Laboratory trials and small- and large-scale external pilot trials have shown promising results for the future of reverse osmosis (RO) membranes treating brackish water. Recent technological developments improve fouling resistance and salt rejection in RO elements with the combination of a novel membrane sheet and optimized feed channel spacer. These improvements are validated by laboratory and field trials.
In systems where colloidal or biological fouling is severe, the advantages of these elements are significant. The optimized feed spacer will reduce the feedside pressure drop by 25% to 30%, reducing system operating costs by reducing clean-in-place (CIP) cleaning frequency.
Dow Water & Process Solutions has organized several field trials for a new extra fouling-resistant brackish water RO element tested in different places with various feedwater qualities.
A biofouling test was set up using city tap water supplemented with 5 ppm of sodium acetate plus 1 million cfu/mL bacteria, comparing the fouling tendency of Dow’s new XFR RO element with the BW30- and XLE-type membranes. In order to quantify the cleanability after fouling, the membranes were cleaned four times with standard caustic-acid cleaning cycles over a 40-day test period— one hour at pH 13.5 (NaOH) followed by one hour at pH 2 (HCl) at a temperature of 25˚C.
The XFR element demonstrated the lowest fouling rate and greatest flux restoration after chemical cleaning among all the tested elements over the 40-day test period. On average, the XFR membrane lost less than 5% of its total expected flow, while the BW30 lost about 10% and the XLE lost more than 20%. Furthermore, the fouling on the XFR is easy to clean, and the flux restoration after cleaning shows that the XFR brings most of the flux back.
Pilot Testing in Australia
The Luggage Point Alliance trial was conducted in the Luggage Point Advanced Water Treatment Plant (AWTP) in Queensland, Australia, in order to compare the performance between four comparable commercial membranes from different manufacturers.
The BW30XFR was selected to represent Filmtec membranes. The feedwater was secondary wastewater pretreated with flocculation (ferric dosing), coagulation and microfiltration. Four pieces of 4-in. membranes from each manufacturer we The trial was controlled by operating the membranes at an approximate flux of 11.8 gal per square foot per day and a recovery of 35%.
The test results over the two-and-a-half-month period show that the XFR membrane was stable and had the best overall performance compared to three other commercial membranes. Its normalized permeate flux is equal to membrane D but has a much lower salt passage value.
Field Trial at TVA
A field trial was conducted to study the effects of a feed spacer on the rate of fouling and the degree of cleanability at a plant operated by the Tennessee Valley Authority (TVA) in Tennessee. The plant produces boiler feedwater from a high-fouling surface source using a combination of conventional pretreatment, two-pass RO and ion exchange. Three vessels located in the first-pass RO were equipped with special instrumentation and control capability to permit side-byside comparison of BW30-400 membrane modules with different feed spacers in an industrial setting.
The trial results showed that the effects of a feed spacer are significant on membrane performance, especially on pressure drop. The membrane with an optimized 34-mil spacer had the best overall performance among the three sets of membrane modules. The pressure drop across the element reduced 20% to 30% compared to a standard 34-mil control and 40% to 60% compared to a standard 28-mil feed spacer.
The feedwater is from a river and has a high biofouling potential. The significant reduction in pressure drop indicates a lower rate of biological fouling by the optimized 34-mil spacer. Pressure drop generally is used as an indicator to clean the membrane system, and CIP cleaning is required when the pressure drop of a one-stage system increases to 10% to 15% of the feed pressure. The reduced pressure drop implies reduced CIP cleaning frequency and element replacement, thus lower operational costs.
A Bright Future
Due to developments in membrane technology, brackish feedwater is becoming less of a challenge for municipal water supplies and various industrial processes. Membrane improvements offer more resistance to fouling with less cleaning and maintenance, lowering operating costs and increasing the affordability of clean water for companies and communities.