Feasibility of glucose recovery from municipal sewage sludges as feedstocks using acid hydrolysis
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In light of rising costs in fossil fuels and petroleum, as well as the strain on this largely non-renewable resource, the conversion of biomass, in this case waste biomass, to value-added products is becoming more attractive. In this study, municipal sewage sludge and biosolids were used to determine their potential for glucose recovery. This research focused on three pretreatment processes including drying/grinding, as well as acid and alkaline pretreatments, followed by acid hydrolysis on primary sludge, activated sludge and biosolids. After each pretreatment under specified conditions, the residues remaining from the sludges and biosolids underwent a 2 % H2SO4 acid hydrolysis at 120oC for 1 hr. Compared with activated sludge and biosolids, primary sludge was found to demonstrate the highest potential for glucose recovery in this study. Primary sludge with 1.0 N HCl pretreatment over a 24 hour contact period yielded the highest glucose conversion result as 5.67±0.24%. The best KOH pretreatment condition for primary sludge was a 0.5 N KOH concentration for a 0.5 hour contact period. However, no consistent glucose recovery trend as a function of reagent concentration or contact time was identified for any of the sludges or biosolids in this study. Drying and grinding were also found to efficiently improve the acid hydrolysis results. Fibre content analysis was also performed on the sludge and biomass feedstocks and their residues following pretreatment and acid hydrolysis during this study, to better understand the conversion of these waste biomass feedstock. The Van Soest methods for neutral-detergent, acid-detergent and acid insoluble lignin analysis and the Weende crude fibre analysis were applied to the sewage sludge and biosolids samples prior to and after acid hydrolysis to determine the fibre content including cellulose, hemicellulose and lignin. A modification to the Weende crude fibre analysis was introduced, where a centrifuge step was added prior to the second filtration after the alkaline digestion of the procedure to reduce filter clogging problems. The centrifuge modification effectively reduced the filtering time from one day to 30 minutes; however, there was an average loss of 46% in crude fibre with the addition of this centrifugation step. It was found that most of cellulose content in the feedstock samples was hydrolyzed to glucose after the acid hydrolysis process and most hemicellulose content was likely to have been solubilized and washed away during acid and alkaline pretreatments and acid hydrolysis. The lignin content did not appear to be affected by the pretreatments applied nor the acid hydrolysis.