Phosphogypsum Leachate: Treatment Feasibility in a Membrane Plant

In landfills for phosphogypsum disposal, a large volume of leachate (PGL) wastewater generated by the residual process water and by the rainfall percolation through the solid material must be managed. The leachate shall be properly treated before its discharge into surface water or its reuse; it has a very low pH and high concentrations of hazardous contaminants such as fluoride, nitrate, phosphate, ammonia, and heavy metals. On the basis of a technical evaluation, a membrane filtration process for the treatment of the leachate was proposed to produce a very good quality permeate, in compliance with the standard limit (CEE 91/271 directive). A membrane pilot plant, composed of an initial nanofiltration (NF) treatment and two reverse osmosis membrane stages placed in series (RO1 and RO2), was employed to produce a permeate that can be discharged or reused. A first campaign was carried out for comparison using the membrane pilot plant in the basic configuration; in the NF step, a recovery of 58% was obtained with high operative pressure and frequent membrane chemical cleanings; at the same time, a maximum overall recovery of 37% was obtained. In a second campaign, a coagulation-precipitation process with lime was introduced as a PGL pretreatment for the membrane plant feeding. Under these conditions, NF and overall recovery increased respectively up to 66% and 43%; furthermore, lower operative pressures were required ensuring more feasible conditions. The results confirmed the expected advantages obtained in a previous experimental study conducted at laboratory scale. In addition, the mass load to be reinjected in the stack was reduced by about 70-95% with a consequent reduction of the injection wells occlusion problems due to the supersaturated conditions of the concentrate. The sludge produced in the process (45 kg/m3 at 45 TS%) has a composition similar to the phosphogypsum present in the stack and can be therefore disposed onsite.