Decision report- Carey's Gully Sludge Dewatering Facility - Greater ...

(ii) ends three months before the expiry of the existing consent;
and
(e) the authority, in its discretion, allows the holder to continue to
operate.
GW exercised this discretion, and thus, the continued operation of the SDF and
associated disposal of sludge contaminants is provided for by Section 124 of
the Act.
4. The proposal
The proposal can be separated into two distinct processes for which consent is
required, the SDF and the sludge disposal at the Southern Landfill.
4.1 Sludge dewatering facility
The SDF is the infrastructure which dewaters the solids out of the waste
stream, producing a liquid and a dewatered sludge mixture which is termed a
‘spadeable’ quality, and can be likened to the consistency of cake mixture.
The process at the SDF is detailed in full in Mr Chang’s report; however, a
brief outline of the process is as follows:
Liquid sludge, a mixture of solids (less than 1% by weight) and water (99% by
weight) is pumped to the SDF via an underground sewage pipeline from the
Moa Point Waste Water Treatment Plant (MPWWTP).
The liquid sludge is kept in one of two storage tanks on site for 13 to 16 hours
before it is processed through the plant. The second tank provides a small
capacity of storage available for contingency purposes.
This liquid sludge is dewatered by centrifuges, spinning some of the liquid
fraction out of the raw liquid sludge. The dewatered sludge is continuously and
automatically transferred from the centrifuges to one of eight skips in the skip
bay. Full skips are then transferred by truck to the landfill on a daily basis. The
disposal of the dewatered sludge is discussed in Section 4.2 of this report.
The air in the entire dewatering building is extracted, putting the building
under negative pressure, and passed through the site’s biofilter.
The liquid (centrate) that is extracted from the liquid sludge is pumped to the
centrate treatment ‘black boxes’. The treated centrate is then discharged to the
sewer, while any settled solids are pumped back to the centrifuges. The gases
from the centrate wet well are contained and extracted to the biofilter, whereas
the gases from the black boxes discharge directly to atmosphere.
4.2 Sludge disposal
The applicant intends to dispose of all dewatered sludge contaminants to the
Southern Landfill for the mid-term, until a regionally-based solution can be
found.
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