Performance Of Pretreatment Anaerobic Digestion For Sewage Sludge And Variation Of Organic Matters During Pretreatment

At present, most wastewater treatment plants in China choose not to construct or operate anaerobic digesters. Regarding restrict discharge standard and requirement for sludge handling, improving treatment performance of anaerobic digestion becomes an urgent problem. Hydrolysis is considered as rate-limiting step of anaerobic digestion, thus pretreatments are adopted to accelerate the hydrolysis rate. Ultrasonic pretreatment, featured at unnecessary of chemical addition and less pollutant production, was well studied. However, there still existed limitation for sludge disruption and energy consumption for ultrasonic pretreatment. Therefore, sewage sludge, including waste activated sludge, mixed sludge and dewatered sludge, was used as raw material for investigation of the effects of ultrasonic pretreatment, ultrasonic +alkaline pretreatment and freezing/thawing pretreatment on sludge disruption. Meanwhile, the treatment performance of combined technology of pretreatment-anaerobic digestion was examined and the optimum conditions for pretreatment and reactor operation were attained. Most researchers have focused on studying the biologic reduction mechanisms instead of chemical changes of reactions and molecular structures, DOM fractionation was adopted to study the variation of organic matter in sludge supernatant and in sludge EBOM (extracellular biological organic matter) during pretreatment.The study of the effects of pretreatment conditions on sludge characteristics showed that the organic matter solubilization and solid matter solubilization of sludge favored at high ultrasound intensity/long sonication time and high pH value/long alkaline treatment time. The effects of cavitation and hydroxyl radical became stronger (over 92%) with an increasing ultrasound intensity. The coagulation performance resulted from organic matter solubilized by pretreatment was weakened with an increasing sonication time.The solubilization of waste activated sludge, mixed sludge and dewatered sludge all favored at long freezing time and high freezing temperature. The addition of primary sludge to waste activated sludge hampered the disaggregation of sludge particles. Freezing/thawing enhanced the settleability and dewaterability of sludge. Physical characteristics regarding settleability and dewaterability changed markedly during freezing stage, while organic matter solubilization and particle size distribution basically relied on curing stage. 72 h freezing with 3 h thawing was recommended as an appropriate operation time for freezing/thawing treatment.At the same degree of sludge disruption, the energy consumption of freezing/thawing treatment was 3 times higher than that of ultrasonic treatment. It makes sense to choose ultrasonic treatment over freezing/thawing treatment. However, within cold climates, under which sludge freezing was processing spontaneously, the energy consumption of freezing/thawing treatment was sharply reduced. Thus the freezing/thawing treatment was suitable to the cold regions.The study of treatment performance of pretreatment-anaerobic digestion technology showed that at hydraulic loading rates of 0.05~0.10 d-1, the TCOD removal rates for sonicated sludge increased by 4.9%~13.5%, the VSS removal rates increased by 5.4%~20.8%, the daily gas production increased by 20.26%~57.9%. At various hydraulic loading rates, the ultrasound + alkali treated sludge showed highest TCOD removal rate, SCOD removal rate, solid matter removal rate and gas production over the alkali treated sludge. The optimum hydraulic loading rates ranged at 0.05~0.07 d-1 for sonicated sludge and ultrasound + alkali treated sludge, which led to satisfaction of discharge standard of 40% of organic matter degradation. The pretreatment played a major role in SS removal, while anaerobic digestion played a major role in TS and VS removal. At stable conditions of anaerobic digestion, the cumulative gas production of freezing/thawing treated primary and secondary sludge increased by 56.2% and 27.5%, respectively. The peak gas production of freezing/thawing treated dewatered sludge appeared in a later time. And the enhancement of gas production for freezing/thawing treated dewatered sludge was not satisfactory.Under various ultrasonic conditions, HPI and TPI-A fractions in sludge showed preferable solubilization performance and HPO-A and HPO-N fractions solubilized at high ultrasound intensity. The solubilization of TPI-N fraction was limited. In terms of ultrasonic + alkaline pretreatment, good solubilization performance was reached for HPO-A, TPI-A, HPI and TPI fractions while solubilization of HPO-N fraction was limited. Freezing/thawing enhanced the percentage of HPI fraction and decreased the percentage of HPO-A fraction in sludge EBOM, thus improved the biodegradation of sludge.The main fluorescent substances in sludge EBOM and in sludge supernatant before and after pretreatment were unchanged, being aromatic protein-like substances and soluble microbial byproduct-like materials. However, the fluorescence intensity of DOM and five fractions increased in sludge supernatant after pretreatment, which showed good solubilization of aromatic protein-like substances and soluble microbial byproduct-like materials by pretreatment. The addition of alkali improved the solubilization of TPI-N related aliphatic compounds. Good solubilization of phenolic compounds and organic matter containing carbonyl and carboxyl unsaturated groups was obtained by different pretreatments.The waste activated sludge supernatant contained DOM of high molecular weight. The low molecular weight organic matter presented in raw sludge supernatant changed to a relatively high molecular weight organic matter in sonicated sludge supernatant, because of organic matter solubilization. The TPI-A and HPO-N fractions solubilized by ultrasound mainly consisted of high molecular weight organic matter, while HPI and TPI-N fractions after ultrasonic pretreatment showed moderate molecular weight organic matter.This research investigated sludge pretreatments and combined technology of pretreatment-anaerobic digestion from the aspects of enhancing sludge disruption and saving pretreatment energy consumption. Furthermore, the variations of organic matter in sludge supernatant and in sludge EBOM during pretreatment in terms of chemical changes were examined. The correlative conclusions provided theoretical basis for economically and effectively choosing pretreatment methods and optimization of operation conditions.