Development And Application Of Straw Type Lightweight Carbon Sustained-release Substrate For Constructed Wetlands

As a country with serious shortage of water resources,in recent years,China’s water environment has been seriously polluted while the conflict between water supply and demand has become increasingly serious.Eutrophication of water bodies is one of the most common water pollution problems,which seriously threatens the safety of water environment and affects human life and activities.One of the main sources of excess nitrogen and phosphorus in water bodies is the confluence of typical low-C/N sewage,such as rural domestic sewage,agricultural surface runoff and urban sewage treatment plant effluent.In order to promote the sustainable development of water resources,the treatment of low-C/N wastewater has become urgent.Artificial wetland,as a common technology for rural wastewater treatment and effluent polishing of municipal wastewater treatment plant,has the advantages of low cost,good treatment effect and environmental friendliness.However,the traditional artificial wetland system not only has the problems of difficult substrate replacement and easy clogging,but also requires additional carbon source addition when treating low-C/N wastewater,which further increases the operation and management costs.Based on the light-weight substrate developed previously by the group,a straw-type light-weight carbon slow-release substrate was developed in this study by combining natural cellulose carbon source,straw,with aluminum sludge and polyurethane material.Its effect as a slow-release carbon source on the denitrification efficiency of artificial wetland treatment of low-C/N wastewater was then investigated.In addition,the microbial community functions in the artificial wetland were explored.The main results of this study are as follows:1)The surface of crushed wheat straw contained a large amount of organic powder.The cumulative release of COD per unit mass of straw was 171.7±0.3 mg/g in 30 days’s time,which was suitable as the core carbon source of this experiment.To develop the lightweight C-slow release substrate,the testing wheat straw addition rate was 0～66.7%（mass ratio of wheat straw/alum sludge）and the best working conditions were:the soaking wet addition of wheat straw without sieving,followed by PM200 addition,which had the best effect on the shaping formation.The cumulative release of TN,NH₄⁺-N and COD in the six groups of substrates showed an increasing trend with the increased proportion of straw,while the TP release was affected by the adsorption of aluminum sludge and its release was low.The physical properties of the six groups of substrates were measured and compared by an established weighted evaluation system.It was concluded that the substrate with 33.33%of straw in mass was the best product under the weighted evaluation system.The compressive strength is 823.50 k Pa,density is 0.47 g/cm3,porosity is 45.20%,and surface dropout rate is 7.00%,which meet the requirements of the specification HJ 2005-2010.2)The physical and chemical properties,such as chemical groups,microscopic morphology,specific surface area and pore size distribution,of the resultant straw-type lightweight carbon slow-release substrates were characterized.It was found that the surface was rougher than that of the previously developed lightweight substrate in our group.The specific surface area was calculated to be 11.21 m2/g,the pore volume was 0.019 cm3/g,and the average pore size was 4.01 nm.The specific surface area was better than that of the previously developed lightweight substrate.3)The results of the CWs with developed lightweight substrate showed that the average removal efficiency of NH₄⁺-N,NO₃^--N,TN,and TP was 19.52%,9.49%,15.33%,and 1.74%,respectively,higher in CW2#（with straw-type lightweight carbon slow-release substrate）than in CW1#（without carbon source substrate）,indicating that The straw type lightweight carbon slow-release substrate owns more superiority for low C/N wastewater nitrogen and phosphorus removal.Moreover,comparing the two CW systems operated separately and in series,the removal effect of each pollutant was significantly enhanced by running two CWs in series.The removal rates of TP,NH₄⁺-N,NO₃^--N,TN,and COD were improved by 6.49%,21.54%,14.09%,20.88%,12.88%,respectively,for CW1#and 7.45%,2.00%,4.60%,5.50%and 14.17%,respectively for CW2#.4)The study of the diversity of microbial communities and the species of dominant microbial genera and phyla as well as their abundance were conducted.The analysis of the variability of microbial communities and the correlation of microbial genera showed that compared to two simulated wetland columns operated individually（condition W）,operation in series（condition D）increased the diversity and abundance of microbial communities,but the dominant microbial communities were significantly larger in condition D than in condition W with 6 species,while the dominant microbial community was significantly larger in the W condition than in the D condition,where 16 species were found in the W condition.The top 3 phyla and genera in terms of relative abundance in both conditions were:Anaplasma,Bacteroidetes,Thick-walled Bacteria;and unclassified Comamonadaceae,Thermomonas,and Aureobasidium,respectvely.The dominant metabolic pathways and COG function annotations were not significantly different in the two conditions（P>0.05）,and the abundance of animal parasites or symbionts and human pathogens all in the microbial community functions predicted by FAPROTAX was significantly higher in W condition than in D condition.