Waste Stabilisation Ponds

Waste stabilization ponds are large man-made basins in which greywater, blackwater or faecal sludge can be treated to an effluent of relatively high quality and apt for the reuse in agriculture (e.g. irrigation) or aquaculture (e.g. macrophyte or fish ponds). They are semi-centralised treatment systems combined after wastewater has been collected from toilets (see also wastewater collection and user interface). For the most effective treatment, WSPs should be linked in a series of three or more with effluent being transferred from the anaerobic pond to the facultative pond and, finally, to the aerobic pond. The anaerobic pond is the primary treatment stage and reduces the organic load in the wastewater. The entire depth of this fairly deep man-made lake is anaerobic. Solids and BOD removal occurs by sedimentation and through subsequent anaerobic digestion inside the accumulated sludge (see also anaerobic digestion general). Anaerobic bacteria convert organic carbon into methane and through this process, remove up to 60% of the BOD.

In a series of WSPs, the effluent from the anaerobic pond is transferred to the facultative pond, where further BOD is removed. The top layer of the pond receives oxygen from natural diffusion, wind mixing and algae-driven photosynthesis. The lower layer is deprived of oxygen and becomes anoxic or anaerobic. Settleable solids accumulate and are digested on the bottom of the pond. The aerobic and anaerobic organisms work together to achieve BOD reductions of up to 75%.

Anaerobic and facultative ponds are designed for BOD removal, while aerobic ponds are designed for pathogen removal (see also pathogens and contaminants). An aerobic pond is commonly referred to as a maturation, polishing, or finishing pond because it is usually the last step in a series of ponds and provides the final level of treatment. It is the shallowest of the ponds, ensuring that sunlight penetrates the full depth for photosynthesis to occur. Photosynthetic algae release oxygen into the water and at the same time consume carbon dioxide produced by the respiration of bacteria. Because photosynthesis is driven by sunlight, the dissolved oxygen levels are highest during the day and drop off at night. Dissolved oxygen is also provided by natural wind mixing.

The major disadvantages of WSPs are a rather long process of days to week (MARA & PEARSON 1998; ROSE 1999) and requirement of large areas of land far away from homes and public spaces for the construction (DEPARTMENT FOR INTERNATIONAL DEVELOPMENT 1998). However, because of the low capital and particularly low O&M costs it is a good option for decentralised treatments in developing countries. In addition, it is one of the few low-cost natural processes, which provides good treatment of pathogens.

Anaerobic ponds are built to a depth of 2 to 5 m and have a relatively short detention time of 1 to 7 days. Facultative ponds should be constructed to a depth of 1 to 2.5 m and have a detention time between 5 to 30 days. Aerobic ponds are usually between 0.5 to 1.5 m deep with a detention time of 15 to 20 days . If used in combination with algae and/or fish harvesting (see Fish Pond), this type of pond is effective at removing the majority of nitrogen and phosphorus from the effluent. Ideally, several aerobic ponds can be built in series to provide a high level of pathogen removal.

Pre-treatment (see Pre Treatment Technologies) is essential to prevent scum formation and to hinder excess solids and garbage from entering the ponds. To prevent leaching into the groundwater, the ponds should have a liner. The liner can be made from clay, asphalt, compacted earth, or any other impervious material. To protect the pond from runoff and erosion, a protective berm should be constructed around the pond using the excavated material. A fence should be installed to ensure that people and animals stay out of the area and that garbage does not enter the ponds.

Only slightly polluted wastewater may be discharged directly into primary facultative ponds. Depending on the requirement for the final effluent in terms of pathogen reduction, only anaerobic and facultative ponds are necessary in some instances.

Comparison of the treatment performance of different waste stabilisation ponds. Source: WSP (2007)

Anaerobic Treatment Ponds (APs)

The main function of anaerobic ponds is BOD removal, which can be reduced 40 to 85 % (WSP 2007).
 As a complete process, the anaerobic pond serves to:

• Settle undigested material and non-degradable solids as bottom sludge
• Dissolve organic material
• Break down biodegradable organic material

BOD removal in anaerobic ponds is governed by the same mechanisms that occur in all other anaerobic reactors (MARA et al. 1992) and anaerobic ponds do not or only rarely contain algae. The process (as in septic tanks) relies on the sedimentation of settable solids and subsequent anaerobic digestion in the resulting sludge layer. During anaerobic digestion, biogas is produced which could be collected by covering the anaerobic pond with a floating plastic membrane (PENA VARON 2004; WAFLER 2008). The recovered biogas can be used for heating, cooking or, if sufficient amounts can be collected for energy production (biogas combustion and biogas electricity small-scale).

Facultative Treatment Ponds (FPs)

Facultative Treatment Ponds are the simplest of all WSPs and consist of an aerobic zone close to the surface and a deeper, anaerobic zone. They are designed for BOD removal and can treat water in the BOD range of 100 to 400 kg/ha/day corresponding to 10 to 40 g/m2/day at temperatures above 20°C (MARA and PEARSON, 1998).

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). Additional oxygen can be introduced by wind due to vertical mixing of the water. Oxygen is unable to be maintained at the lower layers if the pond is too deep, and the colour too dark to allow light to penetrate fully or if the BOD and COD in the lower layer is higher than the supply. As a result of the photosynthetic activities of the pond algae, there is a diurnal variation in the concentration of dissolved oxygen. At peak sun radiation, the pond will be mostly aerobic due to algal activity, while at sunrise the pond will be predominantly anaerobic (ERTAS et al. 2005).

The facultative pond serves to:

• Further treat wastewater through sedimentation and aerobic oxidation of organic material
• Reduce odour
• Reduce some disease-causing microorganisms if pH raises
• Store residues as bottom sludge

FPs loose ammonia into the air at high pH; and settle some nitrogen and phosphorus in the sludge. FPs can result in the removal of 80 to 95% of the BOD5 (SPERLING 2007), which means an overall removal of 95% over the two ponds (AP and FP). Total nitrogen removal in WSP systems can reach 80% or more, and ammonia removal can be as high as 95%. To remove the algae from aerobic pond, effluents’ rock filtration, grass plots, floating macrophytes and herbivorous fish can be used, but most commonly, the effluent flows directly in a final maturation pond.

Aerobic / Maturation Ponds (MPs)

Whereas anaerobic and facultative ponds are designed for BOD removal, maturation or polishing ponds are essentially designed for pathogen removal and retaining suspended stabilised solids (MARA et al. 1992; SASSE, 1998; TILLEY et al. 2008). The size and number of maturation ponds depends on the required bacteriological quality of the final effluent. The principal mechanisms for faecal bacterial removal in facultative and maturation ponds are HRT, temperature, high pH (> 9), and high light intensity. Virus and microorganisms get also removed. If used in combination with algae and/or fish harvesting, this type of pond is also effective at removing the majority of nitrogen and phosphorus from the effluent (TILLEY et al. 2008). Some further information on the physical design is given in ARTHUR (1983) and the international Water and Sanitation Centre (2004).

To prevent leaching, the ponds should have a liner. The liner can be clay, asphalt, compacted earth, or another impervious material. Although effluent from aerobic ponds is generally low in pathogens, the ponds should in no way be used for recreation or as a direct source of water for consumption or domestic use. A berm can protect from erosion or the invasion by vegetation and a fence can protect the lagoons from people and animals and prevent that garbage is thrown in. For the restricted and unrestricted reuse of the effluent in agri- and aquaculture, please refer to the WHO (2006) guidelines.

According to the International Water and Sanitation Centre (IRC), stabilisation ponds are the most cost-effective (semi-)centralised wastewater treatment technology for the removal of pathogenic microorganisms. However, this depends on the availability of land and its price.

Stabilisation ponds also have the advantage of very low operating costs since they use no energy compared to other wastewater treatment technologies and only low-tech infrastructure (see also operation and maintenance and ensuring sustainability). This makes them particularly suitable for developing countries where many conventional wastewater treatment plants have failed because water and sewer utilities did not generate sufficient revenue to pay the electricity bill for the plant (IRC 2004) (see also -financing projects). However, expert design is still required (see also developing human resources).

Further, the ponds can be combined with aquaculture to locally produce animal feed (e.g. duckweed) or fish (e.g. fish ponds). Biogas may also be recovered for use when anaerobic ponds are covered with a floating plastic membrane (PENA VARON 2004) (see also reuse of biogas).

Scum that builds up on the pond surface should be regularly removed. Aquatic plants that are present in the pond should also be removed as they may provide a breeding habitat for mosquitoes and prevent light from penetrating the water column. The WHO (WHO 2005 in MOREL & DINER 2006) does not promote pond systems if appropriate mosquito control measures are not guaranteed.

The anaerobic pond must be de-sludged approximately once every 2 to 5 years, when the accumulated solids reach one third of the pond volume. For facultative ponds sludge removal is even rarer and maturation ponds hardly ever need desludging. Sludge can be removed by using a raft-mounted sludge pump, a mechanical scraper at the bottom of the pond or by draining and dewatering the pond and removing the sludge with a front-end loader.

If the water is reused for irrigation, the salinity of the effluent should be controlled regularly in order to prevent negative impact on the soil structure.