Research On The Optimization Of Key Factors During Municipal Sewage Sludge Biodrying

Sewage sludge, identified as one source of biomass, is receiving muchattention due to its abundant production and high energy recycling potential. Thehigh concentration of water content in original sewage sludge prohibits itsapplication efficiency. Dehydration, harmless deposal and energy recovery ofsewage sludge are key factors of its utilization.Taken the sewage sludge as the study object, a novel technology, bio-drying,was provided in this work for sludge fast dewatering and dehydration, with lowenergy consumption being expected and high-quality energy recycling. The aim ofthe bio-drying treatment （in short-time process） is the exploitation of theexothermic reactions （thermal drying, on the contrary, needs an external heatsource） for the evaporation of the highest part of the humidity in the waste with thelowest consumption of volatile solids. Bio-dried material, which is the productobtained after the bio-drying process, can be used in many ways: as a secondaryfuel in appropriate plants （grate or fluid bed plants, cement factories, etc）, as afeedstock for pyrolysis/gasification systems or as a material to reclaim oldlandfills.The results showed that the sludge had good porosity, water content andnutrition for biodrying when straw was used as bulking agent. A good amount ofproduct reflux could reduce the reactor startup time and improve the temperaturerising rate, and turning could eliminate the temperature and moisture contentgradient caused by one-way ventilation. On the premise of the system warming-uprate was not been restricted, more ventilation could remove more moisture. Thetemperature feedback control system, which was combined with a heataccumulation period （low frequency and few ventilation in warming-up stage） andforced water removal period （high frequency and large ventilation in hightemperature stage）, could effectively improve sludge biodrying efficiency. Theoptimal parameter control conditions for sludge biodrying were: sludge to strawmass ratio8:1, product reflux10%in mass ratio,2days turning frequency,ventilation rate0.06⁰.1Nm³/（h·kgwet material）, temperature control points between55℃and60℃in feedback system.In the process of biodrying, the sludge granule structure became loose, and theheavy metal contents had certain enrichment but were still lower than agriculturalapplication or combustion restricts. With the macromolecular organic matter andPAM been decomposed by microorganism, the sludge leaching solution becamedarken coupled with the increase of water-soluble TOC and the pH value. The CO₂ content （maximum12.9%） and relative humidity （maximum100%） increasedsignificantly in the exhaust air out of the reactor, which was related with the highmicrobial activity and temperature. After the biodrying procedure, the typicalorganic components of carbohydrate, protein, lipid in sludge had a large amount ofconsumption, while hemicellulose, cellulose and lignin appeared a certain degreeof enrichment, which was good for the follow-up disposal of combustion orpyrolysis.The matter and energy balance calculations of the biodrying system found that,after12days biodrying experiment, the sludge moisture content reduced from73.46%to60.31%, and the volatile solid content reduced from67.79%to50.37%,with the moisture removal rate of59.85%and organic matter consumption rate of37.69%. The energy utilization efficiency, which could be described by thebiological metabolic heat production used in moisture evaporation, was65.2%,equivalent to6.48kgwater/kgVS.