The Nitrogen Removal Efficiency Of Several Sewage Treatment Wetlands

Plants play an important direct role in nitrogen removal process of sewage treatment wetland and provide good conditions for nitrogen degradation via microbes. It is obvious that variability of plant species and seasonal temperatures could affect the activity of denitrification processes of the sewage treatment wetland. However, there are still different viewpoints on the contribution of nitrogen removal processes in sewage treatment wetland. Therefore it is meaningful to study the effects of plants on seasonal nitrogen removal in wetland in order to improve the establishment of sewage treatment wetland and its effect in denitrification. In this study, three emergent plant species, Acorus calamus, Typha angustifolia and Phragmites australis, were used to build a subsurface flowed wetland and the same form of non-plant wetland was used as control(CK). Outdoor experiments in spring-summer and summer-autumn seasons under intermittent water distribution were conducted to determine plant growth dynamics and the nitrogen removal effect of plants, microorganisms and the matrix, and to clarify the nitrogen removal contribution via different pathways and in different seasons of the sewage treatment wetland. The results are as follows:1. Plant biomass and N uptake increased as the growing duration prolonged in two seasons. Plant nitrogen uptake was significantly and positively correlated to plant biomass and tissue nitrogen content, and the average biomass in the spring-summer was higher than that of the summer-autumn season. The Biomass and N accumulation of Typha angustifolia was significantly higher than that of Acorus calamus or Phragmites australis. The responses of biomass and N uptake to seasonal change in Acorus calamus was more than that of Typha angustifolia or Phragmites australis.2. In two seasons, the dissolved oxygen(DO) fluctuated, and the effluent DO concentrations in spring-summer season were: CK> Acorus wetland>Typha wetland >Phragmites wetland, and were CK> Acorus wetland > Phragmites wetland > Typha wetland in summer-autumn. The p H value rose rapidly at first and then decline in spring-summer experiment but changed in the reverse fashion in summer-autumn season. The higher DO concentrations and p H values in the spring-summer season than in the summer-autumn implied less oxygen consumption under warm condition.3. The TN removal rate in the spring-summer was higher than that in the summer-autumn. The TN removal rates in the spring-summer were Phragmites wetland > Typha wetland > Acorus wetland >CK, and in the summer–autumn season were Typha wetland > Acorus wetland > Phragmites wetland >CK. In the two experimental seasons, the ammonia nitrogen and total nitrogen removal rates were negatively and significantly correlated to water p H, DO, ammonia, nitrate and total nitrogen contents, while the removal rates of ammonia and total nitrogen was significantly positively correlated. The results showed that the wetland designed in the experiment could meet the needs of bacteria for nitrification and denitrification in different temperature conditions.4. The matrix nitrogen absorption in spring-autumn season was slightly higher than that in summer-autumn, and the N quantity absorbed in the matrix were CK> Acorus wetland > Typha wetland > Phragmites. In the spring-summer season, matrix contents of NH4+-N and NO3--N rose first and then declined, while TN fluctuated first and then stabilized. In summer-autumn season, only the NH4+-N content in the matrix showed rose and declined trend, the NO3--N and TN fluctuated randomly.5. In the two seasons, the number of ammonifying bacteria in the matrix dominated over nitrifiying and denitrifiying bacteria. The total nitrogen removal rate was negatively correlated with ammonifying bacteria, while both ammonia and total nitrogen removal rates were positively correlated with the number of nitrifiying and denitrifiying bacteria, indicating that nitrification and denitrification played important roles in nitrogen removal when NH4+-N was the main form.6. In spring-summer, the total nitrogen removal of Acorus, Typha, Phragmites and CK systems were 3.44 g, 3.67 g, 3.76 g and 3.19 g respectively,the ratios of matrix nitrogen adsorption accounted for 36.74%, 26.11%, 24.15% and 42.57%, and the portion of plant absorption were 15.26%, 41.30%, 33.98% and 0.00%, while the percents of nitrogen removal via microbial degradation were 48.00%, 32.65%,41.78% and 57.43%. In summer-autumn, the total nitrogen removal of Acorus, Typha, Phragmites and CK systems were 3.79 g, 3.84 g, 3.70 g and 3.50 g respectively. Among which the matrix nitrogen adsorption contributed 28.50%, 20.03%, 20.76% and 55.42%, the plant absorption accounted for 6.96%,23.17%, 8.15% and 0.00%, and the microbial decomposition removed 65.54%, 56.81%, 71.09% and 44.58%. in general, the nitrogen removal effect of plant wetland exceeded CK system, and the contribution to nitrogen removal via microbal decomposition was the most, followed by matrix adsorption and plant absorption except that plant absorption contributed the most of the nitrogen removal of Phragmites wetland in spring-summer and the matrix adsorption accounted for the most of ck system in the summer-autumn experiments.