The MBR Book: Principles and Applications of Membrane

on discharged P levels have been imposed (Section 5.2.1.4, Running Springs).
P removal is more commonly achieved by dosing with chemicals, such as metal
coagulants or lime, that can form sparingly soluble precipitates.
2.2.7 Anaerobic treatment
Compared with aerobic processes, anaerobic biological treatment is characterised
by (Stephenson et al., 2000):
● a lower energy demand due to the absence of aeration
● slower microbial growth
● a lower COD removal (generally 60–90%)
● no nitrification
● greater potential for odour generation
● longer start up (months cf. weeks)
● higher alkalinity
● lower sludge production
● biogas (methane) generation.
Fundamentals 53
Conventional anaerobic treatment process configurations are all designed to
achieve both good mixing and sludge separation. A number of configurations exist:
(a) Simple contacting coupled with external sludge separation (by sedimentation,
rotary vacuum filtration, etc.) and/or digestion before returning the clarified
liquid to the reactor. This is a simple and relatively easily controlled process
but is also made expensive by the pumping operations.
(b) Anaerobic filters, which are flooded media filters based on either packed or
structured media.
(c) Upflow clarification using the upflow anaerobic sludge blanket (UASB, Fig.
2.21a) reactor (Lettinga and Vinken, 1980), in which sludge particles settle at
the same rate as the water flows upwards, forming a stationary “blanket” of
sludge in the reactor. This process relies on the formation of a dense granular
sludge bed that is readily retained in the reactor, in much the same way as secondary
clarification in the ASP relies on the growth of large settleable particles.
The process is augmented in the expanded granular sludge bed (EGSB, Fig. 2.21b)
which provides better influent distribution to improve contact between the
sludge and wastewater and so promote more efficient use of the entire reactor
volume (Seghezzo et al., 1998). In this configuration, the sludge bed is expanded
by operating at higher upflow rates and the reactor behaves as a completely
mixed tank (Rinzema, 1988).
(d) Staged reactor systems, based on plug flow (Van Lier, 1995) and using sequentially
operated reactors or compartments within a single reactor. Staged reactors
include anaerobic baffled reactors (ABRs, Fig. 2.21c) in which baffles are
used to direct the flow of wastewater in an upflow mode through a series of
sludge blanket reactors (Metcalfe and Eddy, 2003). The process is more tolerant
to non-settling particles than the UASB and EGSBs whilst still providing long
solid retention times.