Enhancing low-temperature isothermal convective drying of waste municipal sewage sludge with wood-derived biochar in sequential drying cycles

Municipal sewage sludge (MSS) contains a high amount of water, that lowers the calorific value and prevents self-sustaining incineration, making drying critical. This study evaluates softwood-chip biochar (BC) as a structural additive to enhance isothermal convective drying of MSS under natural convection. MSS and MSS/BC blends (1:1, 2:1, 3:1 by mass) were dried as thin layers at 25, 30, 35, and 40 °C to a constant mass. The performance was assessed from the mass balances and endpoint times, and repeated cycles were used to identify an operationally optimal number of drying cycles. BC accelerated drying at all temperatures. At 25 °C, the 1:1 blend delivered the largest improvement, up to ∼ 430% higher drying efficiency (11 h to 1.9 h) relative to MSS; the 2:1 blend showed intermediate gains, and the 3:1 blend modest gains. Diminishing returns across the cycles yielded simple strategies: three cycles for 1:1 and 2:1, and one cycle for 3:1 at 25 °C. The higher-temperature tests confirmed trends while avoiding unnecessary cycling. The fastest run (1:1 at 40 °C) finished in a little over one hour, whereas MSS at 25 °C required ∼ 11 h. A moderate BC dosage combined with a small, well-selected number of cycles can shorten drying substantially under mild conditions, and integrates readily with the existing low-temperature drying units at wastewater treatment plants, reducing time, energy input, and operational complexity without equipment modification or additional chemical conditioning.