3.1 An approved sewage treatment plant should
meet the technical specifications in section 4 and the tests outlined
in these Guidelines. However, section 4.2 on nitrogen and phosphorous
removal applies to passenger ships operating within a special area
intending to discharge treated sewage effluent into the sea. It should
also be noted that, when ships are operating approved sewage treatment
plants, MARPOL Annex IV also provides
that the effluent shall not produce visible floating solids or cause
discolouration of the surrounding water.
3.2 In meeting the effluent standards in section 4, an
approved sewage treatment plant should not rely solely on dilution of wastewater. Where
amounts of dilution are deemed essential to a treatment process, the effluent standards
in section 4 having concentration limits (mg/l) should be adjusted proportionally using
dilution compensation factor Qi/Qe to take account of dilution
Qd. In addition, for effluent standards in section 4 having a percentage
reduction, the geometric mean of the daily percentage reduction values should be
calculated using the accumulated flow Qi and Qe over each 24-hour
test day, in terms of l/day, multiplied by the geometric mean of the corresponding
concentration Ci and Ce for the same 24-hour test day, in terms of
mg/l.
The overall percentage reduction over the entire test period n
is:
where PR
n is the daily removal value:
where:
- n
represents the test day number; and
- s
represents the sample number collected on test day n
3.3 It is acknowledged that the performance of
sewage treatment plants may vary considerably when the system is tested
ashore under simulated shipboard conditions or on board a ship under
actual operating conditions. Where testing ashore demonstrates that
a system complies with the standards, but subsequent onboard testing
does not meet the standards, the Administration should determine the
reason and take it into account when deciding whether to type approve
the plant.
3.4 It is recognized that Administrations may
wish to modify the specific details outlined in these Guidelines to
take account of very large, very small or unique sewage treatment
plants.