Conversion of vegetable wastes to organic acids in leaching bed reactor: Performance and bacterial community analysis

Effect of pH on volatile fatty acid production and the microbial community during anaerobic digestion of Chinese cabbage waste

The effects of pH on the production of volatile fatty acids (VFAs) and the evolution of microbial community structure were studied via anaerobic fermentation of Chinese cabbage waste. The results showed that the concentration of total VFAs was highest at 20,241.4 mg COD/L at pH 6.0, followed by pH 7.0. Ethanol, acetate and butyrate were dominant under the acidic condition. The main products at pH 7.0 were acetate, propionate, and butyrate. Ethanol, acetate and butyrate were rapidly produced during the initial stage. The hexanoate concentration increased quickly from day 6 due to the chain extension between ethanol and butyrate, and was 4,885.1 mg COD/L on day 8, accounting for 30.4% of the total VFAs. As fermentation was extended, Bacteroidia and Clostridia were dominant at pH 6.0 and the uncontrolled pH, respectively. Clostridium IV, Ruminococcus, and Candida, were suspected to be related to hexanoate production.

Effect of different vegetable wastes on the performance of volatile fatty acids production by anaerobic fermentation

Volatile fatty acids (VFAs) are intermediates of anaerobic fermentation with high value and wide range of usage. VFA production from vegetable wastes (VW) is an effective way to dispose of wastes and recover resources. The organic matter composition of the substrate influences VFA yield and distribution, which is related to the separation and purification of the downstream steps and the application of the product. Hence, potato peels, carrots, celery, and Chinese cabbage were selected to investigate the effect of VW types on the performance of the VFA production in a batch anaerobic fermentation reactor with continuous stirring at 37 °C, total solid (TS) of 4.5%. A VFA yield of 452 mg COD/g VS_feed (chemical oxygen demand (COD); volatile solids (VS)) was achieved from potato peels, which was 40.1%, 21.5%, and 124.9% higher than that of carrots, celery, and Chinese cabbage, respectively. The rapid acidification of carrots caused a sharp decline in pH and led to inhibition of VFA production. The acidification of celery started slowly, and the yield of hexanoic acid increased rapidly in the later stage of fermentation. The VFA yield of Chinese cabbage was inhibited due to the low initial pH, but the ethanol concentration reached 7577.04 mg COD/L. According to the VFA profile, the fermentation of potato peels, carrots, celery, and Chinese cabbage can be classified as propionate-type, butyrate-type, mixed-acid type, and ethanol-acetate type metabolic pathway, respectively. The results of this study suggest that a suitable combination of vegetable waste types is important for selective VFA production.

Production of volatile fatty acids and H2 for different ratio of inoculum to kitchen waste

The aim of this study was to evaluate the effect of different inoculum ratio on the dark fermentation of kitchen waste in terms of volatile fatty acids (VFAs) and H₂ production. The experiments were performed in batch bioreactors of effective volume 1 L without pH regulation. The ratio between the DS and KW was being increased from 0.11 to 0.51 on a volatile solids (VS) basis, while the initial content of KW was equal to 34.1 g VS/L. Increase of the DS/KW ratio from 0.11 to 0.28 resulted in the rise of VFAs and H₂ production. Further increase in the amount of added DS did not cause a significant change in the production of VFAs and H₂. In the bioreactor with the DS/KW ratio of 0.28, the production of VFAs and H₂ was equal to 16.0 g/L and 68.1 mL/g VS, respectively. Acetic and butyric acids were produced in the largest amount and their content, for DS/KW ratio of 0.28, were equal 37% and 43%, respectively. At the ratio of DS/KW above 0.4, the caproic acid content attained the level of 25%. Based on the DS and KW microbiological analysis, it was observed that dominant bacteria were Bacteroidetes, Firmicutes, Proteobacteria, Spirochaetes and WWE1 at the phylum level.

Fruits and vegetable-processing waste: a case study in two markets at Rio de Janeiro, RJ, Brazil

Fruits and vegetables (FV) consumed in natura or processed produce a significant volume of waste, causing an economic deficit in the productive chain. FV markets feature a significant production of vegetable residues with potential of use, since they commercialize an increasing amount of minimally processed vegetables and fruit juices. To this end, it is important to identify, quantify, and characterize these wastes and to propose feasible and coherent alternatives for their use at regional and worldwide levels. In this paper, a case study of two FV markets in Rio de Janeiro, Brazil, was conducted to identify and quantify FV processing waste. Over a period of 20 days, the FV residues from 31 vegetables and 17 fruits were identified and weighed. It is estimated by extrapolation that 106,000 kg of FV were processed in 1 year in two units of FV markets and 48.6% of FV were discarded as by-products. This may be a consequence of factors that contribute to waste generation, such as the low preparation and/or training of the manipulators as well as the quality of the equipment and the maintenance thereof. Thus, studies that aim to understand the environmental impact by monitoring the of FV waste are fundamental, since this waste can be used as raw material and converted into value-added products.