Anaerobic Biological Treatment Systems

The anaerobic biological wastewater treatment process is generally practiced if the organic strength of the wastewater is comparatively high (e.g., COD > 4000 mg/L). In this process, anaerobic and facultative microorganisms are utilized to convert the organic pollutants in wastewater to carbon dioxide and methane. Anaerobic treatment consists of two major steps: acid fermentation and methane formation. During the first step, complex organic matters are converted into soluble products such as fatty acids and amino acids, which further leads to the formation of volatile acids and alcohols. The second stage (methane formation) consists of acetogenesis (conversion of volatile fatty acids to acetate, hydrogen gas, and carbon dioxide) and methanogenesis (methane formation by the products of acetogenesis).21

The anaerobic treatment process is pH dependent, especially when it involves methane formation (optimal pH range 6.8-7.2).21,22 Sodium bicarbonate and calcium bicarbonate can be added to provide sufficient buffer capacity to maintain pH in the above range.21 Temperature is another important factor to be considered during the anaerobic treatment process. The acid fermentation stage can withstand a higher temperature range due to the presence of microorganisms that can tolerate different levels of temperature. However, in the methane formation step, the temperature changes should be controlled. Usually, the temperature of anaerobic digesters can be mesophilic (30-40°C) or thermophilic (50-60°C).23 Other than the factors discussed above, maintaining the proper nutrient levels and organic loading rate is important to control the anaerobic treatment process.22

In this section, we will discuss selected anaerobic treatment systems that are in use for food industry wastewater treatment.

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