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

Waste biomass was anaerobically converted to carboxylate salts by using a mixed culture of mesophilic acid forming microorganisms. Municipal solid waste (MSW), paper, sewage sludge (SS), and biosludge were used as substrates. MSW and paper serve as the carbohydrates source, whereas sewage sludge and biosludge are the nutrient source. MSW and SS were combined in an 80/20 ratio, whereas paper and biosludge were combined in 80/20 and 40/60 ratios. MSW and biosludge were treated with lime to increase digestibility before entering the system.; Four fermentors were arranged in series and solids and liquids were transferred in opposite directions. All fermentations were performed at 40°C. Terrestrial inocula were mixed cultures obtained from rumen fluid, compost piles, and swamp material. Marine inocula were obtained from the shores of Galveston Island. Countercurrent fermentation allows both high conversions and high product concentrations. Excellent reproducibility was demonstrated for batch and countercurrent fermentations.; For lime-treated MSW/SS fermentations at the same liquid residence time (LRT) and volatile solid loading rate (VSLR), marine inocula produced higher product concentrations and conversions than terrestrial inocula. For untreated MSW/SS fermentations with terrestrial inocula, intermediate lime treatment between Fermentors 3 and 4 increased the acid productivity, conversion, yield and selectivity.; Continuous Particle Distribution Modeling (CPDM) was applied to predict acid product concentrations and substrate conversions over a wide range of LRT and VSLR. CPDM for lime-treated MSW/SS with terrestrial inocula predicted actual product acid concentrations and conversions within 4.4 and 15.8%, respectively; whereas with marine inocula the model predicted actual product acid concentrations and conversions within 1.1 and 27.8%, respectively. For an 80/20 paper-to-biosludge ratio, CPDM predicted product concentrations and conversions within 12.4 and 2.3%, respectively; whereas for a 60/40 paper-to-biosludge ratio, it predicted actual product concentrations and conversions within 16.7 and 2.9%, respectively. CPDM predicts that product acid concentrations and conversions as high as 50 g/L and 80%, respectively, can be achieved with air-lime-treated MSW/SS using marine inocula.