Reverse Osmosis, Part I: Value proposition, how it works

This is the first part of a five-part series on the design, operation, and maintenance of reverse osmosis systems for powerplants compiled by Wes Byrne, U.S. Water’s consultant on membrane technologies. Parts II through V, listed below, will appear in upcoming issues of CCJ ONsite.

Pure water does not exist in nature. All natural waters contain varying amounts of dissolved and suspended matter. Osmosis is the process in which a solvent (water, for example) flows through a semi-permeable membrane from a less concentrated solution to one with a higher concentration. This normal osmotic flow can be reversed (reverse osmosis) by applying hydraulic pressure to the more concentrated (contaminated) solution to produce purified water.  

There is no perfect semipermeable membrane. A small amount of dissolved salt is also able to diffuse through, but this results in relatively low concentrations compared to the feedwater values.  

The benefits of reverse osmosis (RO) technology should be well understood in water treatment for power generation, particularly because of its potential to reduce O&M expenses. For most sources of water, RO will be the least expensive way to remove dissolved salts.  

The term total dissolved solids (TDS) refers to these inorganic salts with some small amounts of organic matter, present in solution. The salts exist as cations (mostly calcium, magnesium, sodium, and potassium) and anions (mostly bicarbonate, chloride, sulfate, and nitrate). These positively and negatively charged ions can pass electrical flow, thus determining the conductivity of the water as a measurement of its TDS concentration. Pure water is a poor conductor of electricity.  

For plants originally built using only ion exchange, adding RO can reduce chemical regeneration requirements by a factor of 20 or more. Complete removal of regenerable systems might even be considered.  

With RO upstream removing the bulk of the dissolved salts, the polishing ion-exchange systems might be economically replaced with service demineralizer beds that are chemically regenerated by an offsite water service company, or they might be replaced by electrodeionization. EDI units use electricity to continuously regenerate their resins.  

Some new and existing plants are now being required to remove dissolved salts from their wastewater streams prior to discharge. RO may perform this role so well that it may even be possible to re-use the water within the plant. The concentrated salt stream remaining after RO treatment might then be more economically hauled to a region that can better handle the environmental effects, or it could be evaporated or discarded in some other manner. The political and regulatory advantages of becoming a zero-liquid discharge (ZLD) facility can offset part of the capital and operating costs.  

But the superior economics of RO operation are only achievable if the system and its upstream treatment components are correctly designed, operated, and maintained.

Pulling a water sample for laboratory analysis is a good start in preparing an RO design. A comprehensive analysis provides data on the metals in the water (for example, iron, manganese, and aluminum), the dissolved salts (cations and anions), the water pH (acidity), and possibly the inorganic total suspended solids (TSS). A measurement of the total organic carbon (TOC) will often correlate with the potential for biological activity.

A TSS analysis reveals the concentration of filterable solids in the water. The concentration of dissolved metals, such as iron, will change in the water sample as they react with oxygen introduced by contact with air. This will cause some of the metals to oxidize and become insoluble. The metals that stay suspended in the water may cause the TSS value to increase significantly with many well-water sources.

Biological fouling solids will not be well represented in the TSS results. The mass of these solids will typically become negligible when the TSS filter is dried prior to weighing for results. The water could be tested for its silt density index (SDI) if the metals are first separated out of the sample. This test will be highly sensitive to the ability of biological solids to coat and reduce the flow rate through its 0.45-micron test filter. Results will correlate with the fouling tendencies of a membrane system.

No analysis is perfect, and water quality can change over time. Even the characteristics of a well-water source will change if the well is relatively shallow.

Sampling methods affect results. Some concentrations will change between sample pull and analysis. Metals may attach to the container’s inner surface. Ammonia and carbon dioxide may degas or carbon dioxide may dissolve from exposure to air. Any of these changes will cause the water pH to change. An accurate water pH is best measured onsite.

Chemical suppliers can use a water analysis to predict how much purified water (permeate) the RO might safely separate from the source before the dissolved salts become too concentrated in the remaining water and form scale within the membrane elements. The water analysis is also used in designing the RO system, both in projecting the purified water quality and in assessing any effect of the salts on system hydraulics.

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