Experimental Study On Influencing Factors Of Municipal Sewage Sludge Drying Kinetics

With the acceleration of urbanization, the municipal sewage sludge production increased year by year, which caused a series of environmental problems. Sludge thermal drying process cannot only reduce the water content of the sludge, reduce sludge volume and thus reduce their transportation and disposal costs, improve sludge heat value and thus increase the value of its resource utilization, while also reducing the content of pathogenic microorganisms to reduce the environmental pollution. The drying behavior of the municipal sewage sludge is investigated based on a thermogravimetric analyzer and a bench-scale hot air forced convective dryer. The influences of heating methods, additive types and dosages, and pretreatments on sewage sludge drying characteristic are expored based on kinetic analysis theory.The effects of heating methods (isothermal, nonisothermal), additives and pretreatment on the drying kinetic characteristics of municipal sewage sludge are highlighted based on the thermal gravimetric analysis techniques and kinetic theory. During the isothermal drying, the average drying rate increased by about 2 times with the drying temperature rising from 50℃ to 130℃. The average drying rate increased by about 5 times with the heating rate rising from 5℃·min-1 to 25℃·min-1. Within the temperature range 50℃-70℃, the isothermal drying of sewage sludge presented a combination of a short warm-up period, an interim constant rate period, and two distinguished falling rate periods, but without the constant rate period above 70℃. Nonisothermal drying did not show any constant rate period at heating rates of 5-25·min-1. The apparent activation energy of the sewage sludge in the isothermal drying was less than that in the nonisothermal drying.At the same adding dosage of 10%, the average drying rates for sewage sludge blending with lignite, coal fly ash, lime, potassium permanganate, dry sludge and increased by 2.81 percent,3.57 percent,2.65 percent,4.80 percent,3.05 percent, in comparison with the raw sewage sludge between 100℃ and 180℃ . The average drying rates for sewage sludge after freeze-thaw pretreatment increased by 7.08 percent. Apparent activation energies for all sludge/additive blends and freeze-thaw sample were less than that of raw sewage sludge. Apparent activation energies of sludge/lime in two falling rate periods decreased with increasing the lime dosage from 10% up to 30%. Apparent activation energies of freeze-thaw sludge sample in two falling rate periods were the lowest in comparison to all sludge/additive blends. Lignite, coal fly ash, lime, potassium permanganate and back-mixing additives as well as freeze-thaw pretreatment were available for improving thermal drying kinetics of the sewage sludge.Isothermal drying behaviors of the municipal sewage sludge briquettes were investigated based on a laboratory-scale hot air forced convective dryer. The influences of drying temperatures and brown coal as a typical additive on the drying kinetics of the sewage sludge were highlighted. A considerable shrinkage and cracking of the sewage sludge briquettes presented in the drying process, and there were many hollow cavities formed in the final drying stages. The drying processes of the raw sewage sludge and sewage sludge/brown coal blend presented a combination of a short warm-up period, an constant rate period, and two distinguished falling rate periods. The average drying rates of the sewage sludge/lignite blend in the whole falling rate periods were higher than that of the raw sewage sludge. The Midilli models gave good agreement between experimental and predicted moisture ratios of the raw sewage sludge or sewage sludge/ brown coal blend with the highest R2...