| By analysis of oxygen balance and material balance as well as empirical study, contribution of root oxygenation, water-carried oxygen and plant absorbance of nutrient in subsurface wetlands (SW) was examined, and free drainage in substrate was also adopted to check its effect on air transfer in this study. The results showed that plant play important role when SW treating surface water with low concentration of contaminants. In such case, root oxygenation and water-carried oxygen sustains the substrate aerobic and so effluent DO＞0; N and P removal is stable for plant absorbance and media adsorption. However, in the case of higher load, namely domestic wastewater, role of root oxygenation becomes ≤20% and effluent DO≈0. It is also found that amount of nitrate and nitrite bacteria are only 1/1000～1/100 of amonifying bacteria in saturated substrate, which has strong negative effect on anti-clogging and nitrogen removal. Nitrogen removal percentage is often ≤20%. Phosphorous initial removal percentage is 54%, but decreases to 5% after a year operation for the consumption of absorption capacity. Because pollutants load is too high for root oxygenation and plant absorbance and media absorption, limitations do exist in SW for domestic wastewater treatment. It is necessary to take use of air-reoxygenation in SW. The results showed that COD、NH4+-N、TN and TP removal percentage of SW with high water lever was 79%, 34%, 36% and 34% and those of SW with free drainage was 79%, 88%, 14%, 69%, which suggest that only air transfer is used properly, total nitrogen removal could be achieved in single-stage wetland. |
