Evaluation Of Composting Process Using Two Different Kinds Of Sewage Sludge In Small Reactors Under Non-constant Temperature

With the rapid accelerating urbanization and economic development of our country, more and more urban wastewater also will produce. In order to improve sewage collection and treatment systems, a large number of urban sewage treatment plants were constructed rapidly, some are still under construction, which leads to a lot of sludge generated. Due to lack of appropriate treatment and disposal methods, a large number of sludge was piled up in temporary places, and this phenomenon has created a significant threat on our living environment. There is an urgent need for a safe sludge disposal option. Land application is widely recognized as the most effective way of utilization of sludge. In general, sludge needs non-toxic and harmless treatment before land application. Aerobic composting can not only kill most pathogens in high temperature stage but also stabilize organic matter. More and more countries support and favor the method due to its low power consumption advantages. This study proposed that temperature change of the composting pile was simulated based on continuous on-line monitoring microbial respiration, carbon dioxide (CO2) emission rate in order to solve the disadvantages of small-scale composting studies under constant temperature. Composting processes were conducted with forced aeration using excess sludge and digested sludge under mesophilic condition, and dry sawdust was added to the dewatered sludge to adjust the moisture content to ca.60%. The composting of the mixture was performed in a laboratory-scale reactor for 40 days. Periodic sampling was conducted for 40 days to detect and analyze a variety of physical and chemical indicators, including temperature, CO2 emission rate, moisture content, pH, conductivity(EC), carbon and nitrogen ratio(C/N), ammonium nitrogen, nitrite nitrogen, nitrate nitrogen, seed germination index(GI), heavy metals, etc. Mechanisms of aerobic sludge composting was explored through the changes of these physical and chemical properties in composting piles, and sludge compost can be used for land application depending on compost maturity, nutrient contents and the amount of heavy metals. The main conclusions are summarized as follows:(1) Sludge composting reactor system, which was designed for the experiment, successfully simulated temperature change of large-scale composting pile through the use of CO2 emission rate. Monitoring results showed that the fluctuation of CO2 emission rate from excess sludge with sawdust was significant during the composting process. Temperature of the pile, calculated by the four-hour averaged CO2 emission values, experienced warming stage, high temperature stage and cooling stage; temperature in the pile more than 35 ? continued 7 days, more than 45? was 2.3 days, the high temperature stage was a little bit short, and the maximum temperature was 61.5?. The CO2 emission rate from digested sludge with sawdust was much more stable compared to that from excess sludge with sawdust. The pile did not experience high temperature phase and the temperature was maintained below 30? during the whole composting process. Final temperature of the compost pile was 23.6?.(2) During 40 days composting process, moisture contents of the two sludge pile bodies were controlled 53%?75%, and they were roughly within an acceptable range, which did not have a significant impact on the mix composting process. Two kinds of pH values of compost products were controlled between 5.5 to 9, meeting the sludge agricultural standard. The conductivities of the two composting products were less than 3.00 mS/cm, therefore they will not inhibit plant growth or cause toxic effects, and acceptable for land application. Ammonium nitrogen contents were almost same in two kinds of piles at the final time, while nitrite nitrogen contents of both piles were maintained extremely low. Nitrate content in digested sludge with sawdust was significantly higher than the excess sludge with sawdust.(3) After 19 days aerobic composting process, excess sludge with sawdust can reach full maturity, while digested sludge with sawdust was mature from beginning to end. Digested sludge with sawdust pile represented higher levels of inorganic nitrogen than excess sludge with sawdust. But the difference was small, and the inorganic nitrogen content was about 0.3 percent. Heavy metals in the two kinds sludge compost meet the A class sludge of CJ/T309-2009 standard,; in comparison, compost product from excess sludge contained lower levels.(4) Excess sludge with sawdust compost can be used for vegetables, food crops and other food chain crops, and fiber crops non-food chain crops, fodder crops, oil crops etc., after 40 days composting process; judging from CO2 emission rate, digested sludge was stable from the beginning of the composting. Digested sludge might be directly utilized as A grade sludge without composting, and only simple treatment processes such as dehydration and sterilization under high temperature were required. This result was indicated by germination index, EC, pH value, heavy metals and other monitoring data.