Preparation Of PVA-SA-WG(PPF)-GA Enhanced Composite Carriers For Encapsulating Nitrification Bacteria And Its Application On Degradation Of Ammonia-n Wastewater

Immobilized microorganism technology shows several unique advantages in treating industrial wastewater,which can not only reduces the biological toxicity of pollutants to microorganisms and improves the process stabilization,but also prevents the loss of bacteria from wastewater tank and facilitates multiple utilization.At present,polyvinyl alcohol（PVA）carriers that have been widely studied have two important drawbacks e.g.poor chemical stability and damageable property,which leads to a shorter service life of the carriers and an increase in chemical oxygen demand（COD）in wastewater,even causing secondary pollution.Sodium alginate（SA）is often added to the PVA carrier to solve the problem of easy adhesion of PVA particles.Based on investigating the manufacturing process of PVA-SA carriers,this thesis further discussed the stepwise cross-linking method with boric acid and glutaraldehyde（GA）and the method of adding the reinforcing agents:gluten（WG）and polypropylene fiber（PPF）,into the PVA-SA carriers to improve the mechanical strength and chemical stability of the PVA-SA based carriers.In addition,the carriers were used to degrade ammonia nitrogen in wastewater by embedding nitrifying bacteria.The ammonia nitrogen degradation activity and the chemical stability in the wastewater were investigated.The preparation process study of PVA-SA carrier was focussed on investigating the appropriate dosage of PVA,SA and Ca CO₃.It was found that the optimized dosages of PVA and SA were 12%and 2%respectively,the obtained carriers didn’t showed any adhesion and obvious damage,indicating the best mechanical strength and apparent performance.When the dosage of Ca CO₃is 2%,the obtained carrier showed the highest degradation rate of ammonia nitrogen（96%）.The obtained best PVA-SA matrix was further cross-linked by boric acid and GA,to obtain a new PVA-SA-GA carrier.WG or PPF was blended with PVA-SA,to obtain another two PVA-SA-WG and PVA-SA-PPF carriers.In addition,PVA-SA-WG-GA and PVA-SA-PPF-GA carriers were prepared by combining the above two methods（GA secondary cross-linking and blending）.Studies on mechanical strength showed that compared with PVA-SA（tensile strength:1.67 MPa,elasticity:77.1%）,the tensile strength of PVA-SA-GA and PVA-SA-WG-GA increased by 13%and 14%respectively,while the elasticity increased by 61%and 98%respectively.Studies on chemical stability showed that the mass losses of PVA-SA-GA and PVA-SA-WG-GA in water were 11.6mg/g and 18.5 mg/g,respectively,far less than that of PVA-SA（53.5 mg/g）.These results indicate that the addition of WG and GA significantly improved the mechanical properties and chemical stability of PVA carriers.Effects of temperature,pH,aeration,dosage of carriers and substrate concentration on the degradation of ammonia nitrogen by embedded nitrifying bacteria were studied using simulated wastewater.The results showed that the most suitable conditions for ammonia nitrogen degradation were:temperature:30-35°C,p H:8-9,aeration:3.5 L/min and carrier dosage:5%.When the ammonia nitrogen concentration was lower than 550 mg·L^-1,the removal rate of ammonia nitrogen by embedded nitrifying bacteria was higher than 75%;When the ammonia nitrogen concentration was 550 mg·L^-1,the removal rate of embedded nitrifying bacteria reached 96%,higher than that of free bacteria（92%）,proving that the carriers can protect microorganisms from disoperation.The immobilized microbial carriers were used for continuous treatment of the domestic wastewater for 22 days（fresh wastewaster was added everday）,the results showed that the ammonia nitrogen removal rate by the free bacteria was50-60%,while that of the immobilized bacteria by the enhanced carriers（PVA-SA-GA and PVA-SA-WG-GA）was 65-70%.In addition,the COD in the effluent wastewater treated by the embedded bacteria with the enhanced carriers were all lower than that of the PVA-SA.These results indicate that the embedded nitrifying bacteria by the enhanced carriers not only maintained high biological activity,but also these enhanced carriers showed a longer durability than the traditional PVA-SA carrier.