Resource recovery from co-digestion of organic waste with surplus activated sludge via the carboxylate platform

Waste activated sludge (WAS) is an important residue generated from Wastewater Treatment Plants (WWTPs) with a high amount of organic and inorganic resources. In view of this, WAS management systems have changed towards improving the use of waste biomass as a feedstock for bioenergy generation and nutrient recovery and reuse. This study assessed the potential of using WAS as the main feedstock for the generation of high-value chemicals like volatile fatty acids (VFAs), via the carboxylate platform. In order to achieve that, a series of experiments were conducted with the aim to identify the main process variables controlling VFA production in batch and semi-continuous stirred tank reactors (CSTRs). In the first stage, acidogenic fermentations were run for 21 days using iodoform as an inhibitor of methanogenic bacteria, reaching VFAs yields of 0.238 g TVFAs/g TVSWAS with iodoform (CHI3) in a ratio of 6 mg CHI3/g VSS and an Organic Loading Rate (OLR) of 5 g TVSWAS/L. The second stage comprised the acidogenic fermentation of high pressure thermal hydrolysis (HPTH)-WAS under different pH conditions (4-1) with results of 0.415 g VFAs/g TVS at pH 9.0 and C/N=8.77, which emphasize the strong effect that pH has on VFA production and speciation and, on the inhibition of methane (CH4) generation. In order to improve VFAs production from HPTH-WAS, acidogenic co-fermentations at pH 9.0 were conducted using thermally pre-treated food waste and algal biomass (Chlorella vulgaris). Optimum results reported a yield of 0.496 g VFAs/g TVS at C/N=12.72 for fermentations using a blend of 25% HPTH-WAS/75% HPTH-Food waste and 25% HPTH-WAS/75% HPTH-Chlorella vulgaris with VFA yields of 0.378 g VFAs/g TVS, C/N=5.08. This suggests that HPTH pre-treatment and co-fermentation had a positive effect on the final production of VFAs despite of the C/N ratio used. Finally, experiments using semi-CSTR reactors fed with HPTH-WAS at pH 9.0 reported yields of 0.539, 0.328 and 0.364 g VFAs/g TVS for fermentors with OLRs of 0.3, 0.6 and 1.0 g TVS WAS/L·d, respectively. This suggests that increments in OLR have a null effect on VFAs production. Fermentations working with 0.3 g TVS WAS/L·d presented overall VFAs production which stoichiometrically exceeds in 31% the methane produced in AD experiments ran in this project. The OLR presented a null effect on the speciation of the VFAs as acetic acid was present in concentrations above 80% of the carboxylic acids content in all CSTR experiments. These results confirm the potential opportunities for high-value chemicals production from HPTH-WAS as part of the development of the biorefinery concept in existing WWTPs.