The MBR Book: Principles and Applications of Membrane

124 The MBR Book
3.1 Membrane bioreactor system operational parameters
There are essentially three main elements of a membrane bioreactor (MBR) contributing
to operating costs, ignoring membrane replacement. These are:
1. liquid pumping
2. membrane maintenance
3. aeration
Of these by far the most significant, especially for immersed technologies, is aeration.
As already discussed in Chapter 2, aeration is used both for scouring an immersed
membrane and for suspending and maintaining a viable biomass. Design of an MBR
therefore demands knowledge both of the feedwater quality, which principally determines
the oxygen demand for biotreatment, and the aeration demand for fouling
control, which relates to a number system characteristics as summarised in Fig. 2.18.
The various governing principles have been discussed in Chapter 2. Design equations
are summarised below.
3.1.1 Liquid pumping
The key hydraulic operating parameters for MBR operation are obviously flux J litres
per m 2 per hour (LMH) and transmembrane pressure (TMP) �P m (bar or Pa), from
which the permeability K (J/�P m in LMH/bar), kg/(m 2 s) is obtained. In sidestream
MBRs the specific energy demand (the energy demanded per unit permeate product
volume) for continuous membrane permeation relates primarily to the pressure and
conversion, and is roughly given by:
WhP � � �
1
rj ∑
(3.1)
where � is the density (kg/m 3 ), Θ is the conversion achieved by a single passage of
fluid along the length of the module (nominally 100% for submerged systems) and
� is the pumping energy efficiency. In the above equation ��P refers to the sum of all
individual pressure losses which, for a sidestream system, comprises �P m and the
pressure losses in the retentate channels, �P r, all pressure values being in SI units.
As explained in Section 2.3.1, maximising Θ to reduce W h relies on increasing the
total length of the flow path through the membrane channel and increasing the
shear rate, but this has the effect of increasing �P r. The optimum operating conditions
for a given channel dimension (tube diameter or parallel flow channel separation) is
therefore determined by relatively straight-forward hydraulic considerations, perhaps
complicated slightly by the non-Newtonian nature of the sludge (Section 2.2.5.3).
For submerged MBRs, the total pressure loss is of lesser importance, since it is necessarily
very low to sustain operation. In this configuration the pressure loss, principally
the TMP, reflects state of membrane fouling, through K, but contributes very
little to the energy demand. Instead, the specific aeration energy demand is mainly
governed by the aeration intensity (Section 3.1.3.2).