The management of sewage sludge originated from municipal wastewater treatment plants (WWTPs) is an urgent issue. In 2019, the local authority of the Piemonte region started a survey with the aim of collecting recent data concerning wastewater and sludge management in the WWTPs located in its own territory. The survey’s results revealed that 60% of the sludge (51,000 t, as dry substance, d.s.) produced by the local WWTPs was recovered or disposed of outside of the region, and a similar amount of sludge was recovered in agriculture directly or after composting. The increase in the costs to accommodate sewage sludge in recovery or disposal plants, followed to a recent Italian Sentence (27958/2017), and the more and more stringent requirements fixed by lots of European countries for the application of sludge in agriculture, are pushing the Piemonte region authority to re-organize its own network for sludge management, with solutions based onto proximity and diversification. Whether the provisions of the current German legislation are applied in the future also in Italy, approx. 90% of sewage sludge produced into the Piemonte region should be incinerated, with a subsequent step of phosphorous recovery. The new regional plan, according to the Regional Address Deed, should consider a diversification of sludge treatment and recovery practices. On this basis, a need for new plants for around 40,000 t d.s./y could be planned.

Waste activated sludge (WAS) is commonly treated by using anaerobic digestion (AD), which has the advantage of reducing sludge volumes and generating methane. However, the sludge structure limits the efficiency of these AD processes. In fact, most of the organic substance found in sludge is surrounded by cell walls or bound in complex structures [extracellular polymeric substance (EPS)] and can only be made ready for digestion after intense hydrolysis processes. This paper presents the preliminary results of tests carried out to compare the performances of pretreatments and intermediate treatments. They were both performed using a hybrid modality, that is, by combining an alkali agent and heat. Hybrid treatments at low doses [4–8% of the total soid (TS) matrix] of NaOH and $\mathrm{Ca}{(\mathrm{OH})}_2$ were carried out on WAS and on already digested sludge. Batch mesophilic digestion tests were used to assess the methane specific production of samples of WAS and digestate before and after lysis treatment. Digestibility tests revealed a specific methane yield of respectively 0.166 and $0.143{\mathrm{Nm}}^3/\mathrm{kg}{\mathrm{VS}}_{\text{added}}$ for raw WAS and digestate. The production of methane for the WAS sample increased by 40 and 66% when it was treated at 70 and 90°C, respectively, in combination with a NaOH dose of $4\mathrm{g}/100\mathrm{g}$ TS. Intermediate treatments carried out under the same operating conditions caused increases in the specific production of methane of 31% (70°C) and 54% (90°C), respectively, for the samples of already digested sludge. Using these results, it was calculated that if the digestion process was carried out in two steps, with a duration of 10 days each, the production of methane could increase by 27 and 17% (at 70 and 90°C), respectively, compared with the scenario that only used the pretreatment processes.