The MBR Book: Principles and Applications of Membrane

Table 3.1 Main features of aeration systems
provided for the membrane to simplify membrane cleaning operations. During the
process of scouring the membrane, if air is used, there is some transfer of oxygen into
the biomass which raises its DO level in the biomass. Membrane aeration is usually
carried out using coarse bubble aeration because of the increased turbulence and hence
shear forces created (Section 2.3.7.1), whilst biomass aeration is usually performed
using fine bubble devices because of the enhanced oxygen transfer (Table 3.1).
Oxygen transfer is corrected from clean water to process water (i.e. mixed liquor)
conditions by the � factor. Several relationships have been developed that relate
mass transfer to the mixed liquor suspended solids (MLSS) or viscosity of a system
(Section 2.2.5.3), most of them taking the form:
a � � ae bX . ,
(3.14)
where a and b are constants and X is the MLSS concentration in g/m 3 ; a varies from
1 to 4.3 and b from 0.04 to 0.23 (Fig. 2.19).
3.1.3.4 Blower power consumption
The theoretical power consumption for a blower is given by (Appendix A):
⎡
PA,1TK,1l ⎢⎛
P
Power �
⎢ ⎜
2.73�105 jl ( �1)
⎢⎜
⎜
⎝ P
⎣
⎢
Fine bubble Coarse bubble
A,2
A,1
1� 1
�
⎞
⎠⎟
⎤
⎥
�1
⎥
⎦
⎥
(3.15a)
where P A,1 and P A,2 are the inlet (normally atmospheric) and outlet absolute pressures
(Pa) respectively, � is the blower efficiency, � is the ratio of specific heat capacity at
Q A
Design 129
Bubble size 2–5 mm a 6–10 m a
OTE (percentage of O 2 3–10% b 1–3% b
transfer per m depth)
Mechanical component Air blower Air blower
Diffuser type Ceramic or membrane diffuser Steel or plastic disk or tube
disk, dome or tube
Shear rate c Bubble velocity � d 2
(from Stokes Law). The small Bubble velocity, and so shear, is
bubble sizes provide lower velocity higher than fine bubble
and hence smaller shear forces. aeration since the larger bubbles
rise faster than small bubbles.
Diffuser cost d Approximately £40 per diffuser Approximately £15 per diffuser
a EPA, 1989.
b Data from survey of manufacturers and from literature study. The large variation in the efficiency data for fine
bubble aeration is attributed to changes in the distribution of the diffuser nozzles over the tank floor and diffuser
age. Fine bubble diffusers are susceptible to fouling and oxygen transfer efficiency can decrease up to 19%
over 2 years of operation.
c Shear rate is a measure of propensity to ameliorate membrane fouling (Section 2.3.7.1).
d Data obtained from manufacturer quotes for use as a guideline only.