Desalination of geothermal waters using a hybrid UF-RO process. Part I: Boron removal in pilot-scale tests

Abstract

The study assessed the potential of reverse osmosis with low-pressure BWRO membranes to reduce total dissolved solids (TDS) and enhance removal of boron and other microelements (such as iron, fluoride and arsenic), so making geothermal waters suitable for discharge into surface waters or reuse for drinking purposes. Preliminary treatment involved an iron removal system (a reduction in iron concentration from max ca. 4mg/L to 0.013–0.021mg/L, i.e. a reduction of 99%, was obtained) and ultrafiltration modules (the average SDI after pre-treatment was 2.4).The reverse osmosis system was equipped with spiral wound DOW FILMTEC BW30HR-440i polyamide thin-film composite membranes. To optimise boron removal the membrane separation performance was assessed in short and long-term tests at a semi-production scale (production of desalinated water at 1m3/h) over an eight-month period.It was found that the boron removal rate in acidic feedwater depended on the boron concentration. The highest removal rate of 56% was obtained in water with 2.5mgB/L, followed by 48% in water containing 8.98mgB/L and a very poor final result of 12% in 96.73mgB/L. High boron removal rates of 96 and 97% were obtained in water with pH 10 and 11 containing ca. 10mgB/L, regardless of the feedwater uptake ratio. Feedwaters with a high boron concentration of ca. 100mg/L and pH=10 yielded a low boron removal rate of 66%. Efficient and stable performance of the BWRO-membrane equipped desalination system was achieved with geothermal waters containing 7g/L TDS and a boron concentration of up to 10mg/L. Taking account of the low pressure applied in the reverse osmosis process (1.1MPa), a relatively high removal rate was obtained following the first stage of RO (at pH of 5±0.4): 96–97% with respect to conductivity, and 94% with respect to SiO2, 92% for fluoride and not less than 84% for arsenic.