Nitrate Removal From Groundwater Using Sawdust And Ethanol As Mixed Carbon Sources

Nitrate contamination of groundwater has became an environmental and health problem in developed and developing countries. This causes serious concerns because of the link found between nitrate and the blue-baby syndrome, and of the possible formation of carcinogenic compounds in the digestive tract. Nitrate pollution is caused by the intensive use of nitrogen fertilizers, discharge of domestic sewage and industrial waste water. The controlling of pollution of nitrate has been studied in many researches. Nowadays, there are three ways to remove nitrate from groundwater. The ways are physical, chemical reduction and biological denitrification. Biological denitrification, bacteria-mediated reduction of nitrate to nitrogen gas, is harmless to the environment. The denitrifying microorganisms require carbon and energy substrates which may be organic or inorganic compounds. The carbon sources are economical and extensive. The research had demonstrated that ethanol, acetic acid and agricultural wastes could be acted as carbon sources for denitrification. But these carbon sources have some disadvantages, the research key is to selecte appropriated carbon sources.In this study, wheat straw, sawdust and ethanol were selected as carbon sources for the denitrification microorganism to remediate nitrate pollution from the groundwater. The effects of wheat straw, sawdust, ethanol, wheat straw and ethanol, sawdust and ethanol as well as wheat straw and sawdust as carbon sources to remove nitrate from the simulated groundwater were compared. Whilewhile, the factors that affected the process of denitrification and permeable reactive wall were investigated.(1) Wheat straw alone as carbon source had good nitrate removal efficiency. Wheat straw and sawdust, wheat straw and ethanol as mixed carbon sources had the same denitrification rate with wheat straw alone as carbon source. So, wheat straw could be acted as carbon source alone. The effluent from the reactor filled with wheat straw had colour and stink.(2) The reactor packed with sawdust and ethanol had better performance than that packed with sawdust or ethanol alone. The reactor, which was filled with sawdust and ethanol, had lower nitrite concentration. But the reactor packed with sawdust or ethanol had higher nitrite concentration. The experiment indicated that sawdust and ethanol as mixed carbon sources were suitable for biological denitrification. The ratio of carbon and nitrogen 40 was benificial to nitrate removal. 0.5 % ethanol (the percentage of ethanol and the amount carbon sources) had a signification effect on improving denitrification which used sawdust as carbon source.(3) The experimental results indicated that pH between 7.0~10.0 had little effect on the denitrification rate of sawdust and ethanol. When pH was bigger than 7, the denitrification rate was higher. But when pH was smaller than 7, the denitrification rate was lower. With pH increasing, the nitrite concentration increased slowly. The optional pH of sawdust and ethanol for denitrification was 7.0~8.0.(4) Sawdust and ethanol dependent denitrification was markedly affected by the changes in temperature. The denitrification rate of 8.5, 15℃were significant below the denitrification rate of 25℃. When the temperature was 35℃, the reactor had the best nitrate removal performance. The optional temperature of sawdust and ethanol as mixed carbon sources was 25~35℃.(5) The experiment of permeable reactive wall found that the column filled with sand had good performance, the effluent was clear and achromaticity. The column packed with soil had higher effluent nitrate concentration, the effluent was yellow. When sand was used as filled material, the column packed by sawdust and sand mixed had better nitrate removal performance than the column packed by sawdust and sand layer-builted.(6) The column filled with sawdust and ethanol as carbon sources had lower effluent nitrate and nitrite concentraton. The pH of effluent was increased firstly, then decreased, at last the pH was about 7.0, which could meet the requriment of standards for drinking water quality.(7) At the early reaction, the chemical oxygen demand concentration of effluent was large. With the reaction proceeding, the chemical oxygen demand concentration decreased gradually and maintained stability finally. There were lots of reasons which caused the chemical oxygen demand concentration high, for example, the column filled with much sawdust, the column packed with activated carbon. The high chemical oxygen demand concentration of effluent could cause new organic contaminated. Further research need to be done to reduce the chemical oxygen demand concentration.