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1. NIRAS-HALCOM
Operations Manual
Duc Ninh Wastewater Treatment Plant
Quanh Binh URENCO
Do Khac Uan
March 2014

2. Table of Contents
GENERAL OBJECTIVES............................................................................................................5
Objectives.............................................................................................................................5
Structure of the manual......................................................................................................5
1. What goes in ........................................................................................................................6
1.1 Introduction ............................................................................................................6
2. What is an oxidation pond ? ............................................................................................10
2.1 What is an oxidation pond ?..............................................................................10
2.2 Why and where oxidation ponds should be used ?.......................................11
2.3 Who is responsible for what ?...........................................................................11
3. How do ponds work? .......................................................................................................12
3.1 Oxidation pond process description.................................................................12
3.2 Classification of wastewater treatment ponds................................................15
3.3 Use of ponds........................................................................................................16
3.4 The main advantages and disadvantages of stabilization ponds................17
4. What come out ? ...............................................................................................................18
4.1 Effluent characteristics .......................................................................................18
4.2 What affects pond effluent quality ....................................................................18
4.3 Upgrading the quality of oxidation pond effluents..........................................19
4.4 Some impacts of pond effluent on the environment......................................19
5. Practice and problems in process control .....................................................................20
5.1 Staging of ponds .................................................................................................20
5.2 Pond recirculation ...............................................................................................20
5.3 Pond mixing and aeration..................................................................................21
5.4 Odor problem.......................................................................................................22
5.5 Algae problem......................................................................................................23
5.6 Insect problem .....................................................................................................24
6. Maintenance.......................................................................................................................24
6.1 Litter control .........................................................................................................25
6.2 Accumulated material on the wave bands ......................................................25
6.3 Floating material control.....................................................................................26
6.4 Odour control.......................................................................................................26
6.5 Weed control........................................................................................................27

3. 6.6 Vegetable control on the banks ........................................................................28
6.7 Bank maintenance ..............................................................................................29
6.8 Control structure maintenance..........................................................................30
6.9 Seepage control ..................................................................................................30
6.10 Toxic material control .........................................................................................30
7. Monitoring and record keeping........................................................................................31
7.1 Introduction ..........................................................................................................31
7.2 Problems with BOD and SS testing .................................................................31
7.3 Where the sample should be taken from ........................................................32
7.4 Frequency and location of laboratory samples ..............................................32
8. Discharge consents and conditions................................................................................34
8.1 Odour treatment ..................................................................................................34
8.2 Wastewater treatment ........................................................................................34
8.3 Sludge disposal...................................................................................................35
9. Some case histories..........................................................................................................35
9.1 Septic tank cleaner problem..............................................................................35
9.2 Industrial overloading and excess grease problems .....................................35
9.3 Butynol rubber bank collapse............................................................................36
9.4 Concrete wave band cracking and breaking up by wave action erosion ...36
9.5 Fat/oil slick over the first or primary pond surface.........................................36

4. LIST OF ACRONYMS:
ASOP Administrative Standard Operating Procedure
CCESP Coastal Cities Environmental Sanitation Project
CDP Corporate Development Plan
D/S&WW Drainage, Sewerage and Wastewater
GIS Geographic Information System
GOV Government of Vietnam
HRD Human Resources Development
ICT Information and Communication Technology
IT Information Technology
LS Lift Station
O&M Operations and Maintenance
PD Project Director
PMU Project Management Unit
QB Quang Binh
QB URENCO Quang Binh Urban Environment Company
SOP Standard Operating Procedure
SW Solid Waste
TL Team Leader
TOR Terms of Reference
WB World Bank
WWTP Wastewater Treatment Plant

5. GENERAL OBJECTIVES
Objectives
The aim of this manual is to help staffs and operators of the Duc Ninh WWTP to:
 Understand how wastewater treatment ponds work and what factors affect
and control pond treatment processes
 Schedule and conduct normal and abnormal operational and maintenance
duties
 Collect samples, interpret lab results, and make proper adjustments in pond
operation
 Recognize factors that indicate a pond is not performing properly, identify the
source of the problem, and make corrective action
 Determine pond loadings
 Keep records for the wastewater treatment pond facility
Structure of the manual
The Operations Manual for Wastewater treatment has four parts:
 Part A : Wastewater Treatment Theory
 Part B: Operation and Maintenance of Duc Ninh Wastewater System
 Part C: Laboratory Procedures
 Part D: Equipment Suppliers’ Manuals and Instruction Books
This is Part A and it is a stand alone manual

6. PART A: WASTEWATER TREATMENT THEORY
1. What goes in
1.1 Introduction
This section introduces terms which are used in this manual for discussions of
wastewater characteristics and treatment technology.
 Sewerage: This is the system that removes, treat and disposes of sewage
 Sewage: This is domestic and industrial liquid waste matter.
 Wastewater: This term is an alternative for sewage, but is often used
nowadays to refer to a mixture of both domestic or foul sewage and trade
waste.
 A combined sewerage scheme: A sewerage scheme where both the foul
sewage and stormwater use the same piping system as occurs in many older
towns and cities.
 A separate sewerage scheme: The type of sewerage scheme nowadays
conventionally used where the foul sewage only is piped in the sewerage
system with a separate system to handle stormwater.
It should be noted that in Dong Hoi, the wastewater from households, service,
offices, public areas… is collected into the wastewater drainage system
through gas holes connected and then lead to main wastewater collecting
system to wastewater pumping station and then to treating station.
Therefore, wastewater is collected separately without being mixed with rain
water, which lessens the risk of polluting the water and environmental
pollution.
Sewage and effluent terminology: The most important sewage parameters are briefly
described below.
 SS: suspended solids which are removed from sewage sample by filtration
through a very fine filter. A certain portion of sewage solids will settle out
after a certain time in calm conditions, and these are defined as settleable
solids.

7.  Settleable solids: it is applied to that fraction of total solids in sewage which
will settle out in calm conditions within a defined period of time.
 BOD: biochemical oxygen demand, the standard measure of biodegradable
organic matter in sewage. Normally, references are to the standard five day
test.
 COD: chemical oxygen demand, a measurement of the oxygen consuming
capacity of organic matter present in wastewater.
 Total oils and grease: this is a non-specific term used to mean total grease
component of sewage which is recovered via a certain specified test
procedure
 Floatable oils and grease: this term describes that fraction of the total oils
and grease in a sample which will separate out by floatation in calm
conditions in a defined time period.
 pH: a measure of the acidity/alkalinity of wastewater, in a logarithmic scale
from 1 (very acid) to 14 (very alkaline). pH 7 is neutral, and domestic sewage
is approximately this.
 Faecal coliform (FC): a group of bacteria found in large numbers in the gut
and faeces of warm blooded animals, including man, and therefore used to
indicated the presence of sewage contamination and possible presence of
disease causing organisms. Esherichi Coli ( E,Coli ) a particular bacteria is now
uses as a more specific indicator of faecal contamination.
 Nutrients: In some discharge situations (rivers, lakes) the concentration of
plant nutrients in the effluent can have resultant detrimental consequences,
such as aquatic weed infestation, algal scums and slimes. Therefore the
concentrations of nitrate, phosphate and ammonia become of concern, and
may be subject of discharge limits.
 Toxic compounds: many industrial wastewater contain toxic compounds,
namely those inorganic or organic materials which would prove deleterious to
either biological sewage treatment processes, or the receiving environment
for the final effluent.
 Typical composition of raw sewage:
Raw domestic sewage typically contains the following:
BOD 250 – 300 mg/L
SS 200 – 300 mg/L
Faecal coliform 107
– 5x107
/ 100 mL
Oil and grease 50 – 100 mg/L
Terminology in sewage and wastewater treatment technology:

8.  Domestic sewage in a water-borne sewerage system is approximately 99.9%
water and 0.1% contaminants. Wastewater treatment is aimed at protecting
public health and the environment by removing and breaking down these
various contaminants The products of the large variety of sewage treatment
processes usually are a liquid effluent, solids or sludge, and possible gas.
 Treatment and disposal of the liquid are interrelated as the nature of the
environment receiving the treated effluent will, in part, dictate the degree of
treatment required. Selection of the most appropriate methods of treatment
and disposal involves consideration of:
o Water quality including public healthe issues, levels of toxicants and
pathogens, colour and clarity.
o Economic, financial and cost aspects.
o Environmental issues
o Community attitudes and cultural values
o Engineering and operational feasibility and reliability
o Relevant legislation
Sewage and wastewater treatment is conventionally categorised under the following
headings:
 Preliminary treatment: The removal of large solids, grit, and other
problematic material which might interfere with downstream processes and
damage equipment. Screening is usually considered as a preliminary
treatment process. At Dong Hoi, mechanical screening is used to to the
removal of coarse solids bigger than allowable distance between screen bars.
 Primary treatment: The removal of floating and settleable solids by a variety
of means, but normally by physical processes such as sedimentation and
floatation tanks. At Dong Hoi, the aerated grit chamber is used for 3
simultaneous purposes: (i) grit removal, (ii) grease and fatty solids separation
for raw influent, and (iii) odour stripping. This treatment step helps to protect
pumping equipment and to avoid the effects of solids in aerated lagoons.
 Secondary treatment: The removal of dissolved nutrients, organics matter
(BOD) and non-setteable solids by a variety of means, including biological and
chemical processes. The biological processes involve concentrated growth of
microorganisms either on fixed surfaces or in suspension. At Dong Hoi, the
organic pollutant in wastewater is treated in the aerated lagoons. The
facultative lagoons are followed for further removal of organic pollutant.

9.  Tertiary treatment: Further treatment to remove specific elements which
may be of concern (nutrients, trace materials, bacteria) by array of processes,
including disinfection. At Dong Hoi, nutrients (nitrogen and phosphorus)
removal is carried out by aquatic floating system (water hyacinthes) in the
maturation pond. Water hyacinth (Eichhornia crassipes) is widely distributed
in VietNam’s rivers.
 Disinfection: A process designed to reduce the number of microorganism in a
particular effluent, including use of chlorination, UV light, ozone. It is usual to
refer to a disinfected effluent. At Dong Hoi, solar disinfection is carried out a
maturation pond. This is a natural disinfection way then it can avoid expensive
use of UV-radiation and harmful by-products from chlorine disinfection.
 Land treatment: The treatment of sewage effluent on the land. This usually
involves things such as overland flow of effluent over grassed fields, or
wetlands treatment systems. In case of Dong Hoi, only aquatic floating system
(water hyacinthes) in the maturation pond is used.
 Land-based treatment: A common term but not really defined. Generally
means any form of treatment which includes tanks, facilities on land.
 Marine treatment: The use of the marine environment to treat and dilute
sewage by natural processes to harmless meets the acceptable levels. This
treatment system also includes the final disposal of the effluent. This does not
apply to Dong Hoi wastewater treatment.
 Effluent disposal: Is the discharging of the treated effluent from any
treatment process, either to a receiving water such as a river, lake or ocean,
or onto land. At Dong Hoi, the treated wastewater is discharged into the Le Ky
river, then to the East Sea eventually.
 Sludge: Sludge is the by product which is drawn off the bottom when the
liquid in a treatmet stage is allowed to settle. Various processes can then be
used to stabilise and/or dewater the sludes prior to reuse or disposal. At Dong
Hoi, sludge from the wastewater treatment plant is dewatered in the reed
bed system, then can be reused as a rich fertilizer.
 Sewage flows: The flow of sewage into a plant or pond is not constant.
During the night and early morning, flows are very low, and in smaller communities
are made up largely of flushing urinals, groundwater infiltration into the sewers, and
the occasional results of a late night party.Flows start increasing as people wake up,
having breakfast and commence work.
A general pattern of sewage flow over a 24 hour day is shown in the following
diagram. The average daily flow is the flow used for designing sewage treatment
processes, although pipes, tanks etcmust be able to cope with peak flow.

10. Fig. 1. A diagram of sewage flow over a 24 hour day
2. What is an oxidation pond ?
2.1 What is an oxidation pond ?
Oxidation ponds are shallow basins which utilise light and wind action in to allow
natural processes to break down and treat the polluting components of sewage.
Ponds are ideally rectangular or square in shape and are located in open, windy
areas. The inlet and outlet of a pond should be located at opposite sides of corners of
the pond, with the prevailing wind not blowing towards the outlet.
There are various layouts for pond systems:
 A single pond receiving raw sewage. Such a pond provides primary and
secondary treatment, and a significant kill of decrease causing microorganism.
 A single pond can also be used as the final stage in a treatment system,
following conventional in tank processes (primary settlement of solids,
secondary biological treatment). In these instances the pond provided
enhanced treatment and breakdown of organic matter, and better kill of
microorganisms. There are often called tertiary ponds.
 A conventional 2-pond system, where the effluent from one pond enters a
second pond. The idea behind such systems is to provide a major reduction in
numbers of di disease causing microorganism, particularly where the effluent
is to be discharged into well used or sensitive receiving waters.
At Dong Hoi, the oxidation pond is used as the aerated lagoon. The system
includes two aerated lagoons run in parallel.
Hourly fluctuations of flowrate during a 24-h period
for a total of 10 000 m3
/d
0
100
200
300
400
500
600
700
800
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Hours
m3/h

11. 2.2 Why and where oxidation ponds should be used ?
Domestic or municipal and some industrial wastewater may be treated in oxidation
ponds which also can be called stabilisation ponds or lagoons.
Ponds are classified according to the type of bacteria working in them. Treatment in
wastewater in ponds is considered to be inexpensive, straightforward in design,
construction and for maintenance. If maintained properly, they provide reasonably
efficient treatment of wastewater.
One reason why ponds are readily accepted by all concerned, is the belief that the
ponds need little or no operation or maintenance and that they simply go on
forever producing a good quality effluent. Unfortunately this is not the case.
Actually, pond operation may be as difficult as any other form of wastewater
treatment to manage. A poorly operated and maintained pond can become a public
nuisance, producing effluent of low quality.
2.3 Who is responsible for what ?
The operation of the wastewater treatment system is the responsibility of the Owner
as well as the Operator. At Dong Hoi management of the wastewater treatment
system is the responsibility of Quang Binh URENCO under the The Decision no.
1136/QD - UBND dated May 28, 2009 of Quang Binh PPC. Quang Binh URENCO has
establised the Dong Hoi City Environment Sanitation Project Acceptance and
Operation and Management Unit (Decision No. 245/QD-CT, dated November 15,
2013) which has one of the duties to organize for management and operation of Duc
Ninh WWTP meet to the requirements of environment for wastewater after
treatment (TCVN 7222-2002, column B).
URENCO as the Operator is responsible for:
 The effluent and proper operation of the ponds to meet the effluent qualities
stipulated by the regulatory agencies
 Maintaining a safe and healthy environment
 Performing tests and making observations needed for the proper operation of
the ponds
 Properly interpreting and applying laboratory tests and results
 Notifying the Owner far enough in advance so that tools, parts and supplies
will be available when needed
 Keeping the drainage course or pipe to the receiving stream or other location
clear of observations

12.  Keeping records
 Meeting all other requirements of the Contract
3. How do ponds work?
3.1 Oxidation pond process description
The oxidation pond is a self-contained, complete treatment process where everything
that is needed for waste stabilisation is provided by nature. Therefore, this form of
wastewater treatment can be called as natural treatment.
Oxidation pond is one of the simplest forms of biological treatment processes. This
versatile installation serves many basic purposes, including:
 storage or impoundment of wastewater
 settling and removal of suspended solids
 storage or impoundment of settled solids
 equalization
 aeration
 biological treatment
 evaporation
The relative simplicity and low operating costs of a stabilization pond make it the
preferred technology for handling, treatment and disposal of industrial wastewater
as well as municipalwastewater for small communities.
In oxidation ponds bacteria use the organic matter in the wastewater as their food.
There are three types of bacteria, aerobic, anaerobic and facultative, at work in this
type of pond. Aerobic bacteria can only work in the presence of dissolved oxygen,
whereas anaerobic bacteria are actually killed by dissolved oxygen. Facultative
bacteria can operate in both areas.
In the oxidation pond oxygen must be provided for the aerobic bacteria. Nature helps
to do this in two ways: photosynthesis and pond re-aeration.
Photosynthesis is the use of sunlight energy by small green plants called algae to
convert carbon dioxide gas and water into new cells. As the algae produce new cells,
they release oxygen as a by-product. This oxygen dissolves in the water, and is used
by the bacteria to decompose the waste.

13. This all means that bacteria and algae work together in a waste stabilisation pond.
The algae produce the oxygen needed by the bacteria, and the bacteria in turn
produce carbon dioxide and other things needed by the algae.
Aerobic bacteria need dissolved oxygen to decompose organic waste. As aerobic
decomposition takes place, carbon dioxide as well as new bacteria are produced. If
there is sunlight present, then algae can use this carbon dioxide to live and
reproduce.
As the algae grow, oxygen is produced. It’s a cycle. Algae produce oxygen which is
used by the bacteria to decompose waste. The bacteria produce carbon dioxide,
which is used by the algae to produce more carbon dioxide and so on.
The second way of providing oxygen to bacteria is pond re-aeration. Aeration occurs
as a result of turbulence caused by the wind. The amount of oxygen added by the
surface re-aeration depends on the amount of wind there is another condition
affecting surface re-aeration. Oxygen dissolves more quickly in colder water than in
warmer water. Temperature of wastewater determines how much oxygen is
dissolved in it.

14. Fig. 2. Transfer coefficients as functions of various powers of wind velocity
The various, complex processes which occur in a pond are shown in the next
diagrams.
Fig. 3. Processes occur in the oxidation pond
Fig. 4. Ecology and major reactions of wastewater treatment ponds

15. Fig. 5. Basic biological reaction in an oxidation pond
3.2 Classification of wastewater treatment ponds
According to the type of biological transformation and methods of oxygen supply,
ponds can be divided into four general classes.
1. Anaerobic Ponds
These are deep ponds where anoxic condition prevails throughout. Organic loadings
are very high and BOD removal is limited to 80% or below. Further treatment of the
anaerobic pond effluent by aerobic ponds is usually required.
2. Facultative Ponds
Facultative ponds receive medium to low organic loadings. The bottom layer is
usually anaerobic, but the surface layer is kept aerobic through photosynthesis and
surface reaeration. BOD removal is higher than that of anaerobic ponds.
3. Aerated Lagoons

16. Oxygen supply in aerated lagoons for aerobic stabilization relies almost completely
on mechanical aeration devices. Either air diffusion or mechanical aeration can be
used. Depending on the power level used for aeration, aerated lagoons can be
further classified as aerobic and aerobic-anaerobic lagoons. With power levels at
6W/m3
or above, ponds are usually aerobic throughout. At lower power levels, an
anoxic bottom layer can be expected. An aerobic-anaerobic lagoon is therefore
equivalent to a facultative pond, except that the former uses mechanical aeration for
oxygen supply. Aerated lagoons usually are deeper than facultative ponds and
receive medium to high organic loadings. Treatment efficiency can be very high. The
mixing characteristic that separates aerobic (near complete mix) to aerobic-anaerobic
(poorly mixed) is more dependent upon the pond geometric parameters than upon
operating parameters such as horsepower input.
So in summary for Dong Hoi, a series of pond has been used. The system includes two
aerated lagoons (the aeration provided by the air blower) for removal of high organic
loadings, two facultative lagoons for removal of low organic loadings, and a
maturation pond for solar disinfection.
3.3 Use of ponds
Shallow ponds are often used to treat wastewater instead of conventional
wastewater treatment processes. Table 1 present the purpose of each pond parts.
When discharged in to ponds, wastewater is treated by several natural processes
acting at the same time.
Table 1. Purpose of pond parts
Parts Purpose
Flowmeter Measures and records flows into pond
Bar screen Remove coarse material from pond influent
Pond inlets Distribute influent in pond
Pond depth and outlet
control
Regulates outflow from pond and depth of
water in pond. Allows pond to be drained for
cleaning and inspection.
Outlet baffle Prevent scum and other surface debris from
flowing to next pond or receiving waters.
Dike or levee Separates ponds and holds wastewater being
treating in ponds
Transfer line Conveys wastewater from one pond to
another
Recirculation line Returns pond effluent rich in algae and oxygen
from second pond to first pond for seeding,
dilution and process control
Chlorination Applied chlorine to treated wastewater for
disinfection purpose
Effluent line Conveys treated wastewater to receiving
waters, to point of reuse (irrigation), or to land
disposal site or to receiving water

17. 3.4 The main advantages and disadvantages of stabilization ponds
Advantages
 Do not require expensive equipment
 Do not require highly trained operating personnel
 Is economical to construct
 Provides treatment that is equal or superior to some conventional processes
 Is a satisfactory method of treating wastewater on a temporary basis
 Is adaptable to changing loads
 Has few sludge handling and disposal problems
 Low operational and maintenance cost
 Works well in clay soils where conventional subsurface on-site absorption fields
will not work
Disadvantages
 Lagoons must be constructed in clay soil or be lined to prevent leakage.
 May overflow occasionally during extended periods of heavy rainfall
 If there are extended periods of overcast windless days, offensive odors may
occur for a brief time but usually recover rapidly if this occurs.
 Takes up a relatively large space
 Lagoons are not aesthetically acceptable to some people. Some people consider
lagoons unsightly and unsafe.
 As with any other open body of water, there is some potential danger. Although
ponds should be fenced, this does not always prevent access by people or pets.

18. 4. What come out ?
4.1 Effluent characteristics
The reference data showed the following pond effluent data. In general, 2-pond
systems produce better effluent quality than for single pond systems.
Table 2. Reference pond effluent data
Average 90% of samples
Temperature, o
C 15 6 – 24.5
pH 8.2 7.1 – 9.5
DO, mg/L 6.7 0.9 – 9.5
BOD, mg/L 25 7 – 70
SS, mg/L 47 6 – 171
Ammonia, mg/L 3.1 0.001 – 29
Faecal coliform / 100 mL 5500 90-230000
It should be noted that Dong Hoi City is located in the tropical area in the Central
region of Viet Nam. There are two main seasons: winter (from October to March) and
summer (from April to August). In winter, there is more rainfall than in other regions
in Northern Viet Nam. Summer is characterized by hot and dry weather. Frequently
there are Southwest winds flowing from Lao country and increasing hard living
conditions for peasants. It is considered as one of the most severe region in Viet
Nam.
Based on the statistic data from Quang Binh Meteorology Center, the yearly average
temperature was about 25.1o
C (Highest: 40.1o
C, and Lowest: 7. 8o
C). Therefore, the
pond effluent data could refer from the data in the column of 90% sample in the
above Table 2.
4.2 What affects pond effluent quality
 Amount of sunlight
 Wind
 Amount of BOD entering the pond
 Amount of water entering the pond system (retention time)
 Temperature (season)
 Any toxic chemicals entering the pond system
 Excess oil and grease

19.  Pond shape
 Nature: range of bacteria, algae, plants, animals in the pond
Some of these factors can cause problems, either by markedly reducing effluent
quality, causing odours, or even killing the pond’s biological processes.
4.3 Upgrading the quality of oxidation pond effluents
The algae-laden nature of oxidation pond effluents and their associated suspended
solids and BOD, much of which is associated with algae itself, does on occasions
cause problems with the effluent discharge into small watercourses, streams and
rivers where the algae itself exerts a BOD in the receiving water.
The above issues may result in the removal of some of the algae to give a clearer
effluent with lower BOD and SS. Also, there is an increasing need in a number of
situations to further reduce the possible levels of disease-causing microorganisms in
the effluent being discharged.
The above has resulted in increasing attention being paid to methods for upgrading,
or polishing, pond effluents. There are a number of methods available and each has
its own particular features that can apply in any particular instance.
These methods for upgrading oxidation pond effluents include such procedures as:
 Re-arrangement of pond layouts to include more cells, longer travel paths
 Various introduced aeration techniques
 Rock and other types of effluent filters
 Land treatment methods including overland flow and wetlands
 Land disposal methods
 Additional disinfection techniques
 Various combinations of the above and other methods
4.4 Some impacts of pond effluent on the environment
So how does an oxidation pond effluent effect the environment? As mentioned
above, if dilution in a waterway receiving the effluent is not great enough, the BOD
and SS of the effluent might enrich the stream unacceptably, leading to growth of
nuisance algae and slimes, and possibly serious effects on stream life. Water colour
and clarity might also be affected for some considerable distance, well beyond what
could be described safely as a zone of reasonable mixing.

20. Levels of ammonia in pond effluents can be high, requiring far greater dilutions on
order to prevent adverse effects on aquatic life.
Proper design, operation and maintenance will ensure that these potential problems
do not occur.
5. Practice and problems in process control
Although stabilization ponds are simple devices for wastewater treatment, the basic
objectives should be upheld for providing an effective treatment according to design
specifications and not creating objectionable conditions or public health hazards in
the vicinity or downstream of the receiving waters. Common practices in process
control and some problems associated with them are discussed in this section.
Remedies for the control problems are also included.
5.1 Staging of ponds
A common practice in stabilization ponds with mixing is to fractionate the total
design volume to smaller ponds. The validity of belief in increasing the treatment
efficiency by the staging of ponds has been examined previously in this chapter.
Staging of ponds also is not an economical mode of land use because a single pond
occupies less land area per unit volume.
In addition, the staging of ponds adds to the construction costs in excavation,
interconnecting pipes or channels, and requires more maintenance.
Despite the disadvantage of pond fractionation, it adds to the operational flexibility
by recirculation, minimizes upset and nuisance, and maximizes stability. This is a
trade off between a less expensive, simple-in-operation unit and a more expensive,
reliable, sophisticated pond system. The decision usually is in favor of staging the
pond if the system is designed for:
 efficient treatment of raw or primary wastewater to produce an effluent
quality compatible with other types of secondary treatment process, and
 flexibility in being able to switch operations from series to parallel or their
combinations so that various organic and hydraulic loadings can be
accommodated as seasonal load changes.
5.2 Pond recirculation
Recirculation can be practiced for a single pond or multiple-pond system. It dilutes
the wastewater influent and incorporates some photosynthetic oxygen to the feed

21. zone so that odors and anaerobic conditions can be avoided. For n number of ponds
in a series, the dilutioncan be expressed as:
in which Sd is the substrate concentration in the diluted wastewater entering the first
pond, So is the influent substrate concentration, r is the recycle ratio or the ratio of
recycled flow rate R to influent flow rate Q, and S is the effluent substrate
concentration from the nth pond. As r increases, Sd becomes smaller and the
incoming organic load is spread more evenly throughout the pond system. The
overloading of the first eliminated completely. Because the design incorporates low
head losses in interconnecting pipes and channels, recirculation can be accomplished
with high volume, low-head propeller pumps. Siphon discharge flap gates can be
recommended. An auxiliary pump with an air eductor maintains the siphon. Siphon
breaks are provided to insure positive backflow protection. Multiple or variable-
speed pumps can be used to adjust the recirculation rate to seasonal load changes.
5.3 Pond mixing and aeration
Mixing and aeration are accomplished concurrently when compressed air diffusers or
mechanical aerators are used. In high-rate aerobic ponds in which intermittent
mixing is recommended to eliminate a sludge build-up at the bottom, only a high-
volume, low-head propeller pump is needed because aeration is not necessary in the
pond operation.
Where aeration is of prime consideration, diffused aeration is used in ponds deeper
than 3 m. The advantage of diffused aeration is that air can be distributed evenly
over the entire pond of any geometry. It is preferable to mechanical aeration in
northern areas, because ice build-up in the latter might be excessive. Proprietary
devices have been developed to introduce compressed air through slits in piping laid
on the pond bottom. Bubble-gun aerators also combine good mixing characteristics
with aeration. Diffused aeration is also advantageous in that tapered aeration can be
incorporated easily so that air supply can be more efficiently used to meet the
oxygen demand at different locations of the pond.
Mechanical aerators are generally divided into two types: cage aerators and the more
common turbine and vertical-shaft aerators. The former are suitable for shallow
ponds of 1.5 m depth or less, while the latter require a minimum depth for
economical use that depends on the horsepower of the unit. The cage aerator
appears to have a greater pumping capacity than the propeller aerator, because it
does not recycle much the volume pumped.
Mechanical aerators can be installed on platforms in a fixed position or the floating
type can be mounted out in the pond, and spaced about to provide even distribution.
Floating cage aerators may be mounted either in the pond or directly off the dike
slopes. When mounted off the dike slopes, they can be close to the pond inlets.
Better oxygen supply on the inlet side and easy access for maintenance and repair

22. are the advantages of such arrangements. Oxygen saturation in tap water at a given
temperature and altitude, as given in Table 3.
Table 3. Solubility of oxygen (mg/L) at various temperatures and elevations
5.4 Odor problem
It was mentioned that anaerobic zones in stabilization ponds can generate odorous
compounds such as H2S and mercaptans that escape the pond. The oxygenated upper
layer normally serves as a protective shield in keeping these odorous end products of
anaerobic decomposition from escaping the pond. This is because the compounds
are chemically or biochemically oxidized and rendered innocuous before they reach
the surface. During periods of upset, i.e., when high H2S concentration exists, algal
growth and the associated oxygen production may be inhibited. Odorous compounds
may reach the atmosphere as a result.
Other sources of odor are the end-products of the metabolism of certain blue-green
algae, diatoms and pigmented flagellates, and also the oil products released by dead
algal cells. These sources are highly unpredictable and generally are not considered a
major odor problem.
The most common method used to evaluate odour nuisances is an odour panel,
includes a group of people, typically eight or more, divided equally between men and
women. Samples of odourous gas are collected, diluted several times, and delivered
to the odour panel for sniffing. The average person can report the presence or
absence of an odour with more certainty than its characteristics or objectionability.

23. Table 4 presents specific odourous compounds, their odour threshold numbers and
characteristic smells.
Table 4 - Typical odourous compounds, odour threshold concentrations
andcharacteristic smells
5.5 Algae problem
The suspended solids concentrations in the ponds effluent could be higher than those
of activated sludge or trickling filter processes unless a physical solid-separation step
is included in the pond system. Because the discharge of suspended solids
contributes BOD and nutrients to receiving waters, a reasonable amount of solids
removal from the effluent is desirable and, in fact, is required by regulations for high-
quality receiving waters. Algal cells in the effluent constitute only a part of the
suspended solids. Even in high-rate aerobic ponds, algal production is only 50% to
60% of the effluent’s suspended solids. Any physical or chemical separation process
will remove both algal cells and other microbial solids.
Many processes have been investigated for the removal of algal cells from suspended
solutions. Algal cell could be removed by using ion exchange columns or sand
filtration. These processes are not economical. It would be advisable to find a market
for algae harvested from stabilization ponds. The harvested algae can be used as a
livestock feed because it has high nutrient and protein content, and it is reported to
be highly acceptable to livestock. Another potential use of the cultivated algae is to
produce biomass to be used for distilling alcohol fuels, or producing methane gas.

24. 5.6 Insect problem
With improper control or lack of consideration in design, stabilization pond
environments often become a nuisance and public health hazard. Of primary
importance are the breeding places that ponds and their effluents can provide for
mosquitoes. Many of the predominant species of mosquitoes found in ponds are the
primary vectors of encephalitic diseases.
Their breeding takes place in any pond that provides a protected area for oviposition.
Egg rafts and larvae can be found along the shoreline in vegetation, in water-filled
gullies created by wheels of mowing equipment at the water’s edge, in some
overflow structure, in floating sludge that is allowed to accumulate in corners.
Overflow boxes also harbor mosquito egg rafts and larvae because when water is not
flowing through such boxes they provide a quiet place for oviposition. Emergent
vegetation also provides a site for oviposition. Mosquito breeding is extremely heavy
where overflow water is allowed to drain across a flat area before reaching a
drainage ditch. The swampy areas with vegetation are ideal places for breeding by
providing cover and food. Screens on effluent boxes, careful construction, and proper
maintenance can eliminate all of these problems.
Certain midges breed prolifically in stabilization ponds, with adults becoming a
nuisance around residential areas because of their intolerable numbers. The problem
is one of nuisance rather than of health hazard, although mechanical transmission of
pathogen by midges is also possible. Emergent vegetation also has significant
influence on the occurrence of many aquatic insects other than mosquitoes. The
most obvious is the increase in the number of beetle larvae and adults, dragonfly
naiads, and Hemiptera. Near shore and shallower water where sponge-like algal mats
are found are places showing dense populations of midge larvae. At a distance
greater than 1.5 to 2.4 m from shore and about 0.75 m in depth these algal mats and
the dense populations of midges associated with them are seldom found. An
effective control of mosquito breeding therefore is also effective for other insects.
These problems could be appeared during operation of the Duc Ninh Wastewater
treatment plant. Each of these could affect the performance of the system. Besides,
they can affect to the operator and population around (i.e. insect, mosquito).
Therefore, checking and maintenance during operation must be done frequently.
6. Maintenance
Proper maintenance of your pond system is essential for efficient treatment.
To properly maintain the pond, the following factors should be concerned:
 Litter control
 Accumulated material on the wave bands

25.  Floating material control
 Odour control
 Weed control
 Vegetable control on the banks
 Bank maintenance
 Control structure maintenance
 Seepage control
 Toxic material control
6.1 Litter control
Domestic sewage has significant amount of plastic and other litter type material that
can cause littering problems around oxidation ponds if it has not been previously
removed by appropriate screens.
Therefore, the raw sewage should be screened out before sewage enters the
oxidation ponds. Once in an oxidation pond, much of this plastic material which floats
or is neutrally buoyant accumulated on the wave band or banks. It is often
concentrated in certain areas or corners of the pond by the wind patterns on the
pond. Once on the wave band it often dries particularly if the top water level of the
pond varies.
6.2 Accumulated material on the wave bands
Accumulated material also includes fat and grease deposits, any floating accumulated
sludge materials and windblown vegetation and other materials.
Methods of periodically cleaning will depend on the degrees of accumulation, the
size of the pond and the equipment that may be available.
Options include hand clearing, hosing back into the pond to accumulate on the
bottom or lower down the bank below the water level, removal by mechanical
means.
If there is a significant build up of material below the water level on the wave bands,
then lowering the pond water level and letting the material dry and the removing by
an excavator with a swamp bucket has been shown to be an effective methods of
removing this material.

26. 6.3 Floating material control
This should not occur in a normally well operated pond but from time to time it does.
Often floating sludge will break up itself and resettle. Other methods include hosing it
to break up if it is near the banks, or mixing the pond by boat to get more water
movement that will help it break up and sink.
Mechanical floating or fixed aerators, stirrers of bubble aeration also assists by more
movement of the pond liquid.
6.4 Odour control
Ponds may occasionally five off odours, no matter how well operated and
maintained. Bad odours are an indication that something is out of balance and you
should determine and correct the causes of the odours.
Usually odours are caused by:
 Anaerobic conditions in the pond
 By the growth of the blue-green algae.
Either of these conditions could be caused by:
 Extensive cloud cover, which reduces the amount of sunlight and therefore
the amount of dissolved oxygen;
 Toxic waste that killed a portion or all of the bacteria;
 A sudden large inflow of high strength organic waste; and
 Floating sludge mats.
Any one of these conditions can upset the balance between bacteria, oxygen, algae
and food and should be corrected.
To remove odours, there are two methods which could be used:
The first method is to add oxygen to the pond by mechanical methods by:
 Using a portable pump, to spray the pond contents into the air
 Installing portable aeration equipment in the ponds
 Recirculating secondary effluent if it has a higher DO

27.  Using a motorized boat to stir up and aerate the top layer of the pond
The second method concerning odour problems is a chemical method:
 The chemical method for correcting odour problems uses sodium nitrate,
which is a source of oxygen. This chemical can be added in quantities of 5 to
15% by weight of the influent BOD.
 If the ponds are small, spread the nitrate evenly throughout the pond by
scattering it in all directions from the edges of ponds, for very large ponds use
a boat to reach the central areas of the pond.
 Break up any floating scum, septic sludge or blue-green algae mats. Repeated
attempts to break up the scum may be necessary because the mat may tend
to reform. If it is possible, it may be enough to let the cell rest by cutting of
any flow to that cell until it corrects itself.
 An algae bloom is a sudden and extreme algal growth. It consists mostly of
blue-green algae. The growth of blue-green algae may be caused by pond
overloading, lack of sunlight in the wastewater, pond turnover, shutdown or
other reason. The blue-green algae give off a characteristics odour. However,
most of the odour is caused by the ‘just as sudden’ die-off of the bloom. This
large amount of dead algae makes a very high BOD loading on the pond. As a
result, the dissolved oxygen is used up, the pond becomes anaerobic and
odour problems are created.
6.5 Weed control
Plants around of in the pond cause problems. Therefore, the control of weeds is one
of the main tasks in pond maintenance.
Some plants have large roots that will puncture the seal and cause leakage problems.
Plants that grow to the surface promote concentration of scum and sludge mats and
therefore encourage insect breeding. If the plants are dense in the pond, the sunlight
will not penetrate deep enough and the DO in the top layer will not be mixed or
distributed by wind action. This will result in a low DO level.
Surface weeds, like duckweed, can cause problems. Although very often confused
with algae, it is actually a small three-leaved plant that looks like clover leaf.
Duckweed floats on the wastewater surface and has long hair-like roots hanging
down into the water. It is green during its normal growth periods, and will turn
brownish-yellow as it dies off. Duckweed grows in large amounts and rapidly. It will
completely cover the wastewater surface in the pond if not controlled. This would
reduce the amounts of sunlight received by the pond and consequently reduce the
growth of the algae and the production of DO in the pond.

28. Besides reducing sunlight, duckweed poses a threat when it dies off. It will cause a
high oxygen demand in the pond in the same way as it would when being killed off by
herbicides.
There are a number of ways to control duckweed and other surface weeds.
 If the area of coverage is not extensive, a few ducks may be able to control
the growth. Ducks will eat duckweed.
 If the pond is exposed to a clean sweep of the wind, duckweed usually will not
flourish.
 On small systems, the operator can skim off the duckweed.
Duckweed removed from the pond should be buried to prevent odour and insect
problems.
Submerged or bottom-roots weeds can also cause problems. Generally, bottom-
rooted water weeds should not be a problem, as long as the wastewater depth in the
pond is not lower than 0.6 m during the summer. If the water level has to be lowered
below this level, the ponds should be continuously inspected for plant growth that
may be developing.
There will be a constant cause damage to the pond seal, reduce mixing in the cell,
and possibly reduce algae activity. There are several ways to control plant growth.
The removal method must be adjusted in terms of the amount of plant growth and
the time of year.
 Pull the weeds out. When the weeds are young, they should be pulled out
before they become established.
 Cut and drown the weeds. It the water level in the pond is low or can be
lowered, the weeds can be cut off. Immediately after they are cut, the water
level should be raised about 0.8 - 1.0 m above the top of the cut weeds.
6.6 Vegetable control on the banks
Banks usually have a protective grass cover and some sort of wave band. Weeds
growing between racks in the concrete, or holes in the rubber, can de-stabilise the
wave band. Weeds on the banks can be removed by:
 Hand or mechanically
 Using chemicals
Removing the plants by hand or mechanically involves much more work but it is safer
for operator and the biological life in the pond and the receiving stream.

29. 6.7 Bank maintenance
Banks are constructed around the ponds to hold back the water and to allow access
to the pond for maintenance. The most important part in bank maintenance is to
make sure that they continue to hold back the water and do not develop any leaks.
Bank erosion can quickly damage the seal and cause wastewater to leak out. Erosion
of the bank is caused by:
 Wave action
 Surface run-off
 Burrowing animals
Therefore, a regular inspection and maintenance routine must be kept to prevent
erosion of the banks. Certain areas are especially weak and easily affected by erosion.
Such areas include: around the control structures, in the corners, entrances to animal
burrows, the side slopes opposite the prevailing winds, are with insufficient
vegetation cover or with insufficient compaction of the bank during construction.
Erosion problems are generally more serious on large ponds due to increase wave
action.
If the action of wind and wave is likely to cause erosion of the bank’s inner slopes,
some form of wave band must be applied to prevent further damage. This is
necessary because once the erosion has begun, the entire bank can fail very quickly.
The wave band may be:
 Concrete
 Any other suitable and available material such as rubber or synthetic line
Do not use asphalt material, which weakens after a few year and has to be replaced,
or any other material that breaks up easily.
In addition, a good well mowed grass cover must be kept on all bank surfaces.
Erosion may cause the following damage:
 Wash away the parent or liner material
 Form areas along the bank that are difficult for mainetance
All of these problems reduce the design life of the ponds. A properly applied wave
band will stop erosion, reduce the growth of large plants along the shallow area of
the bank and discourage animals from tunnelling into the bank.

30. 6.8 Control structure maintenance
Pay attention to the control structures at all times, make sure that they function
properly. Leakage and corrosion are the main causes that affect the control
structures.
Corrosion problems can be reduced by frequently lubrication of gates and vales and
by moving the gates and valves frequently. One method to reduce corrosion
problems is to replace the solid manhole covers on the structures with grated covers.
This will increase ventilation and greatly reduce the corrosion.
Leaks in structures are usually difficult to correct. Gaskets or sewer plugs can be used
to stop the leak. If necessary preplace the gates, slides or valves to eliminate
problems.
6.9 Seepage control
Inability to maintain adequate wastewater level in the pond is a great problem. Most
ponds have natural clay, bentonite or vinyl layers to prevent pond seepage. Usually a
percolation test of the pond bottom is done as part of the design and construction,
but if leakage occurs, it is unwise to believe that wastewater solids will seal the pond.
Wastewater affects the surrounding land, rivers, lakes and ground-water, causing
potential health hazards and pollution problems.
When considering the seepage problem, take into account the losses of water due to
evaporation first, then investigate for leakage. Seal damage can be caused by erosion,
roots, burrowing animals, heavy equipment, cattle and, it the ponds are allowed to
dry, by cracks.
Proper seal maintenance consists of preventive measures and of correcting existing
problems. It involves total care. The most common seal material used is natural clay.
Normally, the finished clay seal is at least 0.15 m thick. Membrane liners need proper
bedding material and a proper cover material. The cover material protects the liner
and at the same time anchors the liner down. Exposed liners are easily damaged by
animals, equipment, vandals and sunlight.
6.10 Toxic material control
The problem of toxic material in the influent can not be solved in the pond itself. Any
discharges to the sewage system are best controlled by eliminating them at their
source. Therefore, it is important to identify any waste source that may be
discharging toxic wastes.

31. 7. Monitoring and record keeping
7.1 Introduction
Operation of oxidation ponds requires regular laboratory tests to ensure proper
performance and to satisfy the requirements of the regulatory agency. At Dong Hoi, a
labroratory at the WWTP with basic analysis equipments will carry out these tests.
Each pond system is different. Even identical adjacent ponds receiving the same
amount often have different values of pH and the dissolved oxygen content at any
given time. For this reason, sampling and testing for each pond should be done
separately. Once the characteristics of each pond has been know, the performance of
the pond will be judged.
In a properly functioning facultative pond, the pH and DO values will be different
from season to season, day to day and hour to hour. There will also be a decline in
DO from the surface of the pond to the bottom. The DO level will be lowest in the
anaerobic region at the bottom. If the DO drops toward zero throughout the pond
and the pH drops below 7, this is an indication that algae are dying off and the pond
is going anaerobic. Measurement of the pond can warn of impending problems
because the middle DO will decrease before the surface DO.
This condition will frequently be indicated by the change in colour of wastewater
from green to grey. Sometimes odours develop too. Therefore, two tests, pH and DO,
are the most important laboratory tests for evaluation of ponds performance and
their operational control. Both tests can be performed with inexpensive colorimetric
test kits or more modern pH and DO meters with probes.
Other tests, important in judging pond performance, are the biochemical oxygen
demand (BOD), total suspended solids (SS), and coliform counts. However, these
tests are more difficult and time consuming, they also require some extensive
laboratory equipment which is not normally provided in small pond installations.
7.2 Problems with BOD and SS testing
The BOD and SS tests are two of the tests often used to monitor performance of an
oxidation pond. However, there is a major problem with these tests which must be
recognised, that is the tests do not distinguish between sewage BOD/SS and algae.
The effluent from an oxidation pond contains large numbers of algae, which turn up
as suspended solids in the standard tests. A high SS result often means that the pond
is performing well.
Similarly, the BOD test will be significantly affected by the fact that algae use up
dissolved oxygen in the dark and produce it in light. Also, if the BOD test is done in
the dark, many algae will die and be broken down by bacteria, creating a large false
BOD value. This may have no bearing on the actual performance of the pond, nor
environmental impacts of the final discharge.

32. Care should be taken with the use and interpretation of these two tests, at best they
should be used only to indicate changes in pond performance over time, rather than
measure actual sewage treatment performance of potential environmental impact.
Sampling is important. If the samples are not taken properly, the test results will not
be accurate. Samples collected must represent the quality of the wastewater. There
are two kinds of samples:
 Grab samples: a grab sample is one that is collected at one location and at
one instant. This is used for DO, faecal coliform count, pH.
 Composite sample: several samples taken at regular intervals during specific
period of time. This is used for SS, BOD.
7.3 Where the sample should be taken from
Samples of raw wastewater should be collected in either the wet well of the lift
station or the inlet manhole at the pond site. Do not sample wastewater in the
bottom of the wet well basin because solids tend to settle immediately after entering
the wet well.
Take effluent samples from the outlet control structure during discharge. Samples
should be taken at the same time and location every day.
7.4 Frequency and location of laboratory samples
This sampling regime would be used for large pond systems. It can normally be
reduced for smaller pond system. The general program in Table 5 will be adjusted to
fit the needs of Duc Ninh WWTP and the regulation agency as TCVN 7222: 2002.
Table 5. Frequency and location of laboratory samples
Test Frequently Location
pH Daily Pond/influent
DO Daily Pond/effluent
Temperature Daily Pond
BOD Weekly Influent/effluent
Faecal coliform Weekly Effluent
Suspended
solids
Weekly Influent/effluent
DO and pH should be taken during the same time each day because conditions in the
pond change during this period.

33. DO measurements should be regularly recorded for surface, middle, and bottom
from a representative point away from the banks of the pond.
DO, pH and temperature are important indicators of the condition of a pond.
BOD, SS and the faecal coliform count indicate treatment efficiency. BOD is also used
to calculate pond loading.
BOD and solids concentration in the effluent will change with the time of day, the day
of the week and probably with the season.
Although the BOD of the influent is constantly changing, it is not generally necessary
to determine the BOD on a daily basis. Most ponds can be run successfully by taking a
BOD test once or twice a month on 3 samples taken during the daily low, average and
high flow periods. The average BOD calculated from these samples will show an
indication of the organic loading to the pond.
If the strength of the influent changes rapidly and significantly, more frequent
sampling may be necessary.
Other more specific tests may be required for discharge consent or other
requirements, e.g. nutrient (N, P) toxins, etc.
Finally, make and record observations daily or based on a regular inspection
programme:
 Ponds appearance
 Any odours
 Unusual occurrences
 Maintenance needs and actions taken
 Weather
 Other matters it would be useful to record
 Record of when samples were taken for discharge consent monitoring
 Record of duplicate testing on a sample of both on-site and laboratory
sampling is undertaken to confirm
 Record of plant operators for period

34. 8. Discharge consents and conditions
Wastewater treatment engineering design has to provide solutions with equal
emphasis on three main issues with equal emphasis: odour removal, treatment and
sludge disposal. These summarizing issues and proposed treatments are presented
in the following sections.
8.1 Odour treatment
Most of domestic influents contain odour producing components such as ammonia
(NH3), hydrogen sulphide (H2S) and traces of mercaptans (sulfo-organic
compounds). The level of odour causing compounds depends on many factors the
most of which is the existence of septic sources within city sewer network.
Odours from raw influent are typically removed:
 physically by air stripping , or
 chemically by wet oxidation, or
 biologically by absorption and bio-transformation into bio-flocs.
It should be noted that the odour emitted from the raw influent pumping station,
the partial reduction of odour could be taken place during the screening and grit
removal steps inside the pre-treatment building. A multipurpose aerated grit
chamber will remove oil & grease, settle grit, and also strip odour. Contaminated air
and odour are then vented and treated by chlorine-oxidation scrubbing and bio-
filtration. The remaining traces of NH3 and H2S (including mercaptans) could be
absorbed into bio-flocs of the aerated lagoon step.
At Dong Hoi the odour and contaminated air come from wet well of pumping
station, pre-treatment building (screening and grit removal by aerated grit
chamber), septic tank sludge reception wet well, and certain parts of aerated
lagoon treatment step are collected and treated by chemical oxidation using
chlorine scrubbing (for NH3 and H2S removal) and a biofiltration step as absorption
on Ca(OH)2 and biodegradation on biomass fixed on wet organic support as peat
moss, compost bulk, soil-type media, etc., for traces removal.
8.2 Wastewater treatment

35. In order to comply with TCVN-7222:2002 standards (organic contents, nutrient N &
P, coliforms), wastewater will be treated through a chain of combined lagoons
consisting in mechanically-aerated basin, facultative naturally-aerated lagoons and
shallow maturation ponds using solar disinfection and nutrient removal. The main
objective of aerated lagoons in a combined system is to allow a flexibility to face
unexpected pollutant loads. Finally, maturation pond with low depth will offer
advantages of zero-cost solar disinfection while aquatic floating system and surface
flow wetland allow low-cost nutrient removal.
8.3 Sludge disposal
Settled sludge can be annually removed from the lagoon bottom and dried during
the dry season. Large amounts of liquid at relative low solid contents (2 to 3%) is
pumped to a reed bed-based dewatering system. Using their live growing roots,
reed plants can progressively increase the sand interstices of the filtration bed and
improve the filtrate flow.
At Dong Hoi, sludge from the wastewater treatment plant is dewatered in the reed
bed system. Dewatered sludge can be reused as compost for agriculture reuse or
city street urban greening purposes.
9. Some case histories
9.1 Septic tank cleaner problem
A wastewater pond system was used to receive septic tank sludge and grease from
the local community. There was only once access/dumping point for the tank
cleaning operator to use. The ponds were sited in a windless gully, with the results
that as island of sludge gradually formed near the dumping point, creating a major
odour problem and seriously reducing the performance of the pond.
9.2 Industrial overloading and excess grease problems
A two pond system for a town about 6000 people also received miliscreened waste
from a small meatworks. This coupled with an excessive grease content in the
meatworks waste, meant that atmospheric oxygen could not enter the ponds, which
regularly became anaerobic in summer.

36. 9.3 Butynol rubber bank collapse
A large pond with a butynol rubber liner was located in a windy situation. The liner
was not keyed in sufficiently at the top, with the result that spray and pond liquid
seeped down behind the liners, partially collapsing the bank. Further wind and wave
action worsened the problem considerably.
9.4 Concrete wave band cracking and breaking up by wave action erosion
A pond was only partly filled following construction and the water level was such that
the wave action on the banks caused erosion of the embankment immediately below
the lower edge of the concrete wave band. The wave band then lost support and
broke up in places.
9.5 Fat/oil slick over the first or primary pond surface
In a busy tourist town which had many restaurants and hotels establishment there
was a much higher than normal cooking oil and fat discharge in the sewage. The fat
and grease traps at these establishments were also not working effectively. This
resulted in a higher than normal discharge of cooking fat and oils to the ponds,
causing an oily film to form on the top layer of the first or primary pond. This had the
effect of reducing the natural aeration effects on the pond water surface. The film
and associated surface tension effects were most noticeable when compared to the
adjacent second pond especially when there was a breeze. At these times the first
pond still had a surface while the second was rippling from the breeze.