Study On Treatment Of High Concentration Of Digested Wastewater From Pig Manure By A Combined Means Of Coagulation And Adsorption

Anaerobically digested wastewater is a by-product in the process of anaerobic digestion,which is rich in organic matter,nitrogen,phosphorus and some metal elements.Digested wastewater is a kind of precious liquid fertilizer resource,but it is inconvenience for storage or transportation.On the other hand,due to the complexity of composition and high pollution loads,there is a potential risk of pollution to the environment if directly utilizing digestate.This thesis takes the high concentration of digestate from pig manure digestion as the research object and aims to recycle nutrients from digestate by means of coagulation,trying to convert liquid fertilizer to solid fertilizer,meanwhile,reduce the pollution loads of effluent after coagulation,and it is advantageous to subsequent treatment of effluent.This article focuses on the coagulation efficiency and mechanism of coagulants,as well as the subsequent treatment of coagulation effluent by means of adsorption.The main research contents and conclusions are as follows:（1）First,by comparing the coagulation efficiency of four kinds of coagulants,the optimum coagulant was determined.The results showed that the removal efficiency of turbidity,COD,NH₃-N and TP for ferric subsulfate（[Fe（OH）SO₄]_n）and poly aluminium chloride（PAC）were superior to that of ferric chloride（FeCl₃）and aluminium sulfate（Al₂（SO₄）₃）.Besides,considering the performance of flocs sedimentation and security to environment,ferric subsulfate（[Fe（OH）SO₄]_n）is preferable than poly aluminium chloride（PAC）.As a result,ferric subsulfate（[Fe（OH）SO₄]_n）was selected as the optimum coagulant.（2）The optimal condition of digested wastewater coagulation using[Fe（OH）SO₄]_n was studied by single factor experiment.The results showed that in the conditions of which dosage of[Fe（OH）SO₄]_n is 12 g/L,the initial pH value of digestate is 8,rapid stirring speed is 200 rpm,rapid stirring time is 4 min,slow stirring speed is 60 rpm,slow stirring time is 20 min,the efficiency of coagulation was optimal.（3）The main effect factors（i.e.dosage of coagulant and initial pH value）to coagulation efficiency of digested wastewater were optimized by the means of response surface methodology.The optimal coagulation conditions were as follows:the dosage of coagulant is 13.22 g/L and initial pH value is 8.24.The final results of vertification experiment showed that the deviations between the predicted and actual results were less than 5%,indicating that the models can be good fitting the experiment data.（4）By studying the characteristics of the sludge under the optimal coagulation condition using[Fe（OH）SO₄]_n as coagulant,it is found that the density of sludge is higher with large specific surface area,and sludge was rich in essential elements for plant growth,suggesting that recycling nutrients from digested wastewater by using the method of coagulation is indeed feasible.According to the changes of Zeta potential,it is found that the mechanism of coagulation under the optimal condition was based on the synergistic effect of electric neutralization,adsorption and sweep flocculation.（5）The effluent after coagulation was treated by using natural zeolite.Total ammonia nitrogen was removed effectively,wheras,the removal efficiency of organic matters was not ideal.According to the experimental results,in the conditions of which the initial pH value of solution is 8,dosage of zeolite is 3 g/25 ml,and contact time is 8 hours,the removal efficiencies of NH₃-N and COD were 89.58%and 10.49%,respectively.Pseudo-second-order kinetics model can better reflect the adsorption process,indicating that the removal of total ammonia nitrogen and organic matters is mainly attributed to chemical adsorption for natural zeolite.