Research On Pollutant Removal And Resource Utilization Of Bio-Eco System For Rural Domestic Sewage Treatment

At present,the treatment rate of domestic sewage in China’s rural areas is still at a low level.Rural domestic sewage has become the main water pollution source.The existing rural domestic sewage treatment models and processes are not suitable for the characteristics and conditions of agricultural in rural areas,and most of the constructed facilities cannot operate normally.It is extremely urgent to develop efficient,low consumption and easy maintenance rural domestic sewage treatment technology.In this study,a new combination process of high-efficiency and low-consumption biological treatment and resource-based ecological treatment was constructed according to the requirements of single-household or adjacent multi-household domestic sewage treatment and resource utilization mode.The scientific orientation is to focus on the removal of organic matter and ammonia nitrogen in the biological unit,and the resource utilization of nitrogen and phosphorus in the ecological unit.A low consumption aeration driving lateral reflux biofilm reactor（ADLRBR）powered by a small flow air pump was designed and developed as the biological treatment unit.The unsaturated vertical flow forage crop wetland（CVFACW）or composite vertical flow aromatic crop wetland（CVFACW）was used as ecological treatment units.The combined process of ADLRBR+UVFFCW/CVFACW was formed,and the efficiency of pollutant removal and resource utilization was studied.The structure of the reactor can be simplified and the energy consumption can be reduced by setting the lateral reflux channel of ADLRBR and using the method of aeration-driven mixing.The start-up process of the reactor was investigated.The results showed that the COD removal rate of the reactor with 20%,35%and 50%filler filling rate（FFR）reached more than80%on the 20th,18th and 17th days respectively.The factors affecting the operation efficiency of the reactor are analyzed.The improvement of hydraulic retention time（HRT）and gas-water ratio（GWR）can continuously improve COD removal in the reactor.The FFR,HRT and GWR of the most efficient operating conditions of the reactor were determined to be 39.67%,6.98h and 7.73 respectively.The maximum NH₄⁺-N removal efficiency could reach 97.70%,and the NH₄⁺-N removal rate could be used as an indicator of the reactor efficiency.The adaptability of the reactor to changes in application conditions was explored.The results show that under high C/N conditions,the pollutant removal ability of the reactor can be enhanced by improving control strategies such as GWR.The removal rate of COD and NH₄⁺-N can still be rapidly restored to more than 95%and 97%after restarting the reactor after 5 days halt.The long-term stable operation also shows that the average removal rates of COD and NH₄⁺-N by ADLRBR remain at 93.70%and 95.54%,and the average removal rates of TN in sewage remain at 51.24%.The constructed ADLRBR can better realize the functional positioning of efficient removal of organic matter and ammonia nitrogen,and provide good conditions for the resource utilization of nitrogen and phosphorus in subsequent ecological units in the combined process.In the UVFFCW resource recovery efficiency study,the potential of forage as a resource ecological treatment crop was analyzed by studying the uptake kinetic characteristics of forage nitrogen and phosphorus nutrient salts,and forage species were selected.The characteristic values of the absorption kinetics fitting equation of NH₄⁺-N,NO₃^--N,PO₄^3--P showed that the ion influx rate index and barrenness tolerance were better for Pennisetum sinese and Pennisetum hydridum.Nutrient retention and removal characteristics of the substrate showed that river sand removed up to 50.83%of PO₄^3--P and zeolite removed up to 57.85%of NH₄⁺-N under rapid drenching conditions.Based on the hydraulic infiltration characteristics,river sand was identified as the fixation layer,zeolite as the substrate layer,and ceramic pellets as the leaching layer.These three substrates filled the UVFFCW according to the thickness of 1:3:1could obtain a lower constant head permeability coefficient,which helps to guide the design of the UVFFCW substrate bed.UVFFCW operation was optimized from the perspective of nutrient removal and utilization,and the optimal operational hydraulic load of the UVFFCW was 0.3 m³/（m²·d）,and the optimal UVFFCW water distribution was 12 times per day.The long-term material balance showed that the nitrogen resource utilization rate was 4.45%and3.17%,and phosphorus utilization rate was 12.65%and 10.28%for the giant fungus grass and royal bamboo grass test groups,respectively.p H did not decrease significantly during the one-year long-term operation,and the effluent DO was in the aerobic state,so the wetland operation was judged to be healthy.The resource efficiency of forage in UVFFCW was analyzed by material balance,and the physical and chemical characteristics of the wetland were analyzed to determine the health status of long-term operations.UVFFCW,as an ecological unit,can achieve efficient resource utilization of nutrients in the tailwater of biologically treated rural domestic wastewater.The kinetics of nutrient uptake by aromatic crops was studied before the construction of CVFACW.The maximum Imax of NH₄⁺-N uptake by the five pre-selected aromatic crops ranged from 188.59 to 856.75μg/（g·h）,so aromatic crops have the advantage of nutrient uptake rate for wastewater ecological treatment.The sequestration and release characteristics of wetland substrate materials for nitrogen and phosphorus were also explored as a basis for the selection and arrangement of wetland substrates to guide the construction of CVFACW from a mechanistic level.The results showed that Ca O in aerated blocks accounted for 57.44%of the phosphorus removal components,with the best performance in phosphorus removal and stable adsorption.The highest percentage of Fe₂O₃in the phosphorus removal component of volcanic rocks was 21.19%,with solid phosphorus removal ability and a high percentage of weakly bound components,which contributes to the release and resource utilization of phosphorus.The isothermal adsorption test,adsorption kinetics test,and nutrient desorption test all show that the adsorption of aerated blocks is stable and can be used as a guaranteed material for phosphorus removal.Volcanic rock adsorbs faster and has good weak desorption performance,which can be used as crop root zone material to buffer incoming phosphorus impact load.This demonstrates the feasibility of spatially sequencing the preferential utilization of nutrient salts in the front part of the wetland and the guaranteed removal in the back part through differentiated substrate settings.By analyzing the spatial characteristics of nutrient removal in CVFACW,the results show that the downstream flow area in the front section of the wetland is the main site for nutrient removal and utilization,which proves that the design of wetland substrate materials based on nutrient retention characteristics and coupled with crop uptake and utilization can enhance the efficiency of nutrients utilization.Eventually,the resource utilization efficiency was tested by the long-term operation of CVFACW,and the resource utilization rates of nitrogen and phosphorus in the two groups of wetlands planted with a single crop were 5.25%and 16.48%,3.03%and 18.61%,respectively.The study on the mechanism of nutrient utilization in CVFACW promoted the understanding of the process of nutrient utilization in wetlands.In the combined bio-ecological process,ADLRBR as a biological treatment unit can achieve stable and efficient removal of organic matter and ammonia nitrogen,and UVFFCW or CVFACW as an ecological treatment unit can achieve efficient resource utilization of nitrogen and phosphorus nutrients.This study analyzed the mechanism of synergistic interaction between crops and substrates in wetlands to promote nitrogen and phosphorus resource utilization.The application of combined process ADLRBR+UVFFCW/CVFACW can achieve low-power and stable removal of pollutants and efficient resource utilization,which provides a new process technology and theoretical support for rural domestic wastewater treatment.