Study On Nutrient Concentration Of Biogas Slurry By Flocculation-Ultrafiltration-Nanofiltration

With the development of agricultural science technology,the scale of livestock ranching is getting larger and larger,the total amount of manure discharged during the cultivation process has increased substantially,and the biogas slurry formed during anaerobic fermentation has polluted the environment seriously,which hindered the further expansion of livestock and poultry farming simultaneously.However,biogas slurry is rich in nutrient elements such as nitrogen,phosphorus,and potassium,humic acid,active enzymes and etc.The treatment of nutrient resources in biogas slurry had important implications for the development of animal husbandry.In this study,the biogas slurry of a large-scale swine farm in Shanwei City,Guangdong Province was sampled and analyzed.Firstly,the modified corn starch flocculant was used to pretreat the pig-breeding wastewater,the he response surface methodology was used to optimize the flocculation process conditions,and the flocculation mechanism was investigated.Subsequently,the pretreated permeate was concentefficencyd using an ultrafiltration-nanofiltration membrane coupling technique.The response surface methodology was used to optimize the ultrafiltration and nanofiltration process conditions,and the mechanism of membrane fouling was preliminarily identified.The main conclusions obtained were as follows:(1)According to the response surface,the optimal flocculation conditions were as follows: the dosage of modified strch flocculant(MSF)30 mg/L,pH 7.70,and the removal efficencyiesof turbidity and SS reached 69.67% and 82.96%,respectively.In the initial stage,the negatively charged colloidal particles quickly attached to the cationic modified starch flocculant surface through charge neutralization.In the later stage,the formation of bridging and roll sweeping(net trapping)occured,colloidal particles and floc aggregate together to form a relatively large,compact and stable floc,resulting in settling.(2)The Box-Behnken response surface method was used to optimize the ultrafiltration process conditions of biogas slurry,and the performance of the ultrafiltration membrane and the penetration of nutrients were studied comprehensively.Under under the condition of pH5.00,temperature 35.00 °C,operating pressure 0.35 MPa,the appropriate increase in initial temperature and operating pressure helped to increase the flux and improve the transmissibility of biogas nutrients.The transmissibility of total nitrogen,ammonia nitrogen,potassium ions,total phosphorus,and humic acid reached 92.13%,89.24%,90.58%,88.36% and 90.17%,respectively.(3)The Box-Behnken response surface method was used to optimize the process conditions for the concentration of nanofiltration membranes of biogas slurry,and the performance of nanofiltration membranes and the concentration of nutrients were also studied.Under neutral conditions(pH=7.00),the initial temperature of the liquid being 32.5 °C,the operating pressure being 0.65 MPa,and the concentration multiple being 8.00 times,the concentration multiples of total nitrogen,ammonia nitrogen,potassium ion,total phosphorus,and humic acid reached 5.51,4.33,4.86,5.73 and 6.12 respectively.(4)According to infrared spectroscopy,three-dimensional fluorescence(EEM),scanning electron microscope,EDS test and fluorescence area integral analysis of the clean membrane and the contaminated membrane,the ultrafiltration membrane was mainly contaminated by aromatic amine compounds(aromatic proteins)and organic phosphorus compounds,the nanofiltration membrane was mainly contaminated by humic acids and microbial metabolites.