This document summarizes a presentation on flotation processes for wastewater treatment. It discusses how flotation works to separate solids from water by decreasing density. It also describes the components of a flotation system and challenges like rising sludge and bulking sludge. The author argues that using a high rate dissolved air flotation unit before sedimentation can help address these issues and improve effluent quality and plant efficiency compared to conventional activated sludge alone.

1. A PRESENTION BY
ABDULLAHI NAZIFI ABDULLAHI
(MS bioengineering student )
AT CYPRUS INTERNATIONAL
UNIVERSITY (CIU)
2014

2.  Introduction
 Flotation principal and process description
 Flotation Biological System
 Operational Difficulties and Remedy
 summary
 Conclusions
 References

3.  Flotation process (sometimes called flotation
separation) is a method of separation widely used
in the wastewater treatment and mineral
processing industries.
 Activated sludge consists of biological flocs that
are matrices of microorganisms, nonliving
organics, and inorganic substances.
 The microorganisms include bacteria, fungi,
protozoa, rotifers , viruses, and higher forms of
animals such as insect larvae, worms, and
crustacean.

4.  Dissolved air flotation is mainly used to float
suspended and colloidal solids by decreasing their
apparent density.
 The flotation system consists of eight major
components: Influent feed pump, air supply,
pressurizing pump , air dissolving tube(retention
tank),friction valve, flotation chamber , spiral
scoop, and effluent extraction pipe.
 Figures1 and 2 show a single cell and a double
cell, respectively, of high rate dissolved air flotation
clarifiers.

7.  The inlet, outlet, and sludge removal mechanisms
are contained in the central rotating section.
 In this section the spiral scoop rotate around the
tank at a speed synchronized with the flow.
 Unclarified water, first passing through an air
dissolving tube and a friction valve, is released
through a rotary joint in the center of the tank.
 It then passes into the distribution duct that moves
backward with the same velocity as the forward
incoming water.
 The settling and the flotation processes take place
in the quiescent state in the flotation chamber.

8.  The spiral scoop takes up the floated sludge,
pouring it into the stationary center section where
it is discharged by gravity for either recycling or
disposal.
 Clarified water is removed by effluent extraction
pipes, which are attached to the moving center
section.
 Wiper blades attached to the moving distribution
duct scrape the bottom and the sides of the tank
and discharge settled sludge into the built in sump
for periodic purging.
 The variable speed gear motor drives the rotating
elements and scoop.

9. Air dissolving tube

12.  Activated sludge is a continuous flow biological
treatment process characterized by the suspension
of aerobic microorganisms, maintained in a
relatively homogeneous state by the mixing and
turbulence induced by aeration.
 The microorganisms oxidize soluble and colloidal
organics to CO2 and H2O in the presence of
molecular oxygen.
 The process is generally but not always , preceded
by a primary sedimentation clarifier.

13.  The mixture of microorganisms and wastewater
formed in the aeration basins, called mixed liquor,
is transferred to gravity clarifiers for liquid solid
separation.
 The major portion of the microorganisms settling
out in the clarifiers can be recycled to the aeration
basins to be mixed with incoming wastewater,
while the excess, which constitutes the waste
sludge, is sent to the sludge handling facilities .
 The rate and concentration of activated sludge
returned to the aeration basins determines the
mixed liquor suspended solids (MLSS) level
developed and maintained in the basins.

14.  During the oxidation process, a certain amount of
the organic material is synthesized into new cells
 some of which then undergoes auto-oxidation
(self-oxidation, or endogenous respiration) in the
aeration basins, the remainder forming net growth
or excess sludge.
 Oxygen is required in the process to support the
oxidation and synthesis reactions.
 Volatile compounds are driven off to a certain
extent in the aeration process.
 Metals will also be partially removed and
accumulated in the sludge.

15.  Activated sludge systems are classified as high
rate, conventional, or extended aeration (low rate)
based on the organic loading.
 In the conventional activated sludge plant, the
wastewater is commonly aerated for a period of 4–
8h (based on average daily flow) in a plug flow
hydraulic mode.
 Either surface or submerged aeration systems can
be employed to transfer oxygen from air to
wastewater.

16.  The most common operational difficulties
encountered in the conventional activated sludge
treatment plant are rising sludge and bulking
sludge, resulting in high suspended solids and 5-
day BOD in the plant effluent.
 The common cause of rising sludge is biological
denitrification, in which nitrites and nitrates in the
wastewater are converted to nitrogen gas.
 When enough nitrogen gas is formed and trapped
in the sludge mass, the sludge in the conventional
secondary sedimentation clarifier becomes buoyant
and floats to the surface.

17.  This phenomenon is called biological flotation.
 Rising sludge can also be caused by internal solids
overloading and hydraulic overloading to
secondary sedimentation.
 Poor sedimentation clarifier design and operation
in terms of flow-through velocity, weir design, etc.
are also possible causes.
 Sludge bulking is another phenomenon that often
occurs in activated sludge plants whereby the
sludge occupies excessive volumes and will not
settle rapidly.

18.  There are two principal types of sludge bulking
problems:
 (a) The growth of filamentous organisms
 (b) The formation of swelling biological flocs
through the addition of bound water to the cells to
the extent that their density is reduced
 Possible causes of sludge bulking include the
following:
 (a) Absence of certain necessary trace elements in
wastewater

19.  (b) Wide fluctuations in wastewater pH
 (c) Limited dissolved oxygen in the aeration tank
 (d) Inadequate food-to-microorganism ratio (F/M)
 (e) Inadequate mean cell residence time (θc)
 (f) Inadequate return sludge pumping rate
 (g) Internal plant overloading
 (h) Poor sedimentation clarifier operation The
problems of sludge rising and sludge bulking,
when serious, cannot be overcome easily.

20.  If rising and bulking conditions continue to persist
after all the aforementioned factors have been
checked, a critical investigation of the behavior of
aeration basin and secondary sedimentation
clarifier should be made.
 It is very possible that the design is at fault, and
either changes or expansions must be made in the
facilities.

21.  In summary, although the present conventional
activated sludge process has been in use for many
decades, there is still a lot of room for
improvement.
 One big area of weakness of the conventional
process is the secondary sedimentation clarifier,
which gives low consistency sludge, shocks the
living biota by holding them for long periods in
anaerobic conditions, and has problems of sludge
rising and sludge bulking.

22.  The use of a high rate dissolved air flotation unit
in series before the final sedimentation clarifier
eliminates these deficiencies.
 The net results are lower solids loading to the
existing sedimentation clarifier, higher hydraulic
capacity and retention time of aeration tank, easier
concentration of waste sludge, more active recycled
sludge, better effluent quality and lower
wastewater treatment costs.
 Besides, there will be no sludge rising or sludge
bulking problems

23.  The flotation cell is a high rate DAF clarifier.
 The unit has minimum volume (less than 16in.
water depth), low cost and flexibility in application
due to its small size.
 It has been very successfully applied for in-
process and secondary flotation in industrial and
municipal applications.
 The flotation cell has the following advantages
when used to separate biological solids from
activated sludge mixed liquor:

24.  (a) Sludge consistency for the floated biological
sludge is about three times higher than
comparable settled sludge.
 This fact has been recognized for years in the use
of flotation in thickening wastewater sludges.
 (b) The floated sludge is an aerobic. There is far
less kill of the biological community due to
anaerobic shock.
 This has been demonstrated in field comparisons
using both floated and settled sludges.

25.  (c) The aerobic floated sludge is returned to the
aeration basin in 8–15min instead of several
hours for the conventional activated sludge
process using secondary sedimentation clarifier.
 (d) Clarified water from a properly operated and
sized flotation unit is comparable in quality to
settled effluent.
 This has been demonstrated in field pilot trials.
Where a settling unit already exists in the
treatment process, the practice is to undersize
the flotation cell and install it in series with the
existing settling unit.
 The flotation cell is then the workhorse or
“harvester” and the settling unit is the final
“polisher” for the effluent.

26.  The problems of high solids loading, high hydraulic
loading, sludge rising, and/or sludge bulking in
existing conventional activated sludge treatment
plant can then be eliminated.
 (e) The small size of the flotation cell greatly
simplifies the issues involved in the upgrading of
existing wastewater treatment plants.
 (f) Capital cost of flotation cells is lower than the
cost for conventional sedimentation clarifiers or
comparable basin expansion.
 (g) Stainless steel and prefabricated construction
increase the economic feasibility of flotation cells.

27. THANK YOU ALL

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