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5 Design Considerations for Industrial Reverse Osmosis

Reverse osmosis (RO) is one of the most cost effective methods of treating boiler feedwater, cooling tower makeup water and industrial process water. Reverse osmosis may also be used to produce demineralized water, negating the need to handle and dispose of hazardous chemicals.

Through technological advancements, reverse osmosis systems have become capable of economically filtering boiler feedwater, cooling tower makeup water and process water at virtually any required flow. For an industrial RO system to operate at maximum efficiency, several factors must be taken into consideration during the design process.

1. Water Source:

The source of feedwater for an RO system has one of the greatest impacts on the overall design of the unit, including the number of required filtration stages for pretreatment. Feedwater must typically be pretreated to remove contaminants that would scale or foul the membranes in a reverse osmosis system. Cutting edge membrane technologies, combined with advancements in antiscalant chemistry, have vastly reduced pretreatment capital cost over the past decade.

Water from a source that is stable in composition and temperature, such as a groundwater supply, will typically require a simpler pretreatment process than water from a seasonal or more complex water source, such as surface water or wastewater.

2. Temperature and Flow Rate:

The flux rate, or rate at which water passes through a membrane, varies markedly with water temperature. Warm water will flow much quicker through the membrane of a reverse osmosis system than cold water. For this reason, a variable frequency drive (VFD) is frequently specified on larger RO systems to maintain constant flow regardless of water temperature. However, this does NOT negate proper sizing of the RO to accommodate the feedwater temperature variable. Most industrial reverse osmosis systems are rated by the factory to treat water at an ideal temperature of 77┬░ Fahrenheit.

Water temperature and flow rate parameters must be taken into consideration during the design of a reverse osmosis system and pretreatment equipment. The coldest potential feedwater temperature must be determined and the reverse osmosis system size selected accordingly. If applied without taking into account actual operating temperature, especially during winter, reverse osmosis output will drop below design specifications.

3. Pretreatment:

Chlorine, while beneficial for disinfecting municipal water, will deteriorate reverse osmosis membranes. Dechlorination is achieved by activated carbon filtration of reverse osmosis feedwater treated with a chlorine reducing agent, like sodium bisulfite, as part of the pretreatment process.

To prevent scaling from calcium and magnesium carbonates, feedwater should be pretreated with a cation exchange water softener or chemical antiscalant, depending on the water hardness level.

4. Microbiological Contaminants:

Although not generally as damaging as scaling, microbiological contaminants trapped in the membrane of a reverse osmosis system can also result in fouling and a loss of production capacity. Pretreatment is important when working with RO and nanofiltration (NF) membranes due to the nature of their spiral wound design, which only allows water to flow one-way through the system. Since accumulated contaminants cannot be removed from the membrane surface through backwashing, a biocide may need to be added to feedwater to control microbiological growth.

5. Silt Density Index:

Significant levels of very fine particles (i.e., silt) in water can result in frequent membrane cleanings, or even premature membrane failure. Oftentimes, particles causing potential membrane fouling average less than 5 microns in diameter and the water may appear clear. To prevent a loss of efficiency and down time, feedwater should be evaluated using a silt density index test (SDI) in the field prior to designing pre-treatment for reverse osmosis systems. Treatment with flocculent upstream of multimedia filtration may be required for feedwater that contains significant levels of silt. However, care must be exercised in selecting the flocculent and establishing the correct dosage since many such compounds will irreversibly foul the membranes. In some cases, membrane-based micro or ultrafiltration is required based on particle size distribution (carried out by laboratory testing) or total suspended solids (TSS).

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