Investigation On Sludge Hydrolysis And Dewaterability By Alkaline-thermal Treatment

As a pretreatment technology of excess sludge, alkaline thermal hydrolysis cansignificantly improve the degree of sludge cell disintegration and enhance the treatmentresults of subsequent process（dewaterability, anaerobic digestion, etc.）. This researchfocused on the sludge drained from secondary sedimentation tank of municipalwastewater treatment plant and alkaline thermal hydrolysis was undertaken under theconditions of50°C to150°C. The influences of alkaline thermal parameters（such as thekind of alkaline, alkali dosage, temperature, reaction time, etc.） on excess sludgehydrolysis effect and dewaterability were investigated. Following conclusions can bedrawn through the study:（1） The concentration of nutrients in the supernatant of sludge was significantlyimproved after thermal hydrolysis. At90°C for5h, the concentrations of SCOD, TN,TP reached7535.1mg/L,1063.6mg/L and128.8mg/L respectively. The concentration ofreleased nutrients increased with the temperature and reaction time, and the temperaturewas more efficient than the reaction time on the release of sludge nutrients. Thedissolution rate of SCOD can reach19.8%at90°C for0.5h and the dissolution rate ofSCOD was18.7%at70°C for5h, but the dissolution rate of SCOD was only11.1%at50°C for12h.（2） Compared with thermal hydrolysis, the addition of alkali can reduce theresistance of the sludge cells to high temperature and enhance the release and thehydrolysis of organic materials from the sludge cells. Alkaline thermal hydrolysispromoted the release of nutrients （SCOD, nitrogen, phosphorus, etc.） in sludge and theconcentration of released nutrients increased with the increase of the alkali dosage,temperature and reaction time. Among these influential factors, the impact degree ofeach type factors is in the order of the kind of alkali> dosage of thealkali>temperature> reaction time. NaOH performed better than Ca（OH）₂indisintegrating the sludge. The dissolution rate of SCOD at50°C for0.5h when adding0.1gNaOH/gVS can reach the effect at90°C for1h when adding0.2gCa（OH）₂/gVS.The increased concentration of SCOD was mainly attributed to the VSS dissolution.The results showed that alkaline thermal hydrolysis can decrease the amount of sludge.（3） Compared with thermal hydrolysis, alkaline thermal hydrolysis could wellenhance the sludge dewaterability, the water content can be further reduced in the rangefrom1.7%to9.3%. The dewaterability improved with the increase of the alkali dosage. The assistance of Ca（OH）₂performed better than that of NaOH in sludge dewateringwhen the temperature was below100°C and the centrifugal moisture content can bereduced to75.8%. However, the temperature over130°C would have the oppositeresults. Alkaline thermal hydrolysis can also decrease the temperature and shorten thereaction time of thermal hydrolysis. When thermal hydrolysis combined NaOH andCa（OH）₂at90°C for5h, the dewaterability obviously enhanced when compared withalkaline thermal hydrolysis with NaOH and the disintegration was better than alkalinethermal hydrolysis with Ca（OH）₂.（4） Alkaline thermal hydrolysis significantly changed floc structure and shape withthe increase of temperature and alkali dosage. Particles of floc structure became smallera lot after alkaline thermal hydrolysis. Through the analysis of infrared and ultravioletspectrum, lipid, polysaccharide, protein, and humic acid were proved to exist in thesupernatant of sludge. The relationship of the SCOD variation and the treatmenttemperature（T）, duration（t） and NaOH dosage can be well described with a kineticmodel:△SCOD=20.81[T]^0.37[A]^0.38[t]^0.11，and the correlation coefficient reached0.969.