Design and Performance of Waste
Stabilization Ponds
Hamzeh
Ramadan and
Victor M. Ponce
Sun Sep 09
2012
20:04:09
GMT+0700
Contact:
info@virginglobe.com
Version 081218
(SE Asia
Standard
Time)
1. Introduction
2. Waste Stabilization
Ponds Systems
3. Waste Stabilization
Ponds Types and
Functions
4. Additional
Technologies Used to
Improve WSP
5. Siting of Ponds and Geotechnical Aspects
6. Design Criteria
7. Conclusions
8. References
1. Introduction
The most appropriate wastewater treatment is that which will produce an effluent
meeting the recommended microbiological and chemical quality guidelines both at low
cost and with minimal operational and maintenance requirements (Arar, 1988).
Adopting as low a level of treatment as possible is especially desirable in developing
countries, not only from the point of view of cost but also in acknowledgement of the
difficulty of operating complex systems reliably. In many locations it will be better to
design the reuse system to accept a low-grade of effluent rather than to rely on
advanced treatment processes producing a reclaimed effluent which continuously
meets a stringent quality standard.
Waste Stabilization Ponds (WSP) are now regarded as the method of first choice for the
treatment of wastewater in many parts of the world. In Europe, for example, WSP are
very widely used for small rural communities (approximately up to 2000 population but
larger systems exist in Mediterranean France, and also in Spain and Portugal) (Boutin
et al., 1987; Bucksteeg, 1987). In the United States one third of all wastewater
treatment plants are WSP, usually serving populations up to 5000 (EPA, 1983).
However in warmer climates (the Middle East, Africa, Asia and Latin America) ponds
are commonly used for large populations (up to around 1 million). In developing
countries and especially in the tropical and equatorial regions sewage treatment by
WSPs has been considered an ideal way of using natural processes to improve sewage
effluents.
Waste Stabilization Ponds (WSP), often referred to as oxidation ponds or lagoons, are
holding basins used for secondary wastewater (sewage effluents) treatment where
decomposition of organic matter is processed naturally, i.e. biologically. The activity in
the WSP is a complex symbiosis of bacteria and algae, which stabilizes the waste and
reduces pathogens. The result of this biological process is to convert the organic
content of the effluent to more stable and less offensive forms. WSP are used to treat a
variety of wastewaters, from domestics wastewaters to complex industrial waters, and
they function under a wide range of weather conditions, i.e. tropical to arctic. They can
be used alone or in combination with treatment processes.
A WSP is a relatively shallow body of wastewater contained in an earthen man-made
basin into which wastewater flows and from which, after certain retention time (time
which takes the effluent to flow from the inlet to the outlet) a well-treated effluent is
discharged. Many characteristics make WSP substantially different from other
wastewater treatment. This includes design, construction and operation simplicity, cost
effectiveness, low maintenance requirements, low energy requirements, easily adaptive
for upgrading and high efficiency.
[Top]
2. Waste Stabilization Ponds Systems
A World Bank Report (Shuval et al. 1986) endorsed the concept of stabilization pond as
the most suitable wastewater treatment system for effluent use in agriculture. Table 1
provides a comparison of the advantages and disadvantages of ponds with those of
high-rate and low-rate biological wastewater treatment processes (note that Aereated
Lagoon and WSP system are considered low-rate biological wastewater treatment
processes). Stabilization ponds are the preferred wastewater treatment process in
developing countries, where land is often available at reasonable opportunity cost and
skilled labor is in short supply.
Table 1. Advantages and disadvantages of various sewage treatment systems (Arthur 1983).
Criteria
BOD removal
Plant
performance FC removal
SS removal
Helminth
removal
Virus removal
Simple and
Economic
cheap
factors
construction
Simple
operation
Land
requirement
Maintenance
costs
Energy demand
Sludge removal
costs
Package Activated
Extended
Biological Oxidation Aerated
plant
sludge
aeration
filter
ditch
lagoon
plant activated sludge
F
F
F
F
G
G
P
P
F
P
F
G
F
G
G
G
G
F
P
F
P
P
F
F
Waste
stabilization
pond system
G
G
F
G
P
P
F
P
P
P
P
P
F
F
G
F
G
G
P
P
P
F
F
P
G
G
G
G
G
G
F
P
P
P
P
F
P
P
G
P
P
P
F
P
F
F
F
P
P
P
F
G
G
FC = Faecal coliforms
SS = Suspended solids
G = Good
F = Fair
P = Poor
[Top]
Wastewater stabilization pond systems are designed to achieve different forms of
treatment in up to three stages in series, depending on the organic strength of the input
waste and the effluent quality objectives. For ease of maintenance and flexibility of
operation, at least two trains of ponds in parallel are incorporated in any design. Strong
wastewaters, with BOD5 concentration in excess of about 300 mg/l, will frequently be
introduced into first-stage anaerobic ponds, which achieve a high volumetric rate of
removal. Weaker wastes or, where anaerobic ponds are environmentally unacceptable,
even stronger wastes (say up to 1000 mg/l BOD5) may be discharged directly into
primary facultative ponds. Effluent from first-stage anaerobic ponds will overflow into
secondary facultative ponds, which comprise the second-stage of biological treatment.
Following primary or secondary facultative ponds, if further pathogen reduction is
necessary, maturation ponds will be introduced to provide tertiary treatment. Typical
pond system configurations are given in Fig. 1, though other combinations may be
used.
Fig. 1 Stabilization pond configurations: AN = anaerobic pond; F = facultative pond;
M = maturation pond (Pescod and Mara, 1988).
3. Waste Stabilization Ponds Types and Functions
WSP can be classified in respect to the type(s) of biological activity occurring in a pond.
Three types are distinguished: anaerobic, facultative and maturation ponds. Usually a
WSP system comprises a single series of the aforementioned three ponds types or
several such series in parallel (see Section 2). In essence, anaerobic and facultative
ponds are designed for BOD removal (Biological Oxidation Demand-see Section 3.1.1)
and maturation ponds for pathogen removal, although some BOD removal occurs in
maturation ponds and some pathogen removal in anaerobic and facultative ponds. In
many instances only anaerobic and facultative ponds are required. In general,
maturation ponds are required only when stronger wastewaters (BOD > 150 mg/l) are to
be treated prior to surface water discharge and when the treated wastewater is to be
used for unrestricted irrigation (irrigation for vegetable crops). Generally, in WSP
systems, effluent flows from the anaerobic pond to the facultative pond and finally, if
necessary, to the maturation pond. However, for better results wastewater flowing into
an anaerobic pond shall be preliminary treated in order to remove coarse solids and
other large materials often found in raw wastewater. Preliminary treatment operations
typically include coarse screening, grit removal and, in some cases, comminution of
large objects.
[Top]
3.1. Anaerobic Ponds
Anaerobic ponds are deep treatment ponds that exclude oxygen and encourage the
growth of bacteria, which break down the effluent. It is in the anaerobic pond that the
effluent begins breaking down in the absence of oxygen "anaerobically". The anaerobic
pond acts like an uncovered septic tank. Anaerobic bacteria break down the organic
matter in the effluent, releasing methane and carbon dioxide. Sludge is deposited on
the bottom and a crust forms on the surface as shown in Fig. 2.
Fig. 2 Operation of the Anaerobic Pond.
Anaerobic ponds are commonly 2-5 m deep and receive such a high organic loading
(usually > 100 g BOD/m3 d equivalent to > 3000 kg/ha/d for a depth of 3 m). They
contain an organic loading that is very high relative to the amount of oxygen entering
the pond, which maintains anaerobic conditions to the pond surface. Anaerobic ponds
don't contain algae, although occasionally a thin film of mainly Chlamydomonas can be
seen at the surface. They work extremely well in warm climate (can attain 60-85% BOD
removal) and have relatively short retention time (for BOD of up to 300 mg/l, one day is
sufficient at temperature > 20oC).
Anaerobic ponds reduce N, P, K and pathogenic microorganisms by sludge formation
and the release of ammonia into the air. As a complete process, the anaerobic pond
serves to:





Separate out solid from dissolved material as solids settle as bottom sludge.
Dissolve further organic material.
Break down biodegradable organic material.
Store undigested material and non-degradable solids as bottom sludge.
Allow partially treated effluent to pass out.
These fermentation processes and the activity of anaerobic oxidation throughout the
pond remove about 70% of the BOD5 of the effluent. This is a very cost-effective
method of reducing BOD5. Normally, a single anaerobic pond in each treatment train is
sufficient if the strength of the influent wastewater is less than 1000 mg/l BOD5. For
high strength industrial wastes, up to three anaerobic ponds in series might be
justifiable but the retention time in any of these ponds should not be less than 1 day
(McGarry and Pescod, 1970). Designers have been in the past too afraid to incorporate
anaerobic ponds in case they cause odor. Formation of odor is strongly dependent on
the type of waste to be treated in the plant, notably its sulphate (SO4) concentration and
volumetric loading rate, respectively. SO4 is reduced to hydrogen sulphide (H2S) under
anaerobic conditions. H2S is the compound mainly responsible for obnoxious odors.
Other components besides H2S and originating from the anaerobic decomposition of
carbohydrates and proteins may contribute to obnoxious odors, too.
However, odor is not a problem if the recommended design loadings are not exceeded
and if the sulphate concentration in the raw wastewater is less than 300 mg SO 4/l
(Gloyna and Espino, 1969). A small amount of sulphide is beneficial as it reacts with
heavy metals to form insoluble metal sulphides, which precipitate out. In the case of
typical municipal sewage, it is generally accepted that a maximum anaerobic pond
loading of 300 g BOD5/m3 d at 200C will prevent odor nuisance (Mara et al. 1992).
However, results obtained from a more recent study in northern Brazil carried out by
Pearson et al. (1996) suggest that maximum design volumetric loadings may increase
to 350 g BOD5/m3d at 25°C rather that restricting it to 300 g BOD5/m3d at 20°C.
Furthermore, Mara and Pearson (1986) propose a maximum sulphate volumetric
loading rate of 500 g SO4/m3 d (equivalent to 170 g S/ m3d) in order to avoid odor
nuisance.
[Top]
3.1.1. BOD Removal Rates and Factors
First, the concept of Biological Oxidation Demand (BOD) should be introduced. Organic
compounds in wastewater may be used as food for bacteria, which can biochemically
digest or oxidize the organic compounds to produce energy for growth. This oxidation of
organic material, if done under aerobic conditions (i.e. in the presence of oxygen),
"consumes" oxygen and produces carbon dioxide. An organic waste can therefore be
said to have a biochemical oxygen demand, i.e. the amount of oxygen required by
aerobic bacteria to oxidize it.
The term BOD is used to refer to the organic material in a waste and can be used in
quantitative expressions relating to organic material, i.e. the expression g BOD or kg
BOD describes an amount of organic material. The amount of BOD in a specific volume
of wastewater is the concentration or strength of the wastewater and is expressed in
terms such as g/m3 or mg/L or parts per million of BOD (all numerically equivalent). The
loading rate of organic waste to a treatment system or a receiving environment (i.e.
land) is expressed as a mass of BOD/volume (or area) of treatment system per unit of
time: i.e., g BOD/m3/day for loading rate of an anaerobic pond; g BOD/m 2/day to a
facultative pond or to land.
BOD is measured in a five-day test of oxygen consumption. The BOD value derived
from this test is usually expressed as the BOD5 of the wastewater.
Small ponds that receive a reasonably high input of plant nutrients generally develop
ecosystems that feature algal populations that produce oxygen in excess of the
respiration requirements of the algae. This "excess" oxygen can be used by bacteria to
oxidize biodegradable organic matter (quantified as BOD5) entering the pond.
This principle forms the basis of natural-aeration waste stabilization ponds, wherein
bacterial degradation of organic waste provides carbon dioxide and nutrients to sustain
algal photosynthesis and production of oxygen that the bacteria then use.
In anaerobic ponds BOD removal is achieved (as in septic tanks) by sedimentation of
settleable solids and subsequent anaerobic digestion in the resulting sludge layer: this
is particularly intense at temperatures above 15oC when the pond surface literally
bubbles with the release of biogas (around 70 percent methane and 30 percent carbon
dioxide); methane production increases sevenfold for every 5oC rise in temperature
(Marais, 1970).
The biochemical reactions that take place in anaerobic ponds are the same as those
occurring in anaerobic digesters, with a first phase of acidogenesis and a second
slower-rate of methanogenesis. Ambient temperatures in hot-climate countries are
conducive to these anaerobic reactions and expected BOD5 removals for different
retention times in treating sewage have been given by Mara (1976) as shown in Table
2. More recently, Gambrill et al. (1986) have suggested conservative removals of BOD5
in anaerobic ponds as 40% below 10°C, at a design loading of 100 g/m 3d, and 60%
above 20°C, at a design loading of 300 g/m3d, with linear interpolation for operating
temperature between 10 and 20°C. Higher removal rates are possible with industrial
wastes, particularly those containing significant quantities of organic settleable solids.
Of course, other environmental conditions in the ponds, particularly pH, must be
suitable for the anaerobic microorganisms bringing about the breakdown of BOD.
Table 2. BOD removals in Anaerobic Ponds loaded
at 250 g BOD5/m3 d (Mara, 1976)
Retention Time (days)
1
2.5
5
BOD5 removal %
50
60
70
[Top]
Anaerobic ponds are normally designed on the basis of a temperature-dependent
empirical value for the permissible organic loading rate. Land requirements will be
lowest if the maximum possible BOD loading can be applied. The upper limit of the
volumetric BOD loading is determined by odor emissions and minimum pH threshold
value at which the anaerobic decomposition processes cease to work. The maximum
BOD loading rate acceptable to avoid odor nuisance was discussed earlier in section
3.1.
However, the effect of pH must be taken into consideration. Concentrations of H2S,
which is the sulphur form responsible for odors, increases sharply as the pH drops
below 7.5, phenomenon which may occur if an anaerobic pond is heavily loaded or
overloaded (based on a BOD loading rate criterion). Sulphide may also impede
methane production in anaerobic ponds if occurring at excess concentrations. The
presence of heavy metals will lead to insolubilisation of sulphides (e.g. iron sulphides).
Since methanogenesis is the rate-limiting factor in anaerobic metabolism, products from
the preceding acidogenesis reaction may accumulate and lead to a pH decrease.
Optimum pH for methanogenesis amounts to 6.0 - 8.0. Based on various anaerobic
digestion studies, McGarry and Pescod (1970) found that pH = 6.0 probably constitutes
the lowest limit for anaerobic tropical ponds. Acidic wastewaters thus require
neutralizing prior to treatment in anaerobic ponds as a low pH can be considered a
toxicant for anaerobic bacteria. Determination of the maximum BOD loading rate
beyond which pH is likely to drop below this threshold value is, therefore, important.
A study on anaerobic pond treatment of tapioca starch waste conducted by Uddin
(1970) revealed that a volumetric BOD loading rate of around 750 g/m 3·d resulted in a
pond pH of 6.0. Fig. 3, which is based on Uddin's results shows that when the BOD
loading rate was increased above this value, the volumetric BOD removal rate was
reduced. Most likely, pond overloading impaired methanogenesis.
Fig. 3 Influence of Retention Time and Volumetric BOD Loading Rate on Volumetric BOD
Removal Rate in Anaerobic Ponds Based on Uddin (1970).
The published BOD elimination rates for anaerobic wastewater ponds range from 50 to
85%. Temperature, retention time and BOD loading rate affect removal efficiency.
Furthermore, the type of substrate; i.e., sewage, septage or public toilet sludge and its
concentration influence the physical and biochemical processes. To achieve high
elimination rates at the start of a new operating cycle, some sludge should be left for
seeding when emptying a pond. Experience with anaerobic pond treatment in tropical
climate reveals that anaerobic digestion is basically completed after about four days
(van Haandel and Lettinga 1994). Highest BOD elimination and, thus, reduction of land
requirements are attained by applying the highest permissible BOD loading rate
(loading limits were discussed before). Multi-stage anaerobic ponds, each operated at a
maximum BOD loading rate, will, therefore, have the lowest land requirements. If the
influent is of high strength (BOD > 8,000 and COD = 20,000-50,000 mg/l), such as
public toilet sludge without co-mixture of septage, removal rates (expressed in g/m3·d)
will be higher in a multi-stage pond than in a single anaerobic pond. When treating
wastewater of low strength (BOD < 2,000 and COD < 10,000 mg/l), high BOD pond
loading rates will lead to very short retention times. This may, in turn, cause a decrease
in the BOD removal rate. Fig. 3, derived from data presented by McGarry and Pescod
(1970) on work performed by Uddin (1970), shows that the BOD removal rates for
tapioca starch waste decrease at decreasing retention times, and increase to a
threshold value if BOD loading rates are increased.
Another factor may affect the BOD and COD removal, which is the ammonia (NH3)
toxicity to anaerobic bacteria. Experiments conducted by Sergrist (1997) showed a 50%
growth inhibition at a NH3-N/l concentration of 25-30 mg/l. Strong ammonia inhibition in
anaerobic ponds can occur at concentrations >80 mg NH3-N/l and may reduce
significantly COD elimination to as low as 10% in primary anaerobic ponds (Data is still
scarce in this matter).
In certain instances, anaerobic ponds become covered with a thick scum layer, which is
thought to be beneficial but not essential, and may give rise to increased fly breeding.
Solids in the raw wastewater, as well as biomass produced, will settle out in first-stage
anaerobic ponds and it is common to remove sludge when it has reached half depth in
the pond. This usually occurs after two years of operation at design flow in the case of
municipal sewage treatment.
[Top]
3.1.2. Pathogen Removal
In natural treatment systems such as WSP, the pathogens are progressively removed
along the ponds series with the highest removal efficiency taking place in the
maturation ponds (Mara et al., 1992). However, the following observations can be
carried out from different studies that discussed anaerobic ponds participation in
pathogen removal:



Knörr and Torrella (1995) reported a higher removal efficiency of total coliforms
in anaerobic ponds when compared to the facultative lagoons (the latter units
were however more efficient at removing faecal coliforms). Some figures from
this research carried out at a WSP system in the Mediterranean coast of Spain
showed removals of one log unit for total coliforms in the anaerobic pond.
Meanwhile, the viral removal efficiency was very poor in the anaerobic pond.
Arridge et al. (1995) working on an experimental WSP complex in Northeast
Brazil found a one log unit removal in the AP for each of the following indicators:
faecal coliforms, faecal streptococci and Clostridium perfringens. Salmonellae
were reduced from 130 to 70 MPN/100 ml and Vibrio cholerae 01 was reduced
from 40 to 10 MPN/l respectively. Anaerobic ponds appear to be essential for
high levels of V. cholerae removal.
Oragui et al. (1995) reported the removal of one log unit for rotaviruses in the
anaerobic pond of the experimental WSP complex located in Campina Grande in

Northeast Brazil.
Grimason et al. (1993) studied the occurrence and removal of Cryptosporidium
spp. oocysts and Giardia spp. cysts in eleven WSP systems located in towns
across Kenya. The results from this study showed that a significantly higher
concentration of Giardia cysts was detected in raw sewage compared to
anaerobic pond effluent.
3.1.3. Nutrient Removal
Nitrogen
In WSP systems the nitrogen cycle is at work, with the probable exception of
nitrification and denitrication. In anaerobic ponds organic nitrogen is hydrolyzed
to ammonia, so ammonia concentrations in anaerobic pond effluents are
generally higher than in the raw wastewater (unless the time of travel in the
sewer is so long that all the urea has been converted before reaching the WSP).
Volatilization of ammonia seems to be the only likely nitrogen removal
mechanism occurring to some extent in anaerobic ponds. Soares et al (1996)
carried found a very low removal of nitrogen in anaerobic ponds.
Phosphorus
The mechanisms of phosphorus removal most likely take place in maturation
ponds (Mara et al. 1992).
[Top]
3.1.4. Environmental Considerations
Physical as well as chemical factors affect the habitat of microorganisms and
consequently the anaerobic sewage treatment process. The most important
environmental factors to take into consideration are: temperature, pH, degree of mixing,
nutrient requirements, ammonia and sulphide control and the presence of toxic
compounds in the influent (Van Haandel and Lettinga, 1994).
Temperature
As temperature rises, the rate of reaction also increases. In order to have a
reasonable methane production rate, the temperature should be maintained
above 20°C. Methane production rates are doubled for each 10°C temperature
increase in the mesophilic range (Droste, 1997).
pH
According to Zehnder et al. (1982), the optimum pH range for all methanogenic
bacteria is between 6 and 8, but the optimum value for the group as a whole is
close to 7. Van Haandel and Lettinga (1994) reported the same observation and
also pointed out that, since acidogenic populations are notably less sensitive to
pH variations, acid fermentation will predominate over methanogenic
fermentation. The latter may result in souring of the reactor contents. Thus, the
system must contain adequate buffering capacity to neutralize the production of
volatile acids and carbon dioxide, which dissolves at the operating pressure
(Droste, 1997).
Degree of Mixing
The separation of digestion from other processes and the application of mixing
were the first major advances in anaerobic treatment. Mixing is an important
factor in pH control and maintenance of even environmental conditions. It
distributes buffering agents throughout the reactor volume and prevents localized
build-up of high concentrations of intermediate metabolic products, which may
inhibit methanogenic activity. On the contrary, inadequate mixing propitiates the
development of adverse microenvironments.
Nutrient Requirements
Acidogenic and methanogenic bacteria have low growth rates for a given amount
of substrate and this feature results in less nutrient requirements compared to
aerobic systems. On the other hand, anaerobic systems produce 20% or less of
the amount of sludge produced in aerobic systems for the same substrate and so
N and P requirements should decrease proportionally.
Ammonia and Sulphide Control
Anaerobic bacteria can acclimatize to high ammonia concentrations, but large
fluctuations can be detrimental to the process. Free ammonia is much more toxic
than the ammonium ion and it occurs more at high pH values. Wastes with high
contents of proteins will generate significant amounts of ammonia that in turn
increases alkalinity. Wastes containing blood can produce enough ammonium
bicarbonate to raise the pH beyond the optimal range and this requires acid
addition for pH correction. In most cases, the protein content of wastes is not
high enough to cause ammonia toxicity problems.
At the same time, sulphide can be formed in the process due to the reduction of
sulphates. Sulphides are inhibitory to methanogens and sulphate-reducers
themselves, but according to results of Rinzema (1988), a sulphide concentration
of up to 50 mg/l (normally expected in anaerobic sewage treatment systems) is
far lower than the minimum concentration causing toxicity problems.
Toxic Compounds
Other compounds such as heavy metals and chloro-organics affect the rate of
anaerobic digestion even at very low concentrations. Apart from sulphide,
oxygen is also a potentially toxic compound, which can enter the reactor together
with influent flow. However, the presence of these compounds at inhibitory
concentrations is unlikely in domestic wastewater.
[Top]
3.2. Facultative Ponds
Facultative ponds (1-2 m deep) are of two types: primary facultative ponds, which
receive raw wastewater, and secondary facultative ponds, which receive settled
wastewater (usually the effluent from anaerobic ponds). They are designed for BOD
removal on the basis of a relatively low surface loading (100-400 kg BOD/ha d at
temperature between 20°C and 25°C) to permit the development of a healthy algal
population as the oxygen for BOD removal by the pond bacteria is mostly generated by
algal photosynthesis. Due to the algae facultative ponds are colored dark green,
although they may occasionally appear red or pink (especially when slightly overloaded)
due to the presence of anaerobic purple sulphide-oxidizing photosynthetic bacteria. The
algae that tend to predominate in the turbid waters of facultative ponds are the motile
genera (such as Chlamydomonas, Pyrobotrys and Euglena) as these can optimize their
vertical position in the pond water column in relation to incident light intensity and
temperature more easily than non-motile forms (such as Chlorella, although this is also
fairly common in facultative ponds). The concentration of algae in a healthy facultative
pond depends on loading and temperature, but is usually in the range 500-2000 µg
chlorophyll a per litre.
How Facultative Ponds Work?
Effluent entering the facultative pond from the anaerobic pond (secondary facultative
pond) is converted into carbon dioxide, water and new bacterial and algae cells in the
presence of oxygen, i.e., aerobically.
Algae populations within the aerobic pond require sunlight. They develop and produce
oxygen in excess of their own requirements. It is this excess of oxygen that is used by
bacteria to further break down the organic matter within the effluent. The algal
production of oxygen occurs near the surface of aerobic ponds to the depth to which
light can penetrate (i.e. typically up to 500 mm). Oxygen can also be introduced by
wind.
Aerobic pond is more accurately termed "facultative", as in practice the pond usually
has an aerobic upper layer and anaerobic lower layer. This facultative condition occurs
because high oxygen levels cannot be maintained to the total depth of aerobic ponds.
So a fully aerobic surface layer develops, along with an aerobic/anaerobic intermediate
layer, and a fully anaerobic layer on the pond bottom. Oxygen is unable to be
maintained at the lower layers if:



The pond is too deep, and the color too dark, to allow light to penetrate fully.
The demand for oxygen in the lower layer is higher than the supply. Demand is
increased with high levels of organic matter. The anaerobic layer will be deeper
in an aerobic pond where there is an extremely high organic matter content of
the inflowing effluent.
The surface layer, rich in oxygen, is not adequately mixed with the bottom layer.

There is a combination of these conditions.
As a result of the photosynthetic activities of the pond algae, there is a diurnal variation
in the concentration of dissolved oxygen. For a typical facultative pond, the water
column will be predominantly aerobic at the time of peak sun radiation and
predominantly anaerobic at sunrise. After sunrise, the dissolved oxygen level gradually
rises to a maximum in the mid-afternoon, after which it falls to a minimum during the
night. The position of the oxypause (the depth at which the dissolved oxygen
concentration reaches zero) similarly changes, as does the pH since at peak algal
activity carbonate and bicarbonate ions react to provide more carbon dioxide for the
algae, so leaving an excess of hydroxyl ions with the result that the pH can rise to
above 9 which kills faecal bacteria. The wind has an important effect on the behavior of
facultative ponds, as it induces vertical mixing of the pond liquid. Good mixing ensures
a more uniform distribution of BOD, dissolved oxygen, bacteria and algae and hence a
better degree of waste stabilization. In the absence of wind-induced mixing, the algal
population tends to stratify in a narrow band, some 20cm thick, during daylight hours.
This concentrated band of algae moves up and down through the top 50 cm of the pond
in response to changes in incident light intensity, and causes large fluctuations in
effluent quality (especially BOD and suspended solids) if the effluent take-off point is
within this zone. The operation of the facultative pond is shown in Fig. 4.
Fig. 4 Operation of the facultative pond (Tchobanoglous and Schroeder 1987).
The facultative pond will remove odor and kill most pathogenic microorganisms. As a
complete process, the facultative pond serves to:


Further treat the effluent anaerobically through separation, dissolving and
digestion of organic material.
Aerobically break down most remaining organic solids near the pond surface.




Reduce the amount of disease-causing microorganisms.
Allow the loss of 20% to 30% of the ammonia, contained within the effluent, into
the air.
Store residues from digestion, as well as non-degradable solids, as bottom
sludge.
Allow treated effluent to pass out into a waterway or additional treatment system
(i.e. an additional pond, wetland system or for land application).
Sometimes two or more consecutive smaller facultative ponds are constructed instead
of a very large one. This may be more practical for effective desludging and stirring or
when the pond is too long for the site and interferes with existing structures.
In primary facultative ponds (those that receive raw wastewater) the above functions of
anaerobic and secondary facultative ponds are combined. Around 30% of the influent
BOD leaves a primary facultative pond in the form of methane (Marais, 1970). This type
of pond is designed generally for the treatment of weaker wastes and in sensitive
locations where anaerobic ponds odor would be unacceptable.
3.2.1. BOD Removal
The activity of further anaerobic oxidation and the aerobic conversion of effluent to
carbon dioxide, water and new bacterial and algae cells can result in removal of 80% of
the BOD5 of the effluent flowing into the facultative pond (which means an overall
removal in the order of 95% over the two ponds). This removal, and the subsequent
quality of the outflow, depends on:




An adequate oxygen supply.
Sufficient retention time.
Warm temperatures.
An absence of high concentrations of chemical pollutants. High concentrations of
cleaning chemicals and drenches will slow the system's ability to break down
effluent solids.
Moreover, as a result of the algal-bacterial activities described in the previous section, a
high proportion of the BOD that does not leave the pond as methane ends up as algal
cells. Thus in secondary facultative ponds (and in the upper layers of primary facultative
ponds) "sewage BOD" is converted into "algal BOD" and this has important implications
for effluent quality requirements. This provides even better BOD quality of the effluent
from a facultative ponds as most of the BOD contained (70 to 90%) will be "algal BOD".
When a facultative pond is used as a primary treatment, BOD removal may be very
efficient. Abis (2002) reported a BOD removal in a pilot-scale facultative ponds in the
United Kingdom (surface loading 51-117 kg/ha d) to an average of 91% (between
67.5% and 98.6%). These values include the contribution of algae in the effluent. With
the algal (and other) solids removed from the effluent, the average removal was 97.2%
(with a range of 89.7-99.7%).
3.2.2. Pathogen Removal: Bacteria, Viruses and Parasites
Faecal bacteria are mainly removed in facultative and especially maturation ponds
whose size and number determine the numbers of faecal bacteria (usually modeled in
terms of faecal coliforms) in the final effluent, although there is some removal in
anaerobic ponds principally by sedimentation of solids-associated bacteria. The
principal mechanisms for faecal bacterial removal in facultative and maturation ponds
are now known to be:




Time (retention time as pathogen attenuation occurs over time),
Temperature (faecal bacteria dies off increases with temperature),
High pH (> 9), and
High light intensity together with high dissolved oxygen concentration.
Regarding viruses removal, Little is definitely known about the mechanisms of viral
removal in WSP, but it is generally recognized that it occurs by adsorption on to
settleable solids (including the pond algae) and consequent sedimentation.
Some parasites can be removed as well. Protozoan cysts and helminth eggs are
removed by sedimentation. Their settling velocities are quite high (for example, 3.4 x10 4 m/s in the case of Ascaris lumbricoides), and consequently most removal takes place
in the anaerobic and facultative ponds. It has recently become possible to design WSP
for helminth egg removal (Ayres et al., 1992).
3.2.3. Nutrient Removal
Nitrogen
In facultative and maturation ponds, ammonia is incorporated into new algal
biomass. Eventually the algae become moribund and settle to the bottom of the
pond; around 20% of the algal cell mass is non-biodegradable and the nitrogen
associated with this fraction remains immobilized in the pond sediment. That
associated with the biodegradable fraction eventually diffuses back into the pond
liquid and is recycled back into algal cells to start the process again. At high pH,
some of the ammonia will leave the pond by volatilization. Mara and Pearson
(1986) point out that under certain conditions some algal species are able to
adapt to and withstand concentrations of up to 50 mg/l.
There is little evidence for nitrification (and hence denitrification, unless the
wastewater is high in nitrates). The populations of nitrifying bacteria are very low
in WSP due primarily to the absence of physical attachment sites in the aerobic
zone, although inhibition by the pond algae may also occur. Total nitrogen
removal in WSP systems can reach 80% or more, and ammonia removal can be
as high as 95%.
Phosphorus
The efficiency of total phosphorus removal in WSP depends on how much
leaves the pond water column and enters the pond sediments. This occurs due
to sedimentation as organic P in the algal biomass and precipitation as inorganic
P (principally as hydroxyapatite at pH levels above 9.5), compared to the
quantity that returns through mineralization and resolubilization. As with nitrogen,
the phosphorus associated with the non-biodegradable fraction of the algal cells
remains in the sediments. Thus the best way of increasing phosphorus removal
in WSP is to increase the number of maturation ponds, so that progressively
more and more phosphorus becomes immobilized in the sediments. From a well
functioning two-pond system, 70% mass removal of total phosphorus may be
expected.
Heavy Metals
Polprasert and Charnpratheep (1989) and Kaplan et al. (1987) examined the fate
of heavy metals in such ponds. Adsorption of metals was increased in attachedgrowth stabilization pond as compared to stabilization ponds without attachedgrowth. Kaplan et al. reports only a slight decrease in total metals concentration,
however the particulate fraction was mostly solubilized.
A study by Moshe (1972) showed that high concentrations of metal ions (Cd, Cu,
Ni, Zn, and Cr) are toxic to Chlorella species, the most common species in
stabilization ponds, and adversely affect pond efficiency. However, high pH
(higher than 8) causes metal ions to precipitate and allows pond purification
processes to occur normally.
3.2.4. Removal of Algae from Facultative Ponds Effluent
Many techniques have been developed to remove the algae from effluents, these
include rock filtration, grass plots, floating macrophytes and herbivorous fish. Also, the
use of maturation ponds can reduce the algal concentration considerably provided the
system is not overloaded.
[Top]
4. Additional Technologies Used to Improve WSP Effluent
The use of anaerobic and facultative ponds system, as the only wastewater treatment
before final discharge, was proven to be satisfactory under different circumstances and
for various agricultural and aquacultural effluent reuses (Mara 2001, Pearson et al
1996). However, when some of the effluent quality limits are not satisfied, choosing a
supplementary (or even alternative technology) in order to improve the effluent quality
will be a serious option. The choice of adding new agents to the existing anaerobic and
facultative ponds or choosing more advanced SWP treatment systems should be taken
in the light of the following factors:

The vital need of effluent quality improvement (especially if used for unrestricted
irrigation).




The feasibility of the total cost of the additional or alternate system (equipment,
land, operation and maintenance) versus the amount of effluent quality
improvement.
Effectiveness of the selected technology (scientifically/practically proven).
Practicality and workability.
Resources availability.
4.1. Integrated Facultative Ponds (Advanced Facultative Ponds)
One possible solution to benefit from the advantages of both anaerobic and aerobic
ponds and suppress their disadvantages is to integrate the best functions of each pond
type into a single pond to allow the symbiotic relationships of related microorganisms to
proceed unrestrained (Fig. 5). The advanced facultative pond is deep to promote
sedimentation of wastewater solids and anaerobic decomposition of methane. Its most
attractive feature is its high capability of wastewater total suspended solids (TSS)
removal, in addition to BOD removal. The pond is designed so that its surface remains
aerobic, thus reducing potential odor problem. Biogas may be collected using
submerged gas canopy and potentially used for energy production. Until these
integrated systems have been fully developed, most designers will continue to rely upon
the traditional stabilization pond treatment systems.
Fig. 5 Integrated (Advanced) Facultative Pond.
4.2. Mechanical Aeration
Aeration introduces oxygen to effluent standing in a facultative pond, so that bacteria
can effectively convert the organic solids to carbon dioxide, water and bacteria
biomass. Mechanically aerated ponds generate turbulence to mix all the effluent in the
pond and introduce oxygen through equipment that either


Introduces air into the effluent by injecting air under the pond surface (floating
pumps).
Exposes more effluent surface area to the air through spraying effluent into the
air or agitating the effluent.
Aerator numbers and configuration are selected to perform the amount of oxygen
generation needed. This technology can significantly reduce the nutrient, ammonia,
odor, and BOD level in the resulted effluent. However, cost of the aerators including
installation, operation and maintenance shall be taken into account in order to assess
the feasibility of using such equipment (this basically varies from one project to
another).
4.3. Anaerobic Digestion
This involves of using microorganisms to turn the complex organic solids less complex
compounds. The end products of anaerobic digestion are biogas (mix of methane and
carbon dioxide) and a stabilized treated liquid. The biogas can be collected and used as
an alternative energy source, but a storage space is required to fulfill this operation.
This procedure reduces BOD but not the nutrient. In addition, Anaerobic digestion adds
more complexity, equipment and cost to the overall effluent treatment system. A
facultative pond treatment would still be required to improve the quality of the effluent.
4.4. Chemical Treatment and Biological Additives
Several kinds of additives are available to control odors and break down crusting and
organic matter. The main ones are the followings:


Bacterial Additives (bioremedation): Using bacteria to degrade solids in ponds so
that they are eventually liquefied. This may result in changes in BOD (may drop
or may rise) and TSS (drop) concentrations and reduce temporary odor
emission.
Electrolytic Methods: It is claimed that copper electrodes immersed in the pond
reduce odors, kill pathogenic microorganisms and prevent build-up of crust. The
cost of this technology is still high (copper probes need to be replaced every 12
to 18 months, in addition to maintenance, operation and energy costs).
4.5. Stabilization Ponds and Supporting Growth Media
In the pond modified by Zhao and Wang (1996), attached-growth media (AGM) or socalled artificial fibrous carriers were installed. This type of media consists of fine strings
of polyvinyl acetate, with specific surface area of 1,236 m 2/m3 and cost only US$ 5/m3.
A pilot-scale investigation has been conducted by them, using three ponds with working
dimensions of 4.0 m in depth, 1.2 m in width and 1.1 m in depth. This study has
confirmed that the incorporation of AGM enhanced the performance of conventional
WSPs by formation of a great number of small stable ecological systems around AGM,
being abundant in bio-species from bacteria and algae to protozoa, increasing the
biomass concentration, improving the biological distribution. Better removal efficiencies
of COD (75.6%), BOD (90.2%) and NH4-N (68.5%) had been achieved in the WSPs
with AGM than in the conventional WSPs, although the total retention time had been
shortened to 7.5 days. Although capital investment in the system may increase, the
system holds the potential to reduce retention times and decrease spatial requirements
of the WSP technology (Yu, et al., 1997).
[Top]
4.6. Advanced Integrated Wastewater Pond System
Developed by Professor William J. Oswald and his co-workers at the University of
California, Berkeley over the past four decades wastewater treatment and algae
production systems called Advanced Integrated Wastewater Pond Systems (AIWPS)
are potentially feasible for application in the developing world (Oswald, 1990).
Although AIWPS may appear to be an adapted traditional pond system, each AIWPS
facility is uniquely designed and incorporates a series of low-cost ponds or earthwork
reactors. Depending on specific effluent characteristics, regulatory requirements,
human resources, and local climatic conditions, a typical AIWPS facility consists of at
least four ponds in series (Fig. 6):




An advanced facultative pond with fermentations pits;
Algal high rate Pond where photosynthetic oxygenation, oxidation, and nutrient
assimilation occurs (with pedal wheel).
Algal settling ponds; and
A maturation pond where final effluent storage and further natural disinfection
occurs.
AIWPS facilities are designed to minimize the accumulation of sludge and to maximize
the production of oxygen through algal photosynthesis. Algal biomass is produced and
can be used as a nitrogen-rich fertilizer, or as protein-rich animal or fish feed (for further
cultivation of high protein foodstuffs), modern medicine and even cosmetics for the idle.
They are cost-effective, require little maintenance and have generally performed well in
terms of BOD5 and solids removal. Moreover, AIWPS require similar land area to
conventional lagoons, virtually eliminate sludge disposal, produce less odor, and may
be adapted to energy (methane) recovery. However, AIWPS cost about $15,000 to set
up, and $100 a year to power the paddle wheel and the algal settling pond needs to be
desludged once to twice a year. In addition, note that this type of technology is not
energy cost free.
Fig. 6 AIWSP system (adapted from NWA website).
4.7. Sheaffer Modular Reclamation and Reuse System (SMRRS)
Sheaffer International markets a variation of the AIWPS described in the preceding
section. The Sheaffer system is described as a Modular Reclamation and Reuse
System producing no sludge, no odor, and enabling 100% recovery of nutrient rich
water for irrigation. The system is comprised of a deep aerated treatment cell, a storage
cell, and three moving parts, described as a grinder pump, a compressor/blower, and
an irrigation system (Sheaffer International LTD., 1998).
The first stage of the process uses the grinder pump to reduce sewage solids influent
and injects it to an anaerobic zone at the bottom of the treatment cell where it
undergoes anaerobic reduction for a 14- to 30-day period. This zone acts as a
mesophilic reactor. Solids settle out of the anaerobic zone to the base of the deep cell,
and are stored for a time period of 20 to 30 years. The second stage of the process, the
compressor/blower, injects air into the treatment cell just above the anaerobic zone to
create aerobic conditions at the surface level of the cell. The cells are designed to
provide 14- to 36-day treatment and further reductions of organic materials (Sheaffer
International LTD., 1998).
Solid components are broken down into simple organic acids, methane carbon dioxide,
sulphide, ammonia, inorganic compounds, and water. The nitrogen, phosphorus, and
potassium are dissolved and remain in solution for use in agricultural irrigation.
Fig. 7 SMRRS (Sheaffer International LTD. 1998).
4.8. Aerated Ponds/Lagoons
A number of facultative ponds have been designed, or more commonly retrofitted, with
surface aerators to boost dissolved oxygen levels and/or to aid mixing.
There is often confusion between these systems and what are typically called aerated
lagoons. Unlike facultative ponds, aerated lagoons are designed to operate at high
bacterial cell mass concentrations. These require a high power input for aeration and in
some cases incorporate biomass return. They operate at much shorter hydraulic
residence times and as a consequence of this, and their increased depth, do not
develop significant algal populations. Aerated lagoons are essentially designed to work
as a form of lowly loaded activated sludge. Mechanically supplied oxygen increases
treatment efficiency and reduces land requirements. However, the high-cost power
input is sufficient only for diffusing oxygen into the pond and not for mixing the contents.
[Top]
4.9. High-rate Algal Ponds
Originally developed by Oswald at the University of California in the sixties, high-rate
algal ponds have continued to be developed and implemented particularly in the United
States. These systems are shallower than a facultative pond and operate at shorter
hydraulic retention times. A paddlewheel is normally incorporated to drive the water
around a "race-track" shaped pond. The oxygen production is reported to be
significantly higher than typical facultative pond designs. The micro algae produced in
these systems are also reported to have good settling properties (Green et al., 1996).
4.10. Rock Filters
Waste stabilization ponds often have high concentrations of TSS in the effluent, which
may or may not be desirable depending on the irrigation delivery method. Several
polishing options are feasible to use in combination with WSPs to upgrade pond
effluents, thereby increasing the options for effluent reuse. Middlebrooks (1995)
suggests that many low-cost methods exist for polishing WSP effluent, which include
intermittent sand filtration and rock filters.
Rock filters, when used in conjunction with WSPs, have been shown to upgrade WSP
effluent. Research at a pilot-scale rock filter demonstration conducted at the Assamra
WSPs in Jordan showed that effluent content reductions could be reduced greatly. TSS
and BOD were reduced by 60%, total faecal coliform count (TFCC) by a maximum of
94% and T-P by 46% at a loading rate of 0.33-0.044 kg/m3 of TSS (Saidam, Ramadan
and Butler, 1995). If high levels of TSS are not an issue in an irrigation scheme and
there is no risk of clogging irrigation equipment, high TSS may be advantageous as
they will add organic matter to the soil matrix.
4.11. Maturation Ponds and Constructed Wetlands
Maturation ponds (low-cost polishing ponds, which succeed the primary or secondary
facultative pond) are primarily designed for tertiary treatment, i.e., the removal of
pathogens, nutrients and possibly algae. They are very shallow (usually around 1 m
depth, although Mara (1997) believes that at this reduced depth emergent plant growth
and mosquito breeding problems can result) to allow light penetration to the bottom and
aerobic conditions throughout the whole depth. The ponds follow a secondary treatment
i.e., a facultative pond. The size and number of maturation ponds needed in series is
determined by the required retention time to achieve a specified effluent pathogen
concentration. In the absence of effluent limits for pathogens, maturation ponds act as a
buffer for facultative pond failure and are useful for nutrient removal (Mara and
Pearson, 1998). Mara (1997) notes that if an anaerobic and secondary facultative pond
system is used, this will produce an effluent suitable for restricted irrigation. Therefore,
additional maturation ponds will only be needed if a higher quality effluent is required.
Another technology that may replace maturation ponds to improve WSP system
performance is the use of constructed wetlands. Wetlands are areas which support the
growth of a variety of plant species adapted to flooded conditions for part of, or the
entire, year. The plants are densely spaced and, together with the shallow water,
provide habitats for animal, bird and insect communities. Constructed wetland systems
are designed to simulate and optimize filtering and biodegradation processes that occur
in natural wetlands. They are a possible solution to improve the performance of pond
systems, as they can "polish" wastewater effluent before its discharge to a waterway.
During summer months, such a system may even result in zero discharge to
waterways, due to evaporation and evapotranspiration of the water component from the
wetland.
[Top]
5. Siting of Ponds and Geotechnical Aspects
When choosing a site to construct a pond system, an area should be selected where
the water table is deep and the soil is heavy and impermeable. Silt or clay soils are
ideal for pond foundations and construction. Building ponds over coarse sands, gravels,
fractured rock or other materials, that will allow effluent to seep out of the pond or allow
groundwater to enter in, should be avoided.
No part of the system to be within 200 m (preferably 500 m) of any dwelling house. If
possible, ponds should be sited downwind from dwellings, roads and other public
places. The greater the distance from a potential complainant the better.
Soil must be suitable for pond stability. Geotechnical aspects, if not taken into
consideration, may cause the WSP system to malfunction. A geotechnical investigation
of the site should be made during the design stage to ensure correct embankment
design and to determine whether the soil is sufficiently permeable to require the pond to
be lined. A stable and impermeable embankment core shall be formed, whether chosen
from an available local or imported soil. After compaction, the soil should have a
coefficient of permeability of 10-7 m/s (Mara and Pearson, 1998). The following
geotechnical considerations should be taken when constructing the embankment:





Embankments must be well constructed to prevent seepage, excessive
settlement and erosion over time.
Embankment slopes are commonly 1 (vertical) to 3 (horizontal) internally and 1
to 1.5-2 externally.
Slope stability should be ascertained according to standard soil mechanics
procedures for small earth dams.
External embankments should be protected from storm water erosion by
providing adequate drainage.
Internal embankments should be protected from wave action erosion by using
precast concrete slabs or stone rip-rap at top water level.
The following are additional general considerations when siting a pond:










Allowing for a straight run of pipelines, tractors and desludging vehicles to the
ponds.
To minimize earthworks, site should be flat or gently sloping.
Siting in an open area so as to take advantage of the sun and wind, which assist
the efficient operation of the facultative pond and thus improve the quality of the
discharge.
If soil is permeable (>10-6 m/s), a plastic membrane plastic may be used to line
the pond.
Keeping systems away from overhead or underground power lines.
Keeping systems from potable water lines.
Avoiding sites that are likely to flood, have steep slopes that run towards a
waterway, spring or bore hole, are pipe drained or mole ploughed, are likely to
freeze over, or have recently been cleared of trees or similarly disturbed.
Constructing the system below the effluent elevation so that gravity can be used
to carry the effluent.
Orientating the longest diagonal dimension of the pond parallel to the direction of
the prevailing wind.
Ponds should not be located within 2 km of airports, as any birds attracted to the
ponds may constitute a risk to air navigation.
[Top]
6. Design Criteria
Wastewater treatment of only anaerobic and facultative ponds is widely considered as
the most pragmatic option (at least as initial treatment). These two types, when used in
series, are proven to be the most economical water treatment system with an effective
performance. Basically, there are four approaches to wastewater stabilization pond
design: loading rates, empirical design equations, reactor theory, and mechanistic
modeling. Loading rates, as a design criterion, is a simple approach, widely used and
recommended in most of the wastewater standard design handbooks worldwide.
6.1. Effluent Limits
Effluent limits represent the maximum amount of pollutants allowed to discharge from
wastewater to its final destination (waterway, reservoir for reuse, etc.). These limits vary
from country to another due to geographical, climatic and socio-economical reasons.
They vary as well with the character of the wastewater final destination. For example,
the effluent quality of wastewater discharged to the ocean would be less stringent than
the effluent quality of wastewater used for agriculture.
Effluent limits characterize the required and accepted quality of the discharged
wastewater. Hence, prior to design, these limits must be known (from local municipal
effluent standards publications) since they will be used as the water quality design
objectives. An example is the European Union quality requirements for WSP effluents
being discharged into surface and coastal waters:
Filtered BOD = 25 mg/l (non-algal BOD)
Filtered COD = 125 mg/l (non-algal COD)
Suspended solids = 150 mg/l
Together with, for discharge into designated "sensitive areas subject to eutrophication":
Total nitrogen = 15 mg/l
Total phosphorus = 2 mg/l
(Although, if the population served is > 100,000, these last two requirements are
reduced to 10 and 1 mg/l, respectively) (Council of the European Communities, 1991a).
Another example is from India. The general standards for the discharge of treated
wastewaters into inland surface waters are given in the Environment Protection Rules
(CPCB, 1996). The more important of these for WSP design are as follow:
BOD 30 mg/l (non-filtered)
Suspended solids 100 mg/l
Total N 100 mg N/l
Total ammonia 50 mg N/l
Free ammonia 5 mg N/l
Sulphide 2 mg/l
pH 5.5 – 9.0
[Top]
6.2. Design Parameters
The four most important parameters for WSP design are:




Temperature: The usual design temperature is the mean air temperature in the
coolest month, quarter or period of the irrigation season.
Net evaporation: Considered in the design of facultative and maturation ponds
but not the anaerobic ponds as the scum layer generated on top of anaerobic
ponds will prevent evaporation (Shaw, 1962). Net evaporation is equal to the
evaporation minus rainfall. The net evaporation rates in the months used for
selection of the design temperatures shall be used. Another way to look at this
parameter is to calculate the rainwater volume using "rainfall less evaporation"
data, area exposed to the rainwater and the degree of runoff/entry actually taking
place. At the end, the rainwater volume falling directly into the pond system
should be accounted for the load calculation. In addition, a hydraulic balance
must be performed to insure the workability of the pond.
Flow: A suitable flow design value is 80 percent of the in-house water
consumption. The design flow may be based on local experience in sewered
communities of similar socio-economic status and water use practice
BOD: If the wastewater exists, its BOD may be measured. If not, it could be
estimated from the following formula (Mara and Pearson 1998):
Li = 1000 B / Q
Where Li = wastewater BOD, mg/l
B = BOD contribution, g/caput d (30 to 70 g/caput d. Affluent communities
produce more BOD than poor communities, Campos and Sperling, 1996)
Q = wastewater flow, l/caput d
Nitrogen, Faecal coliform, and helminth egg numbers are also important if
the final effluent is to be used in agriculture or aquaculture.
[Top]
6.3. Loading and Retention Time
Any pond treatment system requires steady effluent flow to encourage the rapid and
continuous growth of bacteria involved in the biological breakdown of effluent.
It is essential that the daily loading into the ponds is kept to the design standards of the
pond system. A very large load may flush out important bacteria, eventually leading to
system failure. Variation in loads will alter the retention time.
Any attempt to extend the time that effluent remains within the pond system will
increase the amount of disease-causing microorganism die-off. The concentration of
microorganisms within the effluent will be reduced and the effluent will be of higher
quality before discharge into a waterway.
6.4. Loading Rates Design Approach
This approach involves a "black box" type of design, where a ratio of a parameter such
as population, flow or BOD is used in relation to the required volume or area of pond.
This simplified approach to the process design of pond systems has been very
commonly used throughout the world. For example, in the case of New Zealand, a
figure of 84 kg BOD/ha.day (MWD, 1974), has been routinely used for facultative pond
design regardless of the marked differences in environmental conditions throughout the
country.
6.4.1. Anaerobic Ponds Design
Anaerobic ponds can be satisfactorily designed, and without risk of odor nuisance, on
the basis of volumetric BOD loading (lv, g/m3d), which is given by:
lv = Li Q / Va
where Li = influent BOD, mg/l (= g/m3 )
Q = flow, m3/d
Va = anaerobic pond volume, m3

The first step is to select lv. Mara and Pearson (1986) and Mara et al.(1998)
recommend the safely design values given in the following table:
Table 3. Design values for anaerobic ponds (Mara and Pearson 1996).
Temperature T ( oC)
Volumetric Loading
BOD removal (%)
(g/m3 d)
<10
100
40
10-20
20T - 100
2T + 20
20-25
10T + 100
2T + 20
>25
350
70
v can even reach 400 g/m3 d, but in this table the upper limit of 350 is used to
provide an adequate margin of safety with respect to odor. Note that permissible
volumetric BOD loadings lv should not be less than 100 g/m3 d in order to
maintain anaerobic conditions. This is appropriate for normal domestic or
municipal wastewaters, which contain less than 300 mg/l SO4-.

The second step is to evaluate the mean hydraulic retention time which is
determined from:
qa = Va / Q (minimum 1 day should be used, if calculations gives < 1 day, a value
of 1 day should be used and the new value of Va should be recalculated).
6.4.2. Facultative Ponds


When designing facultative ponds, emphasis must be given to the surface area.
Increasing the surface area of the facultative pond will improve the performance
of the system.
It is recommended that facultative ponds be designed on the basis of surface
BOD loading (ls, kg/ha d), which is given by:
Ls = 10 Li Q / Af
where Af = facultative pond area, m2

An early design value of Ls developed by Mara (1976) suggested the use of the
following equation (note that Ls increases with temperature):
Ls= 20T - 120
However, more appropriate global design equation was given by Mara (1987):
ls = 350 (1.107 - 0.002T)T-25

After selecting Ls and calculating the pond area, the next step is to calculate
facultative pond’s retention time (in days) as follows:
qf = Af D / Qm
where D = pond depth, m (see section 3.2)
Qm = mean flow, m3/day
The mean flow is the mean of the influent and effluent flows (Qi and Qe), the
latter being the former less net evaporation and seepage. Thus:
qf = Af D / [1/2 (Qi + Qe)]
If seepage is negligible, Qe is given by:
Qe = Qi – 0.001 e Af
where = net evaporation rate, mm/day. Thus:
qf = 2 AfD / (2 Qi – 0.001 e Af)




A minimum value of qf of 5 days should be adopted for temperatures below
20oC, and 4 days for temperatures above 20oC. This is to minimize hydraulic
short-circuiting and to give the algae sufficient time to multiply (i.e. to prevent
algal washout).
The BOD removal in primary facultative ponds is usually in the range 70-80
percent based on unfiltered samples (that is, including the BOD exerted by the
algae), and usually above 90 percent based on filtered samples. In secondary
facultative ponds the removal is less, but the combined performance of
anaerobic and secondary facultative ponds generally approximates (or is slightly
better than) that achieved by primary facultative ponds.
Adding maturation ponds after a facultative pond will remove additional 25% per
each pond from the facultative pond discharge.
Nutrient removal
Nitrogen
Pano and Middlebrooks (1982) present equations for ammonical nitrogen (NH3 +
NH+4)
removal in individual facultative (and maturation) ponds. Their equation
for temperatures below 20 oC is:
Ce = Ci / {1 + [(A / Q) (0.0038 + 0.000134T) exp ((1.041 + 0.044T)(pH - 6.6))]}
and for temperatures above 20 oC:
Ce = Ci / {1 + [5.035 × 10-3 (A / Q)] [exp(1.540 × (pH - 6.6))]}
where
Ce = ammoniacal nitrogen concentration in pond effluent, mg N/l
Ci = ammoniacal nitrogen concentration in pond influent, mg N/l
A = pond area, m2
Q = influent flow rate, m3 /d
Reed (1985) presents an equation for the removal of total nitrogen in individual
facultative (and maturation) ponds:
Ce = Ci exp{-[0.0064 (1.039)T-20] [q + 60.6 (pH - 6.6)]}
where
Ce = total nitrogen concentration in pond effluent, mg N/l
Ci = total nitrogen concentration in pond influent, mg N/l
T = temperature, oC (range: 1-28oC)
q = retention time, d (range 5- 231 d)
The pH value used in the previous equations may be estimated from:
pH = 7.3 exp(0.0005 Ai)
where Ai= influent alkalinity, mg CaCO3/l
The equations shown can be applied sequentially to individual facultative and
maturation ponds in the series, so that concentrations in the effluent can be
determined.
Phosphorus
There are no design equations for phosphorus removal in WSP. Huang and
Gloyna (1984) indicate that, if BOD removal in a pond system is 90 percent, the
removal of total phosphorus is around 45 percent. Effluent total P is around two
thirds inorganic and one-third organic.
[Top]
6.5. Hydraulic Balance
To maintain the liquid level in the ponds, the inflow must be, at least, greater than net
evaporation and seepage at all times. Thus:
Qi = 0.001 A (e + s)
where Qi = inflow to first pond, m3/d
A = total area of pond series, m2
e = net evaporation (i.e. evaporation less rainfall), mm/d
s = seepage, mm/d
[Top]
6.6. Process Design for Wastewater Discharged in a Waterway




Determine effluent quality requirements in terms of: BOD or COD, (filtered or
unfiltered), suspended solids, ammoniacal nitrogen, and faecal coliforms.
Design an anaerobic and facultative pond.
Determine BOD, ammonia and faecal coliform levels in facultative pond effluent,
as required.
If any of these are more than that required in the final effluent, review your
options to ameliorate the WSP system performance or design a maturation
pond(s) to reduce concentration(s) to required level.
[Top]
6.7. Pond Geometry









To avoid sludge banks forming near the inlet, generally, anaerobic and primary
facultative ponds should be rectangular, with length-to-breadth ratios of 2-3 to 1.
The geometry of secondary facultative and maturation ponds can have up to 10
to 1 length-to-breadth ratios to better approximate plug flow conditions.
Avoid the use of multi-inlet and/or outlet. The inlet should not discharge centrally
in the pond as this maximizes hydraulic short-circuiting.
A single inlet and outlet should be located in diagonally opposite corners of the
pond.
To facilitate wind-induced mixing of the pond surface layers and maximize the
settlement of solids, the pond should be oriented so that its longest dimension
(diagonal) lies in the direction of the prevailing wind.
Although pond depth recommendations have been given, the depth will need to
be related to the site conditions such as whether there are rock strata, or the
height of the water table.
Pond width should be kept less than 24 m because of the reach limitations of
excavator and desludging machinery.
When designing the pond geometry, it is necessary to take into account the
possibilities for the access of machinery used for desludging and emptying both
sides of the ponds.
Baffles should only be used with caution. In facultative ponds, when baffles are
needed because the site geometry is such that it is not possible to locate the
inlet and outlet in diagonally opposite corners, care must be taken in locating the
baffle(s) to avoid too high a BOD loading in the inlet zone (and consequent
possible risk of odor release).


In maturation ponds baffling is advantageous as it helps to maintain the surface
zone of high pH, which facilitates the removal of faecal bacteria.
A 50 cm freeboard should be provided in the design. For ponds between 1 ha
and 3 ha, the freeboard should be 0.5-1 m. For larger ponds freeboard should be
calculated as follow:
F= (log10A)1/2 - 1
where F= freeboard (m) and A= pond area (m2) at top water level.

For dimension calculations for anaerobic ponds, the following formula is used
(EPA, 1983):
Va = [(L W) + (L - 2sD) (W - 2sD) + 4(L - sD) (W - sD)] [D / 6]
where
Va = anaerobic pond volume, m3
L = pond length at TWL, m
W = pond width at TWL, m
s = horizontal slope factor (i.e. a slope of 1 in s)
D = pond liquid depth, m
With the substitution of L as nW, based on a length to breadth ratio of n to 1, the
equation becomes a simple quadratic in W
;
Fig. 8 Geometry of pond (Mara and Pearson, 1998).

The topography may necessitate subdividing ponds into a series of two or more

parallel ponds. Furthermore, for population more than 10,000, this subdivision is
even recommended so as to increase operational flexibility.
The effluent quality and the performance of secondary facultative ponds are
independent of pond geometry, at least within the range of length to breadth
ratios of 1 to 6 and within the depth range of 1 to 2 m (Mara et al 2001).
[Top]
6.8. Land Area Requirements
Approximation of the land areas required per caput for anaerobic and facultative ponds
can be calculated. This would be very beneficial, especially during the planning phase,
when land availability and price are to be considered as a key factor for final decision
on the type of wastewater treatment chosen.
6.8.1. Anaerobic Ponds
The equation presented in section 4.5.1 can be rewritten as:
Aa = Li Q / D lv
Where Aa = anaerobic pond area, m2/caput
Li Q = quantity of BOD, g/caput day
D = anaerobic pond depth, m
lv as described above
6.8.2. Facultative Ponds
The equation presented on 4.5.2 can be rewritten as:
Af = 10 Li Q/ ls
where Af = facultative pond area, m2/caput
Li Q = quantity of BOD, g/caput day
ls as described above
Note that total area calculated (Aa + Af) shall be multiplied by a factor of 1.25-1.5 (i.e.,
additional 25% to 50% land) to take into account the overall land area required for pond
operation and maintenance. 1.25 factor is suitable for large systems while 1.5 factor is
more suitable for small systems (Mara, 1998). When maturation ponds are required the
additional land area required for building and maintaining these ponds shall be added.
[Top]
6.9. WSP Hydraulics Considerations
Finney and Middlebrooks (1980) stated that consistent prediction of pond performance
by any design method without accurate projections of hydraulic residence time is
impossible. Shilton (2001) presented an extensive study on the hydraulics of
stabilization ponds. Twenty experimental configurations were tested in the laboratory
and ten of these experimental cases were mathematically modeled and had good
agreement with the experimental work. Shilton and Harrison (2003) then introduced
broad and informative guidelines for hydraulic design of WSP to "help fill the knowledge
gap in the pond hydraulics area". Although engineering judgment is always required,
and the current understanding of ponds hydraulics is still limited, the following
observations were proven to be useful for the purpose of improving WSP hydraulics,
and consequently ameliorating WSP design, performance and efficiency:


















Short-circuiting (when water enters and leaves the pond in a very short time)
shall be avoided as it results in a large reduction in the discharge quality.
Influent should be mixed into the main body of the pond to avoid localized
overloading, taking into consideration not to create short-circuiting.
The solids deposition within the pond occurs as a result of the flow, rather than
the flow being redirected as a result of the solids.
Inlet position and type has a significant impact on treatment efficiency in ponds.
Dropping inlets from horizontal pipes above the water have similar behavior as
submerged horizontal inlets.
For high-load wastewaters, horizontal inlets may be needed to mix wastewater
into the pond. Consider baffles and outlet positioning to avoid short-circuiting
problems.
For low-load wastewaters, consider a manifold or baffled vertical inlet but only
after consideration of wind influences.
Inlet positioning has a major influence on the flow pattern.
Designers need to consider the effect of inlet position in conjunction with outlet
position and pond shape/baffles.
A pond should maintain a similar and reasonably well defined flow pattern
through a range of different flow rates.
Outlets should be placed out of the main flow path of the incoming wastewater
(close into a corner).
Final outlet positioning can be selected after the inlet position/type and pond/
baffling have been designed.
Outlet manifolds are not recommended.
Long evenly spaced baffles improve pond performance. Baffles of 70% width
gave superior performance compared with 50% and 90% width.
Horizontal baffles were found to be more efficient than vertical baffles.
Longitudinal baffling was found to be no more efficient than transverse baffling.
Localizing baffles close to horizontal (but not other types!) inlets is generally
effective.
A minimum of two baffles in a pond is recommended. A further improvement was
achieved using four baffles and this extra cost may be warranted in some cases.
Based on Shilton and Harrison study (2003), more than four baffles would not be





recommended.
Traditional thinking that, in a long narrow pond, the influent simply flows slowly
from one end to the other is not necessarily correct except at very high length to
width ratio.
Baffles that shield the outlet are beneficial.
A diversion channel should be build around the pond (the topside) to divert storm
water runoff coming from adjacent areas.
PVC pipe, of at least 100 mm diameter is recommended for carrying effluent to
the pond and between ponds.
All ponds should be surrounded by a fence for public safety and health
protection.
[Top]
7. Conclusions






Natural treatment technologies are attracting a significant level of interest by
environmental managers. Natural treatment technologies are considered viable
because of their low capital costs, their ease of maintenance, their potentially
longer life-cycles (when compared to electro-mechanical solutions) and their
ability to recover a variety of resources including: treated effluent for irrigation,
organic humus for soil amendment and energy in the form of biogas. In fact, the
functional sustainability and longevity of any technology to provide services to
the local neighborhood can, and should be, directly correlated to the ability of
that intervention to recycle precious resources and to enable the production and
sale of products that can lead to the recovery of construction and operation
costs, while meeting the sanitation needs.
WSP proved to be one of the most efficient, high performance and low-cost
wastewater treatment technology used around the world. A WSP wastewater
treatment consisting of an anaerobic and facultative pond having a short
retention time and relatively shallow depths can produce high quality effluents.
Removals of BOD greater than 90%, nitrogen removal of 70-90%, and total
phosphorus removals of 30-45% are easily achievable in a series of welldesigned ponds (Mara and Pearson, 1998).
WSPs can attain a 99.999% faecal coliform reduction when operated in parallel,
and are capable of attaining a 100% removal of helminths, thus facilitating the
recovery of the wastewater for agriculture in both restricted and unrestricted
irrigation (WHO, 1987; Mara and Pearson, 1998). The greatest pathogen
reductions occur during the warm months, which coincide with the irrigation
season. During these times, effluent standards that meet unrestricted irrigation
are easily attained (Mara and Pearson, 1998).
The BOD removal in primary facultative ponds is usually in the range 70-80%
based on unfiltered samples (that is, including the BOD exerted by the algae),
and usually above 90% based on filtered samples. In secondary facultative
ponds the removal is less, but the combined performance of anaerobic and
secondary facultative ponds generally is slightly better than that achieved by
primary facultative ponds.
Anaerobic and facultative ponds when designed as a system can produce an




effluent suitable for surface water discharge with significantly less land
requirements than using a primary facultative pond.
An anaerobic pond followed by a facultative pond will produce effluent quality
suitable to be discharged to surface waterways. However, if wastewater will be
used for restricted or unrestricted irrigation, additional maturation pond(s) may be
sometime used (depends on the effluent quality requirements) succeeding the
facultative pond in order to polish the final effluent from faecal coliform, helminth
egg and nutrient excess. Maturation ponds are not designed for BOD removal,
but it is assumed that 25% filtered BOD removal can be achieved per pond for
temperatures above 20°C.
For hot climates, a minimum 25-day, 5-cell WSP system allows for almost
unrestricted irrigation and that restricted irrigation requires a 2-pond, 10-day
detention time for adequate pathogen destruction (Bartone (1991).
There is still some argument concerning the economical feasibility of using WSP
in urban areas where land price is relatively high. Yu, et al., 1997 argues that
WSP requires large land areas and, consequently, lose their comparative cost
advantage over mechanized treatment systems when land prices are greater
than US$ 15-20/m2. However, Mara and Pearson (1998) contend that even at
high land costs, WSPs are often the cheapest option and the question is: "Do
you pay for the required land area up front, or for continuously high consumption
of electricity in the future?" Often, municipalities can consider WSPs to be an
investment in real-estate (Mara and Pearson, 1998). Furthermore, Mara (2001)
argued that the theory of the "extremely land intensive" WSP system is wrong.
His research in northern Brazil (Pearson et al., 1995 and 1996) shows that a 1 to
2-day anaerobic pond and a 3 to 6-day facultative pond can produce an effluent
suitable for restricted irrigation, where the combined area required for both ponds
is as low as 0.35 m2 per person.
Effluent quality requirements vary from one country and another. However, some
common effluent quality limits are widely recommended and used (European
Union, World Heath Organization…etc.). These effluent quality can be
summarized as follows:
Filtered BOD < 25 mg/l
TTS < 150 mg/l
Nematode egg < 1 /l
Faecal coliform count < 1000 per 100 ml (for unrestricted irrigation only).
[Top]
8. References
Abis, Karen L. (2002). The performance of facultative waste stabilization ponds in the
United Kingdom. Ph.D. thesis, U.K. University of Leeds.
Arar A. (1988). Background to treatment and use of sewage effluent. Ch. 2, Treatment
and Use of Sewage Effluent for Irrigation, M.B. Pescod and A. Arar (eds). Butterworths,
Sevenoaks, Kent.
Arridge, H., Oragui, J.I., Pearson, H.W., Mara, D.D. and Silva, S.A. (1995). Vibrio
Cholerae 01 and Sallmonellae removal compared with the die-off of faecal indicator
organisms in waste stabilization ponds in northeast Brazil. Water Science Technology,
31 (12), pp.249-256.
Arthur J.P. (1983). Notes on the design and operation of waste stabilization ponds in
warm climates of developing countries. Urban Development Technical Paper No. 6.
World Bank, Washington DC. 106 p.
Ayres, R.M., Alabaster, G.P., Mara, D.D. and Lee, D.L. (1992). A design equation for
human intestinal nematode egg removal in waste stabilization ponds. Water Research,
26 (6), 863-865.
Bartone, C. R. (1991). International perspective on water resources management and
wastewater reuse - appropriate technologies. Water, Science & Technology, 23: 20392047.
Boutin, P., Vachon, A. and Racault, Y. (1987). Waste stabilization ponds in France: an
overall view. Water Science and Technology, 19 (12), pp.25-31.
Bucksteeg, K. (1987). German experiences with sewage treatment ponds. Water
science and Technology, 19 (12), pp.17-23.
Council of the European Communities (1991). Council Directive of 21 May 1991
concerning urban wastewater treatment (91/271/EEC). Official Journal of the European
Communities, L135/40 (30 May).
CPCB (1996). Pollution Control Acts, Rules and Notifications Issued Thereunder, 4th
edition. New Delhi: Central Pollution Control Board.
Droste, R.L. (1997). Theory and Practice of Water and wastewater Treatment. John
Wiley and Sons. New York, USA.
EPA (1983). Design Manual: Municipal Wastewater stabilization Ponds. Report No.
EPA-625/1-83-015. Cincinnati: Environmental Protection Agency, Center for
Environmental Research information.
Finney, B. and Middlebrooks, E. (1980). Facultative waste stabilization pond design.
Journal of the Water Pollution Control Federation, 52(1): 134-147.
Gambrill M.P., Mara D.D., Eccles C.R. and Baghaei-Yazdi N. Microcomputer-aided
design of waste stabilization ponds in tourist areas of Mediterranean Europe. Public
Health Engineer. 14(2): 39-41.
Glonya, E.F. and Espino, E. (1969). Sulfide production in waste stabilization ponds.
Journal of the Sanitary Engineering Division, American Society of Civil Engineers,
95(SA3), pp.607-628.
Green, F., Bernstone, L., Lundquist, T. and Oswald, W. (1996). Advanced integrated
wastewater pond systems for nitrogen removal. Water Science and Technology.
33(7):207-217.
Kaplan, Drora, Abeliovich, Aharon et Ben-Yaakov, Sam. (1987). The Fate of Heavy
Metals in Wastewater Stabilization Ponds. Water Research, Vol. 21, No. 10, 11891194.
Knorr, A.E. and Torrella, F. (1995). Microbiological performance and Salmonella
dynamics in a wastewater depuration pond system of southeastern Spain. Water
Science Technology, 31 (12), pp.239-248.
Mara D.D. (1976) Sewage Treatment in Hot Climates. John Wiley, London.
Mara, D.D. (2001), Low-cost, high performance wastewater treatment and reuse for
public health and environmental protection in the 21st century. 10th Annual CWWA
conference on Innovation Technologies in the Water and Wastewater Industries for the
21st century, Grand Cayman October 2001.
Mara, D. D., Alabaster, G.P., Pearson, H.W., Mills S.W. (1992). Waste Stabilisation
Ponds - A Design Manual for Eastern Africa. Leeds: Lagoon Technology International
Ltd.
Mara, D. D. and Pearson, H. (1998). Design Manual for Waste Stabilization Ponds in
Mediterranean Countries, Leeds: Lagoon Technology International Ltd.
Mara, D. D. and Pearson, H. (1986). Artificial Freshwater Environment: Waste
Stabilisation Ponds. In: Biotechnology (Rehm and Reeds, eds.). VCH
Verlagsgesellschaft, Weinheim, Germany.
Mara, D.D., Pearson, H.W., Oragui, J.I, Arridge, H. and Silva, S.A.(2001). Development
of a new approach to waste stabilization pond design. School of Civil Engineering,
University of Leeds, Leeds, England.
Marais, G.v.R. (1970). Dynamic behaviour of oxidation ponds. In Proceedings of the
Second International Symposium on Waste Treatment Lagoons (ed. R.E. McKinney),
pp. 15-46. Laurence, KS: University of Kansas.
McGarry M.G. and Pescod M.B. (1970) Stabilization pond design criteria for tropical
Asia. Proc. 2nd Int. on Waste Treatment Lagoons, Kansas City, pp.114-132.
Meiring P.G., Drews R.J., van Eck H. and Stander G.J. (1968) A guide to the use of
pond systems in South Africa for the purification of raw and partially treated sewage.
CSIR Special Report WAT34. National Institute for Water Research, Pretoria.
Middlebrooks, E. J. (1995). Upgrading pond effluents: An overview. Water, Science &
Technology, 31 (12): 353-368.
Moshe, Meir. (1972). Effect of Industrial Wastes on Oxidation Pond Performance. Water
Research, Vol. 6, 1165-1171.
Oragui, J.I, Arridge, H., Mara, D.D., Pearson, H.W. and Silva, S.A. (1995). Rotavirus
removal in experimental waste stabilization pond system with different geometries and
configurations. Water Science Technology, 31 (12), pp.285-290.
Oswald, W. J. (1990). Advanced integrated wastewater pond systems. Proceedings of
the ASCE convention: Supplying water and saving the environment for six billion
people. San Franciso, CA., 5-8 Nov, 1990, pp: 73-80.
Pano, A. and Middlebrooks, E.J. (1982). Ammonia nitrogen removal in facultative
wastewater stabilization ponds. Journal of the Water Pollution Control Federation,
54(4), 344-351.
Pearson, H. W., Mara, D. D. and Arridge H.M. (1995). The effluence of pong geometry
and configuration of facultative and maturation waste stabilization pond performance
and efficiency. Water Science Technology, Vol. 31 No.12, pp.129-139.
Pearson, H. W., Mara, D. D., Crawley L.R., Arridge H.M. and Silva S.A. (1996). The
performance of an innovative tropical experimental waste stabilization pond system
operation at high organic loadings. Water Science Technology, Vol. 33 No.7, pp.63-73.
Pescod M.B. and Mara D.D. (1988) Design, operation and maintenance of wastewater
stabilization ponds. Ch. p, Treatment and Use of Sewage Effluent for Irrigation. M.B.
Pescod and A. Arar (eds). Butterworths, Sevenoaks, Kent.
Polprasert, C. and Charnpratheep, K. (1989). Heavy Metal Removal in Attached-Growth
Waste Stabilization Ponds. Water Research, Vol. 23, No. 5, 625-631.
Reed, S.C. (1985). Nitrogen removal in wastewater stabilization ponds. Journal of the
Water Pollution Control Federation, 57 (1), 39-45.
Rinzima, A. (1988). Anaerobic treatment of wastewater with high concentrations of
lipids and sulphate. Ph.D. thesis, Wageningen Agricultural University, Wageningen, The
Netherlands.
Saidam, M., Ramadan, S. & Butler, D. (1995). Upgrading waste stabilization pond
effluent by rock filter. Water Science & Technology, 31(12): 369-378.
Shaw, V.A. (1962).An assessment of the probable influence of evaporation and
seepage on oxidation pond design and construction. Journal of the Institute of Sewage
Purification, (4), 359-365.
Sheaffer International LTD. (1998). An idea whose time has come. Sheaffer
International LTD., Promotional literature.
Shilton Andy (2001). Studies Into the Hydraulics of Waste Stabilization ponds. Ph.D.
Thesis. Massey University, Palmerston North, New Zealand.
Shilton A. and Harrison J. (2003). Guidelines for the Hydraulic Design of the Waste
Stabilization ponds. Institute of Technology and Engineering, Massey University,
Palmerston North, New Zealand.
Shuval H.I., Adin A., Fattal B., Rawitz E. and Yekutiel P. (1986) Wastewater irrigation in
developing countries: health effects and technical solutions. Technical Paper No. 51.
World Bank, Washington DC.
Soares, J., Silva, S.A., Oliveira, R., Araujo, A.L.C., Mara, D.D. and Pearson, H.W.
(1996). Ammonia removal in a pilot-scale WSP complex in northeast Brazil. Water
Science Technology, 33 (7), pp.165-171.
Uddin, M. S. (1970). Anaerobic Pond Treatment of Tapioca Starch Waste, Master
Thesis. Bangkok: Asian Institute of Technology.
Tchobanoglous, G. and Schroeder, E. (1985). Water Quality Characteristics, Modeling,
Modification. Addison-Wesley; Reading, Massachusetts, USA.
Van Haandel, A.C., Lettinga, G. (1994). Anaerobic Sewage Treatment: A Practical
Guide for Regions with a Hot Climate. John Wiley & Sons.
Yu, H., Tay, J. & Wilson, F. (1997). A sustainable municipal wastewater treatment
process for tropical and subtropical regions in developing countries. Water, Science &
Technology, 35 (9): 191-198.
Zendher, A.J., Ingvorsen, K. and Marti, T. (1982). Microbiology of methanogen bacteria.
In: Anaerobic Digestion. Elsevier, Amsterdam, The Netherlands.
Zhao, Q. & Wang, B.(1996). Evaluation on a pilot-scale attached-growth pond system
treating domestic wastewater. Water Resource, 30: 242-245.