Anaerobic processes

Anaerobic digestion is one of the most promising technologies for disposing of OMWW as it is a process for both decontaminating and valorizing (by producing methane) such wastewaters. Furthermore, this technology produces low amounts of waste sludges and employs bacteria with very low decay rates, which allows the process to be carried out in seasonal operations without requiring to be fed with OMWW all through the year. The process is generally carried out in conventional contact bioreactors; these however are often unable to efficiently remove OMWW phenolic compounds, which therefore occur in the effluents. Bertin et al. (2004) explored the possibility of mitigating this problem by employing an anaerobic OMWW-digesting microbial consortium passively immobilized in column reactors packed with granular activated carbon (GAC) or 'Manville' silica beads (SB). Under batch conditions, both GAC- and SB-packed-bed biofilm reactors exhibited OMWW COD and phenolic compound removal efficiencies markedly higher (from 60% to 250%) than those attained in a parallel anaerobic dispersed growth reactor developed with the same inoculum. The GAC-reactor exhibited increased COD and phenolic compound depletion yields (by 62% and 78%, respectively) compared with those achieved using the identically configured SB-biofilm reactor. Both biofilm reactors also mediated an extensive OMWW remediation under continuous conditions, where the GAC-reactor was much more effective than the corresponding SB-reactor and showed a tolerance to high and variable organic loads along with a volumetric productivity, in terms of COD and phenolic compound removal, that was significantly higher than those averagely displayed by most of the conventional and packed-bed laboratory-scale reactors previously proposed for OMWW digestion. However, anaerobic digestion is not always a satisfactory bioremediation process, in particular because of its inability to efficiently biodegrade low molecular weight phenolic compounds (tannins, lipids and phenolic compounds themselves exert inhibitory effects on the microflora carrying out the process, which therefore tend to persist in the digestors and to occur in the digestor effluents).

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