Wastewaters generated from food — and beverage-processing industries often have little SS but high concentrations of soluble organic compounds (up to 50,000 mg/L of biological oxygen demand, BOD) such as starch, sugars, and proteins. Some common examples of these high-strength wastewaters come from cheese factories, wineries, breweries, distilleries, slaughterhouses, potato processing, and ice cream factories. The organic compounds in these wastewaters can be readily degraded and converted to methane biogas, but the initial major objective of anaerobic treatment of such wastewaters was to degrade and reduce the organic pollutants in the wastew­aters to satisfy governmental discharge requirements. With the push for bioenergy, the focus of anaerobic treatment of high-strength organic wastewaters has been shifted to methane biogas production. High-rate AD (HRT <24 h) has been com­monly used to both reduce the organic strength of the wastewaters and recover the energy as methane biogas. UASB reactors [32] are among the most popular digesters used by many industries. A new variant of UASB reactors is the expanded gran­ular sludge bed (EGSB) reactors. The advantages of EGSB over UASB, such as improved mass transfer and digestion rate, enhanced ability to handle high-strength influents, and high hydraulic loading rates (HRT <2 h), have been well recognized [82]. Therefore, during the last decade the number of EGSB reactors built exceeds that of new UASB reactors [32].

In addition to UASB and EGSB reactors, the following digesters have also been successfully used in the AD of these high-strength wastewater streams: CSTR (sin­gle staged, e. g. [74], or two-staged e. g. [72],), anaerobic contact filter reactor [83], anaerobic filter reactor [1], down-flow fluidized bed reactor [34], internal circulation (IC) reactors [29], and anaerobic hybrid reactors [14]. The sand-bed filter reactor manufactured by NewBio E Systems, Inc, [91] is another promising AD technology for such wastewater (unpublished data). Compared to the digesters used to digest feedstocks with high SS, most of these reactors have much higher loading rates. Thus, they have smaller footprints, but they need to be operated by well-trained digester operators. Detailed descriptions of each of these reactors and vendors is beyond the scope of this chapter, but interested readers should consult other recent books [79, 91] or reviews [10, 77]. Anaerobic treatment or digestion of specific high-strength wastewaters have also been extensively reviewed (e. g., see [56, 57, 66] for distillery wastewaters; and [18] for meat — and potato-processing and dairy wastewaters).

It should be noted that performance data from an existing AD digester can only be regarded as broadly indicative of how a similar AD technology may perform elsewhere, especially with respect to stability, efficiency of organic removal, biogas yield and quality. Only through studies using laboratory — and pilot-scale AD reactors on the feedstock of interest can the most suitable AD technology be identified for that feedstock.