Use of iron oxide particles as adsorbents to enhance phosphorus removal from secondary wastewater effluent

Potential use of iron oxide particles (IOPs) as adsorbents for removal of residual phosphorus from the secondary effluent discharged from activated sludge treatment was investigated under various operating conditions. Substantial phosphorus removal was achieved with IOPs including ferrihydrite, goethite, and hematite, though the adsorption capability for phosphorus depended on the types of IOPs tested. Adsorption isotherm tests and SEM images helped to understand the behavior of the adsorptive properties of phosphorus on different IOPs. Phosphorus removal efficiencies for secondary effluent remained constant or further increased over the wide pH range of below neutral to alkaline values, in contrast to those for a synthetic phosphorus solution. This could be in close association with the interaction of phosphorus with Ca and Mg ions present in secondary effluent leading to chemical precipitation, such as apatite, dolomite, calcite, and brucite. The reactivity of IOPs with phosphorus at neutral pH appeared to be almost independent of the existence of background inorganic species, such as inert salts, alkalinity, and hardness. The phosphorus-removal efficiency was compared during regeneration and recovery of used IOPs using centrifugation and microfiltration (MF) methods. MF following lOP adsorption was found to be efficient enough to accomplish significant phosphorus removal during continuous adsorption/regeneration runs, suggesting that the combination of IOP adsorption with MF would be attractive as a tertiary treatment alternative.