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Those who depend on membrane filtration as a part of their processes understand that the life of the membranes is unavoidably linked to the quality of the feedwater. Poor water quality can lead to unexpected costs and can result in membrane damage. Fortunately, prefiltration can prevent these potential problems. In fact, properly implemented prefiltration can significantly extend the life of the membrane filters.
Ideally, prefiltration would have been part of the design scheme from the beginning. But for those with existing systems, a retrofit may be just what they have been looking for to extend the service life of their membranes, reduce energy costs, protect warranty and improve the overall plant operating costs.
Kansas Ethanol Plant
A new ethanol plant in Phillipsburg, Kan., highlights the importance of prefiltration in membrane feedwater. The 40 million gal per year ethanol plant is owned by Prairie Horizon Agri-Energy, and was designed and built by ICM, Inc. The reverse osmosis (RO) system and its associated prefiltration skid were designed and built by the Membrane division of Layne Christensen Co.
Water for the ethanol plant is drawn directly from the city of Phillipsburg drinking water facility wet well and treated through a 400 gal per minute (gpm) RO system. The permeate is then delivered to the ethanol plant via a two-mile-long pipeline, while the reject water is recovered by blending with the feedwater destined for the city’s lime softeners.
The RO system provides high-quality water for use as make up to the ethanol plant’s boiler, cooling towers and refining process. But the membranes of the RO system must be protected themselves from contaminants in the untreated feedwater. Testing of the raw water drawn from the city’s wet well revealed high levels of both iron and manganese. Iron levels ranged between 0.6 and 0.8 mg/L, and manganese levels were 0.3 mg/L. Membrane manufacturers typically prefer iron and manganese to be less than 0.05 mg/L. Left unchecked, these contaminants would have quickly fouled the RO membranes.
Iron & Manganese Reduction
Prefiltration was needed to reduce contaminants to acceptable levels, so a prefiltration skid composed of three 66-in.-diameter pressure vessels was incorporated into the filtration scheme. The pressure filters were designed to take advantage of LayneOx, a manganese dioxide filter media with unique physical characteristics. In the presence of chlorine as an oxidant, the LayneOx media achieves highly efficient contaminant removal through a combination of oxidation, precipitation and adsorption. An additional benefit is that the pressure filters can be reconditioned indefinitely through backwash while achieving run-times in excess of 24 hours between reconditioning cycles.
Immediately before entering the pressure filters, free chlorine is added to the raw water. The chlorine serves to oxidize the iron, which is removed through filtration while the manganese is removed through adsorption. Permeate from the pressure filters contains a chlorine residual, which is removed with sodium bisulfite to prevent the negative effects of oxidation to the membranes. As a final precaution, a 5-micron filter has been placed at the RO intake.
In operation, the pressure filters easily satisfy prefiltration requirements, providing iron and manganese removal with up to 533 gpm throughput. The system demonstrates high surface loading rates of 7.7 gpm/sq ft. Onsite pilot tests demonstrated that iron and manganese contaminants had been reduced to an average of 0.025 mg/L for iron and 0.016 for manganese, and these same levels were achieved in the full-scale system.
Overall, the filtration system provides complete turnkey operation with low maintenance requirements. The system serves to protect vital down-stream systems while keeping energy requirements down by reducing the pressure needed to drive the RO filters.
Because suspended solids can have a severe negative effect on membrane performance, the membrane industry uses a standardized method to measure filter fouling over time. This method results in a measurement called the Silt Density Index (SDI). For RO feedwater, an SDI of 5 is typically considered acceptable, with lower numbers being desirable.
The benefits of the Phillipsburg prefiltration system are impressive. The pressure filters provide pretreated water to the RO membranes with a SDI of only 1.3. Maintaining high-quality feedwater is one assurance that the Phillipsburg facility will benefit from optimum performance of its RO system while avoiding unnecessary maintenance and operating costs.
All aspects of securing an ample water supply to the Phillipsburg ethanol plant were carried out by a single source, Layne Christensen Co., including system design, permitting, filter manufacturing, and system and pipeline installation. By contracting with a single company, a complete water supply and treatment system was established in only eight months.
More information on membrane filtration, complete water treatment systems and LayneOx filter media is available from Layne Christensen’s Water and Wastewater Infrastructure Division (www.laynechristensen.com). Water treatment systems, components, chemistry and media are also available through Layne’s distribution division, HMI (www.hmiwater.com).