The One－stage Autothermal Thermophilic Micro－aerobic Digestion For Sewage Sludge Treatment: Digestion Process And Stabilization Mechanism

Sludge stabilization is an important step for sewage sludge treatment and disposal.Autothermal thermophilic aerobic digestion (ATAD) has the advantage of rapid volatilesolids (VS) reduction, short sludge retention time (SRT), and efficient pathogeninactivation, so it is a promising aerobic process that produces Class A biosolids. SinceATAD technology was first introduced in the early1970s, significant advances had beenmade in the optimization and adaption of ATAD technology for sewage sludgestabilization, and a number of ATAD units had been constructed in Europe and NorthAmerica. Although ATAD process can stabilize sewage sludge and has merits overtraditional aerobic and anaerobic digestion processes, there is no ATAD system used inChina due to lack of fundamental research in this field.The conventional ATAD systems are normally two－stage process. In China, a one－stage ATAD digestion device has been developed due to its simple units and small areaoccupation, and it is named as ATMAD system due to micro－aerobic operationcondition. Bench－scale experiments have revealed that a one－stage ATMAD systemcould achieve the same stabilization effects as two－stage ATAD process, so it haspresented potential application for sewage sludge stabilization in small－and medium－sized wastewater treatment plants (WWTPs). To better understand stabilizationmechanism of one－stage ATMAD process and provide recommendations for its practicaloperation, this project mainly focused on following aspects:(1) optimizing operationparameters of one－stage ATMAD system;(2) investigating digestion efficacy (includingVS removal and pathogen inactivation) and the release of nutrition components as wellas dewatering characteristics of the digested sludge;(3) examining microbial diversity during digestion process;(4) proposing biochemical metabolic pathway;(5) setting up alarge－scale ATMAD digester to examine digestion efficacy of one－stage ATMADprocess and to analyze heat balance.The optimal parameters of one－stage ATMAD process are5～6%total solids (TS),1.0～1.2L/(h·L sludge) aeration intensity, and50～60oC digestion temperature. Higheraeration moderately favores VS removal, however it has adverse impact on autothermalthermophilic condition. Digestion temperature is a major parameter for sludgestabilization, a one－stage ATMAD system could achieve more efficient digestionefficacy with moderate increase of thermophilic temperature, but ATMAD system shallattain low VS removal as digestion temperature is up to65oC. Thermophilic digestionresults in poor dewaterability of the digested sludge, and longer SRT is helpful tomoderately improve the dewaterability. Semi－continuous operation of one－stageATMAD digester at TS level5～6%, aeration intensity1.0～1.2L/(h·L sludge), andtemperature55oC indicated that at least13－day HRT（hydraulic retention time）isrequired to achieve above38%VS removal, and pathogen inactivation of the digestedsludge also meet the requirements of Class A biosolids.As the concentration of organic matter rapidly increases during ATMAD process,aeration is unable to meet the demand of oxygen, and the system maintains low oxidationreduction potential (ORP). As a result, digestion system could regulate its metabolicpathways to produce acetic, propionic, butyric acids and other volatile fatty acids (VFA)constituents in order to meet the demand for NAD+. Based on the variation of VFA andfundamental theories of aerobic and anaerobic digestions, a biochemical model isproposed. As aeration density is constant, the biochemical metabolic process of substratehas significant impact on ORP level.VS can be regarded as a suitable indicator for biodegradation kinetics of organicmatter, and VS concentration can be expressed asRelative error is less than6%between the determined results and the calculated valuesfor a large－scale ATMAD digester as kinetics equation is used during semi－continuous operation process.Two representative thermophilic strains T1(No: HQ436531) and T2(No: HQ436531) were isolated, and16S rRNA gene analysis indicated that they belongedto Hydrogenophilaceae and Xanthomonodaceae, respectively. Inoculation with thermo－philic strains can speed up the degradation of organic matter during the early stage ofthermophilic aerobic digestion, and both specific thermophilic strains and micro－environment significantly affect VS removal. Strains T2can completely acclimatize itselfto thermophilic digestion and favor digestion process. According to DGGE profiles,aerobic, anaerobic or facultative microbes can co－exist in thermophilic aerobic digester,which jointly constitute microbial community and contribute to sludge stabilization.A large－scale digester was designed with effective volume of10m3, and semi－continuous operation was conducted at10－day HRT to evaluate the effectiveness of one－stage ATMAD process and to analyze the heat balance of digestion system. From11to20d after start－up, digestion temperature fluctuated moderately between58and62oC,average VS removal was37.8%, the pH and ORP were7.8～8.2and－325～－260mV,respectively, and oxygen utilization coefficient was about30%. The heat derived frommechanical mixing energy contributed30.8%of the total heat release, so one－stageATMAD system is not completely self－heating.