Study On The Recycling Of Wastewater Produced From Cellulosic Ethanol-Methane Coupled Fermentation Process

With the price fluctuations in the international energy markets and rising energy dependency, fuel ethanol converted from lignocellulosic materials has become one of the hottest issues. Using lignocellulosic resources to replace grain ones to produce fuel ethanol, which is considered to be the fundamental way for shortage and high costs of raw materials. Recently the technical bottlenecks of cellulosic ethanol production are the difficulties of wastewater treatment, additional high-cost enzyme, purchase and transportation difficulties of raw material and high-energy consumption, thus these factors restrict cellulosic ethanol production. Cellulosic ethanol wastewater, with a high pollution load, complex components, great quantity, is a kind of industrial wastewater with difficult treatment. Thus, various new treatment methods are being developed with effort to decrease the cost burden of cellulosic ethanol production. The cellulosic ethanol wastewater generated from ethanol fermentation was first treated by anaerobic digestion(methane fermentation) and then the anaerobic effluent utilized for the next cellulosic ethanol fermentation batch, to reduce the pollution of environment. It lays a technical foundation for cellulosic ethanol production extension.The quality of wastewater is tested according to the Chinese National Standard methods. The chemical oxygen demand(COD) is 2902 mg/L, the chromaticity is 3021, the content of ammonia nitrogen is 300.9 mg/L, the content of total phosphorus is 16.75 mg/L. According to the results of GC-MS and ICP emission spectrometry technique for the qualitative analysis, about 20 kinds of organic compounds and many kinds of metal ions can be detected. The content of potassium, sodium, calcium is higher.The effects of formic acid, acetic acid, propanoic acid, furfural and ammonia on enzymatic hydrolysis and yeast fermentation are studied, and the critical inhibitory concentrations are obtained. Results demonstrate that the small molecule organic acids in the anaerobic effluent, mainly formic acid, acetic acid and propanoic acid, are inhibitors of ethanol fermentation,which will result in reduction of ethanol. Effects of formic acid, acetic acid and propanoic acid on enzymatic hydrolysis are mainly acidity, which can be avoided by controlling the medium p H. The inhibitory concentrations of formic acid, acetic acid, propanoic acid on ethanol fermentation are 1.6, 3, 3 g/L, respectively. Furfural has no effect on enzymatic hydrolysis, but fermentation will be inhibited when furfural concentration is 4 g/L. Low concentration of ammonia nitrogen(< 0.6 g/L) promotes ethanol fermentation, and high concentration of ammonia nitrogen inhibits ethanol fermentation.Reducing sugar concentration, ethanol yield, residual reducing sugar concentration and weight loss are selected as evaluation index, wastewater produced from cellulosic ethanol-methane coupled fermentation process used as burden water on enzymatic hydrolysis and ethanol fermentation is studied. Results demonstrate that recycling of wastewater has an inhibitory effect on enzymatic hydrolysis and ethanol fermentation. 20% of wastewater has no effect for enzymatic hydrolysis and ethanol production. Wastewater is treated by adsorption of active alumina and active carbon, and wastewater recycle dosage is 40%, 50%, respectively. Amplification experiments show that wastewater used as part of burden water on enzymatic hydrolysis and ethanol fermentation is feasible.