Effects of Low-Temperature Thermal and Alkaline Methods on the Structural Strength of Sludge Flocs and the Co-Slurrying Ability of Sludge and Coal

Abstract

The great emission of sewage sludge (hereafter referred to as sludge) has become a serious constraint for the sustainable development of cities. As a solution, mixing sludge into coal water slurry is an economical and feasible way to recycle sludge. The application of this method addresses the difficulty of sludge disposal resulting from high water content and facilitates the effective use of the energy contained in sludge. In the present study, a low-temperature thermal-alkaline (LTTA) method was employed to pretreat sludge. The macromolecular organic substances in the sludge flocs were degraded, and the shearing resistance of the sludge was weakened, thereby improving the co-slurrying ability of the sludge and coal. The sludge showed significant degradation within 1 h of the LTTA treatment. The degradation degree of chemical oxygen demand reached 7.87%, 10.92%, and 49.29% under neutral, Ca­(OH)2, and NaOH conditions at 90 °C, respectively. Compared with the original sludge that featured a shearing energy consumption per unit volume (W V ) of 35.40 × 104 J/m3, the treated sludge exhibited a decreased W V , which reached 7.19 × 104 and 3.68 × 104 J/m3 after treatment in 90 °C Ca­(OH)2 and 90 °C NaOH for 1 h, respectively. Consequently, slurry viscosity decreased to 974.9 and 712.5 mPa·s when the sludge was mixed with coal to prepare the coal sludge slurry (CSS). By contrast, the slurry viscosity of CSS made of the original sludge was 1663.6 mPa·s. A low slurry viscosity is conducive to application processes for CSS, including mixing, transportation, and atomization. The LTTA method can be developed using industrial waste heat and low-cost alkali reagents; therefore, its operating cost is low.