Quality Control Of Mesophilic Anaerobic Effluent In Ethanol-methane Dual-fermentation Coupled Process And Its Pilot Scale Demonstration

Wastewater pollution is one of the restrictive factors of the development of fermentationindustry. Based on the ecological food chain theory, an ethanol-methane coupled fermentationsystem was developed to solve the wastewater pollution of the cassava-based fuel ethanolfermentation industry, which provided a new solution to the treatment of fermentationwastewater and set a good example for the green production of submerged fermentationindustry. Feasibility of the coupled system was confirmed by the results from lab and pilotscale experiments. The key factors including chemical oxygen demand （COD）, alkalinity ofthe anaerobic effluent, ammonia nitrogen, sulfate ion, volatile fatty acids （VFA）, andsuspended solids（SS） contained in the effluents all affected the feasibility of the process. Thispaper investigated the regulation of sulfate ion, ammonia nitrogen and SS contained in theeffluent, and the operation principles of the coupled system. The main results were asfollows:（1） When mesophilic anaerobic effluent was used as cooking water, the pH of themedium should be adjusted with sulfuric acid to the optimal pH for liquefying enzyme, whichbrought much sulfate ion to the medium, and its concentration in the stillage could reach3.18g/L, which was harmful to biogas fermentation. Sulfate ion in the stillage decreased to2.12g/L by adding Ca（OH）2, and the usage of sulfuric acid could be reduced about33%.8.02%ofsulfuric acid could be saved when10%of the stillage was used to adjust the pH of medium.（2） Ammonia nitrogen in the system accumulated as the recycle proceeded. Themaximum concentration was811and1100mg/L on the lab and pilot scales. On the lab scale,ethanol fermentation was inhibited when ammonia nitrogen in the cooking water was morethan550mg/L. The inhibition could be eliminated by controlling the ammonia nitrogenconcentration below350mg/L by ammonia nitrogen removal method. On the pilot scale,ammonia nitrogen was regulated by stripping tower, and its concentration was controlledbelow300mg/L, and the ethanol fermentation was not inhibited.（3）Activated sludge was the main ingredient of the SS in the mesophilic anaerobiceffluent. High concentration of sludge would reduce the ethanol production, decline the DDGquality, and make the DDG color deeper. cationic polyacrylamide （CPAM） was the bestflocculant for sludge removal. The optimal condition was pH10,50oC,and additive amountof40mg/L. Flocculants remained in the mesophilic anaerobic effluent did not inhibit theethanol fermentation.（4） Using cassava as fermentation medium, the average ethanol production was （11.97±0.95）%（v/v） during the recycle. COD of the thermophilic and mesophilic anaerobic effluentwas4624±1632mg/L and2656±986mg/L respectively. When the biogas fermentationrun stably, the VFA was400-500mg/L and below200mg/L, the alkalinity was2000-4000mg CaCO₃/L and3000-5000mg CaCO₃/L in the thermophilic and mesophilic anaerobiceffluents, respectively.Using corn as fermentation medium, ethanol production slightly fluctuated during therecycles. High concentration of sludge in the mesophilic anaerobic effluent caused thefluctuation of ethanol production. During recycles, COD was4307±1185mg/L and2656±986mg/L, alkalinity was4527±518mg CaCO₃/L and3949±721mg CaCO₃/L, ammonianitrogen was618±201mg/L and478±165mg/L, and VFA was213and50mg/L, respectively in the thermophilic and mesophilic anaerobic effluents. Experiments on the labscale （11recycles） suggested that the stability of the ethanol production could be obtainedwhen the concentration of SS in the mesophilic anaerobic effluent was low.