Remediation Research For Groundwater Nitrogen Contamination Of Livestock- Farms

The groundwater in bordering areas of livestock farms in China has been suffering from worsening nitrogen contamination. There is absolutely a need for research work on groundwater nitrogen remediation technology. This research chose the groundwater contaminated by wastewater from one hoggery in Zhejiang Province as the remediation object. The aim was to explore remediation methods which are suitable for the groundwater nitrogen remediation in South China. The main results were as follows:(1) Soil in the test site mainly consisted of clay, resulting in the low conductivity. The hydraulic conductivity was less than 2 cm/d, and the depth of groundwater was less than 1 m. Such conditions could reduce the conversion from ammonia to nitrate. The groundwater was terribly polluted by ammonia nitrogen, with the peak concentration reaching 500 mg/L. Nitrate pollution was relatively slighter, and the nitrate content was occasionally larger than 10 mg/L. Since the groundwater was unconfined, the distribution of nitrate and ammonia contamination plumes showed the vast temporal and spatial variations. Nitrogen pollutants mainly gathered in the region next to oxidation ponds and in the dry period. The groundwater was weak alkaline, with pH ranging from 7 to 8. The carbon nitrogen ratio was rather low (generally less than 1:1 in the wet season) and unstable, which prevented sufficient denitrification. Due to the small velocity of water flows and high contamination concentrations, in-situ remediation was not suitable. The ex-situ remediation was implemented.(2) Four kinds of native bacteria, i.e., Ochrobactrum lupine, Ochrobactrumciceri, Pseudomonas putida and Enterobacter cloacae, were screened from the oxidation pond sludge and groundwater. According to the results of nitrate removal experiment, with sodium citrate or glucose served as the only carbon source, Pseudomonas putida always showed the best denitrification effects. In the soil column experiment conducted to compare different solid carbon sources, with the initial nitrate content being 50 mg/L, the mixture of bran and bean flour could greatly improve nitrate removal efficiency, with little nitrate detected in the effluent. However, such mixture would also release more than 10 mg/L of ammonia nitrogen into water. Therefore, to use bran and bean flour as carbon sources, other filters are needed.(3) The designed remediation method was as follows:under the effect of denitrifiers as well as multiple media layers consisting of zeolite and organic matters, converted ammonia to nitrate by adsorption and nitrification firstly, and then, converted nitrate to dinitrogen by denitrification. After the remediation system was started, ammonia removal rates were always more than 90%. However, overmuch nitrate was detected in the effluent (more than 100 mg/L), and the problem of blockage arose after a long-term operation. Therefore the remediation system was refilled, and to dredge the blocked packing layers, back flushing devices were installed. After the system was started again, removal rates of ammonia were still over 80%, with the concentration in the effluent was under 5 mg/L. The nitrate content in the effluent was about 27 mg/L. Concentrations of ammonia and nitrate satisfied the discharge standard of pollutants for Livestock farm and agricultural water standard for groundwater, respectively. The stable effect of the remediation system needs to be verified after a long-term operation.