Abstract
Surface waters, though often highly impacted by high turbidity, microbial content, Natural Organic Matter (NOM) and the presence of microcontaminants, such as pesticides and pharmaceuticals, are frequently used as resource for drinking water production. Application of Low-pressure Reverse Osmosis (LPRO) shows to be promising for surface water treatment due to its combined performances of water softening and removal of dissolved organic matter (DOM) and micropollutants but requires a remineralization step of the permeate to ensure water equilibrium prior to distribution.
A novel solution for remineralization of LPRO permeate by salt recovery from LPRO brine was investigated through application of Assisted-Reverse Electrodialysis (ARED) to these two streams in parallel. In contrast to the recovery of energy in RED technology, an electric field is applied in ARED to enhance the spontaneous flux of ions from the concentrate to the dilute solution. The proof of concept of this novel approach was first validated through extensive lab-work and followed by a pilot-scale demonstration to assess the possibility of selectively recovering minerals from the brine (i.e. mainly calcium and bicarbonates) while quantitatively retaining micropollutants and keeping the passage of NOM under control. The data obtained throughout the research program further allowed to develop and calibrate a process-specific model, which was further used to evaluate the total costs of this new application at industrial scale.
Pilot-testing showed the process was able to increase LPRO permeate mineral content from 6 mg/L CaCO3 up to values of 1060 mg/L CaCO3 and from 26 µS/cm up to 1906 µS/cm for hardness and conductivity, respectively. The significant recovery of minerals, resulting in a remineralized permeate hardness well above the final target value of 90 mg/L CaCO3, is a very positive result for industrial application of the system (where the treated permeated could be blended with a large stream of by-passed RO feedwater), as only a small footprint unit applied to a fraction of the permeate would be required for full remineralization. This small footprint is a significant advantage in the techno-economic evaluation of the process, when compared to traditional remineralization units (e.g. limestone media contact filtration).
To ensure that the process is viable for industrial application, its ability to preserve the permeate’s integrity with respect to various contaminants such as pesticides is critical. Pilot testing provided microcontaminant breakthrough data for 32 compounds which highlighted low levels of micropollutant passage with an overall retention of 91%, while natural organic matter (NOM) breakthrough ranged from 12% to 25% with a limited impact on bacterial regrowth as measured by Assimilable Organic Carbon (AOC).