Anaerobic Expanded and Fluidized Bed Reactors

In both systems, small particles are used as carriers for biofilms (bioparticles). Frequently, silica sand with a diameter of 0.2-0.5 mm and a specific density of 2.65 g cm-3 or activated carbon particles with somewhat higher diameters and a lower density are used. The upward flow rate must be high enough to expand the bed and the wastewater must be recycled at a much higher rate than in fixed bed reactors (Fig. 8.10).

Expanded beds are expanded by 15-30% and fluidized beds by 30-300%, relative to the volume at rest. Because of the very high surface area of the biofilms (9000-11000 m2 m-3), their low thickness and the high mass transfer rate in the mobile bed, the biogas production rate is very high and the mean retention time of the wastewater is often only about 6 h and sometimes remarkably lower.

High biomass concentrations of 15-35 g L-1 MLSS are possible, similar to those achieved with the UASB process (Hall 1992), but the load rates achieved of more than 20 kg COD m-3 d-1 can be significantly higher than that of the UASB process (2-25 kg COD m-3 d-1; Grady et al. 1999) because of the reduced limitation of mass transfer by diffusion and convection. The upper segment of the reactors above the expanded and fluidized beds serves to separate the gas bubbles from the water (which is recycled to a large degree) and from the bacterial flocs which have sheared off the biofilms.

In recent times, new anaerobic systems have been investigated and proposed for technical use which are equipped with integrated membranes for retaining and increasing the amount of bacteria.

Fig. 8.10 Anaerobic expanded and fluidized bed reactors: (a) expanded bed, (b) fluidized bed

192 I 8 Anaerobic Degradation of Organics PROBLEM

An industrial wastewater with a substrate concentration of So = 2000 mg L-1 DOC = 5000 mg L-1 COD and a flow rate of Qo = 1000 m3 d-1 is to be treated anaerobically by a one-stage contact process at steady state for a temperature of 35 °C, with a removal efficiency of 90% and a concentration of X = 1 g L-1 MLSS methanogenic bacteria. The wastewater is free of SO2- and NO-. The reaction is neither limited nor inhibited by low or high concentrations of acetate.

Given coefficients: pmax = 0.18 d-1

1. The necessary volume of the bioreactor V.

2. The volume of methane produced daily rCH4 V in Nm3 d-1.

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