Preliminary technical and economic analysis of alkali and thermo-alkali pretratments for the anaerobic digestion of waste activated sludge

Waste activated sludge (WAS) contains various organic and inorganic substances that should be treated for safe disposal. Anaerobic digestion (AD) is the most widely accepted method for stabilizing WAS. This work investigated the technical and economic feasibility of alkali and hybrid thermo-alkali pretreatments for the improvement of WAS anaerobic digestion. Chemicals used for pretreatments were NaOH and Ca(OH)2, with doses in the range of 0.04-0.2 g alkali/g TS. The same substances were used for hybrid pretreatments at low temperature (70 and 90°C). Test outcomes demonstrated that NaOH was a more performing chemical in sludge disintegration and COD liberation than Ca(OH)2. NaOH showed good performances already at low doses (0.08 g NaOH/g TS) with disintegration rates in the order of 20%. The thermal effect improved alkali performance. Biogas yield increased of 13.2% and 26.8% when samples were treated respectively at room temperature and 70°C with 0.04 g NaOH/g TS for 90 minutes. Until nowadays economic issues (mainly the cost of chemicals for alkali pretreatments and pH conditioning) have limited the real-world applications of alkali and hybrid pretreatments. The preliminary economic analysis performed in this work demonstrated that only an increase in the methane yield in the order of 60% could offset the cost of chemicals. On the other hand, if the alkali treatment was coupled to heat, increases in the methane yields in the order of 15-20% were sufficient.