Sludge-recycling enhanced flocculation (SEF) is an effective method for enhancing flocculation. In the conventional SEF process, the settled sludge is recirculated into the flocculation process without any further treatment. However, studies have shown that the efficacy of the SEF process could be improved by pre-treating the sludge. In this work, the acid activation of sludge was performed using a range of pH values (1.0 – 6.0) and charge states, with and without long-chain bridging. The...