Study On Enhancing Sludge Dewatering And Fueling By Adding Pulverized Coal As Filter Aid

Sludge is an inevitable byproduct in the process of sewage treatment. As the rate of sewage treatment in cities rises, sludge has increased greatly. It is worrying how to treat sludge. Actually, it has become a new question in sewage treatment and comprehensive management of urban environment and a new challenge for local governments and the whole society where there build sewage treatment plants. Besides, sewage can be regarded as a renewable resource; it can be fuel to recover heat and treated safely and harmlessly if economical and low energy consumptive dewatering is realized. This thesis proposes a sewage treatment process in which sludge dewatering is enhanced and sludge is fueled by adding pulverized coal after analyzing features of oily sludge and excess sludge in urban sewage treatment plants and considering requirements of enhancing dewatering with granular filter aid and fuel utilization of filter cakes.The object of the study is concentrated sludge, in vertical flow sedimentation tanks of the sewage treatment plant in Petrol Branch Company Liao He Oil Field, in the sewage treatment plant (secondary treatment) in northern Shenyang (small-scale experiment) and in the sewage treatment plant in western Shenyang (pilot-scale experiment&industrial applied experiment). Considering sludge is used as fuel after dewatered, the research chooses pulverized coal as filter aid to enhance sludge dewatering. Excess sludge in the urban swage treatment plants was filtered at two stages with filter aid, i.e. vacuum filtration first and then filter cakes were pressed and filtered with air pressure. In the experiment, effect of condition factors on enhancing sludge dewatering was investigated, such as filtration differential pressure, filtration time, filtration medium, quality of pulverized coal, particle size and grade proportion, adding amount, PH value, temperature, stirring condition, etc.; operation conditions of filtration and aid-filtering plan were optimized; mechanism of enhancing sludge dewatering was analyzed. With mixed filter cakes of oily sewage-pulverized coal and urban sludge-pulverized coal after enhancing dewatering as object, each technical index of fuel utilization of mixed filter cakes was investigated by testing water, ash and volatile content of filter cakes, analyzing heat value and doing industrial analysis. The application prospect was also analyzed. Based on the small-scale experiment in lab, pilot-scale and industrial applied experiment were conducted. That is, in Casting and Forging Workshops of Dongbei University and Shenyang Lu Zheng Heavy Industries Group, by600X600mm advanced plate-and-frame filter press (diaphragm compress and gas dewatering) and self-made gasifier, oily sludge and excess sludge in urban sewage treatment plants were dewatered with enhancement; mixed filter cakes were gasified to recover heat. Based on data in the experiment, feasibility for industrial application and technical economic index were analyzed and discussed.The result shows that water content of oily sludge may decrease to below45%from over95%and oil content of filtrate is about20mg/L; moderate sawdust, oxidant and flocculation agent are helpful for enhancing the effect of oily sludge dewatering; pulverized coal is also applicable to aid urban sludge dewatering; filtration with two stages can reduce water content of urban sludge to about40%from over96%under the optimized condition; aid-filtering principle of pulverized coal is to adjust sludge physically to reduce filter resistance and increase the rate of filter and separate by adding pulverized coal as frameworks of mixed filter cakes to prevent great deformation under differential pressure of filtration, decrease compressibility, keep high porosity and permeability during filtering, especially in the later period of filtration, air can be pressed into gaps of filter cakes and reduce water so that water in filter cakes decreases greatly.With pulverized coal as filter aid, not only filtration dewatering of sludge can be enhanced effectively, but also oil content in filtrate of oily sludge can be reduced, which is helpful for sludge fueling utilization after dewatered. After dewatered with pulverized coal, the index of filtrate can meet the requirement of reflux sewage treatment system and heat value of filter cakes increases greatly. Under experimental parameters and conditions, the value of wet basis filter cakes of oily sludge can increase to about14500kJ/kg after filtered and dewatered with coal aid; that of urban sludge can increase to about10000kJ/kg after filtered and dewatered with coal aid; proportion, pyrolysis and combustion characteristic of dry basis industrial analysis can meet requirements of heat resource utilization.