Evaluating the thermal regeneration process of massively generated granular activated carbons for their reuse in wastewater treatments plants

Abstract

Wastewater treatment plants (WWTPs) use granular activated carbons (GACs) as odor retainers because of their high surface area and porosity. The volatile compounds retained in these GAC beds saturate their pores and are periodically replaced by pristine GACs, which are frequently made from biomass materials such as coconut shell. Here, in order to recycle these exhausted GACs and reuse them again as adsorbents in a WWTP, a thermal regeneration of these wastes is proposed. Aiming to achieve more efficient and cleaner production, the pros and cons of using an oxidizing versus an inert atmosphere are evaluated. This work demonstrates that a simple thermal process at temperatures no higher than 350 °C for 1 h using an oxidizing atmosphere can achieve the regeneration of exhausted GAC with the appropriate characteristics for reuse as an adsorbent for gaseous emissions in a WWTP, evidencing a lower cost against a regenerative process in an inert atmosphere (about 20% lower in an oxidizing atmosphere). Their specific surface area and micropore volume values are around 475 m2/g and 0.264 cm3/g, respectively, which are in the range of characteristics of pristine GAC (406 m2/g and 0.229 cm3/g). The best regeneration yield was 96.8% for the SL-300-N2 sample. Interestingly, a regeneration at 900 °C using an inert atmosphere allows for the production of GACs with optimized textural properties (SBET ≈ 675 m2/g), a dual system of micro/mesopores (Vt ≈ 0.4 cm3/g and Vmicro ≈ 0.27 cm3/g), and also a regeneration efficiency of 90.5% for SL-900-N2, which would make it a raw material of interest for applications with more stringent requirements.