The MBR Book: Principles and Applications of Membrane

128 The MBR Book
However, to simplify calculations in process design the three correction factors are
often combined into a single � factor.
Unlike other flows m A,b is calculated as a mass flow as opposed to a volumetric
flowrate. It is more practically useful to consider the volumetric biomass aeration
demand, for which m A,b must be converted to a volumetric flow rate, normalised to
some selected temperature (usually 20°C) by the air density, to provide air flow rate
per unit permeate flow:
R
b
where J� net is the net flux normalised to 20°C.
(3.11)
3.1.3.2 Membrane aeration
It is necessary to aerate the membrane unit in an MBR to scour solids from the membrane
surface (Section 2.3.7.1). In practice the membrane aeration value is not defined
theoretically since the relationship between aeration and flux decline is not well understood
at present. Membrane aeration values are based on previous experience, and in
many cases the suppliers recommend an appropriate aeration rate. As is the case for
membrane cleaning regimes the most valuable data are that which are collected from
pilot trials and full-scale case studies. The key contributing factor to energy demand
in submerged systems is the specific aeration demand, the ratio of Q A either to membrane
area (SAD m) or permeate volume (SAD p):
SAD m
SAD p
QA,b
�
J′ A
net m
� Q
A
A,m
� Q
JA
A,m
m
m
(3.12)
(3.13)
where the subscript m refers specifically to the individual membrane element or
module. Whereas SAD m has units of m 3 air per unit m 2 per unit time, and thus
nominally m/h, SAD p is unitless. It should be noted that both are temperature
dependent, since gas volume (and thus flow) increases with temperature as does
flux, through decreasing viscosity. If normalised to 20°C the air flow is demoted
units of Nm 3 /h.
3.1.3.3 Diffuser type
There are three types of aeration used in MBR plants: coarse bubble aeration, fine
bubble aeration and, less commonly, jet aeration. The principal differences between
the two main aerator types are given in Table 3.1. Traditionally, fine bubble diffusion
has been used for biomass aeration and a separate coarse bubble aeration system
applied for membrane scouring. In many proprietary systems separate tanks are