The Technology Research Of Recycling Protein From Urban Sewage Treatment Plan Sludge

As the inevitable product of wastewater treatment and the ultimate collection of wastes in water body, sludge, as well as its recalcitrant characters, has become the key issue of environment protection. If not treated properly, pathogens, heavy metal ions, persistent organic pollutants and other wastes in sludge would lead to secondary pollution, posing severe menace for human health and ecosystem. Landfill, incineration, composting and other sludge treatment technologies are always unsatisfactory for their high cost and insufficient use of biomass which contains in sludge. Under the increasing demand of energy saving and the guiding ideology of circular economy, how to make use of the sludge without posing any hazard to human and environment is becoming the hot point of environmental science and technology.Our research focused on making use of the biomass in the sludge. Acid-heating, alkali-heating, microwave, and alkali-ultrasound methods were used to treat sludge while the organism’s protein were recovered at the same time. Energy input, pH value, processing time and water content of sludge were analyzed to explore their effect on protein recovery. In addition, protein recovery efficiency and monetary cost were compared among different treating methods which are all in the optimal technology condition.In order to confirm the optimal recovery plan, the influences of different factors on the protein recovery were researched. The result shows that energy input has significant effect on protein recovery. When sludge has been heated or processed by microwave, liquid temperature significantly affects the protein recovery:in acid-heating method, the best reaction temperature is120℃, alkali-heating method,100℃, and microwave method,110℃. These relatively temperate reaction temperatures have low requirements on treatment facilities, which means that these methods could be applied in low cost. In the process of recovering protein by the ultrasound, effect of energy density is more notable and the best energy density is7W/ml.The system pH value has significant effects in protein recovery, the best pH is12.5in the alkaline process, and in the acid process, is2. In addition, No matter how strong acid or alkali in the beginning of reaction, pH will tend to be neutral after the reaction is completed. In the process of microwave technology in sludge treatment, when sludge mixture is acid, the protein recovery efficiency is very low; when sludge mixture is alkaline, the protein recovery efficiency is relatively high.In the same recovery effect, the processing time which the microwave and ultrasonic need is short, while the heating needs longer time. The best irradiation time of microwave and ultrasonic are3min and20min respectively, while the best processing time of acid-heating and alkali-heating are4h and4.5h respectively. Water content of sludge has relatively small effect on the sludge protein recovery. Besides, dewatering sludge which the current sewage treatment plants produce can be directly used to extract sludge protein, no other pretreatment process.Four factors, four level orthogonal experiments were used to get the optimum schemes of all recovery methods. In the optimum scheme, the protein recoveries of alkali-heating and alkali-ultrasound methods are73.56%and66.30%, respectively; the protein recoveries of acid-heating and microwave are48.12%and54%, respectively. Protein recoveries, protein concentration in the supernatant and sludge decrement rate were combined to analyze the comprehensive treating ability of different methods. The best result comes from alkali-ultrasound method and the worst one comes from microwave method.The foaming properties of the extracted protein were analyzed. It shows that the expansion ratio of the extracted protein by four methods all could reach the standard. The half-life period of protein getting from microwave and alkali-ultrasound methods are higher, about33.58min and29.47min respectively; while the half-life period of protein extracted by acid-heating and alkali-heating are relatively low.Besides, we analyzed the monetary cost of the above methods from drug dosing cost and energy consumption. It shows that the cost of alkali-heating method is the lowest compared with other methods and the energy cost of ultrasound and microwave methods were less than one third of traditional heating methods.