Treatment And Recovery Of Biogas Slurry From Swine Farms With Microalgae

Chlorella vulgaris is a kind of beneficial microalgae which can be used for resource utilization of swine biogas slurry,it can clean biogas slurry and get rich biomass resources which can be used as animal feed,biodiesel and other raw materials.However,due to the small size and their stable suspension in the cultivation broth,the harvesting of microalgae biomass always been one of the major obstacles for the algae-biodiesel.And the discharge with the treated biogas slurry will cause secondary pollution.At present,there are few reports about using microalgae to purify swine biogas slurry and using mycelium to harvest microalgae biomass.In this study,the PAC and PAM flocculant were used to flocculate the biogas slurry.The pretreated biogas slurry was used as the cultivation medium of Chlorella vulgaris,the adaptability and purification efficiency of Chlorella in biogas slurry were investigated.Furthermore,the pelletizaiton condition of Aspergillus niger in biogas slurry and the affecting factors of harvesting microalgae by fungi pellets were investigated,and made a preliminary exploration for the mechanism of microalgae harvested by mycelium pellet.The main results were shown as follows:（1）Polyaluminium chloride（PAC）,polyacrylamide（PAM）were selected as coagulant for coagulation,the COD,ammonia nitrogen,TP and turbidity in the biogas slurry of pig farm were decreased in the range of different dosage of coagulant.The removal efficiencies of COD,ammonia nitrogen（NH₃-N）,TP（total phosphorus）and turbidity were 61.31%-68.55%,11.44%-36.84%,85.86%-90.21%and 79.73%-86.48%%in biogas slurry after flocculation with 5g/L PAC with different dosage of PAM.（2）13 days cultivation of Chlorella vulgaris by biogas slurry after flocculation pretreatment was study.The ICP-AES result show that the biogas slurry contains the necessary nutrients for Chlorella growth.After 13 days cultuvation,the biomass yield of Chlorella can reach 5.14 g/L in the biogas slurry after 5 g/L PAC+60 mg/L PAM flocculation pretreatment,the COD,NH₃-N and TP removal rate of the effluent were 69.84%,71.68%and 83.96%.（3）Different carbon source,nitrogen source and its concentration were explored,the optimal conditions for forming the mycelial pellets in the biogas slurry after flocculation pretreatment were obtained:5 g/L sucrose was added as the carbon source,2 g/L NH₄Cl as the nitrogen source,the spore inoculation amount was 1.4×10⁴spores/ml,the initial p H of the cultival medium was 7,the rotating speed of shaking table was 150 rpm,and the temperature was set at28℃.（4）The harvesting efficiency of microalgae was optimized by three factors:the amount of mycelium pellet,the initial p H of the mixture and the rotating speed of the shaker.The results showed that the harvesting efficiency of microalgae reached 97.12%when the dosage of mycelium pellets was 60 g/L,the speed of shaking table was 150 rpm,p H=8.However,p H has little effect on the efficiency of microalgae harvesting.Hence,the mycelium pellet can harvest microalgae in a wide range of p H.（5）The results of Zeta potential analysis showed that there were potential difference between microalgae and mycelium pellet,indicated that charge neutralization was the main mechanism of flocculation between algae and fungi.Through SEM,it can be found that a large number of microalgae cells are adsorbed on the mycelium,and it can be judged that there is a sweeping flocculation behavior in the process of mycelium pellet harvesting the algae cells.（6）The lipid production of microalgae and algae-fungi pellets can account for 12.3%and 32.9%（biomass dry weight）.According to GC-MS analysis,the SFA,MSFA,PSFA in algae-fungi pellet accounted for 27.07%,32.65%and40.29%respectively,which was conductive to the production of biodiesel.