Pros and cons of WSPs

The TOP authors cite three primary advantages of WSPs over all other wastewater treatment options:

• Simplicity
• Low cost
• High efficiency

Simplicity

A WSP system consists of a series of man-made earth basins in which natural processes remove the polluting load in sewage effluents to produce a chosen quality of effluent, which can be purified enough to use in agriculture or fish farms. The principle construction activity is earthmoving (plus lining of the basin with a membrane if the earth is too permeable). The only other civil works are grit channels for preliminary treatment, pond inlets and outlets, and embankment protection. Routine operation and maintenance involves cutting the embankment grass, removing scum and floating vegetation from pond surfaces, keeping inlets and outlets clear, and repairing any embankment damage.

Low cost

The big advantage that ponds have over all the alternatives is that there is no need for expensive electromechanical equipment. That means savings on capital costs, but also electrical energy costs of virtually zero and a minimal need for skilled labour. The TOP illustrates WSPs’ cost advantage over aerated lagoons, oxidation ditches and biological filters with a 1983 example, used as a model for any analysis of wastewater treatment options. It shows WSPs having a higher initial capital cost than the equivalent oxidation ditch, but coming out with a significantly lower Net Present Value than all the others when annual operating costs and income from treated effluent reuse enter the equation. If the end-of-project value of the land take is considered, there is no contest – WSPs are streets ahead.

High efficiency

WSPs are designed as a series consisting of anaerobic ponds, facultative ponds and, if necessary, maturation ponds. Between them, they can remove more than 90% of the biochemical oxygen demand from wastewaters (pond size increases as the average temperature of the coldest month decreases). They can also remove more than 80% of nitrogen, 90% of ammonia and up to 50% of phosphorus. A huge benefit is the unique facility of maturation ponds to remove faecal bacteria. While even the best designed activated sludge plants achieve a removal rate of 90-99% of bacteria, viruses, protozoan cysts and helminth eggs, requiring chlorination to achieve lower levels needed for effluent reuse, WSP systems record removal rates up to six log units (i.e. 99.9999%) for faecal bacteria 99.99% for viruses and 100% for cysts and helminth eggs. The pond effluent will then meet the WHO guideline for unrestricted irrigation, with no further treatment necessary.

Conditions that may militate against successful application of a WSP system also fall into three categories:

• Geographic
• Temperature
• Raw water quality

Geographic

The land take for a WSP system is one of the primary cost factors and can sometimes be a constraint close to big cities. For mainly political rather than technical reasons, the ponds site should be at least 500 m downwind of the nearest housing (properly designed WSPs produce no offensive odours, but it is not easy to convince local residents ahead of any planned scheme). Though pond depths can be varied to suit available land area, and there is flexibility in choice of length-to-breadth ratios, a WSP site needs to be reasonably flat to keep down construction costs. Low permeability is a big advantage, as it avoids the need for an impermeable lining, and the soil needs to be suitable for building embankments with slopes of 1 in 3 externally and 1 in 2 internally.

Temperature

The treatment efficiency of WSPs is best at high temperatures (the design temperature is the mean temperature in the coldest month of the year). Lower temperatures mean a greater land area. Despite this constraint, WSPs are being used successfully in relatively cool climates such as Canada and New Zealand. Sites prone to high winds can also cause difficulties, as hydraulic short-circuiting is a common factor in the underperformance of WSPs.

Raw water quality

No biological wastewater treatment system copes well with high levels of industrial toxicants, and WSPs are no exception to this rule. The long retention times in ponds do provide a considerable buffering effect, which allows them to cope, for example with heavy metal concentrations up to at least 30mg/l. They thus cope better than the standard alternatives with wastewaters from, for example, abattoirs, canneries and dairies.

Despite these constraints there are very many places in the world where WSPs will be the best available way of treating the huge amount of municipal wastewater that still goes untreated. They have to be seen as a major approach to meeting the challenging environmental improvement targets that are included in the Millennium Development Goals and in many national development plans. But, success depends on proper design, which is why the design, construction and operation guidance provided in this TOP, together with its pointers to more detailed information, could be a crucial help for planners, designers and policy makers.