Page 159 - Introduction to Marine Engineering

Auxiliaries

Automatic level
control for oil
drain
JL

Oil
collection
space

Mixture
Catch inlet
plates

L- ?_.
Figure 7.8 Oily water separator

second-stage filter uses coalescer inserts to achieve the final de-oiling.
Coalescence is the breakdown of surface tension between oil droplets in
an oil/water mixture which causes them to join and increase in size. The
oil from the collecting spaces is drained away manually, as required,
usually about once a week. The filter inserts will require changing, the
period of useful life depending upon the operating conditions.
Current legislation requires the use of a monitoring unit which
continuously records and gives an alarm when levels of discharge in
excess of 15 parts per million occur.

Sewage treatment

The discharge of untreated sewage in controlled or territorial waters is
usually banned by legislation. International legislation is in force to
cover any sewage discharges within specified distances from land. As a
result, and in order to meet certain standards all new ships have sewage
treatment plants installed.
Untreated sewage as a suspended solid is unsightly. In order to break
down naturally, raw sewage must absorb oxygen. In excessive amounts it
could reduce the oxygen content of the water to the point where fish and