A Study On The Extraction Of Protein From Sewage Sludge And The Possibility For Using As Animal Feed Additive

The construction and operation of wastewater treatment plants are important measurements for protecting water resources and preventing water body from pollution. While, since 1990s, the output of the sewage sludge has been increased rapidly, with the acceleration of urbanization and improvement of wastewater disposal efficiency year by year. A great deal of excess sludge occupies limited living space for human because of being short of appropriate treating methods. Water and air will be polluted if the sludge discharged without proper treatement. This will not only reduce treatment abilities of municipal wastewater treatment plants, but also threaten the environment and human activities seriously. According to the request of reduction, harmlessness and recovery, the sludge should be efficiently utilized and become resources. Therefore, more attention has been paid on the disposal and utilization of excess sludge.There are four municipal wastewater treatment plants in Qingdao(Licunhe, Haibohe, Tuandao and Maidao). Maidao wastewater treatment plant adopts primary wastewater treatment(not produce excess sludge) and the others adopt secondary treatment (produce excess sludge). Lots of excess sludge are piled up in the plants everyday and delivered to landfill or incinerated after natural dehydration. In this way, deleterious substances in sludge may cause secondary pollution to environment and nutrients of sludge are losted without being utilized efficiently. So it is necessary to study an appropriate method for the reduction, harmlessness and recovery of excess sludge.Firstly, the physical and chemical characteristic and composition in the excess sludges from Licunhe, Haibohe and Tuandao wastewater treatment plants are determinded. The results show that three excess sludge samples are black, lumpish and viscous. They are alkalescent with a pH value ranged from 8.68 to 8.81, and have a moisture content ranged from 70% to 83%. The samples have plenty of organic matter, N, P, K, but less heavy metals and boron (B) in content being up to the Control Standards for Pollutants in Sludges from Agricultural Use (China). According to the above characteristics of sludges and the problems in disposal and utilization of sludge in Qingdao city, it is proposed that the best way for utilizing these sludges be used as a fertilizer in forest, flower and grassland after compost. Furthermore, according to the principle of extending food chains by"added loop"in eco-engineering, a new way by which the single cell protein(SCP) in the sludge should be extrated as a feed additive before application in land is brought forward.Then, the technology about extraction, precipitation and separation of cell protein in sludge are studied. Three methods, acid hydrolyzation, alkali hydrolyzation and ultrasonic-alkali extraction, are selected to extract protein from sludge. Orthogonal tests are introduced into each method to optimize the operation conditions. For acid hydrolyzation, the optimum conditions are listed as follows: temp. 121℃; hydrolyzing time 5h; pH value 1.25 and addition of deionized water is 2.5 ~ 3 times as sample mass. For alkali hydrolyzation, the optimum conditions are listed as follows: temp. 70℃; hydrolyzing time 5h; pH value 12.5 and addition of deionized water is 4 ~ 4.5 times as the sludge sample mass. For ultrasonic-alkali extraction, the optimum conditions are listed as follows: ultrasonic power 650W; ultrasonic time 35min; pH value 12.0 and addtion of deionized water is 6 ~ 6.5 times as the sludge sample mass. Under the optimal conditions, protein extraction experiments with three methods are operated repeatly. It shows that, ultrasonic-alkali extraction is best for Licunhe sludge, acid hydrolyzation for Haibohe sludge, and alkali hydrolyzation for Tuandao sludge. The extraction efficiency of protein changes from 62.71% to 69.60%, the mass of sludge are reduced by from 29.95% to 34.21%, and the volume of excess sludge is cut down by above 1/3. Three sludge extraction liquor have the same protein isoelectric point of pH5.5. Protein purities(dry weight) in the precipitates formed at pH5.5 are 83.52%(Tuandao sludge), 81.85%(Licunhe sludge), 79.66%(Haibohe sludge) respectively.At last, the nutrition and security of the precipitates are evaluated. The precipitates contain more amino acids which occupy 55% ~ 65% weight of coarse protein. Besides of tryptophan which is destroyed during the detection, other seven kinds of essential amino acids can be detected, which account for 21%~24% of the precipitates in dry weight 50% of total mass of the amino acids. In addition, eight non-essential amino acids can also be detected in the precipitates, which also account for about 50% of total mass of the amino acids. Lower contents of heavy metals in the precipitates are detected. The content both cadium and lead are up to the Health Standard for Animal Feed(China), zinc content is up to the Permittion of Zinc in Animal Feed (China), and copper content meet the demands in practical production.To sum up, considering the nutrition and security, it is feasible for the protein precipitates extracted from the sludge of municipal wastewater treatment plant in Qingdao to be used as animal feed additive.