Sewage treatment

Almost all urban areas have some form of networked, or reticulated, sewerage system to carry sewage away from homes and industrial areas. Such networks are a necessary element in providing a large-scale sewage treatment service, although there are still many areas where such treatment is basic or non-existent and untreated sewage is disposed directly into the oceans. Where it occurs, sewage treatment may have one or more levels.

Preliminary Treatment, which typically just consists of a screen to filter very large particles from raw sewage. Such raw sewage may contain 10 million faecal coliform bacteria per 100ml. Primary Treatment, involves the removal of most solids and suspended material. This also removes some of the biochemical oxygen demand of the effluent. After this treatment the effluent may contain 1 million faecal coliform bacteria per 100ml. Secondary treatment, involves a process of settlement, and the introduction of bacteria which remove some of the components of the effluent. These, and more solids are removed as a sludge and the resulting effluent may contain 10,000 faecal coliform bacteria per 100ml. Tertiary Treatment involves disinfection and may also involve the removal of nutrients and ammonia. Two well-known methods are ultra-violet disinfection and microfiltration (or membrane microfiltration). Following this treatment faecal coliform levels may fall to below 35 per 100ml

Althought tertiary treatment may be ideal there are considerable financial costs, whilst many more remote settlements may lack a sewerage network. In many situations, particularly in the developing world, there may be alternatives, including well-designed septic tank systems, composting toilets, or even disposal in the open ocean. Unlike the shallow and already impacted waters of the continental shelf, the open ocean has an enormous capacity to assimilate both nutrients and oxygen demand. The use of long outfalls over narrow shelves, such as in California and the Pacific Islands, may therefore still be legitimate.