Mill scale as a ballasted flocculant for enhancing the settleability of activated sludge

One of the common biological methods in wastewater treatment is the activated sludge process. The efficiency of this process highly depends on the aggregation of microbes to form flocs that settle in the settling tank. The settleability of the activated sludge is commonly influenced by the biomass diversity, concentration, density and the size distribution of flocs. This study introduces mill scale as a ballasted flocculant (BF). The behavior of activated sludge is studied after the addition of BF, with the objective to improve the aggregation of activated sludge particles and enhance its settleability. The efficiency of BF is evaluated by varying its dosage (mill scale to sludge weight ratio = 0.2–2) and particle size (<45 and 45–75 μm). The zone settling velocity (ZSV) was 4.58 m h⁻¹ for original sludge that was increased to 8 m h⁻¹ when BF was added into the sludge. The microscopic analysis of the ballasted flocs indicated that the BF particles were finely embedded into the sludge particles. A significant increase in floc size was observed after addition of BF. The Vesilind, Cho and Takacs settling velocity models were applied to explain the relationship between settleability characteristics of the activated sludge. The experimental data was well fitted in Vesilind’s exponential model. Followed by sedimentation, the BF particles were finely recovered via magnetic separation technique. The maximum BF recovery is estimated 86 %, when 4 g of BF is used.