The addition of sewage to water can greatly increase fertility by release of nutrients, with possible long-term beneficial effects. However, in most cases the effects are detrimental because of the sheer volume of the discharges. Such effects include foul deposits, deoxygenation, eutrophication, reduced salinity, infection and toxic residues.
Discharge of untreated sewage may result in strandings of recognizable faecal material and other objectionable objects. Unless fully treated, solid deposits around sewer outfalls form a black sludge, sometimes blanketing the substrate. This generally causes some change in bottom fauna; often bivalves decline and worms increase, and this may reduce the value of the area as a feeding ground for fish. Suspended solids reduce light penetration, and increase scouring and silting effects, with possible adverse effects on both plant and animal life. Stinking mud from sewer outfalls, even though sited some distance below low tide level, may sometimes drift along the sea-bed and become deposited on the shore.
Addition of organic solids or solutes accelerates bacterial growth, increases oxygen demand and may lead to deficient oxygenation if the water is not well mixed. Deoxygenation encourages the growth of sulphur bacteria and the production of offensive hydrogen sulphide. Much of the Baltic Sea suffers from low oxygen levels as a result of its enclosed nature, lack of flushing and high input of sewage from its many coastal cities.
High concentrations of plant nutrients may lead to so rapid a growth of phytoplankton and zooplankton that when the bloom dies off and the material sinks, there is an accumulation of organic debris on the sea-bed. Associated bacterial multiplication causes the water to become deoxygenated and foul with severe effects on the bottom fauna. This condition of eutrophication, though more of a problem in fresh waters than the sea, can occur in sea bays, estuaries and enclosed seas if mixing is slow. It is often indicated by the presence of species such as the polychaete worm Capitella capitata, which can survive and do well in polluted conditions.
'Red tides', which are blooms of toxic phytoplankton (see Section 2.2.2), are also associated with excess nutrients supplied by sewage. These can poison fish and shellfish and may indirectly cause paralytic shellfish poisoning in humans. In 1990 and 1991, shellfish farms along large stretches of the north-east coast of England had to be closed due to red tides.
Reduced salinity occurs around sewer outfalls, which may be adverse to stenohaline species.
Sewage-borne infections such as typhoid, viral hepatitis, enteric infections and ear, nose and throat infections have been associated with exposure to sewage-polluted seawater. Infection may be transmitted to humans directly through contact with water or spray, or indirectly by consumption of marine foods. In the UK there have been several recent cases where surfers have suffered severe illness and paralysis thought (but not yet conclusively proved) to be due to infection from faecal viruses. The increased bacterial content of sewage-polluted water favours filter feeders and often results in excellent growth of bivalves such as cockles and mussels. Shellfish from such areas may be much prized but require thorough purification before they are eaten. Some viruses survive longer than bacteria in seawater and may remain in shellfish even after they are bacteriologically clean.
Sewage is increasingly contaminated with heavy metals, insecticides and various persistent organic poisons which may be transmitted via the food chain to humans.
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