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Source: Adapted from L.D. Benefield and C.W. Randall, 1980, Biological process design for wastewater treatment (Englewood Cliffs, N.J.: Prentice-Hall).

Source: Adapted from L.D. Benefield and C.W. Randall, 1980, Biological process design for wastewater treatment (Englewood Cliffs, N.J.: Prentice-Hall).

process. Part A in Figure 7.28.2 shows the process schematic.

The anaerobic contact process is comprised of two parts. The contact part involves thorough mixing of the wastewater influent with a well-developed anaerobic sludge culture. The separation part involves the settling out of anaerobic sludge from the treated wastewater and recycling back to the contact reactor. The process usually has a vacuum degasifier placed following the aerobic reactor to eliminate gas bubbles that cause SS in the clarifier to float.

BIOENERGY and ANAMET are two commercially available, proprietary anaerobic contact processes. BIOENERGY is a conventional anaerobic contact process that uses a thermal shock procedure to facilitate sludge separation. As the mixed liquor, at 35°C, flows from the contact reactor to the settling unit, a series of heat exchangers rapidly decreases its temperature to 25°C. This temporarily interrupts gasification allowing effective sludge-solids separation by gravity. The temperature of the recycled sludge is increased before it is returned to the contact unit.

In the ANAMET process, an aerobic biological treatment polishing step follows the anaerobic contact process to provide near-complete organics removal. The process recycles the sludge produced in the aerobic treatment process back to the anaerobic reactor to reduce excess sludge production across the entire system and increase biogas yield. Also, recirculation of the nutrient-containing sludge from the aerobic reactor reduces external nutrient requirements in the anaerobic reactor (Shieh and Li 1987).

Design considerations for the anaerobic contact unit are similar to those described for the activated-sludge process in Section 7.25. In addition, separation unit design can follow the prescriptions for secondary clarifiers described in Section 7.29.

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