Thermal hydrolytic pretreatment of wastewater sludge followed by mesophilic anaerobic digestion poses significant benefits in terms of sludge minimization and pathogen reduction; however, high levels of volatile fatty acids (VFA) in digester effluent reveal an interruption in the efficient conversion of organic material to methane gas. Residual soluble chemical oxygen demand (COD) not only requires additional liquid side wastewater treatment, but also represents lost potential energy recovery in terms of methane gas. High residual VFA concentrations within the digester are attributable to increased VFA loading as a result of thermal hydrolysis of lipids, or from poor fatty acid metabolism due to inhibition. A laboratory study was performed in order to determine the role that thermal hydrolysis temperature plays on the destruction of protein and lipids as well as the production of ammonia and VFA. VFA production was found to more sensitive to hydrolysis temperature between 150°C and 170°C. VFA and ammonia production during thermal hydrolysis both increased as temperature increased from 130°C to 220°C.
This research showed that solids loading rather than hydrolysis temperature would serve as a more meaningful control against high ammonia concentrations within the anaerobic digester. Additionally, it was found that polyunsaturated fats were more amenable to thermal hydrolysis than monounsaturated or saturated fats. This finding has practical implications for sludges rich in lipid type material (i.e. primary sludges) as well as coupled processes of thermal hydrolysis and codigestion of fats, oils, and grease.