| In view of the problem that the current"terminal treatment"mode of rural domestic sewage is not sustainable,the research group proposed the"source separation"mode of domestic sewage.Blackwater can be recycled through unified collection,greywater by constructed wetland in situ or nearby treatment can significantly reduce the cost of wastewater and treatment,easy to be promoted in rural areas of China.However,the lack of oxygen supply in traditional constructed wetlands often limits the efficiency of sewage treatment,and the current means of enhanced reoxygenation are not suitable for rural areas in China due to their high cost and complex management.Therefore,a new type of unsaturated vertical flow constructed wetland was proposed,which was filled with river sands and zeolites.We investigated the effect of hydraulic parameters（i.e.,specific pulse volume,pulse period,and specific hydraulic load）and substrate ration on rural greywater treatment.The results are as follows:（1）The sewage retention time of S20,S40 and S60 systems in the flooding-drying constructed wetland increases,while the dimensionless variance2and the percentage of dead zone decrease,in turn.The thicker the river sand layer is,the more the hydraulic state of the constructed wetland tends to be the horizontal thrust flow,and the percentage of dead zone is smaller,which is conducive to the removal of pollutants.（2）The specific pulse volume is inversely proportional to the reoxygenation capacity of the wetland system.Within a certain range of specific pulse volume,the three groups of systems can keep in the aerobic environment and have higher removal rates of COD and NH4+-N.When the specific pulse volume reaches 213.75L/m2,the single inflow is too large,and the sewage is deposited on the substrate surface.The three groups of systems have different levels of sewage accumulation phenomenon,and the pollutant removal efficiency also decreases greatly.（3）When the specific pulse volume is 71.25L/m2,changing the pulse period does not affect the effluent dissolved oxygen concentration of the three systems,and both COD and NH4+-N show good treatment efficiency.Under the influent condition with specific pulse volume of106.88L/m2,the shortening of pulse period has little influence on the effluent dissolved oxygen and pollutant removal efficiency of S20 system.However,due to the large thickness of river sand layer in S40 and S60 systems and the slow infiltration of sewage,sewage accumulation phenomenon appeared in the short pulse period,and the removal rate of dissolved oxygen and pollutants in the effluent is also greatly reduced.（4）The change of specific hydraulic loading rate will affect the reoxygenation capacity of S20 and S60 systems,and thus affect the pollutant removal effect of the two systems.S40system is less affected by specific hydraulic load rate,showing strong stability,and the removal efficiency of each pollutant fluctuates little at different specific hydraulic load rate.（5）In the operation mode exploration stage,the S20 system has the optimal comprehensive treatment efficiency when the specific pulse volume is 106.88L/m2,the pulse period is 6h,and the specific hydraulic load rate is 7.13L/m2·min.The S40 and S60 systems have the best comprehensive treatment efficiency when the specific pulse volume is 71.25L/m2,the pulse period is 6h,and the specific hydraulic load rate is 4.75L/m2·min.Under the optimal operating conditions,the effluent concentration of the three groups of systems is above the level-1 A standard of《Discharge Standard of Water Pollutants for Rural Domestic Sewage Treatment Facilities》（DB32/3462-2020）.（6）The greywater resources are sufficient,and the continuous operation of S20 system under long-term high load will lead to the enrichment of more and more microorganisms in river sand layer,which will affect the reoxygenation capacity of wetland and reduce the treatment efficiency.High load operation at low temperature will aggravate the decrease of matrix permeability,which will lead to the occurrence of water shortage in wetlands.After adjusting to the operation strategy of 5-day operation and 2-day bed rest,it can not only keep the effluent to a higher standard under low temperature condition,but also ensure the stable operation of the wetland and prevent sewage accumulation phenomenon.After adjusting the operation strategy,the average effluent concentration is above the level-1 A standard of《Discharge Standard of Water Pollutants for Rural Domestic Sewage Treatment Facilities》（DB32/3462-2020）.（7）In the case of S40 and S60 systems in continuous operation,due to the decrease of matrix permeability coefficient,the matrix layer is filled with water,and the removal efficiency of pollutants is reduced.The operation strategy was adjusted to the mode of 5-day operation and 2-day bed rest.After the adjustment,the pollutant removal rate of the two groups of wetland systems was only affected by temperature,and the sewage infiltration was normal,without obvious accumulation phenomenon.After adjusting the operation strategy,the average effluent concentration during the monitoring period is better than the level-1 A standard of《Discharge Standard of Water Pollutants for Rural Domestic Sewage Treatment Facilities》（DB32/3462-2020）.In general,the flooding-drying constructed wetland can achieve good reoxygenation effect and ensure the removal rate of pollutants without additional conditions.The system is simple and easy to maintain,and it has good application potential in the treatment of rural greywater. |
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