Critical moisture point of sludge and its link to vapour sorption and dewatering

The mechanical dewatering of sludge is important in order to achieve a high dry matter content, thereby lowering the transportation cost and the energy consumption during incineration. Thermogravimetric analysis is sometimes used to estimate the maximum dry matter content obtainable from mechanical dewatering, by measuring the critical moisture point. In this paper, the critical moisture point of digested sludge was measured and compared with vapour sorption curves. The critical moisture point was determined for raw and conditioned sludge to be 3.4 kg of water per kg of dry matter, corresponding to 23% w/w dry matter. This value was lower than the dry matter content obtained from the mechanical dewatering process, indicating that the dry matter content can exceed the critical moisture point. Moisture vapour sorption was measured for raw, conditioned, and dewatered sludges. The Blahovec and Yanniotis sorption isotherm fitted the experimental data well. Between 10 and 12 g of water was adsorbed as a monolayer per 100 g of dry matter. The rest of the moisture content was explained by the non-ideal Raoult's law, by including the effect of dissolved ions. At water activities above 0.95, the moisture content was determined by capillary condensation and cake compressibility. The water activity was higher than 0.95  at the critical moisture point and the capillary pressure was estimated to be 4-6 bars. This pressure was responsible for cake compression during drying, while the relatively low dry matter content at the critical moisture point may be due to the low capillary pressure.