Treatment Of Low C/N Wastewater In Constructed Wetland Using Woodchips As Carbon Source

On account of low C/N ratio characteristics of wastewater,and considering the issue of low denitrification efficiency for lacking of carbon source in conventional artificial wetlands system for treatment of low C/N wastewater,this study set up a baffled subsurface flow constructed wetland(CW)system to investigate the low C/N wastewater treatment.The CW system employed aluminum sludge and woodchips carbon source as matrix.Aluminum sludge used as main substrate in the CW can provide a carrier medium for the growth of microorganisms,and improve the phosphorus removal in the system.The role of the woodchips was mainly to supply organic matter for denitrification process.The experiment was divided into four periods,among which the operation modes were different.The influent COD/TN ratio was around 1.88 in period 1,and the theoretic concentration of pollutants was 150 mg COD/L,30 mg NH4+-N /L,50 mg NO3--N /L and 7.8 mg TP/L;Period 2 employed the effluent recirculation operation with 1:1 of reflux ratio,maintaining the same pollutants concentration with period 1;In period 3,the influent nitrate nitrogen concentration was about 100 mg/L,and the C/N decreased to 1.15,while the other conditions were the same as the period 1;Period 4 was the stage with the same influent pollutants concentration with period 3,but the inlet and outlet positions were changed.This study analyzed and discussed the performance of CW system in four periods for overall treatment performance of low C/N ratio wastewater,and investigated the differences of the spatial distribution of COD and nitrogen contaminant in the system among period 1,period 3 and period 4.The main results and conclusions of this study are as follows:(1)In period 1 to 4,the overall removal efficiency of COD was 43.0±16.5%,81.9±4.7%,80.3±5.2% and 80.5±2.6%,respectively.In period 1,effluent COD concentration was higher,most likely due to the washout of the organic matter released from the woodchips.Effluent recirculation considerably improved the removal of COD in period 2.With increasing influent nitrate-nitrogen concentration in period 3 and period 4,COD removal efficiencies in both periods were improved.This paper further studied the COD removal law in the CW and found that COD in the raw wastewater was removed in the fore-end of the system.Woodchips continually released a large amount of organic matter into the system.High oxidation reduction potential is beneficial to the degradation of COD.(2)In period 1 to 4,the overall removal efficiency of nitrate nitrogen was 96.1±3.8%,94.8±3.1%,79.3±9.7% and 63.6±5.5%,respectively.Both period 1 and period 2 achieved higher nitrate nitrogen removal,demonstrating that woodchips provided sufficient carbon source for denitrification process.In period 3 and period 4,nitrate nitrogen removal was relatively low,most likely due to the carbon source releasing rate from woodchips limiting the denitrification process.This paper further studied the nitrate nitrogen removal law in the CW and found that low oxidation reduction potential is beneficial to the degradation of nitrate.System achieved a high nitrate nitrogen removal efficiency,while woodchips filled in the second one fourth section of the CW.High concentration of nitrite nitrogen was monitored and then dramatically degraded in the woodchips zone,indicating complete denitrification process.(3)Through period 1 to 4,the overall removal efficiency of ammonia nitrogen was 11.8±4.7%,19.0±10.1%,56.0±15.2% and 75.3±2.3%,respectively.Effluent ammonia nitrogen concentration was high in period 1,because the degradation of COD competed for dissolve oxygen(DO)with nitrification.Effluent recirculation considerably improved the removal of ammonia nitrogen in period 2.In period 3 and period 4,with increasing nitrate nitrogen,the removal of ammonia nitrogen was improved,indicating that anaerobic ammonia oxidation occurred in both periods.This paper further studied the ammonia nitrogen removal law in the CW and found that woodchips facilitated the removal of ammonia nitrogen.(4)Throughout the entire operation periods,the overall removal efficiency of total nitrogen(TN)was 67.4±8.4%,61.9±4.0%,74.4±5.1% and 63.7±7.2%,respectively.Influent TN concentration was 75.38 mg TN/L and 145.57 mg TN/L in period 1 and period 3,and effluent TN concentration was close in both periods,demonstrating that the system has a strong capacity of anti-shock loading.TN removal efficiency was different between period 3 and period 4,because of the different location of woodchips in system between both periods.Compared with the previous study,the CW in this study,which uses alum sludge and woodchips as substrate,has a certain advantage for TN removal.(5)The overall removal efficiency of total phosphorus(TP)cross the experimental periods was 98.6±1.4%,75.0±5.0%,98.8±0.6% and 98.9±0.4%,respectively.In this study,the constructed wetland system using alum sludge as main substrate had a strong capacity of TP removal,demonstrating the remarkable advantage of alum sludge for P removal.