Question 11 of 18: Industries Favouring Anaerobic Biological Treatment

Question 11: Industries Favouring Anaerobic Biological Treatment (6 marks)

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Anaerobic treatment is preferred wherever the wastewater is high in soluble, readily-degradable organic strength, because its energy and cost advantages scale directly with the organic load, and it becomes uneconomical to aerate (i.e., to supply the enormous oxygen demand aerobically) at that strength. Three representative industries are: (1) Breweries and distilleries. Their wastewater is very high-strength (BOD₅ often 2,000–6,000 mg/L or higher, from sugars/starches in spent grain liquors), readily biodegradable, and warm — ideal conditions for a high-rate anaerobic reactor (e.g. UASB). Anaerobic treatment avoids the prohibitive aeration energy an aerobic system would need at that BOD loading, and it produces biogas (methane) that can be recovered as an energy credit, while generating far less excess sludge to dispose of than an equivalent aerobic system. (2) Pulp and paper mills. Certain paper-mill streams (e.g. thermomechanical pulping (TMP) whitewater, or spent sulfite liquor) are high-strength and moderately warm process effluents; anaerobic pre-treatment substantially reduces the organic load (and the associated oxygen demand and sludge production) before a smaller aerobic polishing stage handles the remaining BOD and any toxicity, again with methane recovery offsetting mill energy costs. (3) Meat/poultry processing and rendering plants. These generate very high-strength, warm, fat-and-protein-rich wastewater; anaerobic lagoons or high-rate reactors handle the bulk organic load economically, again with the biogas offsetting the plant's own process-heat demand, before a shorter aerobic step polishes the effluent to permit levels. In each case the common thread is the same: high organic strength plus elevated temperature makes anaerobic treatment's lower energy cost, lower sludge yield and biogas energy recovery decisively cheaper than trying to aerate the full load, even though anaerobic treatment alone typically cannot reach as low a final effluent BOD as an aerobic (or aerobic-polished) system.