Sewage Sludge Drying Characteristics At Room Temperature And Energy Consumption Analysis

With the growth of urban population,the continuous improvement of municipal service facilities and wastewater treatment technologies,both the volume of wastewater and the output of sludge are increasing sharply.Sewage sludge contains a lot of water,so sludge drying is one of the main methods to reduce sludge moisture content.Sludge volume reduces significantly after drying and it can also be used as resource.Compared with traditional high temperature drying technologies,sludge drying at room temperature has more advantages,such as low energy consumption,low exhaust gas and condensate treatment load,low requirement for equipment materials,high safety and no risk of explosion and so on.In this paper,dewatered sludge from wastewater treatment plant was used to analysis the characteristics of sludge drying and energy consumption at room temperature.The influence of sludge thickness,air velocity and relative humidity on the moisture content and drying rate of sludge at room temperature was studied first.Orthogonal experiment L9(34)was designed to determine which influenced most among the three factors during the drying process.Besides,the volume shrinkage and cracks generation rules at different sludge thicknesses were analyzed.The results showed that the thinner of sludge thickness,the higher air velocity and the lower relative humidity,the faster decreased of moisture content and the higher drying rate.The sludge thickness influenced most,followed by air velocity and relative humidity.It turned out that the sludge volume shrank and cracks occurred.The cracks differed at different thicknesses,and they developed randomly during the drying process.The kinetic analysis of sludge drying at room temperature showed that Logarithmic model was most appropriate to describe the moisture content changes during the process of sludge drying.The effective diffusion coefficient ranged from 2.27×10-10 m2/s to 1.05×10-9 m2/s,and it increased with the increase of sludge thickness and air velocity and the decrease of relative humidity.The influence of adding ratio of physical additives(coal powder and dried sludge)and chemical additive(lime)on the moisture content and drying rate during the drying process was studied.Besides,the effect of different lime addition ratios on drying condensation properties was investigated.The results showed that the addition of coal powder and dried sludge was beneficial to the reduction of moisture content during the drying process,but the drying rate of sludge increased slightly.Whatever,adding lime to sludge could rapidly reduce the moisture content and shorten drying time.Lime addition ratio not only had a linear relationship with the initial moisture content of sludge after mixing,but also had a quadratic function relationship with the critical moisture content of sludge.The condensate of sludge lime drying at room temperature was weakly alkaline,and the concentration of COD,TOC and NH4+-N in the condensate was so low that it could be directly discharged into sewage treatment system without treatment,which could reduce the cost a lot.Sludge drying experiments at room temperature were carried out in homemade drying device,and the energy consumption during the drying process was tested under full load operation condition.The results showed that the constant drying period made up 50%~70% of the whole drying process,and the drying rate fluctuated.The energy consumption at room temperature changed with seasons and drying conditions.The unit energy consumption of sludge drying at room temperature was 0.77~1.07(kWh/kg water).Intermittent stirring had little effect on the drying effect,but the energy saving effect was obvious.The lowest unit energy consumption could be reduced to 0.73(kWh/kg water),lower than that of traditional thermal drying.