Study On Denitrification Capacity And Denitrifying Bacteria In Bioreactor Landfill

Landfilling is one of the most important methods for refuse disposal in China. However, leachate produced from the landfill is rich in various contaminants, especially ammonia and if not managed well can pose serious threat to the environment. Recently, the Chinese government published a new standard for pollution control of municipal solid waste landfills, which prescribes that the emission concentration of ammonia in leachate must be lower than 25 mg L"1. At present, ammonia removal is mostly practiced ex situ. However, this treatment approach requires large spatial and financial investment, which restricts its utilization. Therefore, development of an in situ nitrogen removal technique would be an attractive alternative. This technique involves ex situ nitrification of leachate which is coupled with the use of the landfill as a bioreactor for denitrification. In this thesis, refuse at different landfill ages were sampled from Hangzhou Tianziling Landfill. The denitrification capacities of bioreactors filled with refuse at different landfill ages were studied. nirS gene libraries were constructed with a PCR-based clone approach in order to study the structure and diversity of the denitrifying bacteria. The information derived from this research will help other researchers carry out further studies. Some principle results of this thesis are summaried as follows.The mixtures which had similar performance with soil comprised the highest proportion in the refuse of different landfill ages, followed by plastics. Refuse in the landfill became relatively stable but not fully aged after being placed for four years primarily due to low water content. Therefore, in order to accelerate refuse decomposition, a reasonable approach is to develop bioreactor landfills characterized by leachate recirculation.In this study, nitrate solution was injected intermittently. Refuse at different landfill ages all had a considerably high denitrification capacity when the refuse body itself could provide enough organic carbon for denitrification. And the nitrate reduction rates could reach up to 70-92% in the first hour. However, when the refuse contained low organic matter, refuse at different landfill ages possessed different denitrification capacity. R1, R6 reduced nitrate much more quickly than R11, and the nitrate reduction rates were 6.80 mg NO3--N kg-TSwaste-1 h'1,3.00 mg NO3--N kg-TSwaste-1 h-1 and 1.10 mg NO3--N kg-TSwaste-1 h-1, respectively. Moreover, the final products in R1 and R6 were N2 and the concentrations were 82.5% and 80.2%, respectively. On the contrary, N2O accumulated in R11 and the final concentration reached up to 19.3%. These results suggested that organic matter played an important role in nitrate reduction and partly-degraded refuse was more suitable to use as denitrification medium.The effects of different ratios of chemical oxygen demand (COD) and nitrate concentrations in the injected leachate on the denitrification capacity of refuse were evaluated. It was found that nitrate reduction rate increased with the increasing COD concentration in the injected leachate. Nitrate reduction rates in tests 1 to 4 was 1.14 mg NO3--N kg-TSwaste-1 h-1,1.24 mg N03--N kg-TSwaste-1 h-1,6.33 mg NO3--N h-1 and 11.40 mg NO3--N kg-TSwaste-1 h-1, respectively. And for rapid denitrification, COD/NO3--N ratio in the initial leachate should be adjusted to higher than 6.0.According to the optimum ratio of COD/NO3--N, R4, R8 and R12 were used to investigate the denitrification capacity when the refuse could not provide enough organic matter. And the results showed that refuse at different landfill ages possessed a high denitrification capacity when the injection contained enough organic matter. Moreover, their nitrate reduction rates were almost the same.The analysis of nirS clone libraries indicated that the denitrifying bacteria fell into Proteobacteria (Betaproteobacteria) in R6. When the COD/NO3--N ratio was between 5.03-9.08, the diversities of denitrifying bacteria were almost the same. But the structures of denitrifying bacteria were quite different when the ratios of COD/NO3--N were 3.11 and 13.08. Moreover, Thiobacillus denitrificans and Azoarcus tolulyticus were dominant species in this reactor. But the clone abundance of Thiobacillus denitrificans was contrary to that of Azoarcus tolulyticus. The clone abundance of Thiobacillus denitrificans was 77.22%,15.38%,17.39% and 72.46%, and that of Azoarcus tolulyticus was 10.13%,75%,59.42%和7.25%. On the contrary, the denitrifying bacteria in R4, R8 and R12 fell into two lineages: Proteobacteria and Uncultured bacterium. The structures and diversities of the denitrifying bacteria in these reactors were different. The dominant species in R4 were Thiobacillus denitrificans,Azoarcus tolulyticus and the uncultured bacterium named AY078277.1. In R8, only Azoarcus tolulyticus were dominant species and in R12, Azoarcus tolulyticus and the uncultured bacterium named AY078277.1 predominated. In general, the diversities of denitrifying bacteria in bioreactor landfill were relatively low. Although the community structures were different, Thiobacillus denitrificans and Azoarcus tolulyticus could be found in all the reactors. This indicated that Thiobacillus denitrificans and Azoarcus tolulyticus grew stablely in refuse and they might play important roles in nitrogen removal.Based on the lab researches, pilot scale study was conducted in Hangzhou Tianziling Landfill. And the results showed that the bioreactor had good effect on nitrate reduction and developing a new way for nitrogen removal with the use of the landfill as a bioreactor for denitrification had certain feasibility.