Innovative two-steps thermo-chemical pretreatment for sludge reduction and energy recovery: cost and energy assessment

Chemical and thermal pretreatments of sewage sludge have proven to be effective to improve anaerobic digestion process by increasing methane production and sludge stabilization. In order to improve this technology towards sludge reduction combined with energy recovery, lab-scale experimentation was carried out to assess the efficiency of a 2-steps acid-alkali innovative pretreatment to enhance the anaerobic digestion process. Lab-scale results suggested to adopt some measures intended to ensure better anaerobic performances and imbalance risks prevention. The process allowed to achieve high sludge reduction efficiencies (up to 66% TSS removal), and to obtain high methane production yields (0.25 vs. 0.18 Nm/kgVS of the control) assuring high process stability. The electric energy recovery of the combined hydrolysis-digestion process, assessed on full-scale scenario, resulted higher with respect to a conventional digestion process. The sludge reduction obtained could imply cost savings of up to 32% compared to traditional scenarios.