Abstract
Sludge acidification is an efficient pretreatment method for achieving effective dewatering. In the specific case of water treatment plant sludge (WTP), which uses some type of ion-based coagulant (e.g. aluminum and iron), these ions can be recovered and intended for a purification step, that will enable their reuse in specific treatments, significantly decreasing waste disposal and chemicals used for water treatment. This study developed a system in pilot-scale for the treatment of sludge generated in a WTP with the combined application of two polymers (dual flocculation) and an acidifier, aiming at the thickening and recovery of metal ions from the coagulant. The sludge used in this study was sampled in settling basins of a conventional WTP and presented the solids concentration (TS) of 5 g·L−1, due to the dilution with tap water. The first part of the study compared the addition of polymers (one cationic and one anionic polymer) in natural and acidified conditions by sulfuric acid (H2SO4) through continuous treatment. The flocculation step was conducted in the helical tubular flocculator (HTF), also responsible for sulfuric acid mixing. It was observed that the decrease in pH made the aggregation easier, contributing to the lower volume of polymers added under mixing conditions provided in the (HTF). The suggested approach for acid solubilization of aluminum ions provided 87% removal results in sludge samples with an initial concentration of 510 mg·L−1, from a total TS of 5 g·L−1 and pH 2. This recovery rate was obtained with the flow rate of 3 L·min−1 (G: 507 s−1) and hydraulic detention time (t) of 24 s. Then, in studies of filtration thickening, in a built static thickener, and by means of sedimentation, we observed the contribution of the pH reduction in the water release in the flake mass formed. In addition, there was a significant decrease in specific filtration resistance (SFR) due to the release of aluminum ions present in the sludge, making the material more easily dehydrable. The study showed that decreasing from pH 6.5 to pH 2 generated a 50% decrease in SFR. This reduction in pH further provided a thickened with 20% solids sludge (from an initial content of 0,5%) obtained through filtration of the mass of the flakes through for a static thickener.
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