Mechanism of red mud combined with steel slag conditioning for sewage sludge dewatering

Red mud (RM), which is a byproduct of alumina refining, is typically stored in landfills. This study investigated the capability of RM combined with steel slag to improve sludge dewatering performance. When the RM dosage was 20 mg/g, the HCl/RM ratio was 0.6, and the steel slag dosage was 3 mg/g DS, RM combined with steel slag exhibited a good conditioning capability as the pH of the filtrate approached neutrality. This result indicates that the coagulation of RM by Fe3+ and Al3+ is critical in sludge conditioning and that the steel slag acts as a neutralizer and a skeleton builder. Thus, four mechanisms of RM and steel slag were proposed for sewage sludge conditioning. First, Fe3+ and Al3+ generated from RM acted as coagulants that agglomerated small sludge particles into large dense particles with low bound water. Second, extracellular polymeric substances (EPS) were degraded into proteins by RM. Third, the released proteins were destroyed by the steel slag. Last, the bound water was converted into free water due to EPS degradation.