Continuous Cultivation - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 639

the composition of the microbial population in the treatment unit. For example, filamentous
bacteria may dominate the normal floc-forming cells, leading to small, light flocs.
Various modifications of activated sludge processes have been developed. Two examples
are the following:

(1) Step feed process : The feed stream is distributed along the length of the reactor. Such
a configuration converts a conventional PFR system to more CSTR-like behavior, which
provides greater stability and more effective distribution and utilization of oxygen and
oxygen transfer.
(2) Solids reaeration ( contact stabilization ): In the conventional activated sludge process, the
dissolved organics are quickly adsorbed onto (or into) the flocs, while the actual
conversion to CO 2 and H 2 O proceeds much more slowly. In contact stabilization, two
tanks are used: one (about 1-h residence time) is used to promote the uptake of the
organics and the second (3- to 6-h residence time) is used for reaeration and the final
conversion of the organic material. By concentrating the sludge before oxidation, the total
required tank volume for aeration is reduced by 50% in comparison to the conventional
system.

A summary of typical values for the kinetic parameters in biological waste treatment is
shown in Table 12.2 . Despite all simplifying assumptions, the pure culture model seems to
fit steady-state experimental data reasonably well, although it does not predict the dynamic
performance very well.
Example 12.5. An industrial waste with an inlet BOD 5 of 800 mg/L must be treated to
reduce the exit BOD 5 level to below 20 mg/L. The inlet flow rate is 400 m 3 /h. Kinetic param-
eters have been estimated as m max ¼ 0.20/h, K S ¼ 50 mg-BOD 5 /L, YF X/S ¼ 0.5 g-CS600/

0.005 h 1 . A waste treatment unit of 3200 m 3

g-BOD 5 , and k d ¼ is available for the process.

Assume a recycle ratio of R 0. If you operate at

a concentration ratio c R of 2.5, find the BOD 5 at the exit and determine if sufficient BOD 5
removal is attained to meet specifications assuming that the activated sludge process is
well mixed. What will be the concentration of biomass exiting the reactor, X , and the sludge
production rate from the process?

¼ 0.40 and the separation is complete, X e ¼

TABLE 12.2 Typical Monod Model Parameters for the Activated Sludge Process

m max ,h L 1 k d ,h L 1 Waste source K S , mg/L YF X/S , g-cells/g-waste Basis

Domestic 0.4 e 0.55 50 e 120 0.5 e 0.67 0.002 e 0.003 BOD 5

Shell finish processing 0.43 96 0.58 0.058 BOD 5

Yeast processing 0.038 680 0.88 0.0033 BOD 5

Phenol 0.46 1.66 0.85 Phenol

K I ¼ 380

Plastic processing 0.83 167 0.30 0.0033 COD

Data from Sundstrom DW and Klei HE, Wastewater Treatment, Pearson Education, Upper Saddle River, NJ, 1979.