The MBR Book: Principles and Applications of Membrane

Works
inlet Inlet
works
Storm
tank
Ferric
dosing
6 and 2 mm
screens
Storm water
4 No.
MBR
tanks
Case studies 223
Permeate
pumping
chamber Permeate
to river
Figure 5.11 Schematic of Coill Dubh wastewater treatment works
is then, on level, pumped to outfall, or can be recycled back to the inlet of the works
via a divert valve, conducted to assist in commissioning.
Excess sludge is removed manually from each reactor via a single progressive
cavity pump and discharged into the inlet sludge storage tank. Fat, scum and grit are
also collected in this tank, and the accumulated waste periodically removed from
site. Also supplied on-site are a control/mess building, ferric storage and a standby
generator. A remote monitoring system provides Brightwater with site feedback.
Operation and maintenance The MBR operates at a maximum TMP of 300 mbar
and a gross flux of 27 LMH (hence �90 LMH/bar permeability) with relaxation
for 5 min every hour. Aeration is supplied by an aeration lateral system located at
the base of each plate providing coarse-bubble aeration for both membrane scour
and process aeration at a rate of up to 118 Nm 3 /h per module, or a SAD m of
1.28 Nm 3 /(m 2 h). No additional fine bubble aeration system is required for the combined
sewer loads supplied to Coill Dubh. Although chemical cleaning is suggested
at 12-monthly intervals, none has been applied over the first 18 months of operation;
the performance of the membranes is maintained with the air scour rate supplied.
Cleaning is to be conducted ex situ by sluicing, followed by a 4–12 h soak in
either hypochlorite or citric acid, depending on the assessed fouling. Flushing of the
aeration system can be conducted at the same time. The design sludge age is 21
days, though this often increases to �30days such that the design MLSS concentration
of 12 g/L increases to �15 g/L. The HRT is �11 h at full flow.
The process at Coill Dubh was built as a low-tech approach to membrane technology
and has performed better than anticipated despite periods of operational stress.
The permeability remains acceptable at flux rates between 22 and 27 LMH and the
permeate water contaminant levels have consistently been below most of the
parameter limits of detection (i.e. �5 mg/L BOD, �5 mg/L SS, �0.5 mg/L N-NH � 4
and �0.05 mg/L TP). While the effluent quality exceeds that required for discharge,
the simplicity of the system and the robustness of the membranes have made this
plant very simple to operate, despite fat and rag levels far in excess of those anticipated.
The continued operation without the requirement for chemical cleaning