Research On The Effect Of Nitrogen Removal From Farmland Drainage With Denitrification Bioreactors

Nitrogen losses with farmland drainage is a major cause for water quality degradation in aquatic environment.As an emerging technology,denitrification bioreactors,which are filled with carbon media and installed at the field edges,may remove nitrate-nitrogen through denitrification process from drainage water that was either partially or fully introduced into the device.In this thesis,with two pilot-scale denitrification bioreactors filled with rice husks and wood chips,experimental studies were carried out to simulate the reduction processes of nitrogen and phosphorus through the bioreactor treatment system.The observations include both the static and the dynamic experiments.The static experiment focused on the effect of inflow concentrations on denitrification rate and the internal environmental factors;the dynamic experiment examined the different factors that affect the reduction and removal rates.Finally,we calculated the size of bioreactors required to treat drainage water from paddy fields along the Grand Canal in the Jiangdu District,Yangzhou,China.The calculations were based on observed field drainage discharge and the nitrogen concentrations,as well as the experimental results of bioreactor reduction rates.Major findings are as below:(1)Short period leaching tests showed that both rice husks and woodchips released a certain amount of nitrogen and phosphorus.The released NO3--N was lower than 0.25 mg g-1,and the released TP was lower than 0.20 mg g-1.Neither nitrogen nor phosphorus extracts from rice husks and woodchips accumulated,indicating that the risk of leaching from bioreactor media was small.The total amount of organic carbon(TOC)leachate from rice husk was higher than that from woodchips;the average TOC concentration was 12 mg·g-1 from rice husks and 7.47 mg·g-1 from woodchips(2)In the static experiment,the 24hour average denitrification rate of NO3--N was 0.09 mg·L-1·h-1,0.34 mg·L-1·h-1 and 0.50 mg·L-1·h-1 for inflow concentrations of 3.3 mg/L,10.8 mg/L and 20.0 mg/L,respectively.The denitrification rate is positively correlated with the inflow concentration.The internal pH of the bioreactor was 6.33 in average,which was slightly acidic;the average dissolved oxygen was 0.49 mg/L,indicating anaerobic state inside the bioreactor The average redox potential was-128.3 mV inside the bioreactor,indicating a reduced condition The internal temperature of the bioreactor was 1.3? higher than the inflow temperature of 25.6?.(3)In the dynamic experiment,with medium loading rate of NO3--N(11-13 mg/L)and PO43--P(0.8-1 mg/L),for long hydraulic retention time of 12 h,the observed reduction rate of NO3--N 93%in the rice-husk bioreactor and 54%in the woodchip bioreactor;the observed reduction rate of PO43--P was 33%in the rice-husk bioreactor and 7%in the woodchip bioreactor.The denitrification rate was 0.93 mg·L-1·h-1 in the rice-husk bioreactor and 0.55 mg·L-1·h-1 in the woodchip bioreactor.The saturated hydraulic conductivity of the rice husk filled bioreactor was 6.9±0.3 cm/s(4)Denitrification reactions in the bioreactors can be described by the Michaelis-Menten equation;the reaction followed the first-order at low NO3--N concentration(Ci<KM),and the zero-order at high NO3--N concentration.The maximum removal rate was 3.17 mg·L-1·h-1,and the temperature coefficient was 1.019.(5)The total NO3--N output through field ditches of paddy fields was 20154 g/ha between July and August in the rice irrigation district along the Grand Canal in the Jiangdu District,Yangzhou,China.For a bioreactor size to treat 90%of the drainage flow and the HRT was 6 hours,the calculated bioreactor size requirement was 41.12 m2(at 1 m deep),accounting for 0.58%of the drainage area.Such installation of bioreactors could remove 5155 g nitrate nitrogen output,which account for 50.8%of the total nitrogen load of the calculation period.