Evaluation of pre-fermentation using confectionery waste products for two-stage anaerobic digestion

Abstract: The finite amount of energy carriers affects all of us. It is important to utilize all available sources and to find new sources of energy. The confectionery industry generates both solid and liquid waste during the production of confectioneries, which could be utilized as a substrate for biogas production. However, problems might arise during the biogas process since these kinds of waste are very rich in carbohydrates. The initial breakdown of the substrate would probably cause an accumulation of fermentation products such as volatile fatty acids (VFA) and a low pH. A solution to this might be to use a two-stage process. The first stage would be a pre-fermentation that should be optimized for production of fermentation products such as ethanol and VFA. The aim of this master thesis is to evaluate the biogas potential of confectionery waste products. The confectionery waste products are evaluated using a continuous two-stage process, batch experiments and theoretical calculations of the methane potential. The potential from process wastewater was examined. Depending on COD reduction for a reactor and COD content of process wastewater, an annual amount of 75 000 m3 or of 857 000 m3 of process wastewater is necessary to produce enough biogas for a gas engine to continuously convert the biogas to electricity. A batch experiment evaluating the methane production potential of nine different confectionery waste products from a large confectionery industry gave a range of 430 - 690 NmL/g VS, which is relatively high. A continuous experiment in two lab-scale reactors with a HRT of two days worked satisfactory. The gas production was stable periodically with a carbon dioxide content above 60%. The pH was low (3.4 - 3.6) throughout the experiment for one of the reactors. However, addition of digester sludge from a methane-producing reactor towards the end of the experiment resulted in a higher pH and more VFA available for utilization in the second stage. The main fermentation products were: acetic acid, lactic acid, ethanol and carbon dioxide. A second batch experiment showed that the methane potential was not affected by pre-fermentation. A carbon balance calculation of the process indicates that 57% of the ingoing organic matter is fermented within only two days and ends up in the known fermentation products. The study shows that confectionery waste products are well suited for two-stage anaerobic digestion.