Thermal alkaline pretreatment (TAP) of waste activated sludge (WAS) before anaerobic digestion (AD) was reviewed. Focus of the review was on impact of TAP process parameters on biomethane yield (BY) and kinetics of AD and downstream dewatering. With higher initial biodegradability of untreated WAS, effect of TAP on BY decreases. Depending on initial biodegradability, BY increase of 22-97% is expected. Treatment temperatures below 100 °C showed to be as effective as temperatures higher than 100 °C in terms of BY increase. Alkali dosage and resulting initial pH have a significant effect on BY increase and showed to have an optimum range of 40-60 mg NaOH per g total solids (TS) of sludge. It is advised that alkali is dosed based on solids content in WAS and monitored by pH. Treatment time of 1.5-5 h is sufficient for an effective low temperature TAP (T < 100 °C), with longer treatment times showing no positive impact on BY increase. Load of sludge liquor with organics and nutrients increases with more intensive TAP conditions. Despite kinetic enhancement of hydrolysis step in AD, more research is needed to clarify if TAP improves kinetics of entire AD process which determines required digester volume. Impact of TAP on dewaterability of digestate is ambiguous and needs more investigation using standardized methods, also with regards to potential effects on polymer demand. Findings of experimental studies were reflected against available data from commercialized TAP process of Pondus®, throughout review. Finally, important process design parameters of TAP such as input TS and point of alkali dosage are discussed and recommendations for future research are presented.
Keywords: Anaerobic digestion; Biodegradability; Dewatering; Pondus; Thermal hydrolysis; Wastewater treatment plant.
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