Forward osmosis application for water reuse

Abstract

Hybrid forward osmosis (FO) processes such as forward osmosis with membrane bioreactors (FO-MBR), electrodialysis (FO-ED), nanofiltration (FO-NF) or reverse osmosis (FO-RO) present promising technologies for wastewater reuse in agriculture. Their niche of application can be found when dealing with high-salinity wastewater in water stress situations, when the removal of trace organics or boron is targeted due to the multibarrier concept, or when water with high fouling potential is compulsory to treat, among other uses. In this case, one of these emerging technologies, FO-NF treatment has been studied at long-term with the aim of evaluating this technology at demonstration scale for irrigation purposes. In order to design the definitive pilot demonstration plant, an intensive evaluation of draw solutions (DS) was performed by focusing on the wastewater reuse applications of hybrid forward osmosis (FO) processes. The substances studied were potassium formate, potassium phosphate, magnesium sulphate, sodium chloride, sodium polyacrylate and polyethylene glycol, and their osmotic pressure, conductivity, pH, thermostability, sunlight exposure, toxicity, FO filtration performance and replenishment costs were determined. Based on these characteristics, three DS, sodium polyacrylate, magnesium sulphate and potassium phosphate, were selected as the most interesting to be evaluated in the demonstration plant. The results also revealed that the most relevant DS properties for wastewater reuse under the studied conditions were the DS regeneration method, DS replacement price, pH adjustment and toxicity. These properties were shown to be more relevant than filtration flux when a maximum DS osmotic pressure value of 10 bar was used, establishing this pressure as the limit for efficient DS recovery. Thus, prioritizing energy consumption, NF 270 4040 from Filmtec (Dow Chemical), were selected as the most suitable recovery nanofiltration (NF) membrane; because they offered the best relationship permeate flow/feed pressure with an adequate rejection. Additionally, to select the most suitable FO membrane, five commercially available FO membrane modules were evaluated and compared, where thin-film composite (TFC) flat sheet membranes from Porifera showed the highest flux and the highest salt rejection, and the lowest permeability and salt rejection values were presented by cellulose triacetate (CTA) hollow fibre membranes from Toyobo. Based on the information obtained, a FO/NF demonstration plant was constructed next to the wastewater treatment plant (WWTP) in San Pedro del Pinatar, in the region of Murcia (Spain). This plant represented the world’s first FO demonstration plant for municipal wastewater reclamation to be evaluated for agricultural purposes. The plant was operated treating 3 m3 h-1 of real wastewater with an average salinity of 3-5 mS

cm-1 and 1.5 mg L-1 of boron in continuous mode for 480 days. Two of the three DS that had been previously selected (sodium polyacrylate and magnesium sulphate) were evaluated in different periods of experimentation. In a later study, magnesium chloride was catalogued as a DS with great potential to achieve economical expenses during the whole operation. The operation with sodium polyacrylate led to reversible fouling on the FO and NF membranes and the permeate was not suitable for irrigation. Although the permeate quality obtained was acceptable using magnesium sulphate as DS, this generated severe irreversible fouling on NF membranes and therefore, it was discarded. Finally, magnesium chloride showed the best performance, with FO-NF membranes presenting a stable permeability and low membrane fouling during long- term operation. The FO-NF permeate showed high quality for irrigation, achieving a conductivity value of 1 mS cm-1, a boron concentration below 0.4 mg L-1 and an average sodium adsorption ratio (SAR) of 1.98 (mequ L-1)0.5.

La ósmosis directa (OD) utiliza los principios de la ósmosis para desalinizar agua. El proceso de ósmosis directa utiliza un fluido denominado solución extractora a partir del cual se extrae agua de la fuente principal a través de una membrana semipermeable, aprovechando las diferencias de presión osmótica. Este proceso tiene la desventaja de que no genera agua de alta calidad en una sola etapa debido a que el agua producto se mezcla con el agente osmótico de la solución extractora. Así, se debe prever un sistema de separación del agente extractor para obtener el agua deseada, lo que se denomina sistema híbrido.

Durante la tesis doctoral se estudió un sistema híbrido OD-Nanofiltración a escala piloto, demostrando que el proceso híbrido OD-NF es una tecnología con baja propensión al ensuciamiento, que puede lograr un permeado estable y de alta calidad para la reutilización de aguas residuales en una operación a largo plazo. Sin embargo, el consumo energético y del agente extractor es relevante. La falta de módulos de membrana de OD optimizados y disponibles en el mercado sigue siendo la principal limitación para la aplicación del proceso de OD a gran escala. En un futuro cercano, se espera que este proceso emergente se vuelva más sostenible y competitivo para el caso de uso estudiado en esta tesis.