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

Waste activated sludge (WAS) contains various organic and inorganic substances that should be treated for a safe disposal. Anaerobic digestion (AD) is the most widely accepted method for stabilizing WAS. This work presents preliminary results concerning 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%. Thermal effect improved alkali performance, in fact the biogas yield increased of 13.2% and 26.8% when samples were treated respectively at 20°C and 70°C with 0.04 g NaOH/g TS for 90 minutes. However, 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, if the pretreatment was carried out with a dose of alkali of 0.08 g NaOH/g TS, 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 dose was of only 0.04 g NaOH/g TS, increases in the methane yields in the order of 15-20% were sufficient.