This study investigated the effect of pH buffer and solids retention time (SRT) on the anaerobic fermentation of vegetable and salad waste (VSW). Experiments were carried out in batch and semi-continuous reactors at 35 °C. In the batch experiments, the effect of pH buffer on the production of volatile fatty acids (VFAs) and ethanol was investigated. Acetate and butyrate were the main fermentation products. The maximum total product concentration was 43.3 and 18.5 g COD l-1 in the buffered and unbuffered batch reactors resulting in a yield of 62 and 27% (CODtotal product/CODfeed) respectively. Volatile suspended solids (VSS) removal was higher in the buffered semi-continuous reactor (57%) compared to the unbuffered-acidic reactor (39%), but similar yields (15%, CODtotal product/CODfeed) were observed because biogas production was stimulated in the buffered reactor. The effect of SRT on the VSS removal and product distribution in unbuffered systems was investigated at 10, 20 and 30 days SRT. The VSS removal increased as the SRT increased, ranging between 18.2 – 49.1%, likewise the total product concentration, 9.1 - 19.4 g COD l-1, and product yield, 7 – 24% (CODtotal product/CODfeed). Acetate and butyrate were the prevalent fermentation products at all conditions followed by caproate although caproate was only detected at 20 and 30 days SRT. Total COD removal ranged between 15.2 and 35.1% with the highest removal observed at 30 days SRT.
Bibliographical noteAuthors acknowledge the financial support of ADNet (Business Interaction Voucher, BIV2015007) and thank Dr Gavin Milligan, William Jackson Food Group, for the discussion of the research and the experimental data. The authors are also thankful to Mrs Liz Hendrie for her technical assistance.
- volatile fatty acid (VFA)
- mixed culture fermentation
- food waste