Conversion of Municipal Organic Waste Into Higher Value Acids and Sugars

Raymond M RedCorn, Purdue University


Food waste and sewage sludge were studied for their ability to be up-convert to higher value products under three different scenarios; food waste conversion to lactic acid, glycogen quantification in waste activated sludge from municipal wastewater treatment facilities performing enhanced biological phosphorous removal, and finally the conversion of either food waste or sewage sludge to rhamnose biofilm via acid phase digestion. In studying the conversion of food waste to lactic acid, three factors where opportunities exist for process improvement; freezing of samples during storage, discontinuous pH control, and holdover of fermentation broth between fermentations, were studied. Freezing samples prior to fermentation was shown to reduce the production rate of lactate by 8%, indicating freeze-thaw should be avoided in experiments. Continuous pH control can achieve both higher lactate accumulation and higher production rate. Finally, holding over fermentation broth was shown to be a simple method to improve production rate (by 18%) at high food waste loading rates (>140 g VS L-1) but resulted in lower lactate accumulation (by 17%). The results inform continued process improvements within the treatment of food waste through fermentation to lactic acid. Glycogen is a chief metabolic storage pool in bacteria performing enhanced biological phosphorous removal (EBPR), and is a potential resource for the production of bio-based fuels and chemicals. These bacteria are settled out in the wastewater treatment process to form waste activated sludge (WAS). To more fully understand the resource potential of glycogen in WAS, two lab-scale quantification methods were compared to ensure suitable assessment; acid treatment (0.9M HCl) and alkaline treatment (5M KOH) both at 100°C for 3 hours. Alkaline treatment recovered only 58% of glycogen in known standards versus 96% for acid treatment. The acid method was successfully applied to WAS from seven different treatment facilities, which ranged from 2.5 to 2.8% of solids as glycogen. The results represent the first broad survey of glycogen in full-scale EBPR systems and indicates that it is a modest resource potential. Finally, the conversion of food waste or sewage sludge to rhamnose was studied. Rhamnose is a high value carbohydrate used as a precursor for flavorings, aromatics, and cosmetics, but also as a biosurfactant in the form of rhamnolipids. A rhamnolipid producing culture was seeded from wastewater sludge, and found to convert carbon from a model acid phase digestate to rhamnose and ribose within the biofilm matrix. The biofilm was characterized to inform rhamnose production potential. Microbial community analysis revealed a highly diverse biofilm community that does not include prior known rhamnose producers. Carbohydrate production in the biofilm was associated with nutrient limiting conditions, and found highest at a C:N ratio of 28 (mmol-C:mmol-C). Rhamnose production was highest from glucose, followed by propionic acid, acetic acid, lactic acid, valeric acid, and butyric acid. Overall, the work indicates that rhamnose production is achievable from mixed organic waste sources via acid phase anaerobic digestion.




Engelberth, Purdue University.

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