The MBR Book: Principles and Applications of Membrane

Case studies 215
area of 15 840 m 2 . This plant operates under similar aeration conditions as that of
Porlock and also operates with manual diffuser flushing with no diffuser air-scour.
The most recent Kubota MBR plants installed for sewage treatment in the UK have
an automated diffuser maintenance programme, whereby they are periodically flushed
with water and air-scoured. It is generally recognised by the operator that these plants
require careful maintenance to suppress clogging (or sludging), since the filling of the
channels with sludge represents a very significant constraint to the viable operation of
these plant. To this end, more conservative peak fluxes of 27 LMH appear to be appropriate,
though mean operating fluxes are much lower, coupled with rigorously cleaned
aerators to maintain the aeration rate, and thus air-scour, in the membrane flow
channels.
5.2.1.3 Daldowie
Sludge liquor and treatment The plant at Daldowie is a sludge liquor treatment plant
owned and operated by Scottish Water. Sludge liquor is the aqueous fraction of
sewage sludge which has undergone dewatering by processes such as belt pressing,
rotary drum vacuum filtration and centrifugation following conditioning with coagulant
and/or polymeric flocculant reagents. It may contain up to 25% of the TN load
in the original sludge and contribute as little as 2% of the total influent flow. It is thus
highly concentrated in ammonia (Table 5.3), as well as in dissolved organic matter.
Composition and flow are extremely variable and dependent on the upstream sludge
handling and treatment processes. Whilst the most economical option is to return
the liquor to the head of works, this is not always possible either due to logistical limitations
or due to the excessive load it would place on the existing sewage treatment
process. Under such circumstances, a number of options for its treatment can be considered
(Table 5.4). Of key importance in choosing the most suitable option are confidence
in a robust solution, cost and environmental factors such as the impact of the
wastes generated. Speed of start-up may also be of importance if the plant will only
run seasonally.
Plant design and operation As is often the case for this duty, the choice of technology
at Daldowie was seen as being between a sequencing batch reactor (SBR) and an
MBR. However, the large fluctuations in organic loading arising from periodical
releases of poor-quality effluent from the centrifuges and the belt presses meant that
operation of an SBR was seen as being more problematic than the MBR. This was
Table 5.3 Sludge liquor properties
Parameter Average concentration Range (mg/L)
(mg/L)
BOD 1221 162–3004
COD 941 470–1411
NH 4-N 750 162–1218
SS 1442 50–5000
Alkalinity 2200 1900–2500
pH 7.2 7–7.5