Purifying Effect For Domestic Sewage And Mechanism Of Denitrification In Integrated Constructed Wetlands At Different Seasons

Nowadays, eutrophication has been a serious problem for water environmental, which was caused by discharging the domestic sewage into rivers and lakes in a large amount without treatment. There are varies technical methods for domestic sewage treatment, and the constructed wetland has been attached increasing significance with the merits of low cost, good purification effect and high ornamental value. It’s an effective method to inhibit the water eutrophication by removing the nitrogen in the domestic sewage. The constructed wetland can remove the nitrogen by adsorption, precipitation and transformation. It’s of great importance to carry out researches about the mechanism of nitrogen removal for domestic sewage in the constructed wetland, which aimed at improving the efficiency of denitrification in the system.The objective of this research is to study the interaction among the wetland plants, seasons and the denitrification microorganism, which may have an influence on the effect of denitrification for domestic sewage in the constructed wetland. Experiments about the physicochemical parameters of the raw water, intensity of nitrification and denitrification, denitrification microorganism count and the root radial oxygen loss (ROL) capacity of wetland plants have been conducted.The influence of temperature and intensity of nitrification and denitrification on the nitrogen removal efficiency was analyzed, and so the possible relationship between the ROL capacity and the intensity of nitrification and denitrification. Then the impact of the ROL capacity on the number and distribution of denitrification microorganism around roots has also been studied. Probe into the study of purification effect and mechanism of denitrification in the integrated constructed wetlands, the main conclusions are as follows:(1) At the beginning, the effect of the pollutant disposal in the system is not stable, but it still had a higher efficiency than it reached a steady state. The system had a certainly buffering capacity on the pH of the wastewater. After running steadily, the ratio of TP removal in the system was around90%for the whole year, and85%,60%for NH3+-N and TN, respectively. The ratio of COD removal was relatively steady, which reached the value of60%. The treatment efficiency of NO2-N and NO3--N was related to the HRT of the system.(2) The efficiency of pollutant removal in the system was affected by the temperature, either a high temperature or a low one will reduce the treatment effect of the system. In a appropriate temperature range, for example, June and September, during which the temperature was rather high and the plants grew well, the highest removal ratio of TP and NH3+-N can be up to100%and95%, respectively, and the value for TN and COD both reached70%. The growth of the plants also affected the pollutants removal efficiency of the system. Generally, the efficiency of the system during the vigorous growth period of the plants had an obvious advantage over the decline period.(3) The change of the temperature had an impact on the intensity of nitrification and denitrification in the integrated constructed wetland, and had a rather heavy impact on the intensity of nitrification. Then the intensity of nitrification in down-flow chamber of IVCW can be up to0.065mg/(kg·h); as well0.06mg/(kg·h) in the up-flow chamber and0.04mg(kg·h) for HSFW, respectively. The high temperature had a subtle influence on the intensity of denitrification, which basically increased as the rise of temperature. And it can be up to21mg/(kg-h) for HSFW, as well8mg/(kg-h) in down-flow chamber of IVCW and8.5mg/(kg-h) in up-flow chamber. In the same unit, the intensity of denitrification was significantly higher than the nitrification, which indicates that the nitrification is the limiting procedure during the process of denitrification in the constructed wetland system for sewage treatment.(4) Different species of plants had a remarkable impact on the intensity of nitrification and denitrification in the wetland system, and the intensity of rhizosphere nitrification was significantly higher than the non-rhizosphere (p<0.01), but there was an opposite result for the intensity of nitrification. The ROL of different plants in HSFW has a significant impact on nitrification and denitrification (p<0.01). The intensity of nitrification in IVCW was significantly higher than HSFW(p<0.01), but it was opposite for denitrification.(5) As the decrease of temperature, the amount of the denitrification microorganism in rhizosphere and non-rhizosphere were both reduced in IVCW and HSFW, in which there grew different plants. Compared with the denitrifying bacteria, the ammonia-oxidizing bacteria and nitrite-oxidizing bacteria were more easily affected by the temperature. The amount of the ammonia-oxidizing bacteria and nitrite-oxidizing bacteria in rhizosphere was significantly higher than the non-rhizosphere, but the amount of denitrifying bacteria was obviously lower than non-rhizosphere. According to the amount of denitrifying becteria, there were relationships as follows:in IVCW, Juncus bufonius>Canna, and in HSFW, Calamus>Scirpus>Reed.(6) The ROL capacity of the plants varied as the species. In this study, The ROL capacity had the relationship as follows:in IVCW, Juncus bufonius>Canna, and in HSFW, Scirpus>Calamus>Reed. A high ROL capacity in rhizosphere resulted in a relatively high dissolved oxygen content, which also leaded to the large amount of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria in rhizosphere and less denitrifying bacteria. As for the different ROL capacity of the plants, it had a direct impact on the the number and distribution of denitrification microorganism in rhizosphere.