Waste liquid discharges
and sludge are disposed of into watercourses and through the sewage system. Organisations
with permission to discharge into rivers are subject to conditions laid down by
the Environment Agency. Local conditions are taken into account when licenses
to discharge are agreed; a fast flowing well-oxygenated river can carry and disperse
more pollution than a slow moving river in a valley bottom.
from an organisation into a water course are known as point
source discharges. The biological oxygen demand (BOD) of the
water course is calculated so that discharges are diluted
sufficiently. However there are often diffuse and unknown
discharges entering the water such as fertilisers and pesticides
run off from farmland. These may be seasonal and contribute
to the overall pollution.