Session 6

Phosphorus removal and recovery from wastewater effluents by nanofiltration
Oded Nir1, Daniel Collignon2, Sengpiel Robert2, Wessling Matthias2,3
1Zuckerberg Inst. for Water Research, Blaustein Inst. for Desert Research, Ben-Gurion Univ. of the Negev, Israel; 2Chemical Process Engineering, RWTH Aachen University, Forckenbeckstrasse 51, 52064 Aachen, Germany; 3DWI-Leibniz-Institute for Interactive Materials, Forckenbeckstrasse 50, 52056 Aachen, Germany

New regulations in many developed countries call for a significant reduction in phosphorus concentration for effluents released to the environment. Meanwhile, recovery of phosphorus, a non-renewable fertilizer, from wastewater is widely recognized as crucial for maintaining future food security.
Most work on phosphorus recovery focused on concentrated waste streams, e.g. sludge-liquor. Nevertheless, diluted solutions such as wastewater effluents often contains a significant portion of the total phosphorus mass.
Here we present a new approach for the removal and recovery of phosphorus from tertiary effluents using nanofiltration membranes characterized by high phosphate rejection and acid resistance. During filtration phosphate is concentrated in the reject, inducing the precipitation of calcium-phosphate which can be further crystallized and recovered downstream. Ca-P crystals accumulated on the membrane are cleaned by strong acid without deteriorating the acid-durable membrane.
Simulated and real effluents were treated by a commercial membrane and membranes self-made via layer-by-layer polyelectrolytes deposition on porous hollow-fibers. Flux and rejections were recorded. The effect of Ca-P and CaCO3 precipitation was studied by filtering supersaturated synthetic effluents.
Phosphorus and NaCl rejections for the commercial membrane were >99% and 70-90% respectively, demonstrating its suitability to both P removal&recovery and salinity reduction (required for different water reuse schemes). Contrarily, rejections for selected polyelectrolyte-membranes were >90% for P and <10% for NaCl, enabling operation at lower trans-membrane pressure (3-4 bars), thus reducing energy consumption. While CaCO3 precipitation resulted in severe surface scaling, calcium-phosphate precipitated primarily in the bulk, forming colloids which were mostly driven out of the membrane module by the tangential flow. Using thermochemical modeling we showed that Ca-P precipitation could be favored over CaCO3 precipitation via feed and/or inter-stage pH adjustments. Acid-cleaning completely restored the membrane performance. The results demonstrated the potential of the suggested approach within the vital effort of closing the anthropogenic phosphorus cycle.