Characterization of Anaerobic Digesters during FOG Co-Digestion Shock Loading

Biogas (a mixture of methane, carbon dioxide and trace amounts of other gases) is produced as a byproduct of anaerobic digestion in wastewater treatment plants. While the methane in biogas can be converted into electrical energy, most often it is flared due to non-economical quantities being produced. The addition of Fats, Oils, and Greases (FOGs) can increase the production of biogas, and thus methane, to levels that make it economical to convert to electricity. However, adding excessive amounts of FOG to an anaerobic digester, referred to as shock loading, can negatively affect the anaerobic bacteria and lead to a decrease in cumulative biogas production. In this study, the effects of shock loading anaerobic digesters were tested for six different FOG sources. The anaerobic digesters, set up as batch rectors, were used to test various loading rates for each FOG source. The FOG sources tested include Food Manufacturing Waste, Bakery Waste, Vitamin Waste, Yogurt Waste, Cherry Waste, and Mayonnaise Waste. Biogas and methane production were measured daily. In addition, total solids (TS) and volatile solids (VS) were analyzed and used to assess the performance of the anaerobic digesters. Of the six FOG sources tested, four did not demonstrate any signs of shock loading when a maximum of 1.50*10² mg VS of FOG was added to 2.25*10³ mg VS of inoculated sludge. An optimal loading rate of 75.8 mg VS FOG per 1.65*10³ mg VS of inoculated sludge was determined for one of the two FOG sources that exhibited signs of shock loading.