Study On The Biodegradation Of Two Nitro-Containing Compounds In The Water

The organic pollution especially the nitrogen pollution in water, resulting from the direct or inacceptable discharge of wastewater, has received considerable public interest in recent years. Due to their chemical toxicity, nitrate compounds are often considered as important signs of nitrogen pollution in water. Urban wastewater has been proved to be a major source of nitrogenous pollutants, so the effective treatment of urban sewage becomes the key step to improve water quality in the water cycle system. p-Chloronitrobenzene (pCNB) is typical and ordinary nitro- compounds. Being high-toxic but hardly degraded, they have been treated as priority control pollutants by the U.S. EPA. It is necessary and crucial to master the biotransformation law of these nitro- compounds in wastewater treatment process.Building on the quality analysis of water and municipal wastewater in a city of northern China, biotransformations of pCNB (nitro-compound) was investigated in the research. Effects of several parameters on the efficiency of pCNB biodegradation were studied; final products of pCNB biodegradation were identified and pathways of pCNB biodegradation in water were proposed, meanwhile, the effect of pCNB in systemic microecology was studied. The research found that nitrogen compound can be translated into each other and the microbial nitrification inhibition of pCNB makes nitrite content increased, when there are precursors such as secondary amine (DMA etc.) or tertiary amine with secondary-amine structures in water, biodegradation byproduct——NDMA can be biosynthesized. With precursor of NDMA formation in water, effects of factors on the existent pattern of NDMA were studied; biosynthesis pathways and biodegradation of NDMA were analyzed.First, quality of source water and municipal wastewater in a city of northern China were surveyed and analyzed. Results show that: conventional parameters (TOC, COD, UV254, TN, TP, etc.) of both source water and municipal wastewater could achieve water standards and effluent sewage standards; a great deal of organic nitrogenous pollutants have been detected in water, they account for 23.4% and 31% of TN in source and wastewater, respectively, and the concentration of pCNB in municipal sewage is about 2.4 times as that in source water; disinfection researches of source and waste water indicate that the generation amount of nitrogen byproduct, such as halogen acetonitriles(HANs), N-nitrosodiethylamine (NDEA) and N-Nitrosodimethylamine (NDMA) have been found largely generated both in source and waste water for over 1 and 10 days'disinfection.With pCNB as the target pollutant, the inhibition of sludge activity produced by pCNB was removed after a period of domestication, and then pCNB was well transformed, degraded and mineralized by microbes in the treatment process. Under appropriate anaerobic conditions, low concentrations of pCNB can be completely biotransformed into chloroaniline (pCA) within an hour; in aerobic conditions, the total pCNB removal was about 85%, including 48% volatile of pCNB into air because of aeration; In the SBR process, 95% pCNB was effectively degraded after anaerobic treatment for 4h and aerobic treatment for 8h. pCNB can be reduced to chloroaniline (pCA) and aniline (AN) by anaerobic sludge; In the biodegradation of pCNB by aerobic sludge, Benzene ring can be opened through some reduction and oxidation reactions, the mineralization was achieved ultimately.In order to provide more theoretical basis for the design and operation of system, the impact of pCNB on the efficiency of the system was investigated. Furthermore, microbial species composition and the biomass changes were ecologically analyzed. Experimental results show that: the inhibition of metabolic activity produced by trace pCNB on anaerobic sludge is stronger than that on anaerobic sludge, especially, the inhitbition of microbial nitrification by pCNB makes nitrite accumulated and the concentration of NO2--N is increased, but these two inhibitions can be reduced after domestication; the coexistence of NDMA and pCNB in water resulted in obvious reduction of microbe species and biomass in sewage treatment system, after a period of domestication, the systematic biomass increased.Because of pCNB in water, the concentration of NO2--N as an intermediate product in wastewater treatment is increased. When there are precursors in water, NDMA as biodegradation byproduct can be biosynthesized in the course of nitrification and denitrification, the maximum of NDMA in the system was obtained at the reaction time of 30min-60min, NDMA produced in the denitrification stage was obviously higher than that in the nitrification stage; The added NDMA can neither be adsorbed nor degraded by activated sludge, but the bio-synthesized NDMA can be efficiently degraded by microbes themselves; The biosynthesis way of NDMA in denitrification are slightly different from that in nitrification. A new pathway of NDMA biosynthesis was proposed: asymmetric dimethylhydrazine (UDMH), products of hydroxylamine (HA) reacting with dimethylamine (DMA), can be oxidized to NDMA. Due to its high water-soluble, NDMA is hardly bio-enriched, so the toxicity of NDMA on microorganisms is much weaker than pCNB.