Biogas production from pear residues using sludge from a wastewater treatment plant digester. Influence of the feed delivery procedure

Clear economic advantages may be obtained from the management of seasonal fruit wastes by codigestion at existing facilities which are working throughout the year with other residues.We have explored the biomethanization of pear residues in a 5 L stirred reactor loaded with sludge from the anaerobic digester of a municipal wastewater treatment plant. Different organic loading rates (OLRs) of fruit waste were tested with two delivery procedures: a discontinuous one (fed once a day) and a pseudocontinuous one. For both procedures, as the OLR increases the pH of the digester drops to acidic values and large OLRs may cause the reactor failure. Nevertheless, the pseudocontinuous delivery allows the treatment of more residue, (10.5 versus 6.0 g of volatile solids per litre of reactor and day), maintaining the specific biogas production (0.44 L of biogas per gram of volatile solids), with some improvement in methane concentration (44% vs 39%).

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► Digestion of pear residues at wastewater treatment plant digesters is feasible.
► The organic loading rate (OLR) is the critical parameter for the biogas production.
► Important differences are obtained from the two feed delivery procedures tested.
► Monitoring of pH and redox potential values allows the prediction of failures.