The MBR Book: Principles and Applications of Membrane

Other contributions to pumping energy demand include the various transfer operations,
such as from the balancing tank to the bioreactor, the bioreactor to the membrane
tank and the bioreactor or membrane tank to the denitrification tank or zone.
All of these operations are secondary contributors to energy demand. However, given
that it provides the highest flow, the recycle for denitrification (Section 2.2.6), if used,
would normally incur the greatest energy demand as determined by the recycle ratio
Q dn/Q p, where Q dn is the recycle flow for denitrification and Q p is the feed flow. In fullscale
systems this value (r) normally takes values between 1 and 3 (WEF, 1996) but can
be up to 4 (Metcalf and Eddy, 2003) depending on the feedwater quality. Pumping
power is given by:
W
p
gH Q
�
1000
r
j
pump
Q
(3.2)
where W p is the power input to the pump (kW), H is the pump head including
the system losses (m of water), � is the density of the pumped fluid (kg/m 3 ) and � is
the pump efficiency. This relationship yields the theoretical power requirement
for all liquid pumping operations, such as transfer from the bioreactor tank to the
membrane tank. In the case of biomass recycle the term Q pump/Q p approximates to r.
Note that � is highly variable and a more accurate measure of pumping power
requirements can be found by consulting data sheets produced by the pump manufacturers.
3.1.2 Membrane maintenance
p
Design 125
Other factors impacting on operating cost relate to physical and chemical membrane
cleaning, which incur process downtime, loss of permeate product (in the case of backflushing)
and membrane replacement. The latter can be accounted for simply by
amortisation, although actual data on membrane life is scarce since for most plants
the start-up date is recent enough for the plants still to be operating with their original
membranes. Membrane replacement costs are potentially very significant, but data
from some of the more established plant are somewhat encouraging in this regard
(Table 5.2).
Physical and chemical backwashing requirements are dependent primarily on
the membrane and process configurations and the feedwater quality. Thus far only
rules of thumb are available for relationships between feedwater quality and membrane
operation and maintenance (O&M); O&M protocols for specific technologies are
normally recommended by the membrane and/or process suppliers and sometimes
further adapted for specific applications. Fundamental relationships between cleaning
requirements and operating conditions, usually flux and aeration for submerged
systems, have been generated from scientific studies of fouling (Section 2.3.7.1).
However, arguably the most useful sources of information for membrane cleaning
requirements are comparative pilot trials – the assessment of different MBR technologies
challenged with the same feedwater (Section 3.2) – and full-scale reference
sites (Chapter 5).