Engineering Research On Safe Irrigation Technology For Rural Domestic Sewage

China is a large agricultural country.The annual water shortage of agricultural production reached 300×108 t.With the rapid development of China's rural construction and the improvement of people's living standards,annual villages and small towns sewage discharge exceeded 100×108 t.Rural sewage has become potential agricultural irrigation water.The nature of rural sewage is relatively stable,with better biodegradability,without heavy metals and toxic substances.Nitrogen and phosphorus and other substances in wastewater are pollutants for water environment,but they are essential nutrients and valuable resources for agricultural irrigation.However,there are a variety of safe problems in the direct irrigation of domestic sewage.First,long-term use of wastewater for irrigation of farmland causes soil compaction and soil anaerobic even soil degradation and plant rot and other problems because of high concentrations of suspended particles and organic matter;Second,the existence of pathogenic microorganisms in direct irrigative sewage may remain on the surface of the crops,ate or contacted by humans may cause harm to their health.Third,fertilization and irrigation period of flowers,vegetables and other crops often have seasonal or time difference.Extensive sewage irrigation and the lack of security measures will bring safe risks to the surrounding environment.This study presented a safe irrigation system of rural domestic sewage which was composed by the nitrogen and phosphorus inorganic reactor consisted of both an anaerobic filter and an aerobic biological filter and the environmental risk management unit which was actually constructed wetland.And a demonstration project of 25m3/d was built.The nitrogen and phosphorus inorganic reactor of rural domestic sewage was generated by reforming the frequently-used rural sewage treatment process consisted of both an anaerobic filter and an aerobic biological filter.It used to safe irrigation is designed to revoe organic pollutant but to keep the nutrients such as nitrogen and phosphorus in tail water at the same time,by adjusting the operation parameters of the combined process of anaerobic tank and aerobic biofilter,which is popular in rural sewage treatment projects for its discharge meeting the first level B standard.The safe irrigation system of rural domestic sewage was aimed at making the concentration of COD,BOD5,SS and fecal coliform bacteria in rural sewage reach the requirements of direct edibel vegetables safely irrigation of the national standards for irrigation water quality(GB5084-2005).The surplus irrigation water produced by different irrigation season and time contains high concentration of nitrogen and phosphorus.It can cause environmental risks when discharged to the environment directly.Therefore,we made the surplus irrigation water reach the first level B standard of Chinese discharge standard of pollutants for municipal wastewater treatment plant(GB 18918-2002)by environmental risk control unit.Based on demonstration project,conclusions of this study in nearly a year were as follows:1.In summer and autumn,the influent concentrations of COD,TN and TP were 115?229mg/L,17.74?25.32mg/L and 1.42?2.43mg/L.Based on the concentration of COD in irrigation water should be less than 60mg/L,It was concluded that the nitrogen and phosphorus inorganic reactor had good nitrogen and phosphorus inorganic efficiency when the HRT of anaerobic filter was 6h and the HLR of aerobic biological filter was 4.8m3/(M2·d).Under this condition,the average concentrations of inorganic nitrogen in irrigation water were 13.6mg/L and 15.08mg/L,the average concentrations of PO43--P were 1.41mg/L and1.25mg/L.2.In winter,the influent concentrations of COD,TN and TP were 155?196mg/L,19.66-22.16mg/L and 0.89?2.23mg/L.We kept the feasible HRT of the anaerobic filter to be 6h and adjusted the feasible HLR of aerobic biological filter to be 4.2m3/(m2 d)because of low temperature.Under this condition,the average concentrations of inorganic nitrogen and PO43--P in irrigation water were 14.85mg/L and 1.17mg/L,and there is a probability of 80%that the average concentration of COD was under 60mg/L.The effluent concentrations of COD of nitrogen and phosphorus inorganic reactor were 31?49 mg/L,when kept the original operating parameters and used 1:1 reflux in winter.They all met the requirements of direct edibel vegetables safety irrigation of the national standards for irrigation water quality(GB5084-2005).3.We used regular water quality indices such as pH,CODcr,BOD5,SS,etc and heavy metal indices such as Cr,Cd,Pb and Cu in irrigation water in different seasons to evaluate the safety of irrigation water.In summer and autumn,the regular water quality indices and heavy metal indices in irrigation water reached the requirements of direct edibel vegetables safety irrigation of the national standards for irrigation water quality(GB5084-2005).In winter,the regular water quality indices and heavy metal indices in irrigation water could also reach the requirements of safe irrigation by keeping the original operating parameters and using 1:1 reflux.It is indicated that the application of nitrogen and phosphorus reactor in agricultural Irrigation is safe and feasible.4.We used statistical analysis method to to evaluate the public health safety of irrigation water in different seasons.It was determined that the concentration of faecal coliforms in the effluent accorded with log normal distribution.The limit value of fecal coliform bacteria of direct edibel vegetables safety irrigation of the national standards for irrigation water quality(GB5084-2005)was used as reference.It was found that the sanity safety of the concentration of fecal coliform bacteria in effluent was 95.9%.It has a quite high public health safety.5.Environmental safety evaluation of the follow-up treatment of surplus irrigation water in different seasons was studied.The concentrations of TN,NH4+-N and TP in surplus irrigation water were 8.39-12.91mg/L,3.70-7.69mg/L and 0.35?0.80mg/L.They all met the first level B standard of Chinese discharge standard of pollutants for municipal wastewater treatment plant(GB 18918-2002)when treated by environmental risk control unit.Even the concentrations of TN reached the first level A.The results showed that the safe irrigation system of rural domestic sewage had good environmental safety.