Study On Recovery Of Nitrogen And Phosphorus By Struvite Precipitation And Filtrations Of Biogas Slurry

In the recent years,most of the researches on the treatment and application of biogas slurry is mainly focus on the methods of returning biogas slurry and eliminating the pollutants to the emission standard.The use of biogas slurry as a resource to recover or transfer the nutrient to make it recycled are also viable treatment methods.Biogas slurry has lots of advantages for the irrigation of fruits and vegetables,but it also have certain limitations.High levels of suspended solids is the main technical barrier for the use of biogas slurry in drip or membrane concentration.Therefore,how to remove suspended solids is the key to ensure the better utilization of biogas slurry.The nitrogen and phosphorus content in the biogas slurry is relatively high,and the nitrogen and phosphorus in biogas slurry can be recovered as slow-release fertilizer by struvite formation.There are many researches on the recovery of nitrogen and phosphorus by struvite precipitation,but the reaction conditions for obtaining both higher nitrogen and phosphorus recovery rate need to be further explored.In this study,there are a screening of different filtration methods of biogas slurry and a trial of struvite precipitation to recover the nitrogen and phosphorus of biogas slurry.Through designing three different filtering methods and comparing their filtering effect,the filtering method with the best filtering effect was obtained.A trial of recovering nitrogen and phosphorus from biogas slurry by struvite precipitation was designed to explore new ways to recycle resources in the condition of the lack of farmland to absord the huge numbers of biogas slurry.Through the design of different treatment levels of the main factors that affect the struvite precipitation,the optimal conditions for the recovery of nitrogen and phosphorus are obtained,providing a certain reference for the promotion of struvite precipitation.The results of the study are as follows:The first experiment is to screen biogas filtration methods.There are three methods to filter:quartz sand filtration,straw filtration and microfiltration.The biogas slurry was derived from a biogas engineering of a farm in Tianjin.There were three kinds of straw diameter: 1cm,2cm and 5cm,and they were fillied from fine to coarse.The particle size of quartz sand were 1mm,3mm and 10 mm.It is also filled from fine to coarse,and the last layer is a bearing layer.The filtering of quartz sand filtration method was generally,the removal rates of ammonia,total phosphorus and suspended solids were all below 10%.Comparing the filtering effects of straw,quartz sand and microfiltration,quartz sand had the worst filtering effect,and microfiltration had the best filtering effect.The effect of microfiltration on the removal of suspended solids and COD was much higher than that of straw and quartz sand.Quartz sand had the worst effect on the filtration of biogas slurry,and the removal rate of several basic indexes is below 10%.The filtering effect is poor and cannot meet the drip irrigation requirements.It is not suitable for the filtration treatment of drip irrigation pretreatment.Straw filtration had a fine filtering effect on biogas slurry,but it still did not meet the requirements of drip irrigation and cannot be used as a pretreatment for drip irrigation.Microfiltration had the best filtration effect,and had the best removal effect on suspended solids.Some of Nitrogen and phosphorus nutrients in biogas slurry was taken away,but the removal rate of suspended solids is up to 40.01%,and the removal effect of fecal coliform bacteria is better.The second experiment is to optimize the parameters of recovering nitrogen and phosphorus in biogas slurry by struvite precipitation.The experiment is divided into three groups of tests.The first set of tests is a single-factor experiment.Five factors are selected: pH,reaction time,stirring speed,Mg:N,P:N ratio.There were five levels of each factor,and three parallel tests for each level.The second set of the tests is the PB test.The single factor analysis can preliminarily obtain the factors that have a greater effect on the recovery rate of nitrogen and phosphorus.According to the principle of PB test design,the difference between high and low levels were appropriately expanded during the test design process,and the impact size can be further analyzed.Select 2 levels from the 5 parameters of pH,reaction time,stirring speed,Mg:N,P:N,screening significant factors.The third set of tests is the Box-Benhken test.Combined with PB test results,The final recovery rate of ammonia nitrogen and phosphate were chosen as the response value,and the Box-Benhken test was performed on the three factors that had a significant effect.The other two factors are fixed at the level determined by the single factor.Using the Design-Expert software to optimize the regression equation to determine the optimal conditions for reclaiming the nitrogen and phosphate from biogas slurry by struvite precipitation.The results showed that the optimum conditions for the recovery of nitrogen and phosphorus in biogas slurry were pH 10,ratio of magnesium to nitrogen 1.1,ratio of phosphorus to nitrogen ratio 0.6.Under the above optimized conditions,the ammonia recovery rate was 65.21%,phosphate recovery rate was 89.47%.In the actual experiments,the recovery rate of ammonia was 65.01%,phosphate recovery rate was 90.81%,with difference values of 0.20%,1.34%,respectively,indicating good fit with the regression model.Under this condition,better recoveries of nitrogen and phosphorus can be obtained,which will provide reference for the biogas slurry to be returned to the field and treatment.However,the composition of the biogas slurry is complex and the test is conducted under laboratory conditions.The test results using directly for actual engineering treatment may have a difference.To solve this problem there will be perfect in the follow-up study.