The Denitrification Research Of Eutrophic Lake's Overlying Water Through Water Mixing And Carbon

Currently,the problem of lake's eutrophication is more serious,in order to achieve the goal of treating water bodies and restoring lakes ecological environment,it need to find a economical and viable way urgently.Currently,constructed wetlands,ecological ponds and others had been widely used in lake management.To improve the efficiency of these denitrification process,in this paper,it studys the effect on the denitrification rate of water at different substrates and water mixing;then,selects methanol as external carbon source to study the effect on the denitrification rate of sediment-water interface at different C/N;additionly,it analyzed and compared the nitrogen removal effect under the conditions:planting submerged plants Elodea on sediment surface,adding decomposing water hyacinth and ethanol,sediment itself without any change.The main conclusions are as follows:(1)Test of beaker,there are four groups of beaker,their initial concentrations of TN and NO3--N were about 9.0 and 7.5mg/L respectively.The result showed that:when water mixing,the removal rates of TN and NO3--N were the highest,after 12d,their values are 85%and 91%respectively;followed by sediment condition,they were 69%and 73%respectively;while under the substrate of sediment-zeolite compound or sediment-gravel compound,the removal rate of TN and NO3--N was poor,they were 56%and 61%respectively.As far as NH4+-N,although the zeolite had good adsorption effect on NH4+-N,but not completely removed from the system,it would continue to release when NH4+-N concentration of the overlying water is low.As an intermediate product,NO2--N increased firstly and then decreased,but the peak appeared earlier when the water is mixing,and the removal rate was faster.(2)In the device of beaker,methanol was selected as external carbon source,it could reach steady state after acclimating about 20d,and meet denitrification basically when the C/N = 4,further increase of the C/N ratio,the denitrification rate does not increase obviously,when the overlying water was as high as 15cm,the promoting effect on denitrification was not obvious through the vertical mixing of water.NO2--N would accumulation in denitrification process,so in order to complete gentrification radically,it needed to ensure enough reaction time.When C/N= 4,testing the NO3--N concentration of the overlying water,the results showed that the denitrification process does not show zero-order reaction,when the overlying water NO3--N concentration was 15.3mg/L or more,the average removal rate of NO3--N was 1.1 mg/(L·h);at 7.4?15.3mg/L,the average removal rate was 0.8mg/(L·h);at 2.3?7.4mg/L,the average removal rate was 0.4 mg/(L·h).(3)In the device of water column,methanol was selected as external carbon source,when C/N=8,after acclimating about 20d,the nitrogen removal rate of control group and mixed group reached a steady state basically,the NO3--N removal rate of control group was about 3.1 mg/(L·d)while the mixed group was up to 5.5 mg/(L·d).From the above rate,it could include that denitrification rate can be increased 1.77 times by mixing the overlying water;when changing the C/N ratio is 4,it found that the control group whose water was not mixing,NO3--N removal rate is only 2.3 mg/(L·d),compared to the C/N=8,it reduces by 25%,NO3--N removal rate will still be 5.5mg/(L·d)after mixing the water.What was more,when C/N=8,the overlying water itself also have gentrification effect,whose NO3--N removal rate was about 1.Omg/(L·d).(4)In the device of water column,selecting different types of carbon on the basis of water mixing,the test results showed that:only the sediment itself providing organic carbon,NO3--N average removal rate was 0.08mg/(L·d);adding decomposing water hyacinth,the NH4+-N concentration of the overlying water would increase,and it could not increase denitrification rate effectively,the average removal rate of NO3-N was about 0.08 mg/(L-d)in the entire testing process;through planting Elodea,which has a better removal effect on NO3--N,whose average removal rate is up to 0.14mg/(L·d),DO and pH of the water would increase After planting submerged aquatic plants.(5)In the water column experiment,when the temperature was about 20?,methanol as an organic carbon source,C/N=4(the ratio of methanol and nitrate mass),the N03--N removal rate of overlying water was 5.5 mg/(L·d);when the temperature was about 30?,ethanol as an organic carbon source,C/N=4(the ratio of ethanol and nitrate mass),the N03--N removal rate of overlying water was up to 14.0mg/(L·d),whose rate was 2.54 times of methanol,although the carbon source type was not the same,but the promoting effect of temperature on denitrification should not be ignored.In summary,we could include that when using the treatment process of pre-library and ponds sewage,additional zeolite or gravel on the surface of sediment,it might inhibit the denitrification process,then reducing the removal rate of nitrogen.When the C/N of the water is low,in order to achieve better effect on dentrification,carbon must be applied.In the water,when the concentration of nitrogen pollutants is lower or the depth of water is deeper,it can improve nitrogen removal efficiency effectively through vertical mixing of water.