Waste stabilization ponds for wastewater treatment

FAQ sheet on waste stabilization ponds, prepared by Cinara, Colombia

Introduction

Waste water stabilization pond technology is one of the most important natural methods for wastewater treatment. Waste stabilization ponds are mainly shallow man-made basins comprising a single or several series of anaerobic, facultative or maturation ponds The primary treatment takes place in the anaerobic pond, which is mainly designed for removing suspended solids, and some of the soluble element of organic matter (BOD5). During the secondary stage in the facultative pond most of the remaining BOD5 is removed through the coordinated activity of algae and heterotrophic bacteria. The main function of the tertiary treatment in the maturation pond is the removal of pathogens and nutrients (especially nitrogen). Waste stabilization pond technology is the most cost-effective wastewater treatment technology for the removal of pathogenic micro-organisms. The treatment is achieved through natural disinfection mechanisms. It is particularly well suited for tropical and subtropical countries because the intensity of the sunlight and temperature are key factors for the efficiency of the removal processes. [1].

Water treatment in waste stabilization ponds

Anaerobic ponds

These units are the smallest of the series. Commonly they are 2-5 m deep and receive high organic loads equivalent to100 g BOD5/m3 d. These high organic loads produce strict anaerobic conditions (no dissolved oxygen) throughout the pond. In general terms, anaerobic ponds function much like open septic tanks and work extremely well in warm climates. A properly designed anaerobic pond can achieve around 60% BOD5 removal at 20° C. One-day hydraulic retention time is sufficient for wastewater with a BOD5 of up to 300 mg/l and temperatures higher than 20° C. Designers have always been preoccupied by the possible odour they might cause. However, odour problems can be minimised in well designed ponds, if the SO42- concentration in wastewater is less than 500 mg/l. The removal of organic matter in anaerobic ponds follows the same mechanisms that take place in any anaerobic reactor. [1]; [2].

Facultative ponds

These ponds are of two types: primary facultative ponds receive raw wastewater, and secondary facultative ponds receive the settled wastewater from the first stage (usually the effluent from anaerobic ponds). Facultative ponds are designed for BOD5 removal on the basis of a low organic surface load to permit the development of an active algal population. This way, algae generate the oxygen needed to remove soluble BOD5. Healthy algae populations give water a dark green colour but occasionally they can turn red or pink due to the presence of purple sulphide-oxidising photosynthetic activity [3]. This ecological change occurs due to a slight overload. Thus, the change of colouring in facultative ponds is a qualitative indicator of an optimally performing removal process. The concentration of algae in an optimally performing facultative pond depends on organic load and temperature, but is usually in the range 500 to 2000 μg chlorophyll per litre. The photosynthetic activity of the algae results in a diurnal variation in the concentration of dissolved oxygen and pH values. Variables such as wind velocity have an important effect on the behaviour of facultative ponds, as they generate the mixing of the pond liquid. As Mara et al. [1] indicate, a good degree of mixing ensures a uniform distribution of BOD5, dissolved oxygen, bacteria and algae, and hence better wastewater stabilization. More technical details on the efficiency of the process and removal mechanisms can be found in Mara et al. [1]; and Curtis [4].

Maturation ponds

These ponds receive the effluent from a facultative pond and its size and number depend on the required bacteriological quality of the final effluent. Maturation ponds are shallow (1.0-1.5 m) and show less vertical stratification, and their entire volume is well oxygenated throughout the day. Their algal population is much more diverse than that of facultative ponds. Thus, the algal diversity increases from pond to pond along the series. The main removal mechanisms especially of pathogens and faecal coliforms are ruled by algal activity in synergy with photo-oxidation. More details on these removal mechanisms in maturation ponds can be found in Curtis [4].

On the other hand, maturation ponds only achieve a small removal of BOD5, but their contribution to nitrogen and phosphorus removal is more significant. Mara et al. [1] report a total nitrogen removal of 80% in all waste stabilization pond systems, which in this figure corresponds to 95% ammonia removal. It should be emphasised that most ammonia and nitrogen is removed in maturation ponds. However, the total phosphorus removal in WSP systems is low, usually less than 50% [1]; [3].

Operation and maintenance

Starting up the system. Once the construction of the system has been completed it should be checked that all ponds are free of vegetation. This is very important if the waste stabilization pond is not waterproof. Facultative ponds should be filled prior to anaerobic ponds to avoid odour release when anaerobic pond effluent discharges into an empty facultative pond [5]. Anaerobic ponds should be filled with raw wastewater and seeded whenever possible with biosolids from another anaerobic reactor. Later, the anaerobic ponds can be gradually loaded up to the design’s loading rate. This gradual loading period can be from one to four weeks depending on the quality of the digester used or in case the pond was not seeded during the start-up procedure. It is important to measure the pH in the anaerobic pond and maintain it above 7 to permit the development of the methanogenic bacterial population. During the first month it may be necessary to add lime, to avoid the acidification of the reactor.

Initially, facultative and maturation ponds should be filled with freshwater from a river, lake or well, so as to permit the gradual development of the algal and heterotrophic bacterial population. If freshwater is unavailable, facultative ponds should be filled with raw wastewater and left for three to four weeks to allow the aforementioned microbial populations to develop. A small amount of odour release is inevitable during the implementation of the latter method in the facultative pond.

Routine maintenance

Once the waste stabilization ponds have started to operate, it is necessary to carry out regular routine maintenance tasks. Although simple, these tasks are essential to the good operation of the system. According to Mara and Pearson [5], routine maintenance tasks are as follows:

• Removal of screening and grit retained in the inlet works during the preliminary treatment.
• Cutting, pruning and removing the grass and vegetation that grows on the embankment to prevent it from falling into the pond and generating the formation of mosquito breeding habitats. The use of slow-growing grass or vegetation is recommended to minimise the frequency of this task.
• Removal of floating scum and macrophytes (e.g. Lemna spp.) from facultative and maturation ponds to maximise photosynthesis and surface re-aeration, and prevent fly and mosquito breeding.
• Spraying the scum on the surface of anaerobic ponds (which should not be removed as it aids the treatment process). In the event fly breeding is detected this material should be sprayed with clean water.
• Removal of any accumulated solids in the pond’s inlets and outlets.
• Repair of any damage to the embankments caused by rodents or other animals.
• Repair of any damage to external fences and gates or points of access to the system.

The operator responsible should register these activities in a pond maintenance record sheet. Usually this operator is also in charge of taking samples and measurements of the pond’s effluent flow. Monitoring and evaluation activities can be consulted in a specialised bibliography [1].

Cost considerations of waste stabilization pond systems

A World Bank study carried out by Arthur [6] gives a detailed economic comparison of waste stabilization ponds, aerated lagoons, oxidation ditches and trickling filters (Figure 1).

The data for this study, which produced the curves in Figure 1, were taken from the city of Sana’a in the YemenArabRepublic [6]. If the cost of land is variable for a constant discount rate (opportunity cost of capital) of 12%, then the net present value varies as shown in Figure 1a. Note that waste stabilization pond technology is a cheaper option up to a land cost of U.S $ 7.8/m2. Above this cost, oxidation ditches become the cheapest option. For discount rates between 5 and 15%, the choice will always be between waste stabilization ponds and oxidation ditches. The other two systems considered are more expensive. Figure 1b shows the variation of the land cost as a function of the discount rate for the interval in which waste stabilization ponds are cheapest, this is between US $ 5 and 15 per m2 (US $ 50.000 and 150.000 per ha).

Fig. 1a. Net present value versus land cost
Text next to the lines from top to bottom: Areated lagoon system; Biological filter; Waste stabilization pond system; Oxidation ditch

Fig. 1b. Net present value (US$ per square metre) versus discount rate
Waste stabilization ponds are the cheapest treatment option

With these levels of costs, even the most productive land in developing countries is reasonably priced, which ensures the feasibility of waste stabilization ponds as a sustainable alternative for wastewater treatment. In real terms, in specific situations the main constraint against selecting this technology is not land cost but land availability. Thus, the argument of the high costs of land—due to the large areas of land required for waste stabilization ponds—cannot stand when a rigorous economic evaluation of all the project's costs is carried out.

References

[1] Mara, D.D., Alabaster, G.P., Pearson, H.W. and Mills, S.W. (1992). Waste Stabilization Ponds: A Design Manual for Eastern Africa . Lagoon Technology International. Leeds, England.

See also: powerpoint presentation of Prof. D.D. Mara on waste stabilization ponds [PDF file], http://www.leeds.ac.uk/civil/ceri/water/tphe/publicat/wspwarm/wsp-slides.pdf

[2] Peña, M.R. (2002). Advanced primary treatment of domestic wastewater in tropical countries: development of high-rate anaerobic ponds. Ph.D thesis. School of Civil Engineering, University of Leeds. Leeds, United Kingdom. Url: http://www.leeds.ac.uk/civil/ceri/water/tphe/publicat/theses/penavaron/penavaron.html

[3] Mara, D.D. and Pearson, H.W. (1986). Artificial freshwater environments: Waste stabilization ponds. In: Biotechnology. Vol 8. (ed. W. Schoenborn), pp. 177-206. Weinheim: VCH Verlagsgesellschaft.

[4] Curtis, T.P. (1994). The effect of sunlight on mechanisms for the die-off of faecal coliform bacteria in waste stabilization ponds. Ph.D thesis. School of Civil Engineering, University of Leeds. Leeds, UK. URL abstract: http://www.leeds.ac.uk/civil/ceri/water/tphe/publicat/monog/Res-mon1.doc

[5] Mara, D.D. y Pearson, H.W. (1998). Design manual for waste stabilization ponds in Mediterranean countries. European Investment Bank. Lagoon Technology International. Leeds, United Kingdom. URL: http://www.leeds.ac.uk/civil/ceri/water/tphe/publicat/pdm/med/medman.html

[6] Arthur, J.P. (1983). Notes on the design and operation of waste stabilization ponds in warm climates of developing countries. Technical paper No 7. Washington D.C: The World Bank. URL: http://www-wds.worldbank.org/servlet/WDS_IBank_Servlet?pcont=details&eid=000178830_98101904165457 incl. link to scanned document.

Bibliography and additional reading

Comisión Nacional del Agua; Instituto Mexicano de Tecnología del Agua (IMTA) (1994). Manual de agua potable alcantarillado y saneamiento. Libro II. Proyecto 3ª sección: potabilización y tratamiento. Tema: tratamiento. Subtema: lagunas de estabilización. IMTA, Mexico.

Sanitation Connection - wastewater treatment technology [website]. Partnership of organisations working in environmental sanitation. Their wastewater treatment topic provides publications, websites, and a mailinglist on stabilization ponds. URL: http://www.sanigate.net/titles/topicintro.php3?topicId=6

Shilton, A.; Harrison, J. (2003). Guidelines for the Hydraulic Design of Waste Stabilization Ponds. Institute of Technology and Engineering, Massey University, Palmerston North, New Zealand. URL: http://www.leeds.ac.uk/civil/ceri/water/tphe/publicat/pdm/nzealand/nzealand.html

Waste stabilisation ponds in warm climates [website]. Website of the University of Leeds, School of Civil Engineering, Tropical Public Health Engineering . Provides links to publications and presentations on the topic. URL: http://www.leeds.ac.uk/civil/ceri/water/tphe/publicat/wspwarm/wspwarm.html

Contact persons

• Prof. David Duncan Mara. Escuela de Ingeniería Civil, University of Leeds. United Kingdom
• Prof. Marcos von Sperling. Departamento de Ingeniería Sanitaria y Ambiental, Universidad Federal de Minas Gerais. Brasil
• Dr. Miguel R. Peña Varón. Instituto Cinara, Universidad del Valle. Cali, Colombia

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Date written: 10 Oct 2003
Date revised:
Author: Dr. Miguel Ricardo Peña Varón
Organisation: Universidad del Valle, Instituto Cinara. Cali, Colombia
Reviewed by: Arlex Sánchez T., Instituto Cinara. Cali, Colombia. At that time working as Junior Professional Officer at IRC International Water and Sanitation Centre
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