NaCl
N97% CaCl2
Polyamide thin-film composite
99% Polyamide

99% Polypiperazineamide
95% Polyamide
60% Polypiperazineamide
55% Cross-linked polyamide composite
5 94% MgSO4 Cellulose acetate
98% MgSO4 Polyamide
96% MgSO4 Cross-linked aromatic polyamide
98% MgSO4 Cross-linked aromatic polyamide
.5 99% MgSO4
40% NaCl
Proprietary polyamide thin-film composite
.5 97% MgSO440%
20% NaCl
50% MgSO4
10% NaCl
N99% Proprietary thin-film composite polyamide
N99% Proprietary thin-film composite polyamide
99% Proprietary thin-film composite polyamide
89% Composite polyamide
50% Sulfonated polyethersulfone
228 A.W. Mohammad et al. / Desalination 356 (2015) 226–254
reference book on NF. Other recent reviews covered the
chemical modifi-

cation of NF membranes [4], fouling [5,6], effect of pH and salt
on NF [7],
NF for textile dye wastewater treatment [8], and NF modeling
[9].
Advances over the last 6 years have shown a significant growth
of
papers published on NF membranes in many different areas.
Based on
a quick search using Scopus database, a total of 1642 papers
have
been published on NF membranes from 2008 to present. As
shown in
Fig. 1, the main areas of focus in these published papers on NF
mem-
branes were in environmental applications (25%), membrane
fabrica-
tion (19%), miscellaneous topics (11%), fouling (10.68%),
desalination
(8.95%), pharmaceutical and biotechnology (7.49%), modeling
(7.12%),
fundamental studies (4%), non-water applications (3.65%), food
appli-
cations (2.83%) and economics and design (0.55%). Such
classifications
may assist in understanding the overall view of current research
efforts
on NF membranes, thereby providing indicators and evidence of
new
applications and research direction.
The purpose of this paper is to critically review the advances in
NF
membrane research based on the important focus areas as shown
in
Fig. 1. The review will start by looking at the fundamental of

NF mem-
branes which include membrane characterization and modeling,
followed by other important aspects such as membrane
fabrication
and modifications, applications of NF for environment,
desalination,
pharmaceutical and biotechnology, food and finally NF
membrane foul-
ing and fouling mitigations. The wide scope of the review
signified the
huge potential of NF for future research and development. The
review
will then look at the prospective recommendations for future
works
that should be addressed in order to make significant
improvement in
NF membrane processes.
2. Fundamental of NF
2.1. Separation mechanisms
NF is an extremely complex process and is dependent on the
micro-
hydrodynamic and interfacial events occurring at the membrane
sur-
face and within the membrane nanopores. Rejection from NF
mem-
branes may be attributed to a combination of steric, Donnan,
dielectric
and transport effects. The transport of neutral solutes is via the
steric
mechanism (size based exclusion) and has been well established
through numerous studies of UF membranes [10]. The classical
Donnan
effect describes the equilibria and membrane potential
interactions be-

tween a charged species and the interface of the charged
membrane
0
5
10
15
20
25
P
e
rc
e
n
ta
g
e
(
%
)
Categories
1642 Published Articles
(2008-Present)
Fig. 1. Categorization of NF membrane process articles

published from 2008 to present.
[11]. The membrane charge originates from the dissociation of
ionizable
groups at the membrane surface and from within the membrane
pore
structure [12–14]. These groups may be acidic or basic in nature
or in-
deed a combination of both depending on the specific materials
used
during the fabrication process. The dissociation of these surface
groups
is strongly influenced by the pH of the contacting solution and
where
the membrane surface chemistry is amphoteric in nature, the
mem-
brane may exhibit an isoelectric point at a specific pH [15]. In
addition
to the ionizable surface groups, NF membranes have a weak
ion-
exchange capacity and in some cases ions from the contacting
solution
may adsorb to the membrane surface causing a slight
modification of
the membrane charge [16,17]. Electrostatic repulsion or
attraction
takes place according to the ion valence and the fixed charge of
the
membrane that may vary depending on the localized ionic
environment
as a result of the aforementioned phenomena. The phenomena of
di-
electric exclusion are much less understood and there are two
main
competing hypotheses as to the exact nature of the interaction.
These
are the so called ‘image forces’ phenomenon [18] and the

‘solvation en-
ergy barrier’ mechanism [19]. Both exclusion mechanisms arise
as a re-
sult of the extreme spatial confinement and nano-length scales
that are
present in NF membrane separations and are effectively charge
based
exclusion phenomena. These interactions have been reviewed in
detail
[20]. Solutes moving in free solution experience drag forces
exerted by
the solvent flowing through the confined pore structure. The
movement
of solutes in this confined space is greatly affected by the local
environ-
ment and the transport of the solute is considered as hindered.
Hin-
dered transport can be expressed in terms of both a convective
and
diffusive element which contributes to the overall transport
effect. The
fact that the dimensions of the NF active layer are at near
atomic
length scales, coupled with limitations in current measurement
technologies, has delayed a detailed knowledge of the physical
structure and electrical properties of real NF membranes and
has re-
sulted in uncertainty and significant debate over the true nature
of
the separation mechanisms [3] and the role of dielectric
exclusion
is particularly contested [20].
2.2. Characterization of NF membranes
There is sufficient evidence now available to conclude that a

porous
active layer is indeed present in most NF membranes [20] and
charac-
terization of these nanopores in terms of pore size and
distribution is
key to understanding the steric partitioning that may be
achieved by a
given membrane. This characterization can be achieved using a
variety
of methodologies, namely;
▪ Gas adsorption–desorption technique, also known as the
Brunauer–
Emmett–Teller (BET) method, allows the direct measurement of
a
pore size distribution [21–23].
▪ Atomic force microscopy (AFM), allows the direct
measurement of
pore size and distribution, surface roughness, topography and
force
interactions between membrane and colloids [24–27].
▪ Use of neutral solute rejection studies and models, allows the
indi-
rect measurement of pore size (and distribution if coupled with
other techniques) [28–30].
▪ Reverse surface impregnation coupled with Transmission
Electron
Microscopy (TEM), allows the direct measurement of pore size
and
distribution [31].
Each of the above mentioned methodologies can provide useful
in-

formation if employed correctly. However, due to the length
scales in-
volved and the various imperfections associated with each
method, a
combination of methodologies is always recommended as well
as the
application of logical rationale. Measurement of the charge
properties
of NF membranes is the other key variable required for process
under-
standing and these vary depending on the nature of the contact
solu-
tion, the concentration and pH. Often these charge properties
are
229A.W. Mohammad et al. / Desalination 356 (2015) 226–254
characterized across a range of process conditions using a
variety of
methodologies, namely;
▪ Streaming potential allows for the determination of the
membrane
surface zeta (ζ) potential which is a pseudo measurement of the
Donnan potential [32–35].
▪ Streaming current, analogous to the measurement of streaming
po-
tential [36–38].
▪ Electro-osmosis allows for the determination of the zeta (ζ)
poten-
tial through the membrane pore, i.e. perpendicular to the
membrane
surface [39].

▪ Use of charged solute rejection studies and models allows the
indi-
rect measurement of the effective membrane charge density (re-
quires other characterization techniques in-situ) [32,40–42].
Other useful characterization measurements of NF membranes
can
be performed and the most useful of these being scanning
electron mi-
croscopy (SEM) for imaging of the membrane surface,
membrane cross-
section and fouling layers [43–45]. Other techniques include
simple
methods with relatively low cost equipment such as contact
angle for
hydrophilicity/hydrophobicity [46–48] to more advanced and
increased
cost techniques for membrane morphology and structural
analysis such
as spectroscopy methods [49–52]. In essence there is a plethora
of pos-
sible characterization techniques available for NF membranes
and the
key to success is choosing the right technique, with the right
resolution
for the desired end purpose.
2.3. Predictive modeling
The prediction of membrane performance has been an active
area of
research over the last two decades. During that time, the
emphasis has
shifted from empirical black box models based on irreversible
thermody-

namics [53] to models based on the extended Nernst–Planck
equa-
tion [54] due to the ability of the latter to provide information
related to
properties of both the membrane and the process stream. The
main pur-
pose of such models is to incorporate as much physical realism
of the
membrane process as possible in order to better match
measurable quan-
tities to adjustable model parameters. One should always be
mindful that
the major limitation of NF modeling is the requirement for
characteristic
model parameters, such as pore radius and membrane charge,
that are
not readily measured at the near atomic length scales
encountered. The
nano-scale phenomena involved in neutral solute and charged
electrolyte
separations by NF membranes are extremely complex and, as
such, are
likely to be a rigorous test of any macroscopic continuum
description of
ion transport and partitioning. Similarly, the development of
rigorous
physical descriptions (such as molecular dynamics simulations)
has
been limited due to the lack of detailed knowledge of the
physical struc-
ture and electrical properties of real NF membranes and process
streams.
As a direct consequence, developments in modeling have moved
in paral-
lel with improvements in the measurement techniques employed
for the

characterization of NF membranes and process streams, as only
then will
a check of the appropriateness of model parameters be possible.
The application of the extended Nernst–Planck equation was
origi-
nally proposed by Schlogl [55] for the description of transport
of electro-
lytes in RO through ion-exchange membranes and is arguably
the most
commonly used model of modern times. The equation is
particularly
useful for NF as consideration is given to the mechanisms of
transport
and the adjustable fitting parameters required are based upon
real mea-
surable membrane properties.
The transport of ions is typically described using the extended
Nernst–Planck equation
ji ¼ −
ciKi;dDi;∞
RT
dμ
dx
þ Ki;cciV ð1Þ
where ji is the ionic flux, c is the concentration, V is the solvent
velocity
and Ki,c and Ki,d are hindrance factors to account for the
convection and
diffusion inside a confined space. This equation can
subsequently be

manipulated to derive the full extended Nernst–Planck equation.
ji ¼ −Di;p
dci
dx
−
ciDi;p
γi
dγi
dx
−
ciDi;p
RT
Vsi
dP
dx
−
ciDi;p
RT
zi F
dψ
dx
þ Ki;cciV ð2Þ
Eq. (2) represents the full extended Nernst–Planck equation and
governs the transport of an ion in solution. In this case the
equation
has been specifically developed to describe the transport of an
ion

through a membrane pore, typical of that found in NF and loose
RO
membranes. In order to solve the transport equation, the solute
concen-
tration at the feed side and permeate side, ci(0) and ci(Δx), of
the mem-
brane must be known. The entrance and exit of an ion from such
a pore
are an equilibrium process and this relationship is expressed as
γici
γoi Ci
¼ Φi exp −
zi F
RT
ΔψD
� �
exp −
ΔWi
kBT
� �
: ð3Þ
The first two terms on the right hand side of Eq. (3) are the
classic
expressions for both steric and Donnan effects respectively
[10,11,56]
and are generally well accepted throughout the literature [20].
The
third term on the right hand side of Eq. (3) represents
partitioning as

a result of dielectric exclusion. There has been much debate
over the na-
ture of this effect and this has been summarized in a recent
review [20].
A detailed description of the theory outlined above is reported
by
Bowen and Welfoot [19]. The theory was then further expanded
to con-
sider the pore size distribution of the membrane [57] and then
simpli-
fied by the consideration of an average pore concentration
which
allows for the linearization of the transport model and avoids
the com-
putationally intense numerical integral required [58]. The
solution to
the transport equations then becomes a simple arithmetic matrix
of
equations. The application of this modeling approach to
prediction of
NF performance was first introduced to a separation of real
industrial
importance by considering the isolation of N-acetyl-D-
neuraminic acid
(Neu5Ac) [59], an example of an equilibrium-controlled
biotransforma-
tion reaction and precursor for the antiviral agent 4-guanidino-
Neu5Ac-
2-en (zanamivir, Relenza™). In this work, a comparison was
made
between the full model and the linearized model and the
agreement
was found to be satisfactory for engineering purposes and the
linearized
model was then used for prediction of the diafiltration process

to
remove pyruvate from the reaction mixture. A further example
of an
industrial separation was provided by considering the recovery
of sodi-
um cefuroxime, an important cephalosporin antibiotic having
activity
against most gram-positive cocci, from spent crystallization
liquors
[60]. The full-scale recovery process was modeled theoretically
and
demonstrated that NF was more than adequate for the separation
required. An estimate of the industrial scale process operating
condi-
tions was made and the addition of the NF process as a hybrid
technol-
ogy to the crystallizer was suggested as a favorable
modification to
existing plants.
There are many variations on this basic modeling theme in
existence, however, in essence they are all intrinsically related
to the
Nernst–Planck equation and are either variations in solution
methodol-
ogy, simplifications or mild extensions in particular parameters.
For
example, the model described above is an extension of the
original
Donnan-Steric-Pore-Model (DSPM) first proposed by Bowen et
al. [61].
The version of the NF model described here is more rigorously
derived
from the extended Nernst–Planck equation by considering a
Hagen–
Poiseuille flow pattern through the nanopore. The model also

includes di-
electric exclusion by ion solvation phenomena by incorporation
of the
Born model into the equilibrium partitioning expression Eq. (3).
The sim-
plest description of dielectric exclusion by image forces
phenomena (for
slit-like pores) was first incorporated into NF models by
Vezzani and
Bandini [62]. This model was essentially an extension of the
original
DSPM model by inclusion of a description of dielectric
exclusion by
image forces derived from the theories of Yaroshchuk [63],
aptly called
230 A.W. Mohammad et al. / Desalination 356 (2015) 226–254
the DSPM & DE model. This model was further updated by
Szymczyk and
Fievet [64] to produce the Steric Electric and Dielectric (SEDE)
model. This
included descriptions of both image forces for cylindrical pores
(although
these are mathematically intense) and ion solvation via the Born
term, i.e.
both mechanisms of dielectric exclusion. Both the DSPM & DE
and SEDE
models suffer from inclusion of the fitting parameter thickness
to porosity
ratio (Δx/Ak) which is irrelevant to membrane rejection. Some
recent de-
velopments and applications of these NF models have been
made. Saliha
et al. [65] tested the ability of the SEDE model to describe the

separation of
multi-ionic solutions (3 and 4 ions) by NF and concluded that
the exper-
imental solute rejections were not well described when the two
dielectric
exclusion mechanisms were incorporated into the model.
However, the
model did work well when only one of the dielectric
mechanisms was in-
cluded, either ion solvation or image forces. This result shows
that only
one of the dielectric mechanisms is most likely apparent in NF
mem-
branes, but either model produces calculated values for the
phenomena
to the same order of magnitude. Thus, in earnest, either model
could po-
tentially be used to describe dielectric exclusion irrespective of
which
mechanism is fundamentally more appropriate. One concern
raised by
this work was the failure of the model to appropriately describe
the effec-
tive membrane charge density. Estimates of the membrane
charge densi-
ty were made from measurements of the tangential streaming
potential
for the membrane across a range of pH. This methodology
substitutes
the measured zeta-potential for the Donnan potential at the
membrane
surface (pore interface) which has never been demonstrated to
be truly
representative and the authors state that this method provides
only a
rough estimate of the value. However, this does emphasize that

the
coupled effect of the membrane charge density and dielectric
exclusion
phenomenon remain the most elusive aspect in the complete
under-
standing of the surface phenomenon occurring in NF processes.
Deon et al. [66] investigated a novel approach to modeling the
mem-
brane charge density by considering the membrane charge
density to
vary along the length of the pore. The authors named this the
Pore
Transport Model (PTM) and introduced this variation in charge
by
means of adsorption isotherms, which were determined
beforehand
from tangential streaming potential measurements. In some
respects
this model is a variation on the classic Space Charge Model
(SCM) orig-
inally proposed by Gross and Osterle [67], which is a far more
rigorous
(and complex) model that takes into account a radial
distribution of
both concentration and electric potential within the nanopores.
Howev-
er, in this case the variation in charge is not in the radial
direction but is
in the axial direction. The authors concluded that the membrane
charge
evolution with concentration was successfully assessed from
streaming
potential measurements and Freundlich adsorption isotherms
were
found to be reliable to calculate the charge densities inside the

pores.
However, charge densities inside a nanopore have very little
physical
relevance to tangential streaming potential measurements across
the
surface of the membrane (as described previously) and
adsorption of
ions inside a nanopore is simply non-quantifiable. For these
reasons,
one would have to …
Science of the Total Environment 595 (2017) 567–583
Contents lists available at ScienceDirect
Science of the Total Environment
journal homepage: www.elsevier.com/locate/scitotenv
Review
A review of reverse osmosis membrane fouling and control
strategies
Shanxue Jiang a, Yuening Li b, Bradley P. Ladewig a,⁎
a Barrer Centre, Department of Chemical Engineering, Imperial
College London, United Kingdom
b College of Environmental Science and Engineering, China
H I G H L I G H T S G R A P H I C A L A B S T R A C T
• RO membranes are prone to fouling in
different forms.
• Current control strategies can mitigate
fouling but cannot prevent fouling
completely.
• Novel membrane materials have great

potential to control fouling effectively.
• Statistical analysis revealed strong re-
search interest in RO fouling and
mitigation.
Abbreviations: AA, acrylic acid; AOM, algal organic ma
DAF, dissolved air flotation; DOC, dissolved organic carb
acetic acid; EfOM, effluent organic matter; EIS, electrical im
NF, nanofiltration; NIPAM, N-isopropylacrylamide; NOM,
mosis; SC, surface coating; SDS, sodium dodecyl sulfate;
ultrafiltration.
⁎ Corresponding author.
E-mail address: [email protected] (B.P. Ladew
http://dx.doi.org/10.1016/j.scitotenv.2017.03.235
0048-9697/© 2017 Elsevier B.V. All rights reserved.
a b s t r a c t
a r t i c l e i n f o
Article history:
Received 8 February 2017
Received in revised form 23 March 2017
Accepted 25 March 2017
Available online 8 April 2017
Editor: D. Barcelo
Reverse osmosis (RO) membrane technology is one of the most
important technologies for water treatment.
However, membrane fouling is an inevitable issue. Membrane
fouling leads to higher operating pressure, flux de-
cline, frequent chemical cleaning and shorter membrane life.
This paper reviews membrane fouling types and
fouling control strategies, with a focus on the latest
developments. The fundamentals of fouling are discussed
in detail, including biofouling, organic fouling, inorganic

scaling and colloidal fouling. Furthermore, fouling mit-
igation technologies are also discussed comprehensively.
Pretreatment is widely used in practice to reduce the
burden for the following RO operation while real time
monitoring of RO has the advantage and potential of pro-
viding support for effective and efficient cleaning. Surface
modification could slow down membrane fouling by
changing surface properties such as surface smoothness and
hydrophilicity, while novel membrane materials
and synthesis processes build a promising future for the next
generation of RO membranes with big advance-
ments in fouling resistance. Especially in this review paper,
statistical analysis is conducted where appropriate
to reveal the research interests in RO fouling and control.
© 2017 Elsevier B.V. All rights reserved.
Keywords:
Reverse osmosis
Membrane fouling
Pretreatment
Surface modification
Antifouling
tter; BSA, bovine serum albumin; CC, chemical coagulation;
CEOP, cake enhanced osmotic pressure; CNTs, carbon
nanotubes;
on; DTAB, dodecyltrimethyl ammonium bromide; EC,
electrocoagulation; ED, electrodialysis; EDTA, ethylene
diamine tetra
pedance spectroscopy; EPS, extracellular polymeric substances;
EXSOD, ex-situ scale observation detector; MF, microfiltration;
natural organic matter; PDA, polydopamine; PEI,
polyethylenimine; PV, pervaporation; PVA, polyvinyl alcohol;
RO, reverse os-
SG, surface grafting; SWRO, seawater reverse osmosis; TEP,
transparent exopolymer particles; TFC, thin film composite; UF,

ig).
http://crossmark.crossref.org/dialog/?doi=10.1016/j.scitotenv.20
17.03.235&domain=pdf
mailto:[email protected]
http://www.sciencedirect.com/science/journal/00489697
www.elsevier.com/locate/scitotenv
568 S. Jiang et al. / Science of the Total Environment 595
(2017) 567–583
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 568
2. Membrane fouling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . 569
2.1. Biofouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . 569
2.2. Organic fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 571
2.3. Inorganic scaling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 571
2.4. Colloidal fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 572
3. Membrane fouling control strategies . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 572
3.1. Pretreatment technologies. . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . 572
3.2. Membrane monitoring and cleaning . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 574
3.2.1. Membrane monitoring . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . 574
3.2.2. Membrane cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . 575
3.3. Surface modification and novel membrane materials. . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 576
3.3.1. Surface modification . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . 576
3.3.2. Novel membrane materials . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . 577
4. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 578
Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 578
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 578
Fig. 1. Number of publications per year and cumulative number
of publications on RO
fouling over the past 25 years.
1. Introduction
Water shortage is one of major challenges in many places
around the
world (Adeniji-Oloukoi et al., 2013; Avrin et al., 2015; Garcia-
cuerva
et al., 2016; Hibbs et al., 2016). It is exacerbated by water
pollution
from agricultural residues, sewage as well as industrial waste
(Yao
et al., 2016). In order to meet the rising demand for fresh water,
strate-
gies like water reuse and seawater desalination have already
been ap-
plied (Bartman et al., 2011; Gu et al., 2013). Membrane
technology is

one of the most promising ways to produce high quality water
(Lin
et al., 2016; Ochando-Pulido et al., 2016; Tang et al., 2016b).
The common membrane technologies for water treatment
include
but are not limited to microfiltration (MF) (He et al., 2016),
ultrafiltra-
tion (UF) (Sun et al., 2015), nanofiltration (NF) (Ribera et al.,
2014), re-
verse osmosis (RO) (Yang et al., 2017), forward osmosis (FO)
(Boo et al.,
2012), membrane distillation (MD) (Bush et al., 2016),
electrodialysis
(ED) (Zhang et al., 2015c) and pervaporation (PV) (Subramani
and
Jacangelo, 2015). RO membrane technology is widely used in
seawater
desalination, drinking water production, brackish water
treatment and
wastewater treatment. RO is currently the most energy-efficient
tech-
nology for desalination, with energy cost about 1.8 kWh/m3,
which is
much lower than that of other technologies (Xu et al., 2013b).
Also,
RO membrane has the advantages of high water permeability
and salt
rejection, fulfillment of the most rigorous rules for public
health, envi-
ronmental protection and separation process (López-Ramírez et
al.,
2006).
However, RO membrane fouling is a main challenge to reliable
membrane performance. Fouling is a complicated phenomenon

which
involves different mechanisms under different circumstances
(Khan
et al., 2014). For example, a lot of RO projects reusing
wastewater
with high levels of phosphate are in operation worldwide
(Chesters,
2009). In these plants, calcium phosphate scaling on membrane
sur-
faces is a big problem, resulting in poor plant operation and
high
cleaning and maintenance cost (Chesters, 2009). Membrane
fouling
could significantly reduce productivity and permeate quality
while in-
creasing operation cost due to increased energy demand,
additional
pretreatment, foulants removal and membrane cleaning,
maintenance,
as well as reduction in membrane lifetime (Al-Amoudi, 2010;
Eric
et al., 2001; Kochkodan et al., 2014; Tang et al., 2011). In order
to control
membrane fouling, a variety of methods such as pretreatment,
mem-
brane monitoring, membrane cleaning, surface modification, as
well as
developing novel RO membranes have been studied (Al-Juboori
and
Yusaf, 2012; Brehant et al., 2002; Henthorne and Boysen, 2015;
Nguyen
et al., 2012; Robinson et al., 2016). The application of different
methods
could result in different control effects and therefore, in
practice these
techniques are usually applied together to reduce RO membrane

fouling.
Statistical analysis revealed that in the last 25 years, over 3000
pa-
pers were published to address the issue of RO membrane
fouling
(shown in Fig. 1, see Supplementary Information for more
details on
statistical analysis method), indicating researchers' great
interest in
this area. Specifically, the number of SCI papers published in
2016 in-
creased by around 20 times compared to papers published in
1992
and was around twice as the papers published 5 years ago (i.e.,
year
2011). A polynomial model was derived to describe the
cumulative
number of publications from 1992 to 2016, with the equation P
=
0.3735 ∗ Y3 − 6.881 ∗ Y2 + 67.139 ∗ Y − 83.109 (R2 N
0.999), where P
is the cumulative number of publications and Y denotes the
number
of years since 1992. Based on this model, and assuming that no
revolu-
tionary breakthroughs in RO membrane technology and
alternative
technologies as well will be made in the next ten years, then it
can be
predicted that by the year 2022, the cumulative number of
papers pub-
lished will possibly be about twice that of 2016. Although the
research
trend may not be predicted precisely simply by this model, it
can at

least give us an indication that research interest in this field
will contin-
ue to bloom.
569S. Jiang et al. / Science of the Total Environment 595 (2017)
567–583
Therefore, it is necessary to provide an up-to-date review of RO
foul-
ing and its control. This paper reviews membrane fouling and
fouling
control strategies, with a focus on the latest advances. The first
objective
of this paper is to elucidate the types of fouling. The second
objective is
to discuss state of the art strategies for fouling mitigation,
including pre-
treatment, monitoring, cleaning, surface modification as well as
novel
membrane materials and synthesis process. Especially,
statistical analy-
sis (bibliometric method) is adopted where appropriate in this
review
paper to reveal researchers' interest in related sub-fields. This
compre-
hensive review may provide an avenue for future research work
related
to RO membrane fouling.
2. Membrane fouling
Generally fouling is the accumulation of undesired deposits on
the
membrane surface or inside the membrane pores, causing
decrease of
permeation flux and salt rejection (Malaeb and Ayoub, 2011).

Since
water is the operating environment for most RO applications, it
is im-
portant to understand the behaviors of water as well as ion
transport
through RO membrane, which could indicate how RO fouling
occurs.
Water permeation through membrane could take place in the
form of
Brownian diffusion, flush and jump diffusion (Gao et al., 2015).
The in-
termolecular interactions of water and ions with membrane are
strong-
ly affected by the structure of membrane such as the free
volume size in
the membrane. In other words, if the membrane structure is
more com-
pact, then more energy will be required for water permeation,
and as a
result, it will be easier for fouling to occur since particles are
more prone
to accumulate on membrane surface, known as surface fouling
which is
discussed below.
Fouling can be divided into surface fouling and internal fouling,
in
terms of the fouling places (Lin et al., 2014; She et al., 2016;
Yu and
Graham, 2015). The fouling mechanisms of low pressure
membranes
(i.e., MF and UF) are some kind of different from those of high
pressure
membranes (i.e., NF and RO). For MF and UF, pore adsorption
and clog-
ging are more common while for NF and RO, surface fouling is

compar-
atively more frequent due to the relative compact and nonporous
nature of RO membrane (Greenlee et al., 2009). This does not,
however,
mean that surface fouling is more “dangerous” than internal
fouling for
RO membrane. Actually, compared with internal fouling,
surface
fouling can be controlled more easily through improving feed
water hydrodynamic conditions or chemical cleaning (Hoek et
al.,
2008; She et al., 2016). Therefore, it is usually more reversible
than
internal fouling (Arkhangelsky et al., 2012). Nevertheless, it
should
be clarified that depending on feed water compositions and their
in-
teractions with membrane, both surface fouling and internal
fouling
can be irreversible.
In terms of foulants types, fouling can also be classified into
bio-
fouling, organic fouling, inorganic scaling and colloidal fouling
(Hakizimana et al., 2015; Weinrich et al., 2016; Xu et al.,
2010).
Fig. 2 shows scanning electron microscope (SEM) images of
four
fouling types on membrane surfaces. More specifically, Fig. 2A
shows the surface of a RO membrane contaminated by bacteria
(Xu
et al., 2013a) while Fig. 2B displays the RO membrane surface
that
is fully covered by an organic foulant, sodium alginate (Shafi et
al.,
2017). Fig. 2C clearly demonstrates calcium sulfate (CaSO4)

scaling
on a RO membrane surface (Hu et al., 2013) and Fig. 2D reveals
a
RO membrane surface fouled by a common colloidal foulant,
silica
(Ho et al., 2016). In practice membrane fouling is usually
caused by
a combination of different foulants and membrane autopsy
method
is widely used to study the origin and extent of membrane
fouling
and distribution of foulants because it can provide precise
informa-
tion about foulants compositions and properties (Gorzalski and
Coronell, 2014; Kim et al., 2015; Tang et al., 2014; Tran et al.,
2007). However, fundamental understanding of formation
mecha-
nism of fouling layer could not be obtained through autopsy.
2.1. Biofouling
Biofouling is the process of microorganism adhesion and
prolifera-
tion on membrane surface. In other words, it is the formation of
biofilm
to an unacceptable degree which could cause huge operational
costs.
Biofilm formation is essential in this process (Creber et al.,
2010a). Bio-
fouling is more complex than other fouling types. There are two
key
components of biofilms, namely the bacteria and the
extracellular poly-
meric substances (EPS) which are excreted by bacteria during
the me-
tabolism process (Yu et al., 2016). In marine environment, the
bacterial community is highly diverse and distinct, with

proteobacteria,
bacteroidetes, firmicutes and cyanobacteria being the typical
ones
(Belila et al., 2016; Khan et al., 2013). Depending on different
water en-
vironment and bacteria community, EPS could have different
sub-
stances, but are mainly made up of polysaccharides, proteins,
glycoproteins, lipoproteins or lipids and nucleic acids (Drews,
2010;
Matin et al., 2011; She et al., 2016). The surface morphology of
biofoul-
ing layers could be different under different environments and it
is a
good way to observe and analyze the biofouling vividly on a
micro
level (Karkhanechi et al., 2014; Leterme et al., 2016; Weinrich
et al.,
2016).
According to Flemming (1997), biofilm development could
undergo
three stages, namely induction, logarithmical growth and
plateau
stages. From another perspective, biofilm formation could be
briefly di-
vided into three phases in terms of bacteria activity and
mobility, and
the three phases are bacteria attachment, reproduction, and
detach-
ment. Bacteria attachment is a dynamic process consisting of
bacteria
approaching and then adhering to the membrane surface, which
is ex-
pected to be the most important stage in biofilm formation. The
exis-

tence of dead or low flux zone in the pipe system could have an
important effect on bacteria growth. A lot of other factors could
also af-
fect this process, and these factors could be classified into
microbial
properties (Camesano and Logan, 1998; Tang et al., 2016a),
membrane
surface characteristics (Nguyen et al., 2016), and surface-
bacteria inter-
actions (Kang et al., 2004; Walker et al., 2004), as well as
operational
conditions (Habimana et al., 2014). In brief, as illustrated by
Fig. 3, mi-
crobial properties include hydrophobicity, surface charge and
surface
structure, etc. Membrane characteristics include surface
hydrophobici-
ty, surface charge, chemical compositions, roughness, surface
morphol-
ogy, etc. Operating conditions include permeate flux, crossflow
velocity,
temperature, pressure, pH, salt concentration, presence of
certain mole-
cules, feed spacer, etc.
The next stage is bacteria reproduction, and during this period
the
attached microorganisms consume nutrients in the water and
undergo
proliferation and meanwhile excrete EPS (Matin et al., 2011).
The EPS
could make the biofilm structure stronger, making it more
difficult to
clean the biofilm (Ben-Dov et al., 2016; Leterme et al., 2016).
Also, EPS
could function as a barrier to protect bacteria from bactericide

(Belila
et al., 2016). The final stage is the bacteria detachment, and
during
this period the bacteria leave the mature biofilm due to lack of
nutrients
as well as the increase of population density. The bacteria find
new sites
to grow and the process repeats and new biofilm forms. Later
stage of
biofouling is more difficult to be controlled compared to earlier
stage
(Creber et al., 2010b).
Biofouling is widely regarded as one of the most formidable
fouling
(Al-Juboori and Yusaf, 2012; Hibbs et al., 2016; Ni et al., 2014;
Xu
et al., 2013b). Statistical analysis revealed that around 500
papers
were published in the past 10 years to address the issue of
biofouling.
Unlike other fouling types, membrane biofouling is difficult to
eradicate
by pretreatment methods. As analyzed above, biofouling is
formed by
microorganism, and microorganism can grow and multiply. As a
result,
unless pretreatment can remove 100% of the bacteria, the
remaining or-
ganisms can grow gradually on membrane surface and cause
mem-
brane fouling. On the other hand, for a biofilm to form, two
conditions
are essential, namely the presence of bacteria as well as the
nutrients.
So the logic is that if all the nutrients are removed from the

water
Fig. 2. SEM of four fouling types on membrane surfaces. (A)
Biofouling. (B) Organic fouling. (C) Inorganic scaling. (D)
Colloidal fouling.
Adapted from Ho et al. (2016), Hu et al. (2013), Shafi et al.
(2017), Xu et al. (2013a).
Fig. 3. Factors affecting bacteria attachment to membrane
surface.
570 S. Jiang et al. / Science of the Total Environment 595
(2017) 567–583
Table 1
Some representative work on RO organic fouling under different
situations.
Organic matter Category Main findings
Alginate Polysaccharide Membrane fouling aggravated
with decreasing pH, increasing
ionic strength, and addition of
calcium ions (Lee et al., 2006).
Octanoic acid Fatty acid Both pH and calcium ions
could affect the octanoic acid
fouling behavior (Ang and
Elimelech, 2008).
Humic acids Humic substance Fouling was mainly due to
hydrophobic interactions

between organic matters and
membrane as well as
interactions between the
organic matters (Yu et al.,
2010).
Hydrophilic carbohydrates,
EPS, aquatic humic
substances
EfOM Hydrophilic carbohydrates
and EPS made more
contributions to membrane
fouling than aquatic humic
substances (Zhao et al., 2010).
Alginate, BSA, NOM, octanoic
acid
Polysaccharide,
protein, humic
substance, fatty
acid
Membrane fouling by alginate
was dominated by foulant
aggregate size. Furthermore,
smaller and more compact
aggregates could result in
more significant flux decline
(Ang et al., 2011a).
BSA Protein Membrane properties had no
effect on long term flux
behavior, the latter was
mainly controlled by

foulant-deposited–foulant
interaction (Wang and Tang,
2011).
Transparent exopolymer
particles (TEP),
biopolymers,
proteinaceous compounds
NOM in seawater Concentrations of TEP and
proteinaceous compounds
were closed related to
membrane fouling levels
(Miyoshi et al., 2016).
571S. Jiang et al. / Science of the Total Environment 595 (2017)
567–583
through pretreatment technologies, then the remaining cells
could not
proliferate due to lack of food sources. Based on this principle,
Weinrich et al. (2016) investigated the relationship between
membrane
fouling rate and the content of assimilable organic carbon
(bacteria nu-
trient). By observing operational changes such as increased
differential
pressure and decreased permeate flux, they found that
membrane bio-
fouling was more serious when nutrient level was higher.
2.2. Organic fouling
Just as its name implies, organic fouling is caused by organic
matters.
These organic matters usually consist of humic substances,
polysaccha-

rides, proteins, lipids, nucleic acids and amino acids, organic
acids, and
cell components (Cho et al., 1999; Jeong et al., 2016). For
surface
water or seawater, natural organic matter (NOM) is often used
while
for wastewater effluent organic matter (EfOM) is often adopted
(Kim
and Dempsey, 2013). In wastewater treatment, organic fouling
is a
main problem in RO treatment because the EfOM concentration
(10–20 ppm) is much higher compared to typical NOM
concentration
in surface waters (2–5 ppm) (Malaeb and Ayoub, 2011).
Fig. 4 shows the cumulative number of publications related to
three
common RO organic foulants studied in the past 10 years. As
indicated
by Fig. 4, there are strong research interests in bovine serum
albumin
(BSA), alginate and humic acid as RO organic foulants. BSA is
a type of
protein while alginate is a typical representative of
polysaccharide.
The fouling mechanism of BSA for RO is different with other
membranes
such as MF. For example, BSA could deposit inside MF
membrane pores
and cause pore blocking as a result. As RO membrane is non-
porous, the
fouling behavior of BSA is different. BSA fouling of RO
usually occurs on
membrane surface, with the first step of depositing on the
surface via
foulant-surface interactions followed by BSA-BSA interactions,

the latter
of which could cause more BSA to deposit on membrane surface
and fi-
nally result in serious membrane fouling if no action (e.g.,
cleaning)
takes place (Ang and Elimelech, 2007). Furthermore, when
other pollut-
ants such as alginate exist together, BSA fouling could be
intensified due
to foulant-foulant interactions (Yu et al., 2012). The fouling
behaviors of
alginate and humic acid are similar to that of BSA.
Some representative work on RO membrane organic fouling
behav-
iors under different situations is summarized in Table 1. As
revealed by
Table 1, the contributions of different organic matters on RO
fouling
could be different in different situations, with one kind of
organic matter
being the dominant foulant in one situation but replaced by
another or-
ganic pollutant in another situation. However, one conclusion
that could
be safely drawn is that feed water chemistry, foulant-surface
interac-
tions as well as foulant–foulant interactions are three important
factors
Fig. 4. Cumulative number of publications related to three
common RO organic foulants
studied in the past 10 years.
affecting organic fouling. Organic fouling could result in
significant flux
decline of RO membranes and it is hard to eliminate due to the
complex

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