The MBR Book: Principles and Applications of Membrane

50 The MBR Book
a-factor
1
0.8
0.6
0.4
0.2
0
a � e�0.08788 MLSS
(Krampe and
Krauth, 2003)
a � 1.5074e�0.0446 MLSS
(Muller et al., 1995)
0 5 10 15 20 25 30 35 40
MLSS (g/L)
Figure 2.19 �-factor vs. MLSS concentration
a � 4.3177e�0.2331 MLSS
(Muller et al., 1995)
a � e�0.083 MLSS
(Gunder, 2001)
In a number of studies of sewage treatment, an exponential relationship between
�-factor and MLSS concentration has been observed. Muller et al. (1995), recorded
�-factor values of 0.98, 0.5, 0.3 and 0.2 for MLSS concentrations of 3, 16, 26 and
39 g/L, respectively, yielding an exponential relationship with an exponent value of
�0.045 and an R 2 of 0.99 (Fig. 2.19). Günder (2001) and Krampe and Krauth
(2003) observed the same exponential trend with exponent values of �0.083
and �0.088, respectively, whereas an even higher exponent value of �0.23 was
recorded by Germain (Germain, 2004). Studies on model or simplified systems by a
number of authors (Freitas and Teixeira, 2001; Ozbek and Gayik, 2001; Verlaan and
Tramper, 1987) appear to indicate that the principal impact of solids concentration
is on the interfacial area a, which decreases with increasing solids level whilst leaving
the mass transfer coefficient k L largely unaffected. This has been attributed to the
promotion of bubble coalescence by suspended solids (Klein et al., 2002), and the
effect is also aeration rate-dependent (Freitas and Teixeira, 2001). Since MBRs run
at a high MLSS the aeration demand for biotreatment operation of an MBR is somewhat
higher than that of an ASP.
The impact of particle size is more complex than particle concentration, since aeration,
mass transfer and particle size are interrelated. For fine particles, �0.01 mm,
k La has been shown to increase with increasing solids concentration up to a certain
level and remain stable, before decreasing with further increased solids concentration
(Saba et al., 1987; Smith and Skidmore, 1990). With larger particles, 1–3 mm, k La
appears to decrease with concentration (Hwang and Lu, 1997; Koide et al., 1992;
Komaromy and Sisak, 1994; Lindert et al., 1992; Nakao et al., 1999). Experiments
examining excess sludge production, in which the DO concentration was adjusted independently
of aeration intensity, indicated that higher mixing intensity and DO concentration,
created by raising the airflow, had almost the same impact on floc break-up and
therefore on particle size. At a sludge loading of 0.53 kg BOD 5/kg MLSS day, the excess
sludge production was reduced by 22% by raising the oxygen concentration from 2 to
6 mg/L (Abbassi et al., 1999). However, the principal impact of particle size in an MBR
is on filter cake permeability, as indicated by the Kozeny Carman equation.