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The chief health risk from sewage discharges to sea is undoubtedly
through the ingestion of contaminated seafood.
Filter feeders , most conspicuously bivalve molluscs , flourish in waters
enriched by
a neighbouring
input of organic waste , but they
accumulate the human pathogens on their gills and these may be
transmitted to the consumer.the risk is obviously greatest if the
bivalves are freshly harvested and eaten uncooked.
Biotoxins :
A considerable variety of marine organisms produce toxins that may
be harmful and sometimes fatal to humans who encounter them by
contact with the organisms or by ingesting them in seafood. Most such
cases are unrelated to pollution , but the incidence of blooms of toxic
dinoflagellates has increased in recent years , with serious public health
consequences , and it is suspected that nutrient enrichment of the
waters has been a contributory factor.
Sewage treatment :
By far the greatest volume of oragnic waste to be disposed of is human
sewage and , in areas of intensive farming , animal wastes. Some of
this material may be spread on farmland as a fertilizer, but the
quantity is far to great for this to be realistic way of disposing of it. It
may be discharged to water courses or the sea , but the high organic
loading creates problems in these environments.
The answer to this problem is to provide facilities where bacterial
action can reduce the organic content of the waste under controlled
circumstances .
Various stages of treatment may be used , depending on the quality of
effluent that is required.