Kinetics and drainage index in function of pH, in the dewatering of arsenic iron sludge

Water for human consumption containing arsenic from natural and anthropogenic sources is a public health problem worldwide. Therefore, different technologies must be used to remove it from the water (coagulation-filtration assisted with ferric chloride, adsorption, membranes, etc.). While these technologies produce water that is free from arsenic, they also produce toxic residuals with high arsenic concentrations, which must be treated in order to decrease their volume and thereby facilitate transport and final disposal. Thus, the main purpose of this investigation was to study the physical and chemical properties of arsenic iron sludge in thickening, chemical conditioning, and dewatering processes, as well as to propose new kinetic criteria for obtaining the drainage index (Eg) based on polymer dose, mesh permeability, specific resistance to filtration, and pH. We found a significant improvement in the physical and chemical properties when thickening the sludge, in particular, floc size increased and specific resistance to filtration and Z-potential decreased due to weakened repulsive forces, resulting in more sedimentation. The polymer AN913VHM (PF2) had the best behavior in the chemical conditioning and dewatering tests, with an optimal dose of 8 mg/L and a mesh permeability of 80–100 ft3/min ft2, which made it possible to retain more flocs (60% in 3.74 min). The pH affected the amount of total solids in the cake, with more solids obtained at a pH of 6 (25.93 g/L) and 6.5 (21.81 g/L), and with rapid drainage of surface water (69.28%). Furthermore, new kinetic criteria were obtained for a drainage time of 120 s in order to eliminate 60% of the total volume of the filtrate mass, with Eg of 3.05 at a pH of 6. This value is considered medium drainage for this type of sludge, which is difficult to treat. Study of the physical and chemical properties and the kinetics of drainage according to the pH of the iron sludge with arsenic from the water treatment that contains arsenic. [Display omitted] •The amount of the As in the iron sludge was 371.54 mg As/kg, which are above the Mexican standards.•The use of high molecular polymers in chemical conditioning improves the dewatering.•The best dewatering conditions were obtained for a pH of 6.•It has been obtained new kinetics criteria for the sludge dewatering: time 120 s to drain 60% of water.