SEA OUTFALLS 653
15.7 Sea outfalls
15.7.1 GeneralThe oceans have a great capacity to assimilate wastes and to make them
harmless and they are being exploited as receivers of wastes from outfalls.
A typical outfall consists of a pipe discharging effluents from the land to
the sea and laid on or buried in the sea bed (or housed in a tunnel). It is
extremely important that the design of outfall should take account not
only of its hydraulic function but also of the environmental impact. Some
of the short outfalls constructed have failed from the environmental stand-
point since effluents have found their way back polluting the beaches.
Even long outfalls may sometimes be unsatisfactory since effluents can
return to the beach depending upon the direction of currents that are pro-
duced by the tidal flow, waves and wind.
There are essentially two outfall types. One consists of a pipe with a
diffuser at its seaward end, both above the sea bed, and the wall ports,
through which the effluent discharges, are just holes through the wall of
the pipe; the second type, buried under the sea bed, has a number of risers
from the diffuser. The outfall pipe and the risers must be protected from
ship anchors and fishing lines.
In large outfall schemes and in schemes involving confined waters,
the concentrations of the pollutants discharged should be checked periodi-
cally by sampling the quality of water which is usually characterized by the
biological oxygen demand (BOD), bacterial content, suspended matter,
turbidity, pH, temperature, toxic chemicals, minerals and organic and
inorganic matter. The impact of these various constituents on the environ-
ment is often complex and not necessarily immediate.
When the effluent is discharged into the sea, it is diluted rapidly by
initial dilution and subsequently by the secondary dispersion. The BOD
and the suspended solids are reduced quickly by these processes. It is,
however, necessary that the ambient currents are favourable and that the
site is at a sufficient distance from the shore to reduce the bacterial
content still further through dilution, mortality and sedimentation before
the diluted sewage may reach the shore.
Apart from the above environmental aspects, the design of the
outfall system will depend on the characteristics of the sewage system,
storm overflows, the outfall site, the headworks and the outfall itself. The
storm overflows on combined systems (collecting both sewage and rain-
water run-off ) are designed to reduce the flow in the sewers to about 6
times the dry weather flow and may also be a cause of pollution whose
impact on the environment has to be assessed.
The preliminary treatment of the wastewater on land and the sub-
sequent marine treatment should complement each other in order to