Increase in biogas production in anaerobic sludge digestion by combining aerobic hydrolysis and addition of metallic wastes

The objective of this work was to determine the effect of a controlled micro-aeration as a pretreatment or hydrolytic stage of mixed sewage sludge and the incorporation of solid wastes as a source of trace metals in the anaerobic digestion of this pretreated sludge. Three experimental runs were carried out under the same conditions in laboratory-scale anaerobic reactors, to which a previously aerated mixed sludge was added as a substrate and anaerobic sludge as the inoculum. Two anaerobic digesters (blank) were also operated without aerobic pretreatment and without the addition of metallic wastes. The aerobic pretreatment was performed during 48 h at 35 °C with an aeration flow of 0.35 vvm. All anaerobic reactors were operated at the mesophilic temperature of 35 ± 2 °C. Fly ash or Copper mining residues were added to the anaerobic reactors as trace metal supplementation. The aggregated concentrations were 250 mg L−1 fly ash, 25 mg L−1 Copper mining residues and 0 mg/L. The blank reactors produced 38% less methane than those generated in the reactors operating with the pre-aerobic treatment without addition of metallic wastes (controls). It was found that the reactors with micro-aerobic pretreated sludge and the addition of fly ash gave the best yields of methane, producing a 201.6% increase in methane with respect to the blank reactors. On the other hand, the pretreatment of micro-aerobic hydrolysis and the addition of mining residues generated an increase of 185.8% in methane production compared to the blank reactors.