Disposal at sea

Sewage discharges give rise to problems for bathing water and shellfish marketability, though invariably on local scales in the vicinity of untreated or incompletely treated discharges. Such compromises, however, are widespread and, therefore while not a truly "global" problem, the ubiquity of the adverse effects of sewage discharge make it a problem of global socio-economic dimensions. Net reductions in nutrient discharges to the marine environment depend on higher levels of sewage treatment because primary treatment alone does little to reduce nutrient releases. Such additional treatment does not always require reliance on conventional techniques that may be appropriate to urban sewage streams, but can be achieved through the use of natural coastal wetlands as treatment systems.

Historically, it was commonly believed that the introduction of organic carbon and nutrients to the marine environment in sewage was a good thing, resulting in increased biological production. [...] [Such views] clearly did not foresee the growth and concentration of population in coastal areas that occurred in the latter half of the 20th Century with the resultant overloading of coastal waters though [this view] is still applicable to the open ocean.

The sheer rate and ubiquity of nutrient discharge has overwhelmed the capacity of many inshore coastal areas to assimilate nutrients and oxygen demand.

From GESAMP Report 71 without harm. This is not a problem for the open ocean because of its enormous capacity to assimilate oxygen demand and its oligotrophy. The use of long outfalls over narrow shelves, such as in California and the Pacific Islands, to deliver sewage to the offshore ocean is therefore still legitimate.