| Summary: | 碩士 === 國立暨南國際大學 === 土木工程學系 === 92 === In the natural water body, heavy metals can not be decomposed as nutrition by microorganism and, therefore, are usually accumulated in the bodies of organisms and make them poisonous. Although many techniques are developed to treat heavy metal containing wastewater by microorganism, there are seldom studies focus on the effects of heavy metals on biochemical reaction and microbial communities of nutrition removal. Thus, this study used two A2O pilot-plants with adding cadmium into one of the plants and utilized molecular biotechnology to investigate the effects of cadmium on microbial kinetics and communities of nutrient removal system.
Results showed that Cd 2ppm firstly affected the reaction of phosphate removal. Both removal efficiencies of phosphate (PO43- ) and total phosphorous(TP) were dropped suddenly from 78.5% to 40% and 15%, respectively. Comparing with the system without cadmium, the microorganism of activated sludge were inhibited and decomposed by the toxicity of heavy metal while Cd concentration was increased up to 5ppm. Effluent SS concentration was increased obviously and resulted in the decrease of the removal efficiency of total COD from 92% to 88%. At the same time, the removal efficiency of total nitrogen was also substantially dropped from 69% to 17%. Nitrification rate was also dropped from 99.9% to 29%. When Cd concentration was continually increased up to 35ppm, both removal efficiencies of total COD and soluble COD were dropped from 92% and 95% to 61% and 66%, respectively. Moreover, this study also found that there was no obvious inhibition on anaerobic ammonification and anoxic denitrification reactions even though the Cd concentration was increased up to 35ppm.
In the aspects of kinetic parameters, the yield coefficients of heterotrophs (YH), ammonium oxidizer (YAOB) and nitrite oxidizer (YNOB) were inhibited about 33%, 27% and 68%, respectively while Cd concentration was increased up to 5ppm. And, the corresponding specific growth rate (μH、μAOB、μNOB) were inhibited about 27%, 59% and 44%, respectively. The biomasses were also inhibited about 70%, 1.5% and 71%, respectively. Those results indicated that the removal mechanisms of nitrogen and phosphate nutrition of activated sludge were greatly inhibited by Cd heavy metal. It can be proved by the inhibition of those kinetic parameters.
In the microbial community structure, it can be divide into 45 OTUs in without Cd situation, but only 17 OTUs under Cd 5ppm situation, and only 11 OTUs under Cd35ppm. It appeared that the diversification of microbial community was obviously affected by cadmium and the amounts of predominant strains were reduced. The results also showed that Proteobacteria was the predominant bacteria both in without Cd, Cd 5ppm and Cd 35ppm system. The first one system contained 39.6%, the second contained 35.1% of that strain, and the last contained 48.9%. The greatest discrepancy of microbial communities in those systems occurred in Beta-subclass. There was 30.7% in without Cd system, 2.1% in Cd 5ppm system, and 17.9% in Cd 35ppm system. It demonstrated that the main bacteria responding in nutrition removal, such as Beta-subclass, were greatly affected by the toxicity of Cd. Moreover, Gamma-subclass made up 5.4% in without Cd system, but substantially increased to 25.1% and became the predominant strains in Cd 5ppm system. The major strains of this sub-class were filamentous bacteria (Thiothrix spp.). Thus, it demonstrated that filamentous bacteria could resist the toxicity of Cd 5ppm. However, it was disappear in Cd 35ppm system and instead of Pseudoxanthomonas japonensis. The amount of CFB group was also obvious different. There contained 4% of CFB group in without Cd system but significantly increased to 14.7% in Cd 5ppm system. This group is harmful to the removal of phosphate. On the other hand, nitrifying bacteria (Nitrosomonas communis) and floc-forming bacteria existed in without Cd system, but not appeared in Cd 5ppm system. It demonstrated that Cd 5ppm is harmful to both nitrifying and floc-forming organisms.
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