以棉花为碳源去除地下水硝酸盐的研究

Denitrification of Groundwater Using Cotton as Energy Source

采用室内试验装置,研究了以棉花为碳源和反应介质的生物反应器去除地下水中的硝酸盐。结果表明,以棉花为碳源的反应器启动快。在室温25℃±1℃,进水硝酸盐氮浓度为22.6mgN·L-1,水力停留时间不小于9.8h时,反应器对硝酸盐氮可以100%去除,出水未检出亚硝酸盐。反硝化反应受温度变化及水力停留时间影响大:14℃的反硝化速率不到25℃的1/2;当水力停留时间为7.2h,N去除效率只有45%。反硝化反应受pH值和DO的影响小,当pH值在6～9,进水DO在2～6mg·L-1范围变化时,反应器去除效率没有变化。在反应进行过程中,棉花也被消耗掉。

Removal of nitrate from groundwater was studied in laboratory columns packed with unprocessed cotton. Laboratory reactors we used were cylindrical PVC columns, 50 cm in height and 8.5 cm in diameter. The columns were packed with 220 g cotton up to 45 cm high. The cotton served as the sole physical support for bacterial growth and was apt to attached bacteria. Startup of the reactor was fast. Complete removal of 22.6 mgN· L- 1 was achieved (nitrite was not detected), at temperature of 25 ℃ ± 1 ℃ and HRT=9.8h on day 16. The speed of reactor startup was proportional to the amount of inoculum. When inoculum reached 50% of the amount of cotton in a fresh reactor the influent nitrate (22.6 mgN· L- 1) was completely removed in the first day with temperature of (25± 1)℃ and HRT=9.8h. High temperature helped to stimulate the denitrification. The denitrification rate at 29℃ was more than twice as that observed at 14℃ , which could be explained by the bacteria increasing at high temperature. Cotton dependent denitrification was markedly affected by changes in HRT: breakthrough of nitrate (about 4.6 mg N· L- 1) appeared at HRT=8 h; the rates of nitrogen removal were only 45% at HRT=7.2 h . PH and DO had little on denitrification: the rates of nitrogen removal were not changed at pH6～ 9, or influent DO ranging between 2～ 6 mg· L- 1. These experiment results indicated an important fact that the bacteria activity in the reactor was stable and the reactor had the ability to accommodate a wide range of pH. It also indicated that 6 mg· L- 1 DO in the influent was consumed in the lower part of the reactor by bacteria, and could not bring negative effects on denitrification. Ammonia was never detected in the effluent. The concentrations of dissolved organic carbon were usually below 20～ 25 mg· L- 1. The number of colony forming units in the denitrified water was in the order of 105· mL- 1. The characteristics of effluent showed that some post treatment would be required to oxidize the residual organic carbon and to remove microorganisms before being used as potable water. In addition, among the total N removed per day, 90% of the nitrate was reduced to N2 when the operating reactors were stable. Clogging of denitrification reactors due to entrapment of N2 bubbles can be a serious problem and clogging by N2 was not a critical problem in the current study. The cotton was consumed in the process, which simplifies a long- term continuous operation. In such cases, emptying and disposal of waste or spent substrate will not be required. The high denitrification rate, easy operation and low expenses of the running reactor (cotton was a cheaper readily available carbon source) made the application of the reactor was possible in the nitrogen- loss from the groundwater.