Primary, secondary and blended sludges were collected from the municipal WWTP in Reus (Tarragona, Spain) with a capacity to daily process 25,000 m3of wastewater. Fig.1a shows a schematic diagram of the WWTP,illustrating the step where these different types of sludges
are generated. Primary sludge was collected after partial gravity thickening. Secondary sludge, produced by an activated sludge process,was collected after partial thickening by flotation. Blended sludge was collected after the combination of primary and secondary at a ratio of
65:35, v/v. The collected sludges were immediately stored at 4 °C prior to use. Because the sludge properties could be changed during long storage time, fresh sludge was always used for each experiment.The inoculum used in anaerobic digestion tests was sludge collected from a mesophilic anaerobic digester in the same facility.