Highlights

• Supercritical CO₂ extraction was proven to efficiently dry municipal sludge.
• SCO₂ removes the free and interstitial water with water loss up to 65% in 15 min.
• HHV of treated sludge was higher than brown coal at same moisture level.
• SCO₂ treated sludge can be compliant to environmental regulations.

Abstract

Eighty percent of the world’s wastewater is discharged back into nature without reuse and further treatment and solid byproduct of wastewater is either spread on land or landfilled. The valorization of municipal sewage sludge for energy use is a major opportunity for sustainable energy applications. This includes effective dewatering and drying of the sludge in addition to producing biogas during the anaerobic digestion. Supercritical CO 2 extraction process was investigated for the purpose of drying municipal sewage sludge. Remarkably our results showed that after sCO2 processing the higher heating value (HHV) of sludge at wet basis moisture content of 52% was 14 MJ/kg and thus higher than the HHV of brown coal with similar moisture level (12 MJ/kg). Our research also shows that under specific conditions sCO2 extraction reduces the moisture content of the municipal sludge from 89% to 53% in 15 min. Although, more research is needed this dewatering process could be considerably less energy intensive than conventional thermal and other drying processes. Another benefit is extraction of micro pollutants by sCO₂ treatment and thus making the dried sludge more useable in energy applications as per regulations. Our results show an alternative and novel pathway for highly effective dewatering and drying of municipal and other types of sludge.

This article appeared on the Journal of Environmental Management website at https://www.sciencedirect.com/science/article/pii/S0301479721002103