The MBR Book: Principles and Applications of Membrane

neo-exponential increase at fluxes closer to the notional critical flux (Fig. 2.14,
10 LMH line). Sub-critical fouling is discussed further along with MBR membrane
fouling mechanisms in Section 2.3.8.
2.2 Biotreatment
2.2.1 Biotreatment rationale
Biological treatment (or biotreatment) processes are those which remove dissolved
and suspended organic chemical constituents through biodegradation, as well as
suspended matter through physical separation. Biotreatment demands that the appropriate
reactor conditions prevail in order to maintain sufficient levels of viable (i.e.
living) micro-organisms (or, collectively, biomass) to achieve removal of organics. The
latter are normally measured as biochemical or chemical oxygen demand (BOD and
COD, respectively); these are indirect measurements of organic matter levels since both
refer to the amount of oxygen utilised for oxidation of the organics. The microorganisms
that grow on the organic substrate on which they feed derive energy and
generate cellular material from oxidation of the organic matter, and can be aerobic
(oxygen-dependent) or anaerobic (oxygen-independent). They are subsequently separated
from the water to leave a relatively clean, clarified effluent.
The most attractive feature of biological processes is the very high chemical conversion
efficiency achievable. Unlike chemical oxidation processes, aerobic processes
are capable of quantitatively mineralising large organic molecules, that is, converting
them to the end mineral constituents of CO 2, H 2O and inorganic nitrogen products, at
ambient temperatures without significant onerous byproduct formation. In doing so
a variety of materials are released from the biomass in the reactor which are collectively
referred to as extracellular polymeric substances (EPS) and which contain a
number of components which contribute to membrane fouling in an MBR (Section
2.3.6.1). The relative and overall concentrations of the various components are
determined both by the feed characteristics and operational facets of the system, such
as microbial speciation. Anaerobic processes generate methane as an end product, a
possible thermal energy source, and similarly generate EPS. Biotreatment processes
are generally robust to variable organic loads, create little odour (if aerobic) and generate
a waste product (sludge) which is readily processed. On the other hand, they are
slower than chemical processes, susceptible to toxic shock and consume energy associated
with aeration in aerobic systems and mixing in all biotreatment systems.
2.2.2 Processes
Fundamentals 37
Processes based on biodegradation can be classified according to the process configuration,
feeding regime and oxidation state (Table 2.3). Process configuration defines
the way in which the water is contacted with the biomass, which can form a layer on
some supporting media to form a fixed biofilm or be suspended in the reactor, or
sometimes a combination of these. Suspended growth systems provide higher mass