Study On Removal Methods Of Nitrogen And Phosphorus And Heavy Metals From Irrigation Drainage And Aquaculture Wastewater

In recent years,with the improvement of China’s industrial wastewater discharge standards and the continuous progress of wastewater treatment technology,the main sources of water pollutant discharge have gradually shifted from industry to agriculture.Due to the large amount of water,relatively low concentration of pollutants and difficulty in treatment,agricultural polluted water has attracted more and more attentions.Among them,the pollution of farmland backwater and livestock and poultry breeding wastewater is the most common and the most harmful.Sixty-five percent of the total phosphorus in the Nansihu river basin in China comes from livestock and poultry breeding industry.In this paper,the removal methods and mechanisms of ammonia nitrogen,phosphorus and heavy metals,which are the representative pollutants in agricultural non-point source pollution,such as farmland backwater and livestock and poultry wastewater,were studied.The removal effects of natural and modified zeolite on ammonia nitrogen and phosphorus in field wastewater were studied under different conditions.The removal effect of heavy metal pollutants in livestock and poultry wastewater was discussed by flocculation sedimentation method,and two kinds of macromolecular heavy metal flocculants with remarkable flocculation effect on heavy metal ions in livestock and poultry wastewater were prepared.The mechanism and process conditions of removing heavy metal ions by flocculation method were investigated.Finally,the optimum conditions for the treatment of ammonia nitrogen and phosphorus in aquaculture wastewater by magnesium ammonium phosphate precipitation were obtained.The contents and main conclusions are as follows.1.Properties of natural and modified zeolite for removal of nitrogen and phosphorus in field effluent.Firstly,the surface morphology,specific surface area,porosity and phase composition of zeolites were characterized by SEM,BET,XRD and EDS respectively.The Cation Exchange Capacity（CEC）of the natural and NaCl modified zeolites are also determined.The artificially simulated nitrogen and phosphorus wastewater were treated with Baiyin zeolite as adsorbent material.The static adsorption method was used to study the nitrogen and phosphorus removal performance of the zeolite under different conditions.The main conclusions of the study are as follows:（1）The results of electron scanning electron microscopy show that the natural zeolite surface has developed pore structure.The specific surface area of natural zeolite is determined by BET method,and the specific surface area of the natural zeolite is 9.29m²/g.XRD analysis shows that the main mineral species of the ore used in the test are clinoptilolite,illite and quartz.EDS determination results show that the content of aluminum is 11.36%,m _Si/m_Al=4.62,and the iron content is as high as 14.55%,so that the appearance of the zeolite is brick red.The CEC of natural and NaCl modified zeolite was determined by pH meter indication potentiometric titration method respectively.The cation exchange capacities of natural and NaCl modified zeolites are 80mmo1/100g and 87.5mmo1/100g,respectively.（2）The optimum conditions for the removal of ammonia nitrogen and phosphorus were obtained by orthogonal experiment.The optimal conditions of NaCl modified zeolites are about NaCl solution at a concentration of 8%,modified time 6h,solid-liquid ratio of4g/100mL,modified temperature 35℃.The optimum conditions of FeCl₃ modified zeolites are about the temperature of 200℃,the particle size of 80¹00 mesh,the roasting time 4h,and the concentration of FeCl₃ is 4.5mol/L.The orthogonal experimental analysis shows that the calcination temperature is the main modified factor,followed by the particle size of zeolite,roasting time,ferric chloride solution concentration.（3）Zeolite particle size,initial concentration of nitrogen and phosphorus,contact time,temperature,pH value and zeolite dosage can influence the removal of nitrogen and phosphorus from the natural and modified zeolite.The adsorption capacity of natural and modified zeolite on nitrogen and phosphorus increases with the initial concentration increases rapidly at first,and then gradually flatten.The adsorption equilibrium time of natural and modified zeolite is slightly different.When the pH value ranges from 4 to 9,natural zeolite and modified zeolite have good removal effect on ammonia nitrogen in water.When the pH value is less than 8,the amount of phosphate adsorption increases with the increases of pH value.The adsorption capacity of zeolite to phosphorus was maximum when the pH value of the solution is 8.（4）The effect of coexist substances with chlorine cation is K⁺>Na⁺>Ca²⁺and the effect coexist substances with sodium anion is SO₄^2->NO₃^->Cl^->H₂PO₄^-.（5）The adsorption of phosphorus by zeolite is dominated by multi-layer adsorption with uneven surface,while the adsorption of ammonia nitrogen by zeolite is dominated by single-layer adsorption.The denitrification and dephosphorization of zeolite are a spontaneous process of entropy increase.Compared with the quasi-first-order kinetic model,the quasi-second-order kinetic model can better describe the kinetic process of nitrogen and phosphorus removal by zeolite adsorption.（6）The removal rate of phosphorus in aquaculture wastewater by modified zeolite can reach 75%.2.Removal of heavy metals from aquaculture wastewater by flocculation and sedimentation.Macromolecular heavy metal flocculants MAC and MCC are prepared by introducing the strong coordination group of heavy metal ions,sulfhydryl group,into the natural macromolecular flocculant chitosan through the amidation reaction between chitosan and thioglycolic acid and l-cysh,respectively.The preparation mechanism,preparation conditions and flocculation performance under different conditions of heavy metal ions of MAC and MCC flocculants are studied in detail.At the same time,the stability of the modified flocculant and the recovery of the valuable metals in the flocculation are also discussed.The main conclusions are as follows:（1）The strong ligand of the heavy metal ions-thiol can be introduced into the molecular chain of natural macromolecular flocculant CTS by the amidation reaction.The optimal preparation conditions of the flocculant are related to the properties of the treatment target,such as the preparation of MCC,and the optimal preparation conditions are very different when the removal rate of Cu²⁺and Cd²⁺different targets are selected as evaluation indexes.（2）Compared with CTS,the water solubility and the ability to remove ions of MAC or MCC are significantly enhanced.（3）The removal efficiencies of heavy mental ions and the turbidity are greatly promoted by the cooperation effects for the water sample containing heavy metal ions and turbidity causing substances.The maximum removal rate of turbidity is up to 100%.（4）The morphology of the aggregates varies with the dosage of flocculant and the changes of the properties of water samples.The smaller the fractal dimension of flocs,the more voids in flocs,the better the removal effect of heavy metal ions.（5）The effect of coexisting substances on the flocculation of removing heavy metal ions can be affected by a certain degree.The influence degree is related to the nature of coexisting substances,concentration and dosage of flocculant.（6）The settling performance of the floc produced by MAC/MCC and heavy metal is good.The residue is stable and it is not easy to produce second pollution.Furthermore,the recovery of valuable metals can be realized under certain conditions.3.Simultaneous removal of ammonia nitrogen and phosphorus from aquaculture wastewater by magnesium ammonium phosphate precipitation（MAP）method.The single factor test,orthogonal test and response surface are used to optimize the process conditions of simultaneous removal of nitrogen and phosphorus from aquaculture wastewater by MAP method.The influence of external conditions on the precipitation purity of magnesium ammonium phosphate is discussed.The main conclusions are as follows.（1）Among the factors affecting the removal of nitrogen and phosphorus by ammonium phosphate precipitation method,the pH value is the most influential factor.When the pH value is 10.5,the MAP method has the best effect on the removal of nitrogen and phosphorus.Stirring speed,stirring time,static time,the ratio of nitrogen and phosphorus content,the amount of magnesium salt and other factors will affect the removal of nitrogen and phosphorus in MAP process.When the molar ratio of magnesium,nitrogen and phosphorus is1.2:1:1,ammonia nitrogen and phosphorus can achieve a better removal effect.（2）When the pH value is 10.5,proper turbidity is beneficial to the removal of nitrogen and phosphorus,while low concentration of HCO₃^-can promote the application of MAP method to remove nitrogen and phosphorus.The presence of SO₄^2-is not conducive to the removal of ammonia nitrogen,but slightly promotes the removal of phosphorus.（3）The pH value has a great influence on the purity of magnesium ammonium phosphate.The purity of magnesium ammonium phosphate can be affected by other factors either.（4）The response surface is designed by Box-behnken Design with pH value,n（P）:n（N）,n（Mg）:n（N）as the influencing factors.Response surface models of nitrogen and phosphorus removal rates are obtained.The removal rate model of ammonia nitrogen is extremely significant,and that of phosphorus is significant.Both models are accurate and reliable（5）The optimum condition of removing ammonia nitrogen and phosphorus by MAP method is pH=9.5,n（P）:n（N）=1.08,n（Mg）:n（N）=1.34;The optimal condition for phosphorus removal is:pH=10.2,n（P）:n（N）=0.99,n（Mg）:n（N）=1.11.In the process of nitrogen and phosphorus removal by magnesium ammonium phosphate precipitation,the optimum conditions for nitrogen removal and phosphorus removal are not consistent.The results of this paper can provide technical reference for reducing the pollution of farmland backwater and aquaculture wastewater and protecting rural ecological environment.