Evaluation of volatile fatty acids production and dewaterability of waste activated sludge with different thermo-chemical pretreatments

The effects of three kinds of waste activated sludge (WAS) thermo-chemical pretreatments, including thermo-NaOH, thermo-mixed alkali and thermo-CaO2 pretreatments, on volatile fatty acids (VFAs) production and sludge dewaterability were investigated in a large-scale WAS anaerobic fermentation project. Performances of VFAs production of sludge pretreated with thermo-NaOH (7.48 ± 0.64 g VFAs/L) and thermo-CaO2 (7.91 ± 0.56 g VFAs/L) were proved to be better than that with thermo-mixed alkali (6.93 ± 0.63 g VFAs/L). Sludge pretreated with thermo-CaO2 presented the best dewaterability and the moisture content of the sludge cake could be reduced to as low as 63.4 ± 4.4% after dewatering by plate-and-frame filter press. Conversely, the dewaterability of sludge pretreated with thermo-NaOH was significantly deteriorated and the VFAs were hard to be recovered. Furthermore, the economic feasibility comparison of thermo-NaOH, thermo-mixed alkali and thermo-CaO2 pretreatments for WAS anaerobic fermentation showed that the net profits were 1.55, 34.44 and 38.69 USD/ton dewatered sludge, respectively. Thermo-mixed alkali and thermo-CaO2 pretreatments are promising pretreatment methods for the wide application of WAS anaerobic fermentation. •Different pretreatments on sludge fermentation were studied in a large-scale project.•The recoveries of VFAs by thermo-chemical pretreatments were quantified.•Thermo-CaO2 pretreatment improved VFAs production and sludge dewaterability.•Thermo-mixed alkali and thermo-CaO2 pretreatments were economically feasible.