Effect of mild microwave pretreatment on characteristics and mesophilic digestion of primary sludge
Creators: name: Zheng, Jian
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2006
Subjects: subject: Engineering, Sanitary and Municipal.

description: Wastewater treatment produces a large amount of contaminant-containing sewage sludge, disposal of which is of great concern and is tightly regulated. Anaerobic digestion of sewage sludge is used in most large-scale wastewater treatment plants prior to sludge end use or ultimate disposal mainly due to its advantages of low energy consumption and potential energy recovery. Thermal pretreatment has been studied and successfully applied to improve the quality of the digestion product. Microwave (MW) irradiation has become of interest with some advantages over conventional thermal pretreatment. The objective of this research is to investigate whether MW pretreatment can enhance the anaerobic digestion of primary sludge (PS). The influence of MW irradiation on the characteristics of pretreated primary sludge was studied in terms of MW intensity, sludge solid concentration, and temperature achieved. The experimental range of sludge of sludge characteristics was sludge solid concentration of 1-4% (w/v) total solids (TS), temperature 35-90°C, and MW intensity of 40 and 80%. MW irradiation was found to increase the concentration of soluble COD (SCOD) in the sludge. The ratio of SCOD/TCOD increased from 2.5% to around 6-7% for 4% TS sludge and MW pretreatment temperature of 90°C. Both sludge solid concentration and MW irradiation temperature were shown to be the most important MW pretreatment parameters in solubulizing primary sludge. MW intensity in the range of temperatures studied had no impact on primary sludge solubilization. Mesophilic biochemical methane potential (BMP) assays were applied to primary sludge pretreated at MW scenarios of 1-4% TS, temperatures 35-90°C, and MW intensity of 40 and 80%. Again, the BMP assay indicated no obvious difference in samples with the same TS concentration and pretreated to similar temperatures but at different MW intensities. Both higher MW pretreatment temperatures and higher TS concentrations significantly improved biogas production rate and reduced required digestion time to achieve 85% of ultimate digestion. For 4% TS primary sludge samples pretreated to 90°C, biogas production rate increased 37% or resulted in a 28% reduction in digestion time to achieve 85% of the ultimate biogas production. There was some indication that MW pretreatment may have caused some very mild inhibition of the whole (soluble and suspended) sludge sample based on the existence of a 2-3 day lag phase for pretreated sludge samples. While MW pretreatment increased the rate of digestion of primary sludge it resulted in no significant increase in the ultimate biogas production and biodegradation of organic matter in terms of VS and TCOD removal. Analysis of biogas production results from the BMP assays indicated that they could be described by a first order reaction. The reaction rate constant increased with increase of TS concentration and MW pretreatment temperature, but was not affected by MW intensity. BMP assay of the soluble fraction of pretreated primary sludge was carried out on 4% TS primary sludge pretreated at MW intensity of 80% and temperatures of 65°C and 90°C. The soluble fraction was shown to exhibit no methanogenic inhibitory effects. In general MW pretreatment at the conditions tested did not result in any significant microbial inhibition and resulted in increased rate of primary sludge digestion without increase of the ultimate degradability of the sludge.