Activated Sludge Dewatering,Drying And Mixed With Sawdust To Prepare Biomass Fuel

Activated sludge,one of the most massive solid wastes,will bring about huge damage to the environment if not disposed reasonably.The dewatering is a key step in the treatment and disposal of activated sludge.After dewatering,the volume of the activated sludge is still pret-ty large.Hence the drying technology is usually used to further reduce its volume before its utilization.Here we study the dewaterability of activated sludge under microwave irradiation and its combination with conditioners,design the sludge drying facilities for the sewage plants in Jiande County of Zhejiang Province,and then use the dried sludge to prepare bio-mass fuel by mixing with sawdust to provide a simple,safe and efficient way for the utiliza-tion of activated sludge.The dewaterability of activated sludge was improved by microwave irradiation under the optimal conditions of 7 W/mL and 100 s.The microwave irradiation stimulate the growth of sludge flocs,which neutralizes the increase of SB-EPS and LB-EPS that deteriorates sludge dewaterability.Microwave irradiation combined with one of Ferric chloride(FeCl3),Polyalu-minum Chloride(PAC)and Chitosan(CTS)can all enhance the sludge dewaterability and FeCl3 has the best synergistic effect.The increase of sludge flocs and the reduction of EPS are responsible for the sludge dewaterability improvement.The sludge conditioned by adding conditioner first has better dewaterability than that conditioned by microwave irradiation first.The calorific value of the sludge gradually decreases with the time under microwave irradiatio,while it increases first and then decreases with the increase of ferric chloride dosage.The cal-orific value is negatively correlated with the content of SB-EPS and LB-EPS in the activated sludge.Rotary disc drying process,three kinds of belt drying process,paddle process,solar dry-ing process and thermal drying process are investigated for activated sludge drying.The belt drying process(Norman)is finally selected for the engineering design after a comprehensive evaluation based on the investment,energy comsumption,secondary pollution,operating properties and the economy.An activated sludge drying facility is designed with a treatment scale of 13.2t/d for water removal from sludge to achieve dried sludge with lower than 20%of water content.The water removal efficiencies are about 89.3%,and the cost for each ton of sludge with water content of 70%varies from 128.2 to 138.9 Yuan.The shatter resistance and the relaxed density of biomass fuel ware increased with the molding pressure and activated sludge ratio.The moisture content of biomass fuel is prefera-bly controlled in the range of 15%to 20%.The molding pressure and moisture content have a strong influence on shatter resistance,while the molding pressure and activated sludge ratio significantly determine the relaxed density.The thermogravimetric analysis reveals that the combustion stages of biomass fuel rely on the blend ratio of activated sludge.Biomass fuels with different ratio of activated sludge(except the ratio of 0%)all have three stages of dehy-dration,combustion of low-temperature volatile,and decomposition of inorganic matter.Re-sidual volatiles and fixed carbons are precipitated when the ration of activated sludge is less than or equal to 40%.The general combustion performance of biomass fuel is improved by the increase of sawdust ratio,while the flammability is bettered by the increase of activated sludge.The biomass fuel with better combustion characteristics depends on an optimized process and recipe.The bioavailable forms of the elements of Ni,Pb,Zn,Cu,As and Cr are reduced greatly in the residue compared with them in the sludge,while the residual forms of these elements are increased remarkably.Hence the migration and the bioavailability of these heavy metals are restrained significantly if they are disposed in the nature.The leaching toxicity of these heavy metals in the residue is obviously lower than the values legally required by the National Standard,making sanitary landfill feasible for the residue disposal.In conclusion,the method of microwave irradiation combined with conditioners such as ferric chloride is suitable for the improvement of activated sludge dewaterability,and the preparation of biomass fuel using dried sludge and sawdust is a feasible method for sludge utilization.