Effect of Thermal Pretreatment on Digestibility of Thickened Waste Activated Sludge and Primary Sludge in Two-stage Anaerobic Digestion

Abstract

This study investigated the effect of high pressure thermal (HPTH) pretreatment on the biodegradability of a mixture of primary sludge (PS) and thickened waste activated sludge (TWAS) in single-and two-stage continues anaerobic digestion. The HPTH was applied to the TWAS only at 1500C and 3 bars for 30 minutes. All the systems were operated at mesophilic temperature (350C). The objective of this study was to evaluate the impact of TWAS pretreatment on hydrolysis and fermentation in acid phase digesters and to evaluate potential for increased methane production in the subsequent methanogenic digester. For the two-stage systems, the impact of hydraulic retention time (HRT) on performance was evaluated at two different conditions.

The biodegradability of TWAS after pretreatment was evaluated through BMP tests and an increase of 16% biodegradation was observed with pretreated TWAS (PTWAS) as compared to raw TWAS. The HPTH also increased the solubilisation of TWAS by 25-34%. The biodegradable products generated in the pretreated stage were further hydrolyzed and acidified in the acidogenic stage and produced 2 fold higher VFAs in the pretreated digester compared to the control digester. To evaluate the impact of PT on hydrolysis dynamic tests were employed and a two-component hydrolysis model was fit to the data. The readily biodegradable hydrolysis coefficient (Kr) for the control and pretreated digesters were 3.09 d-1 and 2.53 d-1 respectively. Thus, the overall rates of solubilisation were higher for the control than the pretreated digester. However, the advanced pretreatment stage was capable of producing 10% more VFA/TCOD compared to the control. The dynamic tests showed that despite slower hydrolysis rates in the pretreated digesters this PT was capable of producing higher fermentation products in the pretreated digester of the acidogenic phase.

The results of this study showed that the HPTH pretreatment resulted in about a 30% increase in the methane production in both single and two-stage processes. Also, same yield value of 0.16 L CH4/ g TCOD added was found for the pretreated digester in two stage system with 10 day HRT and in the control digester of single stage system with 13 day HRT. Thus, integration of pretreatment with two stage digestion yielded similar methane production to that observed with a control that had an extended HRT.