INTRODUCTION

Anaerobic processes are used to treat strong organic wastewaters (biological oxygen demand (BOD) >500 mg l ^?1), and for further treatment of primary and secondary sludges from conventional wastewater treatment. Liquid wastewaters rich in biodegradable organic matter are generated primarily by the agricultural and food-processing industries. Such wastewaters are difficult to treat aerobically mainly due to the problems and costs of satisfying the high oxygen demand and maintaining aerobic conditions. The majority of the organic carbon removed from solution during conventional wastewater treatment is converted into waste sludge (Section 14.2). This can be further stabilized either aerobically or, more conventionally, using anaerobic digestion. Anaerobic treatment, although slow, offers a number of attractive advantages in the treatment of strong organic wastes. These include a high degree of purification, ability to treat high organic loads, production of a small quantity of excess sludge that is normally very stable and the production of an inert combustible gas (methane) as an end product (Sterritt and Lester, 1988). Unlike aerobic systems, complete stabilization of organic matter is not possible anaerobically and so subsequent aerobic treatment of anaerobic effluents is normally necessary. The final effluent produced by anaerobic treatment contains solubilized organic matter, which is amenable to aerobic treatment indicating the potential of using combined anaerobic and aerobic units in series. The advantages and disadvantages of anaerobic treatment are outlined in Table 19.1.