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This system is not designed to produce a
discharge suitable for a controlled area, but as a holding system for discharge
ashore or in a non‑controlled area.
Sewage enters a first stage
chemical dosing tank where it is treated with chemicals to improve its odour
and colour. It then passes through a comminutor to break it into small
particles before entering a second treatment tank where chemicals are added to
sterilise the sewage and to form flocs that easily settle.
A recirculation pump returns
the heavier constituents and some of the chemicals to the first treatment tank.
The sewage has a dwell time of about
five minutes in this section, before overflowing to the settling tank. After
settling, the clear liquid is drawn through a filter to a pressure tank and
used as a supply of flush water to the toilets, hence keeping the volume
required for storage small.
The sterile sludge at the
bottom of the settling tank may be removed to a storage tank, incinerated or
pumped overboard in a de‑controlled area. Daily sampling and testing of the
clear liquid is required to avoid over‑dosing with chemicals which can lead to
a high pH and corrosion in the pipework.
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