| Pollution problem caused by wastewater is the key factor that restricts the fastdevelopment of the fermentation industry. The constructed ethanol and biogasdual-fermentation coupled process effectively solved the discharging problem of massivewastewater in the fuel ethanol production process and reduced the fresh water consumption,thereby achieving the goal of cleaner production in the submerged fermentation. In thecoupled process, stability of the digestate composition is crucial to the successful operation ofthe recycling process. Addition of sludge to the anaerobic digestion system causes thatmassive suspended solid is contained in the digestate and then affects mass transfer and yeastmetabolism in the ethanol fermentation. As a result, effect of the suspended solid in thedigestate on ethanol fermentation was studied and economically feasible solution wasproposed. The main research results were shown as follows:(1) In the ethanol and biogas dual-fermentation coupled process, suspended solidconcentration of the digestate gradually increased as recycling proceeded, and then inhibitedethanol fermentation. This inhibition was caused by two reasons. On the one hand, thesuspend solid contained some ammonium nitrogen, which can be used as nitrogen source forgrowth of yeast and also react with reducing sugar under high temperatures, thereby reducingthe total sugar of the culture medium. On the other hand, massive anaerobic microorganismsin the suspended solid changed the metabolic flux of yeast. And to maintain an intracellularredox balance, a large number of byproducts were produced and synthesis of ethanol wasinhibited. The critical inhibitory concentration to ethanol fermentation of suspended solid inthe mesophilic and thermophilic digestate was0.35g·L-1and0.5g·L-1, respectively.(2) Flocculation could remove the suspended solid and colority of the digestate. Sevenflocculants and differnet flocculation methods were used and compared to remove thesuspended solid from the digestate. Results showed that300mg·L-1of PAC and2mg·L-1ofCPAM were the optimum combination of flocculation for the mesophilic digestate, underwhich the turbidity and colority removal rates were92.4%and23.7%, respectively, andsettling time of the floc was25s, besides the flocculant input cost was about0.51yuan percubic meter of wastewater. For the thermophilic digestate,300mg·L-1of PAFC and5mg·L-1of APAM were the optimum combination of flocculation, under which the turbidity andcolority removal rates were87.3%and22.5%, respectively, and the flocculant input cost wasabout0.6yuan per cubic meter of wastewater. In addition, flocculation could not influenceother physicochemical property of the digeatate, which ensured that the characteristic ofdigestate was stable.(3) Feasiblity of flocculation used in the coupled process was investigated. Resultsshowed that inorganic flocculant, PAC and PAFC presented in the culture medium did notcause adverse effect on the synthesis of ethanol, while organic flocculant could inhibit ethanolfermentation. The critical inhibitory concentrations to ethanol fermentation of CPAM andAPAM were2mg·L-1and5mg·L-1, respectively. However, concentrations of the two organicflocculant detected in the digestate were only0.03mg·L-1and0.16mg·L-1, which far always from the inhibitory concentration. Furthermore, ethanol fermentation with the digestatetreated by flocculation was slightly better than the fermentation with the digestate treated bycentrifugation. These results suggested that flocculation was feasible in the coupled process.(4) The application of flocculation in the coupled process was verified in the pilot scaleexperiment. Flocculation could efficiently remove the suspended solid in the digestate, whichimproved ethanol fermentation performance. The solid-liquid separation process thatconventionally used in the coupled process was replaced by flocculation, thus the investedcost was reduced by about25%when the obtained flocculation project was used. |
PDF Full Text Request