Research On Coupled Technology Of Reduction And Condition Of Sludge In Municipal Wastewater Treatment Plants

The activated sludge process is the most widely used biological wastewater treatment for both domestic and industrial plants in the world, but it has a serious drawback of producing huge amount of excess sludge. Due to its high moisture content, larger volume and complex components, excess sludge may cause serious secondary pollution problems if improperly disposed. This paper reviewed the technologies on sludge treatment and disposal, and focused on sludge reduction and stabilization process with anoxic/aerobic digestion(A/AD). Comparing coal fly ash, diatomite conditioner and CaO on settleability and dewaterability of excess sludge. Finally, the integrated effect of coupled technology with anoxic/aerobic digestion and coal fly ash conditioning method was studied. The main conclusions are shown as follows: (1) In conventional aerobic digestion(CAD) system, when dissolved oxygen(DO) value of mixed sludge rose to6.0mg/L, and sludge retention time(SRT) reached to12days, the degradation ratio of mixed liquor volatile suspended solids(MLVSS) was up to40%, which had met the demand of Discharge Standard Of Pollutants For Municipal Wastewater Treatment Plant (GB18918-2002)(the degradation ratio of organics^40%). There was positive correlation between MLVSS and dehydrogenase activity(DHA), therefore, it's conducive to determine the degree of stability of sludge by investigating the variations on both MLVSS and DHA.(2) In CAD system, some microbes in activated sludge were lysed by hydrolyzation and oxidation, and a large amount of nutrients (especially nitrogen and phosphorus) released and accumulated in the supernatant, pH level went down. Therefore, it should pay more attention on digestion supernatant treatment while excess sludge was digested and stabilized in CAD system.(3) Different aeration modes influenced sewage sludge degradation. A/AD process was relatively inferior to CAD process on the degradation of sludge organics. However, if considering energy consumption, in order to meet the MLVSS degradation ratio≥40%(GB18918-2002),4hours aerobic and4hours anoxic cycles(4/4),8hours aerobic and8hours anoxic cycles(8/8),12hours aerobic and12hours anoxic cycles(12/12) actually needed aeration time of9days,9days and8days, respectively, which saved25.0%,25.0%and33.3%energy cost compared with12days of aeration time of CAD process, A/AD process show some potential in low cost operation. In addition, aerobic and anoxic environment appear alternately in A/AD system, nitrification and denitrification turn up in one system, A/AD process could remove partial N element in digestion supernatant(TN concentration in A/AD system was about60%of which in CAD system). Alkalinity produced by denitrification could make up for alkalinity consumption at nitrification, and digestion supernatant in A/AD system could keep neutral.(4) Excess sludge was conditioned with coal fly ash, CaO and diatomite conditioner at room temperature, and the best dosage (conditioner/dry sludge) was100%,50%and100%respectively, the best gravitational settling time was90mintues. In addition, the pressure was desirable to be controlled at0.5Mpa when sludge conditioned by coal fly ash and CaO was dewatered by filter-pressing mechanical equipment, while that conditioned by diatomite conditioner was rational to be controlled at0.6Mpa.(5) Effects of coal fly ash, diatomite conditioner and CaO on settleability and dewaterability of activated sludge were compared. The order was as follows:CaO> coal fly ash> diatomite conditioner. As an industrial solid waste, coal fly ash performed well in improving dewaterability of sludge.(6) During the digestion process, bacterium extracellular polymer substances(EPS) were hydrolyzed and sludge flocs were disintegrated at endogenous respiration phase, therefore, the average particle size of sludge decreased and the turbidity and viscosity of digestion supernatant increased. However, since some organisms were oxidized and lysed during the digestion process, partial intracellular water released, which was relatively easy to remove by mechanical dewatering equipment. It could be concluded that the mechanical dewaterability to digestion sludge was better than that of raw excess sludge. Adding coal fly ash as conditioner could increase the removal amount of moisture in digestion sludge, and the best dosage was100%.