| Horizontal subsurface flow constructed wetlands are low-cost treatment systems which have been successfully employed to remove classical contaminants from wastewaters, such as the organic load, nutrients and pathogens. The main processes of nitrogen removal in the horizontal subsurface flow constructed wetlands are nitrification-denitrification, ammonia-volatilizing, adsorption and plant uptake. Nitrification-denitrification is widely accepted as the main process for nitrogen removal. The denitrification potential is considered to be high in the horizontal subsurface flow constructed wetland as its anoxic and anaerobic habitats. Nitrification-denitrification is greatly affected by the availability of easily degradable organic compounds. Other factors influencing nitrogen removal in constructed wetlands are pH, dissolved oxygen, temperature and so on.To enhance the nitrogen removal efficiency in horizontal subsurface flow constructed wetland, Canna indica Typha latifolia and rice straw were hydrolyzed by dilute sulphuric acid under different conditions to get relevant carbon extracting solutions. Orthogonal test indicated that the increase of dilute acid concentration and time of the hydrolysis led to the increase of COD concentration. Rice straw hydrolyzed in5%dilute sulphuric acid over30minutes had the highest COD concentration. According to the result of orthogonal test, rice straw was collected for the preparation of carbon extracting solution. In the lab-scale constructed wetland and horizontal subsurface flow constructed wetland, after calculation, rice straw extracting solution was added to the influent at various concentrations to access the effect of COD/N ratio on nitrogen removal.The experimental results suggested that the removal rate of nitrogen was high in the initial two days of the experiment. The removals of NH4+-N and TN obviously increased as the COD/N ratio increased while NH4+-N was controlled by DO and its removal was limited. Carbon source further restrained reaction of nitrification by competition for DO. Adding carbon source significantly enhanced the removal of TN and NO3--N with the removal of TN increased from54%to95%and the removal of NO3--N increased from48%to96%.The accumulation of intermediate product "NO2--N" was dependent on the removal rate of NO3--N. When the removal of NO3--N was relatively high, no accumulation of NO2--N was found. Furthermore, denitrification in matrix showed an increasing tendency as the COD/N ratio increased and denitrification in the fine sand layer was relatively higher than that in rubble layer.In the horizontal subsurface flow constructed wetland, combinations of different hydraulic retention time (HRT, especially for2-day and4-day) and different influent COD/N ratios were designed to prove the enhancement on nitrogen removal by carbon source supplement. The results indicated that nitrogen removal efficiency was higher in the condition of4-day HRT than2-day HRT in constructed wetland; The variation of DO concentration with COD/N ratios had significant effect on the removal of NH4+-N. NH4+-N removal efficiencies in the wetlands with low COD/N ratios (2and4) were higher than those with high COD/N ratios (6and9);A low influent COD/N ratio caused incomplete denitrification, and the accumulation of NO2--N occurred while the NO3--N removal efficiency was relatively low; Strong relationships existed between TN removal and the accumulation of NO3--N and NO2--N, and TN removal efficiency increased with the COD/N ratios. Appropriate control of COD/N ratio in the influent was crucial to achieve the optimal nitrogen removal in the denitrification process. |
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