Volatile fatty acid production from hydrolyzed sewage sludge: effect of hydraulic retention time and insight into thermophilic microbial community

The disposal of sewage sludge potentially reaches 50–60% of the total operation cost of a wastewater treatment plant. Given its high content of organic material, adopting effective technologies for sewage sludge treatment minimizes its environmental impact and the parallel conversion of the organics into recovered bio-products. Hence, the such stream can be viewed as a renewable carbon source to produce high-value products such as volatile fatty acids (VFA). Short-time (8 h) alkaline (pH 9–11) and thermal (70–85 °C) hydrolysis were applied to enhance the acidogenic fermentability of thickened sewage sludge. Mild thermal hydrolysis (70 °C) was chosen as the best performing method to increase the soluble chemical oxygen demand (CODSOL) and boost the VFA production in the following dark fermentation process, designed at three different hydraulic retention times (4.0, 5.0, and 6.0 days). The highest acidification yield (0.30 g CODVFA/g VS) and CODVFA/CODSOL ratio (0.73) were obtained at 6.0 days as hydraulic retention time. Microbial community analysis performed at the end of semi-continuous tests showed the occurrence of several fermentative bacteria (i.e., Coprothermobacteraceae, Planococcaceae, Thermoanaerobacteraceae) responsible for the fermentation of complex organic matters mainly into acetic, propionic, and butyric acids, which dominated the VFA spectrum.