Study On Nutrient Recovery Performance Of Biogas Slurry Based On Biomass Composites

Biogas slurry is a main by-product of anaerobic fermentation of livestock manure,which is rich in nitrogen,phosphorus,potassium,humus,amino acids and other nutrients.A large amount of biogas slurry produced by large-scale farms in China can not be absorbed nearby,which has potential secondary pollution problem.It is an effective way to reduce and utilize biogas slurry,especially to recycle nitrogen and phosphorus.However,it is difficult to recover nitrogen and phosphorus from biogas slurry because of the low adsorption efficiency of phosphorus adsorption materials and the volatilization of ammonia nitrogen.In this paper,aiming at the problem of low efficiency of phosphorus adsorption and recovery,metal oxide/biochar composites were prepared from waste materials such as mushroom residue and eggshell.The effects of modification temperature,modification method and concentration of modification reagent on phosphorus adsorption performance were investigated,and the results were as high as 159.69 mg/g.In order to solve the problem of ammonia nitrogen volatilization and high energy consumption,low-energy solar photothermal evaporation material was used to concentrate ammonia nitrogen in biogas slurry,and polyacrylamide with rich positive charge was modified to improve the ammonia nitrogen interception efficiency of photothermal evaporator to 93%.Finally,the bacteriostatic experiment of photothermal evaporator was carried out on Escherichia coli and the real biogas slurry of the farm,and it was found that polyacrylamide modification also had bacteriostatic effect.The specific conclusions are as follows:1.Preparation and characterization of biochar materials for phosphorus adsorptionCao/biochar composites with high phosphorus adsorption performance were prepared by mixing eggshell with waste mushroom residue and carbonizing at high temperature.It was found that the high-temperature decomposition of eggshell had a pore expansion effect on the biochar of bacterial residue,and had a better phosphorus adsorption effect than the direct use of calcium chloride/magnesium chloride mixed with bacterial residue.The best ratio of eggshell to mushroom residue is 1:1,and the best carbonization temperature is 800℃.Under the optimal conditions,the phosphorus adsorption capacity of Ca O/biochar can reach 159.69mg/g,which is 5.3 times higher than that of unmodified biochar（EC-0）.2.Preparation and characterization of biogas slurry photothermal evaporator materials:（1）Acetobacter cellulose hydrogel was used as substrate material for photothermal evaporator.By adding chitosan,starch and carbon black into the growth liquid of Acetobacter,the pore structure of photothermal film was regulated to improve the evaporation efficiency of photothermal evaporator.It was found that chitosan had the best effect on improving the efficiency of water evaporation,and the enthalpy of water evaporation could be reduced by32.8%.The results of sum frequency spectrum showed that the incorporation of chitosan could reduce the crystallinity of cellulose by destroying the Iαisomer of cellulose,which made it easier to form low binding energy intermediate water in the photothermal evaporator.（2）In order to reduce the ammonia nitrogen loss in the water evaporation of photothermal evaporator,the bacterial cellulose photothermal evaporator was further improved,and a composite photothermal evaporator with Janus structure was designed.The combination of porous bacterial cellulose photothermal evaporator and dense polyacrylamide hydrogel,using the charge rejection effect of the positively charged amine group of Polyacrylamide in ammonia nitrogen,can increase the ammonia nitrogen rejection rate from63%to 93%.It was found that BC/Chi/PAM evaporator showed good ammonia nitrogen retention performance in the concentration range of 50-800 mg/L NH₄Cl.The BC/Chi/PAM evaporator with Janus structure showed good persistence in the evaporation and concentration of biogas slurry,and the performance of continuous evaporation for 72 h did not decrease significantly.（3）The inhibition zone of BC/Chi/PAM membrane on E.coli was 5 mm,which was much higher than that of BC membrane.The results showed that BC/Chi/PAM membrane could inhibit the growth of bacteria significantly,and the number of colonies was 53%less than that of BC,while the composite membrane with chitosan had a significant inhibitory effect on the growth of fungi in biogas slurry,and the number of colonies in BC/Chi membrane and BC/Chi/PAM membrane was one order of magnitude less than that of BC.