Combined alkaline and ultrasound pre-treatment of thickened pulp mill waste activated sludge for improved anaerobic digestion

Samples of pulp mill waste activated sludge were thickened by centrifugation, from approximately 25 g kg(-1) TS to approximately 65 g kg(-1) TS, and subjected to combined alkaline (NaOH dosage of 0.206 and 0.261 g g(-1) TS respectively) and ultrasonic (39.6 and 16.8 MJ kg(-1) TS, respectively) pre-treatment. Pre-treatment increased the soluble TS, VS, and COD of the pulp sludge 3-14 times over non-treated sludge. Batch anaerobic digestions were used to compare the effects of sludge pre-treatment. Pre-treatment did not significantly improve biogas production over 28 d, but did increase VS reduction (21 +/- 3 to 30 +/- 0.8% for the 25 g kg(-1) TS sludge and 23 +/- 0.5 to 27 +/- 0.7% for 65 g kg(-1) TS sludge). Pretreatment increased the initial rate of methane production. 80% of the total methane production was reached 5-6 d earlier when pre-treated. Overall methane production was found to be 404 +/- 205 and 222 +/- 123 mL g(-1) VSconsumed for 25 g kg(-1) TS sludge non-treated and pre-treated sludge and 384 +/- 183 and 354 +/- 93 mL g(-1) VSconsumed for 65 g kg(-1) TS non-treated and pre-treated sludge. Pre-treatment reduced the dewaterability of both sludges. Thickening by centrifugation reduced the total S content of the dried sludge. Overall, biogas production from pulp sludge was unstable and inconsistent. Further research is required to determine the cause of inhibition. (C) 2012 Elsevier Ltd. All rights reserved.