Removal Of Nitrate In Source Water By Biological Denitrification With Solid Carbon Source

Source water, especially groundwater, has been contaminated by nitrate in many countries and regions. Nitrate in source water is harmful to health. As an effective and energy-saving means of controlling nitrate pollution of source water, biological denitrification techniques have been studied intensively in recent years. Biological denitrification with solid carbon source can avoid the adding difficulties of carbon source in traditional biological denitrification processes, and create the steady and maintainable environment for denitrifying bacteria as well.This work employed a novel biological denitrification process of source water as the pretreatment for drinking water. Four solid carbon sources, agar, corn, sweet potato and potato, were examined with regard to their denitrification efficiencies. The results showed that the release rates of organic matter of the carbon sources were as follows: sweet potato> potato> corn> agar, indicating that both corn and agar have satisfactory denitrification rates. Since the excessive COD in the effluent after denitrification would inevitably lead to problems for the following water purification processes and the quality of drinking water. Therefore, based on the experimental data, the agar was selected as the most suitable solid carbon source for the biological denitrification of source water.Three reactors, agar-ceramic granule reactor (reactor 1), agar reactor (reactor 2), and biological aerated filter (reactor 3), were designed and used in this work. In the steady operating stage, when the temperature was 25 ?C, HRT was 1.5h, and the influent NO3--N concentration was 25mg/L, the NO3--N removal rates of reactor 1 and reactor 2 could reach 70%. Besides, Reactor 3 could reduce the CODMn and NO2--N to less than 5mg/L and 0.1mg/L, respectively. It was found that the temperature had obvious influence on the operating effect of the process. While the temperature rose from 22 to 26?C, the NO3--N removal rate increased accordingly. At 22 ?C, the denitrification rates of reactors 1 and 2 were 1.32 and 6.87 mg NO3--N L-1h-1, respectively; whereas at 26 ?C, they were 2.25 and 12.00 mg NO3--N L-1h-1, respectively.The kinetic study showed that the relation between the denitrification rate and the influent NO3--N concentration obeyed a zero-order reaction. Thus the denitrification rate increased with the rise of the influent NO3--N concentration. When the HRT of the agar part of the reactor varied from 1.5 to 5 hours, the NO3--N removal rate slightly increased with the rise of the HRT. In the range of 7 to 8, the influent pH value had no apparent effect on the denitrification rate; and the influent DO value varying from 3 to 5 mg/L had a limited effect on the denitrification rate.It was also found that the ceramic granule part of reactor 1 played a limited role in the denitrification. Therefore the investigations on promoting the denitrification efficiency were conducted by introducing different startup mode. The preliminary results showed that some improvements in the stability of the startup stage and in the denitrification rate could be achieved, when the agar reactor and the ceramic granule reactor were started up respectively by inoculating large quantity of microorganisms with the big influent flow of source water.