Study On Excess Sludge Reduction Using Lysis-Cryptic Growth System Integrated With Low Density Ultrasonication And Alkaline Treatment

Currently, the large amounts of sludge produced in wastewater treatment and itshigh cost for treatment and disposal have resulted in an increasing concern on thein-situ excess sludge reduction technologies. Lysis-cryptic growth strategy is thepromising technology with great potential, because the excess sludge reduction andthe improvement of nutrients removal can be achieved simultaneously in alysis-cryptic growth system. Effective lysis is the key of this technology. It has beenfound in our previous research that the effective sludge lysis with low cost can beaccomplished using the combined technique of low-density ultrasonication andalkaline treatment, and a desirable sludge reduction efficiency （SRE） has beenachieved in a related bench-scale investigation. However, the applicability andfeasibility of this combined method for large-scale system still needs furtherinvestigation.In this study, a large-scale lysis-cryptic growth system with a capacity of400m³/dwas built and operated continuously to treat municipal wastewater for excess sludgereduction. Low density ultrasonic/alkaline disintegration and hydrolysis/acidogenesiswere integrated to pretreat its waste activated sludge （WAS）. Conventional activatedsludge （CAS） process was used for its biodegradation.The investigation of continuous operation indicated that the observed biomassyield of the large-scale system was0.27kg VSS/kg COD-consumed, and the SREreached56.45%. The pretreatment promoted significantly the biodegradation of WAS.The water quality of effluent was satisfactory, and the COD_Cr, BOD₅, SS and pHvalue of effluent wastewater could steadily meet the first class B criteria specified inthe Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant （GB18918-2002）. There was no significant effect of sludge pretreatment on the bioactivityof activated sludge in the CAS system. The SVI was80¹20mL/g. The sludgepretreatment system performed well. Under the optimized conditions of53.3mmsludge depth, which corresponded to an ultrasonic density of0.089W/mL and anultrasonic duration of154s, and66.0g NaOH/kg DS, the degree of disintegration（DD） was12.16%, and the SCOD, TN, NH₄⁺-N and TP in the supernatant ofultrasonicated sludge increased dramatically to2280.7,287.27,15.90and126.33mg/L, respectively. The pH value of ultrasonicated sludge was11.84. With thepretreatment of post-reaction and hydrolysis/acidogenesis, the biodegradability ofultrasonicated WAS could be improved. The specific VFA production, sludge acidogenesis degree （SAD） and ammonification efficiency （AE） were2.38mol/kgVSS,19.05%and19.59%, respectively. The pH of pretreated sludge decreasedto about8.70.The sludge depth in the ultrasonic trough and alkaline dosage were importantparameters to affect disintegration effectiveness. It was necessary to modify thestrategy of thin sludge layer for ultrasonic disintegration from bench-scaleinvestigation. With an ultrasonic intensity of0.476W/cm², the desirable sludge depthwas between40and70mm and the optimal value of53.3mm could be determined.The preferred alkaline dosage was66.0g NaOH/kg DS.The COD, nitrogen and phosphorus balances of the large-scale lysis-crypticgrowth system were determined as91.30%,97.25%and71.49%, respectively.Besides the assimilation of microorganisms, there must be other important approachesto remove the nutrients in wastewater. The operation cost of the large-scalelysis-cryptic growth system was1.210Yuan （￥）/m³wastewater, which was11.49%less than that of a typical CAS system. The cost for wastewater and sludge treatmentof CAS system was predominant.It can be concluded that the combined low-density ultrasonic/alkalinelysis-cryptic growth process for excess sludge reduction was both technologically andeconomically feasible.