Nitrate Removal From Drinking Water By A Three-dimensional Biofilm-electrode Reactor

China is seriously short of water resources and suffers from severe water pollution since the reform and opening up, although great successes have been achieved in its economic construction. Drinking waters have been polluted by large amount of industry and agriculture wastewater, and the safety of drinking water is threatened. The contamination of nitrate in drinking water is becoming an ever-increasing environmental problem, and it has been paid more and more attention in recent years. The bio-electrochemical denitrifying technology combining electrochemical process and autotrophic biological denitrification, is considered to be a new technology more suitable for the treatment of nitrate pollution of drinking water compared with the heterotrophic denitrification process.A three dimensional Biofilm-Electrode Reactor (3d-BER) was developed in this dissertation, and the performance of the 3d-BER was tested after inoculating, cultivation and acclimation periods. Effects of current intensity, temperature, influent pH, flow rate and N/P ratios on the denitrification characteristics were investigated. Main results were as follows:1. Results showed that the effect of the electric current intensity on denitrification was significant. Nitrate removal efficiency increased as the current increased from 0 mA to 100 mA. However, with a further increase of the current intensity from 100 mA to 150 mA, the nitrate removal efficiency decreased slightly. With current intensity increased from 0 mA to 150 mA, the nitrite generation first increased then decreased, The accumulation of nitrite was most serious when the current intensity was 60 mA. With the current intensity increased from 0 mA to 80 mA, the ammonium nitrogen generation gradually increased, and a maximum concentration value of 4.91 mg L-1 was achieved at the current intensity of 80 mA. The ammonium nitrogen generation decreased with a further increase of the current intensity above 80 mA.2. In the temperature range of 16℃to 35℃, the denitrification performance of the 3d-BER improved when temperature was raised, and nitrate removal efficiency up to 85.5% was achieved at 35℃. The nitrite accumulation first increased then decreased in the process. Ammonium nitrogen can be detected in effluent in this temperature range.3. The 3d-BER had a good pH buffer capacity. Effluent pH was maintained between 7.6 and 8.2 when influent pH was increased from 5.5 to 9.0. Nitrate removal efficiency increased at first, and the highest removal efficiency reached 80.2% at the pH of 7.2. The nitrate removal efficiency decreased with a further increase of the pH above 7.2. In the process, nitrite accumulation first decreased then increased later, a minimum concentration value of 0.15 mg L-1 was achieved at the pH of 8.2. Ammonium nitrogen generation first increased then decreased later, a maximum concentration value of 2.09 mg L-1 was achieved when the influent pH was 8.2.4. The rate of influent flow substantially affected the performance of the 3d-BER. When flow rate was increased from 95 mL h-1 to 195 mL h-1, the nitrate removal efficiency droped from 89.1% to 52.7%. The nitrite accumulation increased dramatically in the process. With the increase of flow rate, the generation of ammonia nitrogen increased at first, and then decreased. The maximum concentration value of 4.50 mg L-1 was attained when the flow rare was 140 mL h-1.5. Probable reasons of the ammonia nitrogen generation were also discussed. Two reasons was proposed:denitrification through the assimilation of microorganism and the electrochemical reduction of nitrate. According to the experimental data achieved by comparing the 3d-BER with an electrochemical reactor, the generation of ammonia nitrogen was mainly caused by the assimilation.6. The denitrifi cation performance of the 3d-BER was tested at different nitrogen to phosphorus (N/P) ratios. Results showed that nitrate removal efficiency was not significantly affected by the N/P ratios. However, the nitrite generation was obviously influenced by this parameter. When the N/P ratio was raised from 5:1 to 10:1, the nitrite generation decreased from 0.79 mg L-1 to 0.27 mg L-1, and it tended to be more serious with a further increase of the N/P ratio. The N/P ratios also had a significant effect upon the ammonia nitrogen generation. When the N/P ratio was increased from 5:1 to 100:1, the generation of ammonia nitrogen decreased from 4.50 mg L-1 to 0.26 mg L-1. 7. Phosphorous removal was also realized in the 3d-BER. Effects of current intensity, temperature, influent pH, flow rate and the N/P ratio on the phosphorus removal were investigated. With currents increased from 0 mA to 150 mA, the TP removal efficiency was between 51.3% and 93.0%. When the temperature was raised from 16℃to 35℃, the TP removal efficiency kept a steady increase. TP removal efficiency increased gradually when the influent pH was increased from 5.5 to 9.0. Generally speaking, the TP removal efficiency decreased with the increase of the flow rate and the N/P ratio. Experimental results indicated that the phosphorus removal was mainly brought about by the biological action.