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Marine scientists on sewage non-point sources of pollution

The construction of artificial wetlands [...] increases the extent of coastal wetland habitat, often generating cross-benefits, As a result, non-point sources such as septic fields, and pit or overwater latrines, are a significant source of sewage contamination in many areas. In many countries significant reductions in sewage contamination could be achieved by converting pit or overwater latrines to septic tanks, by better design and construction of existing septic tanks, or by better provisions for septic sludge disposal.

The failure of on-site systems because of poor ongoing operation and maintenance (e.g., not emptying tanks or pits) is a common reason given for needing sewerage. Septic tanks can also be linked to stepped digestion tanks that produce effluent suitable for irrigating home gardens. There are also simple technologies, such as composting toilets and biogas generation, that are suitable for application in individual households or to small groups of them.

Depending on circumstances, measures concerning such on-site systems can have significant advantages over centralized reticulation and treatment systems. They are less expensive than conventional sewerage systems, especially at relatively low population density and can be implemented in smaller increments and with shorter lead times. They can also be implemented at the community or even individual level, while ongoing operation and maintenance are often less financially and technically demanding. Furthermore, equipment can often be manufactured locally. Even when reticulated sewerage and sewage treatment is the best long-term approach to sewage management, on-site systems may be useful interim measures, and may enhance the system in the long term. In "settled sewerage", for example, septic tanks are used to pre-treat wastewater before it is discharged to a central system, reducing the load on it.

On-site systems do have disadvantages, however. Soils have a finite capacity to absorb septic effluents. This varies widely with soil characteristics, and in some places soils are unsuitable for septic tanks. Septic tanks are also relatively poor at disinfection. They can lead to microbial contamination of ground water – a negative impact, especially where wells are an important source of drinking water. Sewage contamination of wells, for example, has been identified as the highest regional priority in Eastern Africa.

Another useful measure in managing the impacts of diffuse sources of sewage is to take advantage of the capacity of artificial or natural wetlands to assimilate and retain wastes and remove pathogens. Again, however, the [capacity] is not unlimited; when it is exceeded, the wetland can be degraded. Such assimilative capacities are poorly known, particularly in the context of long-term variability. For this reason, and because natural coastal wetlands are both ecologically very important and widely threatened, using existing wetlands for sewage treatment should be approached with considerable caution.