Conversion of municipal solid waste into carboxylic acids by anaerobic countercurrent fermentation

Municipal solid waste (MSW) and sewage sludge (SS) were combined and anaerobically converted into carboxylate salts by using a mixed culture of acid-forming microorganisms. MSW is an energy source and SS is a source of nutrients. In this study, MSW and SS were combined, so they complemented each other. Four fermentors were arranged in series for a countercurrent fermentation process. In this process, the solids and liquid were transferred in opposite directions, with the addition of fresh biomass to fermentor 1 and fresh liquid media to fermentor 4. An intermediate lime treatment of solids exiting fermentor 3 before entering fermentor 4 was applied to improve the product acid concentration from the untreated MSW/SS fermentations. All fermentations were performed under anaerobic conditions at 40°C. Calcium carbonate was added to neutralize the carboxylic acids and to control the pH. Iodoform was used as a methanogen inhibitor. Carboxylic acid concentration and gas composition were determined by gas chromatography. Substrate conversion was measured by volatile solids loss, and carboxylic acid productivity was calculated as the function of the total carboxylic acids produced, the amount of liquid in all fermentors, and time. The addition of intermediate lime treatment increased product concentration and conversion by approx 30 and 15%, respectively. The highest carboxylic acid concentrations for untreated MSW/SS fermentations with and without intermediate lime treatment were 22.2 and 17.7 g of carboxylic acid/L of liquid, respectively. These results confirm that adding a treatment step between fermentor 3 and fermentor 4 will increase the digestibility and acid productivity of the fermentation.     

Conversion of municipal solid waste into carboxylic acids by anaerobic countercurrent fermentation: effect of using intermediate lime treatment

Municipal solid waste (MSW) and sewage sludge (SS) were combined and anaerobically converted into carboxylate salts by using a mixed culture of acid-forming microorganisms. MSW is an energy source and SS is a source of nutrients. In this study, MSW and SS were combined, so they complemented each other. Four fermentors were arranged in series for a countercurrent fermentation process. In this process, the solids and liquid were transferred in opposite directions, with the addition of fresh biomass to fermentor 1 and fresh liquid media to fermentor 4. An intermediate lime treatment of solids exiting fermentor 3 before entering fermentor 4 was applied to improve the product acid concentration from the untreated MSW/SS fermentations. All fermentations were performed under anaerobic conditions at 40 degrees C. Calcium carbonate was added to neutralize the carboxylic acids and to control the pH. Iodoform was used as a methanogen inhibitor. Carboxylic acid concentration and gas composition were determined by gas chromatography. Substrate conversion was measured by volatile solids loss, and carboxylic acid productivity was calculated as the function of the total carboxylic acids produced, the amount of liquid in all fermentors, and time. The addition of intermediate lime treatment increased product concentration and conversion by approx 30 and 15%, respectively. The highest carboxylic acid concentrations for untreated MSW/SS fermentations with and without intermediate lime treatment were 22.2 and 17.7 g of carboxylic acid/L of liquid, respectively. These results confirm that adding a treatment step between fermentor 3 and fermentor 4 will increase the digestibility and acid productivity of the fermentation.     

Fermentation of rice straw/chicken manure to carboxylic acids using a mixed culture of marine mesophilic micoorganisms

Countercurrent fermentation of rice straw and chicken manure to carboxylic acids was performed using a mixed culture of marine mesophilic microorganisms. To increase the digestibility of the biomass, rice straw, and chicken manure were pretreated with 0.1 g Ca(OH)₂/g biomass. Fermentation was performed for 80% rice straw and 20% chicken manure at various volatile solid loading rates (VSLR) and liquid residence times (LRT). The highest acid productivity of 1.69 g/(L·d) occurred at a total acid concentration of 32.4 g/L. The highest conversion (0.69 g VS digested/g VS fed) and yield (0.29 g total acids/g VS fed) were at a total acid concentration of 25 g/L. A Continuum Particle Distribution Model of the process predicted the experimental total acid concentration and conversion results with an average error of 6.41% and 6.15%, respectively. Results show how total acid concentrations, conversions, and yields vary with VSLR and LRT in the MixAlco process.     

The effect of initial cell concentration on xylose fermentation by Pichia stipitis

Xylose was fermented using Pichia stipitis CBS 6054 at different initial cell concentrations. A high initial cell concentration increased the rate of xylose utilization, ethanol formation, and the ethanol yield. The highest ethanol concentration of 41.0 g/L and a yield of 0.38 g/g was obtained using an initial cell concentration of 6.5 g/L. Even though more xylitol was produced when the initial cell concentrations were high, cell density had no effect on the final ethanol yield. A two-parameter mathematical model was used to predict the cell population dynamics at the different initial cell concentrations. The model parameters, a and b correlate with the initial cell concentrations used with an R(2) of 0.99.     

Fermentation of Acid-pretreated Corn Stover to Ethanol Without Detoxification Using Pichia stipitis

In this work, the effect of adaptation on P. stipitis fermentation using acid-pretreated corn stover hydrolyzates without detoxification was examined. Two different types of adaptation were employed, liquid hydrolyzate and solid state agar adaptation. Fermentation of 12.5% total solids undetoxified acid-pretreated corn stover was performed in shake flasks at different rotation speeds. At low rotation speed (100 rpm), both liquid hydrolyzate and solid agar adaptation highly improved the sugar consumption rate as well as ethanol production rate compared to the wild-type strains. The fermentation rate was higher for solid agar-adapted strains compared to liquid hydrolyzate-adapted strains. At a higher rotation speed (150 rpm), there was a faster sugar consumption and ethanol production for both the liquid-adapted and the wild-type strains. However, improvements in the fermentation rate between the liquid-adapted and wild strains were less pronounced at the high rotation speed.