Experimental Study On Nitrogen Removal With Denitrification Bioreactors Of Rice-Husk And Woodchip Mixture

Due to extended use of chemicals,pollutants from agricultural land are leached with drainage water and degrade water quality of the receiving water bodies.As one of the strategies to overcome this problem,denitrification bioreactors have been advocated as a promising technology that needs less maintenance and are cost efficient.To study the effect of hydraulic retention time on nitrogen removal through bioreactors having different carbon media and the environmental factors effecting nitrogen removal through bioreactors of different carbon media a lab experiment was done in this research.A lab experiment using column bioreactors having different combinations of carbon media（ricehusk and woodchips）was allowed to operate under different influent nitrate concentrations at different HRTs.Three different influent nitrate concentrations（1,5,15mg/L）were tested at three different hydraulic retention times（4,8,12hr）using five combinations（1:0,1:1,0:1,3:1,1:3）of rice-husk and woodchips as carbon substrate.Effluent samples were collected maintaining the time interval between two sampling event at least 2*HRT.After measuring the in situ values（i.e.Temperature,pH,DO）samples were stored in refrigerator to study the change in nitrate concentration and the factors affecting the performance of denitrification bioreactor.Major findings are as below:1)The results showed that low HRT is needed for low influent concentration to remove the nitrate to higher extent in denitrification bioreactor.For lmg/L influent concentration the nitrate reduction efficiency at 4hr HRT was up to 80%whereas for 5mg/L and 15mg/L influent concentration at 8hr HRT was up to 90%.2)A lower HRT than the appropriate HRT needed to reduce nitrogen up to 80%will not provide enough time to microbes to reduce nitrate to much extent and high HRT will lead to nitrogen limited conditions in the bioreactor.3)There were some fluctuations in the effluent nitrate values at the start of experiment but with the passage of time a state of equilibrium was established in all cases.4)Effluent pH was lower than that of influent and was near to neutral pH 7,showing no harmful effect to environment.5)DO values of influent were generally around 8mg/L and were significantly reduced to less than lmg/L.This low DO value is much less than that needed for aquatic life（10mg/L）and need to be focus before discharging effluents to the water bodies.6)Effect of different filling medias on nitrate removal efficiency of bioreactor was analyzed and the results showed that a combination of rapid carbon releasing media（rice-husk）and a slow carbon releasing media（woodchips）is the best for quick startup and higher longevity of denitrification bioreactor.