Comparative mass and energy evaluation of pyrolysis and hydrothermal carbonization of dairy processing sludges: Effect of dissolved air flotation sludge inclusion

Hydrothermal carbonization (HTC) and low-temperature pyrolysis (LTP) were carried out on two dairy processing sludge (DPS) samples from WWTPs in milk factories: waste-activated sludge and a mixture of waste-activated sludge with dissolved air-flotation sludge (DAF) (containing fat, oil and grease). Both thermal treatments were conducted at three temperatures – 180, 200 and 220 °C for HTC; 350, 450 and 550 °C for pyrolysis. Mass balances were calculated, and energy recovery analyses from gas and liquid products only were undertaken with the assumption that biochars and hydrochars are not used for energy recovery. The cooled pyrolysis gases were analysed for composition, HTC liquids from waste-activated sludge were tested for biomethane potential. Theoretical energy calculations for coupled drying and pyrolysis of DPS showed that the process has the potential to be self-sustaining when DAF is incorporated into the feedstock. Depending on feedstock and processing conditions, the energy transferred into the hot volatiles comprises 22% to 126% of the energy required for pyrolysis. More energy-dense gases and volatiles are produced from the decomposition of the fat-rich sludge. HTC will not provide enough energy to sustain itself; however, it remains less energy-demanding than combined drying and pyrolysis and may potentially be used for dewatering. Anaerobic digestion of HTC liquids from waste-activated sludge does not provide a high biomethane yield but could be considered for anaerobic digestion as a co-substrate.