Anaerobic Filter

In an anaerobic filter reactor, the growth-supporting media is submerged in the wastewater. Anaerobic microorganisms grow on the media surface as well as inside the void spaces among the media particles. The media entraps the SS present in the influent wastewater that can be fed into the reactor from the bottom (upflow filter) or the top (downflow filter) as shown in the process schematics in Part C of Figure 7.28.2. Thus, the flow patterns in the filter can be either PF or completely mixed depending on recirculation magnitude. Periodically backwashing the filter solves bed-clogging and high-head-loss problems caused by the accumulation of biological and inert solids. BACARDI and CELROBIC are two proprietary anaerobic filter processes currently available.

The anaerobic filter process is effective in treating a variety of industrial wastewater (Shieh and Li 1987). An advantage of using the filter process for industrial wastewater treatment is that the filter reactor can retain the active biomass within the system for an extended time period. The long sludge-retention time maintained by the reactor allows ample time for aerobic microorganisms to remove organics in the wastewater, and there is no appreciable loss of the active biomass from the system until the filter is saturated (Shieh and Li 1987). In addition, the anaerobic filter minimizes operational concerns of sludge wasting and disposal because the synthesis rate of excess biomass under anaerobic conditions is low (Young and McCarty 1968).

Because it can retain a high concentration of active biomass within the system for an extended time period, the anaerobic filter can easily adapt to varied operating conditions (e.g., without significant changes in effluent quality and gas production due to fluctuations in parameters such as pH, temperature, loading rate, and influent composition). Also, intermittent shutdowns and complications in industrial treatment will not damage the filter since it can be fully recovered when it is restarted at a full load (Shieh and Li 1987).

A problem associated with the filter's ability to retain the biomass for a long time period is the close control of biomass holdup. Although periodic backwashing of the filter is a feasible method for maintaining the biomass holdup at the required level, more efficient techniques are needed.

Environmental engineers can determine the size of an anaerobic filter from the volumetric loading approach or from the biofilm kinetic theory approach described next. From the design approach commonly used in heterogeneous catalytic processes, the following expressions describe the overall substrate utilization rate for a completely mixed anaerobic filter:

Ro = (kSXs)/(Ks + S) + fok'S)/(K + S) 7.28(1) k' = pkAS 7.28(2)

where:

Ro = the overall substrate utilization rate, mass/volume-time

Xs = suspended biomass concentration, mass/volume ^ = the effectiveness factor that defines the degree of diffusional limitations of the biofilm k = the maximum substrate utilization rate in the biofilm, mass/volume-time p = the biofilm dry density, mass/volume A = total biofilm surface area per unit filter volume, l/length

S = biofilm thickness, length

Expressions for the effectiveness factor have been developed by researchers Atkinson and How (1974) as follows:

V = 1/$ - [tanh(B)/B]{[1/tanh($)] - 1} for $ > 1

$ = [0.709(S/Ks)]/[1 + (S/Ks)] {(S/Ks) - ln[1 + (S/K)]-05}

where D = the effective diffusivity of substrate in the biofilm, area/time.

Hence, the filter volume can be calculated from the following equation:

For a PF anaerobic filter, environmental engineers can calculate the overall substrate utilization rate using the CF-STR-in-series model (see Section 7.25) as follows:

Applying Equations 7.28(1) to 7.28(6) to each reactor i calculates Roi.

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