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Enhancement of Sludge Reduction and Methane Production Rates Using Different Pretreatment Methods Applied Prior to Small Scale Laboratory Anaerobic Digesters

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Rapidly increasing sludge quantities and limitations on reliable disposal methods compelled the researchers to look for methods to minimize the generation rates. Sludge pretreatment prior to anaerobic digestion emerged as a potential application. Pretreatment using physical, chemical, biological, mechanical or combined methods when applied prior to anaerobic digestion, not only increases the biogas production potential but also reduces the sludge quantities. The suggested mechanism is the solubilization of both extracellular polymeric material matrix around the cells and the microbial cell membranes in waste activated sludge to produce easily degradable organics and hence accelerate the rate of hydrolysis step during anaerobic digestion.

This study aims to compare five different pretreatment methods for their effectiveness in solubilizing the sludge floc components and further effects on anaerobic digestability of waste activated sludge (WAS) considering both the ultimate methane production potential and the methane generation rates. Study concentrated on two chemical (acid and alkali), one physical (microwave), one mechanical (ultrasound) and one combined (microwave and alkali) method of pretreatment prior to batch anaerobic digestion of WAS in 250 mL serum bottles. Soluble COD (sCOD) increases were measured after each pretreatment method before the reactor set-up. Two series of anaerobic digesters were then put together as pretreatment and control reactors using pretreated and untreated sludge, respectively. On anaerobic digesters total gas and methane productions were measured routinely for approximately 50 days of reactor operation.

Results showed that in all the methods employed, sCOD increased significantly right after pretreatment. The control samples which had not gone through any pretreatment had about 10 mg sCOD/gVSS, whereas the pretreated samples by acid at pH 1.5, microwave, ultrasound, alkali at pH 12 and microwave combined with pH 12 yielded 95, 280, 325, 425 and 550 mg sCOD/gVSS, respectively following pretreatment.

Results further indicated that methane productions in anaerobic reactors improved significantly after the application of pretreatment compared to control reactors. The cumulative methane generations, the projected ultimate methane generations and the calculated first order methane generation rates improved appreciably due to pretreatment. The residual sludge following digestion was reduced in varying degrees depending on the method employed.

Keywords: Acid treatment; alkali treatment; anaerobic digestion; methane; microwave; sludge pretreatment; sludge reduction; ultrasound

Document Type: Research Article


Publication date: January 1, 2010

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