Research On Bio-eco Treatment System For Domestic Wastewater In Rural Area Based On Source Separation

The rural environment in China is not opyimistic.Nowadays,the domestic wastewater in rural area is still not effectively treated.There're several problems,like low coverage rate,low operating rate and low removal efficiency of the treatment facilities in rural areas.One of the keys to develop rural wastewater treatment is designing the treatment facilities based one the rule:"local conditions friendly,technically feasible,and well adapted to the demands at rural area".According to the rule,a biological-ecological combined system was investigated based on source separation.The combined system includes“black water pretreating Modified Anaerobic Baffled Reactor(MABR)-Anoxic filter(ANF)-multi-stage water driving rotating biological contractors(ms-wd RBCs)-economy-friendly constructed wetland(ef-CW)”.The main research contents and results demonstrate as following:Study on MABR treating blackwater indicated that MABR can degrade the organic pollutant in blackwater effecitively,which helps reduces the organic load in subsequent units.MABR was operated at mesophilic condition(36±1?)and started from a low organic load.The start-up strategy can help MABR start successfully in short period.The study on HRT influence shows that longer HRT helps better COD removal in MABR.About 94%COD can be removed when MABR was operated stably at 48h HRT.Analysation on the forms of pollutants demonstrated that the MABR performed well on removal of particale contaminants.The alkalinity in each compartment was high,thus it has strong buffer capacity.The 16S analysis indicates that there's phase separation in MABR.The hydrotrophic methanogens occupied a large advantage in all compartments.The higher abundance of non-hydrotrophic methanogens in the second compartment indicated the degaration of VFA,thus the possibility of MABR acidification is low.Influent of ANF/ms-wd RBCs was the mixture of pretreated black water and raw grey water.Study on this section indicated that it achieved COD removal,ammonia oxidization,odor decreasing and TN partly removal.The influence of recirculation ration,HRT and rotation rate was investigates.Appropriate increase of recirculation ratio and HRT will help improve the pollutant removal performance of ANF/ms-wd RBCs.ANF/ms-wd RBCs achieved well performance under 150%RR,7.11h HRT_ANF and 1h HRT_{wd RBC}.When operating stably,the average removal efficiency of COD,NH₄⁺-N,TN and TP were 88.40%,88.14%,52.33%?34.11%,respectively.Removal of COD occurred mainly in ANF but removal of NH₄⁺-N occurred mainly in ms-wd RBCs,particulally in RBC2 and RBC3.Except TP concentration,the effluent can reach the Class A standard in the discharge standard of water pollutants for rural domestic sewage treatment(DB32/T 3462-2020)well.Considering the application at rural area,reducing the reflux ratio or shortening the HRT would help energy-saving and cost-control.The remained nitrogen and phosphorus is conducive to the growth of the subsequent ef-CW.Considering the further absorption and utilization of nitrogen/phosphorus in ef-CW,further shortening the HRT_ANF and reducing the reflux ratio is considerable when the land area is sufficient.Samples of biofilm/sludge at different locations in ANF/ms-wd RBCs were taken for16S analysis.The dominant bacteria in ANF were a variety of denitrification related bacteria and digestion and degradation of organic matter related bacteria.Sulfur autotrophic denitrifying bacteria have a relatively high abundance,which is beneficial to the removal of odor.High abundance of Ammonia Oxidizing Bacteria(AOB)and Nitrite Oxidizing Bacteria(NOB)were detected in ms-wd RBCs.The presense of AOB and NOB was coincide with the NH₄⁺-N removal perdormance.The abundance of NOB reached higher and higher from the first stage to the last stage.It was coincide with the removal of NH₄⁺-N in ms-wd RBCs,it also helps prove that setting three stage is reasonable.Ms-wd RBCs combined water dropping system with RBC,hence achieved efficient oxygenation,which is conducive to ammonia nitrogen oxidation.In each RBC,water driving waterwheel was used to replace the conventional driving motor.It helps reduce the energy consumption and is easier for maintaining and management.In view of the potential problems of ms-wd RBCs during long-term operation,the configuration of ms-wd RBCs was improved.The main improvement included:(1)Position of the driving waterwheel was moved from the side to the middle of the shaft,and the disks were evenly distributed on both sides of the waterwheel.This helps to avoid possible bending problems of the rotating shaft.(2)A water distribution plate with a triangular overflow weir was added to disperse the water column.This helps to increase the oxygenation capacity of the water drop process;(3)The contact water tank of the driving waterwheel was improved to a corner type.It helps to increase residence time of driving water in the sink,thereby reducing drive flow and saving energy consumption.The oxygenation capacity of ms-wd RBCs was proved mainly relating to the drop height,drop flow rate,disks rotation speed,etc..The suggested height and rotation speed was 0.5-0.6m and 4-8 rpm,respectively.In order to further understanding the oxygenation performance of ms-wd RBCs,a model for oxygen mass transfer process was established.The model simplifies the oxygen and mass transfer process of ms-wd RBCs into two parts:oxygenation in the process of water dropping and rotation of the disk.Based on the two-film theory,the section 1 of the model for water dropping was established through mass balance.The section 2 of this model for disks rotation was established by modifying the Kim&Molof model.Based on the two section,the final ms-wd RBCs oxygenation model was established through theoretical derivation and experimental correction.The final model was decided by initial DO,height,rotation rate,design of wd RBCs and temperature.The model calculation shows that the oxygenation capacity of single-stage wd RBC can exceed 4mg/L even at 35?,so the oxygenation capacity of ms-wd RBCs is sufficient to support the nitrification during operation.Research on how ef-CW enhanced phosphorus removal was launched from slection of substrates and plants.The main purpose of ef-CW is enhancing phosphorus removal.To enhance phosphorus removal,several substrates and plants were compared and selected.Aerated concrete block was selected as its good performance on phosphorus removal.The subsurface flow CW with aerated concrete block as substrates can improve the phosphorus removal capacity significantly,compared with using gravel as substrates.The study on plants selecting was conducted in palnts filter beds.The recommended plants for summer to autumn were Ipomoea aquatica Forsk,Gynura cusimbua,and the recommended plants for winter to spring were Oenanthe javanica(Bl.)DC.,Nasturtium officinale R.Br.,Lactuca sativa.These economic plants can help remove nitrogen and phosphorus in wetlands.Besides,they also have economic values.MABR can effectively reduce the organic load of subsequent units and reduce the possibility of pathogenic microorganism pollution;ANF/ms-wd RBCs can achieve efficient nitrification of ammonia nitrogen,further reduce the concentration of organic matter and reduce the odor but retain most of nitrogen and phosphorus;ef-cw can reduce nitrogen and phosphorus efficiently,in the meanwhile,produce certain economic benefits by screening plants with large biomass and high economic value.The whole process is low-cost,easy to maintain and manage,which meets the needs of rural areas.