| Agricultural non-point source pollution is one of the important causes of water environment pollution.Rural domestic sewage is one of the main sources of agricultural non-point source pollution.The Nitrogen and Phosphorus Elements contained in rural domestic sewage,is the main reason for the eutrophication of lakes.Therefore,combining with the characteristics of sewage,designing and selecting reasonable technology to treat rural domestic sewage can improve and beautify the rural ecological environment and meet the requirements of "the strategy of rural vitalization".This paper takes aquatic plant Canna indica L.(canna)and vegetable aquatic plant Oenanthe javanica(water celery)as the main research objects,using artificial floating islands and subsurface constructed wetland modes to treat rural domestic sewage,and studies:(1)Design of subsurface constructed wetland system for purifying rural domestic sewage;(2)Investigate the effect of different sewage concentration and hydraulic retention time factors on the treatment of rural domestic sewage of canna and water celery;(3)Investigate the effect of different hydraulic retention time factors on the treatment of rural domestic sewage in subsurface canna constructed wetland system and the change of removal effect with time.The main results were as follows:(1)There was a certain correlation between the removal rate of rural domestic sewage and the influent concentration of pollutants of canna and water celery.When HRT was 3 days,the removal efficiency of the two plants to the sewage with different influent concentrations of NH4+-N(10,25 and 40 mg/L)were 21.50%,9.35%,-2.79%and 91.57%,12.40%and 0.48%respectively,which decreased with the increase of influent concentration of NH4+-N.The removal rates of sewage(2,4 and 6 mg/L)with different P influent concentrations were 33.47%,21.75%,38.28%and 34.36%,32.58%and 20.47%respectively.The removal rates of Canna increased first and then decreased with the increase of P influent concentration,while that of water celery decreased with the increase of P influent concentration;The removal rates of sewage with different TOC influent concentrations(10,20,40,60,80 and 100 mg/L)were-141.20%,-34.88%,41.24%,87.21%,87.77%,90.62%and-103.40%,-70.18%,46.10%,84.59%,87.40%,84.15%,respectively,which increased with the increase of TOC influent concentration.(2)The removal rate of rural sewage by two plants had a certain correlation with the length of HRT.Under different HRT(3,4,5 d)conditions,the two plants,canna and water celery,treated domestic sewage with NH4+-N influent concentration of 25 mg/L and P influent concentration of 6 mg/L.The removal rates of P were 8.38%,8.68%,52.24%and 15.08%,42.50%,65.33%,all increased with the increase of HRT;The removal rates of water celery for NH4+-N were 23.64%,56.42%,42.46%,which increased first and then decreased with the increase of HRT.The removal rates of P for cress were 33.25%,29.42%,44.92%,and the increase of HRT decreased first and then.increased..(3)During the operation period of sub section treatment of rural domestic sewage in the subsurface Canna constructed wetland system,the removal effect of pollutants in the sewage had three stages with time,which were adaptation period,treatment period and stability period.①When the HRT was 2 days,the treatment effect of NH4+-N in rural domestic sewage would not play a role until the constructed wetland system continues to operate to 7 T(T was the cycle);For the treatment effect of P in rural domestic sewage,the constructed wetland system could play a role from the beginning of T,and the effect gradually stabilizes by 4 T.After treatment,the P concentration in the wetland section reached the first level B standard in the discharge standard of pollutants for urban sewage treatment plant(GB18918-2002)after 3 T,and the P concentration in the effluent section reached the first level A standard after 4T;For the treatment effect of TOC in rural domestic sewage,the constructed wetland system could play a role from the beginning of T,and the effect gradually stabilizes by 3 T.② When HRT was 3 d,the treatment effect of NH4+-N in rural domestic sewage would not play a role until the constructed wetland system continued to operate to 6 T;For the treatment effect of P in rural domestic sewage,it could play a role from T.The P concentration after treatment in the wetland section reached the first level a standard in the T～6 T period,the first level B standard in the 7 T～10 T period and the first level a standard after 10 T period,after the treatment of the effluent section,the P concentration reached the first level A standard in the T～6 T period,the first level B standard in the 7 T～9 T period,and the first level A standard in the 10 T period;for the treatment effect of TOC in rural domestic sewage,the constructed wetland system could play a role from T,and the effect tended to be stabilized after 2 T.③ When HRT was 4 d,the treatment effect of NH4+-N in rural domestic sewage was not obvious in constructed wetland system;The treatment effect of P in rural domestic sewage could be effective from the beginnning of T.After the treatment of the effluent section,the concentration of P reached the level B standard in the period of T-3 T.For the treatment effect of TOC in rural domestic sewage,the constructed wetland system could play a role from the beginning of T,and the effect dropped to less than 0 after 3 T.(4)There were differences in the treatment effect of each section on each pollutant in the rural domestic sewage strength in the subsurface Canna constructed wetland system.In addition to ranking the average removal rate of NH4+-N,under 2 d conditions:effluent section>inlet section>wetland section,under 3 d conditions:Wetland section>effluent section>inlet section,under 4 d condition,the order of average removal rate of TOC was:inlet section>wetland section>effluent section.In other cases,the average removal rates of NH4+-N,P,and TOC are ranked as follows:effluent section>wetland section>inlet section.Among them,the structure of the inlet and effluent sections was similar to that of coarse gravel,but the removal effect was significantly different.Gravel as the base material was more suitable to be set in the tail of the constructed wetland.(5)The average removal efficiency of NH4+-N,P,TOC in rural domestic sewage by each section of subsurface canna constructed wetland in the whole operation cycle shows that HRT 3 d mode was better than HRT 2 d mode.In terms of NH4+-N removal rate,the average removal rate of NH4+-N in the 2 d mode of the inlet section was-5.75%less than 16.51%in the 3 d mode,the average removal rate of NH4+-N in the 2 d mode of the wetland section was-14.23%less than 32.47%in the 3 d mode,and the average removal rate of NH4+-N in the 2 d mode of the effluent section was-17.20%less than 19.61%in the 3 d mode;In terms of P removal rate,the average P removal rate of 2 d mode in the inlet section was 9.63%,which is less than 21.42%of 3 d mode;the average P removal rate of 2 d mode in the wetland section was 58.06%,which is less than 65.26%of 3 d mode;the average P removal rate of 2 d mode in the effluent section was 68.25%,which is less than 72.21%of 3 d mode;In terms of TOC removal rate,the average removal rate of TOC in the 2 d mode of the inlet section was 6.13%,less than 13.1%of the 3 d mode;the average removal rate of TOC in the 2 d mode of the wetland section was 9.86%,less than 30.32%of the 3 d mode;the average removal rate of TOC in the 2 d mode of the effluent section was 10.94%,less than 35.65%of the 3 d mode. |
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