Dear Faraj,

The principal gross misconceptions concerning WSP are: odour, insect (especially mosquito) breeding, high effluent suspended solids, high land take, high water loss due to evaporation, and groundwater contamination.

Odour. Well-designed, well-maintained and not overloaded WSP do not give

rise to odour nuisance. Odour from anaerobic ponds can be prevented at the

design stage . Odour does occur with overloaded WSP, but not to a greater extent than with other forms of treatment. 

Mosquito breeding does not occur in well-maintained ponds. Mosquitoes

need both water and shade for breeding, and shade is only provided in WSP by

emergent vegetation or grass growing down the embankment into the pond.

This does not occur if the WSP are properly maintained .

High effluent suspended solids. The EU Directive on urban wastewater

treatment (Council of the European Communities, 1991a) permits WSP

effluents to contain up to 150 mg suspended solids per litre, since it is recognised

that most (70-90 percent) of the suspended solids are algae and hence

environmentally less damaging .

High land take. WSP do, of course, require more land than other treatment

processes. However, as noted in Box 1, an honest economic or financial

appraisal will indicate that WSP, even with a high land take, are often the

cheapest option. The question generally is: do you pay for a large land area now,

or for a continuously high consumption of electricity in the future? Land

purchased for WSP can often be a real-estate investment. In member States of the European Union farmers are frequently paid not to farm part of their land. This so-called “set aside” land could clearly be leased by sewerage authorities for WSP.

High water loss. In the arid and semi-arid areas of the Region water is a

valuable commodity, and a high evaporative loss from WSP reduces the quantity

of treated wastewater available for crop irrigation (Section 12). However, this

loss is rarely more than 10 percent even in desert areas (see Design Example No.

3 in Annex I). This lost water does, of course, have a value, but the proper

question should be: is its value higher or lower than the energy costs of

alternative electromechanical treatment?

Groundwater contamination is not a problem if the physical design of WSP

has been properly done. The key parameter is the coefficient of soil

permeability as this determines whether or not the ponds need to be lined. If the coefficient of permeability is > 10–9 m/s and the groundwater is used as a source of potable supply, specialist hydrogeological advice should be obtained. A further important perceived disadvantage of WSP is that they are sometimes considered inferior in some way to other treatment technologies –despite their clear advantages of simplicity, low cost and high efficiency. Aerated lagoons used to be frequently promoted as a better alternative to WSP, but their energy costs are high, and it is not uncommon for the aerators to be permanently switched off. The result is that the aerated lagoon then functions as an anaerobic pond. Provided this is recognised and the resulting anaerobic pond is not overloaded and regularly desludged, BOD removal efficiency can be as high as in the aerated lagoon but

without, of course, its high energy costs.

A more recent technology which is being increasingly promoted is the upflow

anaerobic sludge blanket (UASB) reactor (see, for example, van Haandel and

Lettinga, 1994). This should really only be considered as an alternative to

anaerobic ponds as some form of subsequent treatment is required (and WSP are

noted as being especially suitable for this purpose – see van Haandel et al.,

1996). However, while UASB reactors have been found to be appropriate for

the treatment of high strength industrial wastewaters, there is less experience of

their treatment of domestic wastewaters, especially at temperatures below 20oC

(van Haandel et al., 1996). A two-stage UASB system has been recommended

for the treatment of domestic and municipal wastewater below 20oC, but this is

an even more complex technology and further research and development is

needed before it can be considered a viable option (van Haandel et al., 1996).

High-rate algal ponds (HRAP) hold considerable promise for their primary

purpose, which is the production of large quantities of high-quality algal protein,

rather than wastewater treatment per se (Oswald, 1995). However, despite

many years of research, principally in California and Israel, HRAP are not yet at

the stage of large-scale application. Little has changed since their review in the

WHO Waste Stabilization Pond Design Manual for Mediterranean Europe

(Mara and Pearson, 1987).

Similar arguments can be made in the case of the Advanced Integrated Pond

System (AIPS) recently developed in California (Green et al., 1995b). With

AIPS the wastewater enters a 4-5 m deep facultative pond containing a “digester

pit”, which functions much like an anaerobic pond but, in this case, within the

facultative pond, rather than preceding it. The facultative pond effluent is

discharged into a stirred high-rate pond, then into a settling pond to remove most

of the algae produced in the high-rate pond, and thence into maturation ponds

for biological disinfection. Recirculation of some of the high-rate pond contents

back to the surface layers of the facultative pond ensures odourless conditions in

the latter. Operation and maintenance are thus greater than with conventional

WSP, to which AIPS have not been shown to be superior.

The same is largely true of macrophyte ponds both floating and rooted (the

latter sometimes being called gravel bed hydroponic systems – see Williams

et al., 1995) and also of constructed wetlands (which generally comprise a

selection of both types of macrophytes). Reed beds are

about twice as expensive as WSP. Their operation and maintenance

requirements are significantly higher (Mara and Pearson, 1987) and mosquito

(especially Mansonia spp.) breeding is a serious problem if there is a free water surface.

For more on this topic, please use the following link:

http://www.efm.leeds.ac.uk/CIVE/Sewerage/articles/medall/medall02.pdf

Hoping this will be helpful,

Rafik

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 pondto 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 laststep 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 in ROSE 1999) and requirement of large areas of land far away from homes and public spaces for the construction (DFID 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 ofpathogens.