Effects Of Carbon Sources, Temperature And Hydraulic Retention Time On Nitrate Removal From Groundwater

Groundwater is not only an important component of the hydrologic cycle but also an important drinking water source. Excessive nitrogen fertilizer and organic manure application has caused severe nitrate pollution in groundwater, which threats human health. Bioremediation of groundwater nitrate using denitrifying forms of bacteria found in groundwater is one of the effective methods for nitrogen removal.Carbon source is one of the necessities for heterotrophic denitrification, and water temperature and hydraulic retention time can also significantly influence the nitrate removal efficiency. Based on economical and safety considerations, ethanol and acetic acid are two excellent choices. But for in situ groundwater nitrate remediation technology, the liquid carbon source will move with the groundwater, which will cause resource waste, as well as the potential risk of secondary pollution in groundwater. Currently some new types of carbon sources, such as poly-β-hydroxy butyrate (PHB), poly butylene succinate (PBS), and polylactic acid (PLA), also have been used for nitrate removal. Such carbon sources have high denitrification rate but with high cost. The main objectives of this study are to:(1) investigate the performance of denitrification by using different single carbon source;(2) evaluate the effect of temperature, HRT and pH on denitrification performance and screening out the best solid carbon for nitrate removal.Rice straw, corncobs and newspaper were selected as the solid carbon sources in this study because of their low cost and easy accessibility. This research focuses on nitrate removal using commonly available agricultural solid organic residues as carbon sources. Removal of nitrate in water was investigated using corncob, rice straw, newspaper, and liquid alcohol as carbon sources at different temperatures and HRTs by a column study. The results showed that nitrate could be effectively removed using solid-liquid mixed carbon (adding a moderate amount of alcohol in the mixture of corncob and rice straw as the carbon sources). Meanwhile, a combination of carbon sources (solid and liquid) can further lower the nitrite accumulation of the treated water. It showed that95%of the nitrate could be removed from the water spiked to30mg/L nitrate after one day in the columns under four different HRT treatments (i.e.,6.7h,10h,20h, and40h). Better nitrate removal was achieved at longer HRT. When the HRT was40h, nitrate concentration reduced fastest and after10hours of running the columns, almost nitrate was removed. Meanwhile temperature was an important factor for denitrification. Higher temperature and sufficient carbon sources accelerated denitrification rate, and consequently resulted in lower nitrite concentration in the treated water. This study indicated that complete nitrate removal could be achieved readily using solid and liquid mixed carbon sources by adjusting HRT or temperature in water. When the HRT was10h and the temperature was28±2℃,800g mixed carbon (400g rice straw and400g corncobs) can treat747L nitrate wastewater when experimental apparatus were fed with tap water spiked using KNO3to an initial concentration of30mg N/L, which can be potentially implemented in environmental engineering practices.