Wastewater Feedstocks
Biogas from Wastewater
The anaerobic treatment of liquid wastes or wastewaters provides the opportunity to rapidly reduce the organic content of the waste while minimizing treatment process energy consumption and production of microbial biomass or sludge. The conversion of organic compounds in wastewater into sludge produces a by-product which requires further treatment or disposal. Reduction in sludge and energy consumption are the two attributes which have made consideration of direct anaerobic pretreatment of wastewater economically attractive for municipal and industrial waste streams. For relatively warm wastewaters containing significant degradable organic compounds, direct anaerobic treatment may also provide excess energy. But even with low strength wastewaters the energy savings which can be achieved by avoiding most of the cost of aeration is significant. However, effluents from anaerobic treatment are often not suitable for direct discharge into receiving waters without further treatment which may require aerobic polishing. Yet, the reduced aeration demand and sludge production in aerobic treatment following anaerobic pretreatment may justify this treatment scheme.
Characteristics of Liquid Wastes
The characteristics of a waste stream influence the potential for anaerobic treatment, as well as the selection of reactor design. Of these characteristics, the most important are suspended solids concentration, organic strength (BOD or COD), temperature, pH, and presence of inhibitors. Suspended solids and grit can accumulate and impair certain reactor designs. For the purposes of this section, liquid waste or wastewater is considered to have a suspended solids concentrations less than 1000 mg/l with negligible amounts of grit (inorganic non-soluble solids) which can be mostly removed by simple pretreatment. Defined as such, wastewaters can be classified as low, medium or high strength based on BOD (or degradable COD) concentration. Table 1 indicates the range of BOD concentrations associated with this classification and provides examples of wastewater sources.
Table 1. Classification of wastewater strength and examples.
Wastewater Strength |
BOD Range (mg/l) |
Examples of Sources |
Low |
<1,000 |
municipal, agricultural (including flushed manures), pulp and paper |
Medium |
1,000-10,000 |
food processing, canning, citrus processing, dairy processing, juice processing, brewery |
High |
10,000-200,000 |
ethanol production, distilleries, biodiesel production, petrol-chemical, slaughterhouse |
The temperature of the wastewater, as well as the average ambient temperature, impact the selection of anaerobic treatment design. Some low and medium strength wastewaters are relatively cool (< 20 oC) and the energy required to heat them to mesophilic temperatures is significant and not economical. For a wastewater with a temperature of 20 oC and a COD of 20 g/l, the production of biogas yields around the same amount of energy required to raise the temperature of the liquid to 35 oC. Thus, for low and medium strength wastewaters, treatment only at ambient temperatures is practical. Successful anaerobic treatment of wastewaters as low as 15 oC is possible but below 12 oC the application of anaerobic digestion should not be considered. At the other extreme, there are many industrial wastewater streams which are quite warm and application of mesophilic (food processing) and in certain cases thermophilic (distillery wastes) anaerobic digestion can be considered.
Finally, the impact of wastewater pH or the presence of inhibitors can influence process selection and the cost of operation, through the requirement of chemicals for controlling alkalinity or precipitating metals. These factors are best explored through the use of treatability and pilot studies to determine the economic feasibility of chemical pretreatment and anaerobic digestion.