Pretreatments and intermediate hybrid treatments for the improvement of the anaerobic digestion of sewage sludge: preliminary results

Waste activated sludge (WAS) is commonly treated by using anaerobic digestion (AD), which has the advantage of reducing sludge volumes and generating methane. However, 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 (EPS, extracellular polymeric substance) 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 pre- 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 TS matrix) of NaOH and Ca(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 Nm3/kgVSadded for raw WAS and digestate. The production of methane for the WAS sample increased by 40% and 66% when it was treated at 70°C and 90°C respectively in combination with a NaOH dose of 4 g/100 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°C and 90°C) respectively, compared with the scenario that only used the pre-treatment processes.