If an open coastline is concerned with favorable geographic situations and a good
water exchange pattern, one could as well argue that no sewage treatment plant is
necessary at all. As regards pathogens, one could rely on the bactericidal properties
of seawater and the effects of time and dilution. As regards oxygen demand, if
enough seawater rich in oxygen flows by, then the degradation of organic wastes could
as well be done in the sea as within a biological sewage treatment plant. Calculations
have been made that the cost of a biological waste water treatment plant for about
4000 m 3 of effluents per day is comparable to the expenses for a piping system that
could bring untreated sewage 4 km away from the shore into the sea; for a 38,000 m 3
plant, expenses are equal to a 16-km-Iong piping system (Pearson 1975).
Under conditions of good oxygen supply and good seawater transparency, there is
little difference between the effects of biologically treated and untreated sewage
when emitted into the sea. Nutrients which leave the treatment plant in soluble form
cause an increased development of plankton in the seawater that receives the effluents. When the plankton dies, demand for oxygen results and the effect on the oxygen
situation may then be almost the same as when the effluents are released untreated
into the sea (Officer and Ryther 1977). Moreover, nutrient-rich effluents from treatment plants seem to especially enhance the growth of U/va and Enteromorpha,
green algae which cover tidal flats and cause anaerobic conditions in the sediment
beneath.
There is one important difference between emitting either untreated or biologically
treated domestic sewage into the sea: biologically treated sewage effluents contain
only a fraction of the original content of heavy metals and persistent organic compounds. The rest has settled down with the sediment and can be found in the sewage
sludge. From a number of coastal cities sewage sludge is dumped into the sea (see
Chap.4.2).
2.7 Detergents
Detergents, or tensides are defined as substances which reduce the surface tension of
water. They are effective in wash powders. For several decades they have played a
large role in households and industry. As a result of bad experiences with persistent
compounds, laws now exist specifying that wash powders must be to a great extent
biologically degradable. It is, however, a fact that various compounds of tenside are
poisonous in concentrations over 0.1 mg/I. This is, among other effects, because the
change in the surface tension of water also influences the transfer of substances in
and out of organisms.
But marine life proves to be very resistant in classical toxicity tests. It is only at concentrations above 5-100 mg/l that half of the mussels (Mytilus edulis) studied over
periods of 4 days died; at 1.5 ml/l of the tenside TAE 10 EO even over a period of
5 months mussels are not hindered in their biological functions and produce offspring
(Granmo and J9rgensen 1975). But even in concentrations of only 0.5 mg/l, the percentage of fertilized eggs is defmitely smaller and the development of the larvae proceedes slower. The transition from trochopora larva to the veliger stage is only successful in concentrations of under 0.5 mg/I. Such concentrations do not occur in the
28
water exchange pattern, one could as well argue that no sewage treatment plant is
necessary at all. As regards pathogens, one could rely on the bactericidal properties
of seawater and the effects of time and dilution. As regards oxygen demand, if
enough seawater rich in oxygen flows by, then the degradation of organic wastes could
as well be done in the sea as within a biological sewage treatment plant. Calculations
have been made that the cost of a biological waste water treatment plant for about
4000 m 3 of effluents per day is comparable to the expenses for a piping system that
could bring untreated sewage 4 km away from the shore into the sea; for a 38,000 m 3
plant, expenses are equal to a 16-km-Iong piping system (Pearson 1975).
Under conditions of good oxygen supply and good seawater transparency, there is
little difference between the effects of biologically treated and untreated sewage
when emitted into the sea. Nutrients which leave the treatment plant in soluble form
cause an increased development of plankton in the seawater that receives the effluents. When the plankton dies, demand for oxygen results and the effect on the oxygen
situation may then be almost the same as when the effluents are released untreated
into the sea (Officer and Ryther 1977). Moreover, nutrient-rich effluents from treatment plants seem to especially enhance the growth of U/va and Enteromorpha,
green algae which cover tidal flats and cause anaerobic conditions in the sediment
beneath.
There is one important difference between emitting either untreated or biologically
treated domestic sewage into the sea: biologically treated sewage effluents contain
only a fraction of the original content of heavy metals and persistent organic compounds. The rest has settled down with the sediment and can be found in the sewage
sludge. From a number of coastal cities sewage sludge is dumped into the sea (see
Chap.4.2).
2.7 Detergents
Detergents, or tensides are defined as substances which reduce the surface tension of
water. They are effective in wash powders. For several decades they have played a
large role in households and industry. As a result of bad experiences with persistent
compounds, laws now exist specifying that wash powders must be to a great extent
biologically degradable. It is, however, a fact that various compounds of tenside are
poisonous in concentrations over 0.1 mg/I. This is, among other effects, because the
change in the surface tension of water also influences the transfer of substances in
and out of organisms.
But marine life proves to be very resistant in classical toxicity tests. It is only at concentrations above 5-100 mg/l that half of the mussels (Mytilus edulis) studied over
periods of 4 days died; at 1.5 ml/l of the tenside TAE 10 EO even over a period of
5 months mussels are not hindered in their biological functions and produce offspring
(Granmo and J9rgensen 1975). But even in concentrations of only 0.5 mg/l, the percentage of fertilized eggs is defmitely smaller and the development of the larvae proceedes slower. The transition from trochopora larva to the veliger stage is only successful in concentrations of under 0.5 mg/I. Such concentrations do not occur in the
28