Research On Enhanced Nitrogen And Phosphorus Removal Technology And Application Of Integrated Rural Domestic Sewage

China’s rural sewage features in difficult collections,large distribution,strong vibration of water quality,etc.It is urgent to promote the treatment of rural domestic sewage and improve the quality of rural life based on local conditions.Small-scale integrated wastewater treatment units are well-suited for decentralized management and have been widely adopted,yet they face issues with nitrogen and phosphorus emissions.This study focuses on enhancing denitrification and phosphorus removal in rural domestic wastewater treatment,utilizing agricultural waste to prepare biochar with improved removal capabilities.Investigating Luffa sponge and corn cob as solid carbon sources,the research examines their effect on denitrification enhancement.Further,it explores how combining these materials with biochar,Fe S,sulfur,and iron-carbon can strengthen the process.The aim is to identify cost-effective techniques in nitrogen and phosphorus removal.Alongside this,the development of integrated modified AO equipment and comprehensive systems for advanced removal is undertaken,incorporating these techniques.Key findings include:（1）The practical application effect of 21 purification tanks in the Yangtze River Delta plain river network area was investigated.It was found that COD and suspended solids removal effect was better,and the effluent compliance rate was higher.Total nitrogen and total phosphorus removal effect is poor,effluent compliance rate is low.The changes of pollutant concentration and operating conditions of each treatment unit in five purification tanks were investigated in detail.It was found that there were two main reasons for denitrification:one was the extremely low influent carbon nitrogen ratio,and the carbon source was extremely insufficient;second,the redox potential of inclusion removal tank and anaerobic filter bed tank is too high to maintain anoxic conditions.It is not conducive to heterotrophic denitrification.（2）Biochar,serving as an electron transfer medium,facilitates denitrification in low carbon ratio wastewater.Biochars prepared from corn cob,rice husk,and sawdust at different pyrolysis temperatures of 300,500,and 700°C were all effective in enhancing denitrification,with corn cob biochar produced at 300°C showing the most significant enhancement.With a COD/TN ratio of 2 and TN（NO₃^--N）concentration at40mg/L,after 88 hours of denitrification,the addition of 300°C corn cob biochar increased the TN removal efficiency from 45.3%（with only microbes present）to 80%.（3）Agricultural waste was used to create biochar through a simple one-step pyrolysis method,modifying wood chips with eggshell and rice husk with oyster shell.Eggshell and wood chips,sieved through a 100-mesh screen at a 2:1 mass ratio and pyrolyzed at 900℃,resulted in eggshell-modified wood chip biochar with a maximum phosphorus adsorption capacity of 301 mg/g.Similarly,oyster shell and rice husk,sieved through a 40-mesh screen at a 2:1 mass ratio and pyrolyzed at 900°C,produced oyster shell-modified rice husk biochar with a maximum phosphorus adsorption of196.2 mg/g.Both types of modified biochar exhibited effective selective adsorption of phosphorus across a wide p H range of 3-11.The Langmuir model and pseudo-second-order kinetics appropriately describe the adsorption processes of these adsorbents.The primary mechanism of adsorption involves the biochar’s Ca O or Ca（OH）₂ reacting with phosphorus to form Ca₅（PO₄）₃OH precipitates.In treating actual rural domestic wastewater,the removal rates for total phosphorus（TP）reached 95.3-99.5%and 93.9-99.4%,meeting and even exceeding the secondary（TP<3 mg/L）and primary（TP<2mg/L）discharge standards of the DB 33/973-2021 Comprehensive Wastewater Discharge Standard.The phosphorus-rich biochar,post-adsorption,could potentially be used as a soil amendment or fertilizer,contributing to a beneficial phosphorus cycle in the ecosystem.（4）Luffa sponge was selected as a slow-release carbon source and polyurethane as the filler for the integrated modified AO（Anoxic/Oxic）equipment used in treating actual rural domestic wastewater.This integrated system achieved significantly higher average removal rates of 67.1%for total nitrogen（TN）and 98.7%for ammonia nitrogen compared to traditional purification tanks,which had average removal rates of10.2%for TN and 62.8%for ammonia nitrogen.The anoxic zone of the integrated AO equipment maintained a dissolved oxygen（DO）level below 0.5 mg/L.Additionally,the combination of luffa sponge and sulfur was chosen to enhance denitrification through acid production,coupled with an iron-carbon process for advanced phosphorus removal,leading to a comprehensive denitrification and phosphorus removal technique.The slightly acidic environment created during the luffa sponge-sulfur denitrification process was favorable for the iron-carbon phosphorus removal.According to the results of high-throughput sequencing analysis,the mixed nutrient type of luffa sponge and sulfur reactor has abundant species.The dominant genera of heterotrophic,autotrophic and polytrophic groups are Heterotrophic and autotrophic denitrifying bacteria such as Thauera and Thiobacillus.It also produces sulfate-reducing bacteria such as Desulformicrobium,which can achieve simultaneous denitrification and desulfurization.Function to reduce the formation of sulfate by-products.（5）The integrated modified AO and advanced nitrogen and phosphorus removal process demonstrated strong resilience to water quality and volume fluctuations.Application demonstrations of the 1 ton per day integrated modified AO and advanced nitrogen and phosphorus removal process showed high pollutant removal rates even in lower temperatures from January to March.This combined process achieved average removal rates of 65.8%for COD,88.1%for TN,95.3%for ammonia nitrogen,and88.2%for TP.The effluent quality of the standalone integrated modified AO equipment met the’Rural Domestic Wastewater Treatment Equipment Water Pollutant Discharge Requirements’（DB33/T 2377-2021）for discharge into other water bodies.Meanwhile,the effluent from the combined process met the more stringent standards required by Zhejiang Province for discharging into key or sensitive areas,as specified in the same regulation.The enhanced nitrogen and phosphorus removal rural household wastewater treatment technology and equipment developed in this study can solve the problem of household wastewater discharge that is difficult to cover by urban sewage pipelines and centralized treatment facilities,improve the rural wastewater treatment network,achieve water quality standard compliance for sensitive areas with high requirements for total nitrogen and total phosphorus,and provide certain scientific reference for the improvement of rural wastewater treatment technology.