Study On Nitrogen Transformation In Two-stage Anaerobic Treatment Of OFMSW

With the development of clean energy mechanism, anaerobic digestion has already been a preferred method for the treatment of organic fraction of municipal solid waste (OFMSW). Digestive effluent reuse is the important method of reducing the effluent in the anaerobic digestion process. However, reuse of the complete digestive effluent will bring negative influence to anaerobic digestion process that a continuous accumulation of ammonia nitrogen decreases the activity of methanogens. To solve the problems due to the accumulation of ammonia nitrogen and obtain an appropriate key controlling parameter, a batch hydrolysis-UASB two phase anaerobic digestion process was adopted to treat OFMSW in this paper. The following aspects were concentrated on.The effluence experiments of digestive effluent reuse to two-phase anaerobic digestion process were carried out. The results show reuse of the entire digestive effluent in two-phase anaerobic digestion will lead to the continuous accumulation of ammonia nitrogen. After a 49d stable operation, the influent and effluent ammonia nitrogen concentration of the methanogenesis reactor increased from 262.2 mg/L,261.5mg/L to 828.7mg/L,1024.5 mg/L, respectively; While the effluent alkalinity of the methanogenesis reactor increased from 1881.1 mgCaCO₃/L to 5240.7mgCaCO₃/L, and influent / effluent pH maintained stable. The accumulated ammonia nitrogen did not influence the activity of methanogenesis negatively in the anmmonia nitrogen concentration, whereas it developed the resistant to a certain acid-alkaline shock. The removal of COD was about 93.25%, the maximum gas production / COD reached 0.54L/gCOD and the methane content in the gas was relatively high (maximum value: 82.1%). Additionally, the degradation rate of OFMSW was improved.On the conditions that the digestive effluent was reused completely, the principles of nitrogen leachating, transformation and existence in the hydrolysis-acidification phase and methanogenic phase were studied. The experimental results indicate that after the marinating of OFMSW with 11.11¹6.67% solid content, the nitrogenous substances in OFMSW follow the first-order reaction dynamics to hydrolyze and dissolve out organic nitrogen. The transformation efficiency of organic nitrogen to ammonia nitrogen fluctuated, and the first-day conversion efficiency of 60.15% was the highest. The oxygen in the internal OFMSW oxidized ammonia nitrogen to nitrate nitrogen and nitrite nitrogen in the hydrolysis-acidification reactor and the first-day ammonia oxidation efficiency of 92.2%was the highest, gradually decreasing. The average transformation efficiency of organic nitrogen to ammonia nitrogen was 78% in the methanogenic reactor, and the inorganic extent of organic nitrogen in effluent maintained at an average value of 86%, relatively stable, while the inorganic extent of organic nitrogen in influent was relatively low and gradually increased. The transformation of organic nitrogen to ammonia nitrogen made the ammonia nitrogen increase in the digestive effluent; at the same time, the ammonia nitrogen, nitrate nitrogen and nitrite nitrogen were simultaneously removed accompanying the anaerobic ammonium oxidation. But the quantities of produced ammonia nitrogen from organic nitrogen were more than those of consumed ammonia nitrogen. Consequently, the concentrate of ammonia nitrogen in effluent was higher than influent ammonia nitrogen, leading to the ammonia nitrogen accumulation when the digestive effluent was totally reused.Forecast model of ammonia accumulation in the two-phase anaerobic digestion with digestive effluent reuse was established. Key controlling parameter (reflux ratio) is considered as an analysis tool to prevent ammonia inhibition efficiently.