When reverse osmosis (RO) membranes were first introduced into the commercial and industrial water treatment markets, the understanding of flows and pressure related to rates of flux (the amount of water passed through a square foot of membrane area) was not considered. The idea then was: the more pressure exerted, the more water produced.
By the mid-1980s it had become apparent that pressures were a byproduct of flow and not the inverse of it. Levels of pretreatment were clearly misunderstood as well, which also contributed to membrane failure and fouling issues. These design misconceptions led to many years of poorly operated and failing RO systems, leaving many customers and potential customers with horror stories and ill will toward RO.
Basic RO is not complicated and can be accomplished with a pump, a membrane, and plumbing fittings. However, to maximize the life of an RO system and optimize productivity, more should be invested into the design than just a pump and membrane.
In this first part of a two-part article, we’ll discuss basic design and component elements of an RO system.
Understand limitations
Before working with any RO system, you must understand the limitations of membranes and their respective removal capabilities. Before design or installation, you should know what the water is being used for and how much of it will be required.
A full water analysis is required to determine the compatibility of the membrane and its fouling potentials. This information will help with system design parameters like recovery (amount of water product from the amount fed the RO system), flux rate of permeate, quality of permeate, and levels of pretreatment. It will also eventually affect the economics of that RO application.
Once a water analysis has determined level of pretreatment and the type of RO system, accessories for system maintenance and operation can be determined. These accessories should be provided based on the level of expertise the operator has and the level of monitoring required.
Not all systems require a full array of meters, gauges or telemetry, but should have at least the minimum equipment to provide information for sound maintenance and operating decisions.
Feed and water analysis
Using sound design principles, you must decide if the product water will be stored or fed directly to where it is used.
If stored, the size of the RO may be designed to handle normal daily requirements, and the storage can be sized for peak usage. If fed directly, the system size must be able to meet peak demands and pressure requirements. This could add substantial cost to the system and make it cost-prohibitive.
Post-treatment options should also be considered, depending on the water requirements and parameters.
At a minimum the following parameters should be tested to determine scale and fouling potential of the membrane(s): hardness, alkalinity, sodium, sulfate, potassium, chloride, barium, nitrate, iron, fluoride, pH and total dissolved solids (TDS).
When salts dissolve in water and water is diffused through an RO membrane, the cationic and anionic components of the salts are left behind along the membrane surface. Without water as a buffer between them, they come into close proximity with each other and form solids, blocking the ability of water to diffuse through to the permeate tube.
Recalling my initial observation about flows and pressures, good flow practices will help carry salts off to drain before scale can form. A good RO manufacturer should be able to provide you with a membrane projection model based on your water analysis. The projection will provide a guideline for the membrane’s scale potential based on required recovery and flows (sidebar).
Pretreatments
Water feeding the RO should be pretreated to provide the best quality for optimal RO performance. Never under-design or underestimate the importance of proper pretreatment. RO is not black-box technology and is not a cure-all for water.
The removal or control of calcium, iron, suspended solids, oxidizers, silica, and bacteria in water before it enters the membrane(s) will ensure longer life and operation.
Standard treatment methods for iron, chlorine and suspended solids can be applied. For hardness, there are the options of using traditional softening or antiscalants.
In some situations, softening (less maintenance-intensive) will not be the optimal choice. An example would be when the TDS of the feed supply exceeds 3,000 parts per million (ppm) due to high levels of sodium. The softener will not exchange all of the hardness ions and will form hardness scaling on the membrane. In these situations, the use of an antiscalant may be preferred.
When designing size and flows for pretreatment, keep in mind that you are designing for the inlet feed flow, not the permeate flow. So it’s important to know what recovery will be.
Vessels, pipes, pumps
After the water analysis and a determination of pretreatment, you can begin to determine the style and component requirements for the RO.
Higher-TDS systems will require more pressure (approximately 1 pound per square inch [psi] osmotic back pressure for every 100 ppm TDS). This will give you an idea of the pressure the pump will have to produce, as well as components that can handle the higher pressure and higher TDS.
Most stainless steel pressure vessels are made of 304SS (a stainless steel type). When used on water with a TDS that exceeds 1,200 ppm, the level of TDS in the concentrate could exceed 4,500 ppm, creating pitting and leaking through housing. Fiberglass-reinforced plastic (FRP) housings are priced compatibly with the stainless steel and offer a broader application spectrum.
All the piping material needs to match up with pressure and TDS ratings for corrosion and burst. The pump should be a minimum of 304SS at all times, and at higher TDS, I recommend using 316SS. If acid is used in the pretreatment, all downstream components should be 316SS and/or compatible with low pH.
With all centrifugal pumps, you need to "give to get." That is, in order to give pressure, you need to get the right amount of water. By cutting corners on the pump size and price, you may be sacrificing the membranes by starving them of the right amount of cross-flow required to carry away the particulate.
Some RO manufacturers have increased the volume output in gallons per day (gpd) of their units, so what may have been a 1,500 gpd RO last year is a 3,000 gpd today. What happened? It’s the same number of membranes — in fact, the pump may have gone from 1 horsepower (hp) to 1/2 hp. How can this be?
It’s possible they may be using low-pressure membranes that could be applied under the wrong conditions. The low-pressure membrane was designed for cold water applications, in which you can get the same permeate throughput at temperatures of 55 degrees F that standard membranes obtain at 77 degrees F. They were not designed for pushing the pressure up to 200 psi and getting twice the flow.
Flux rate and water quality
When considering RO design, take into account not only the water supply but where it is coming from. Deep wells typically will allow for flux rates of 17 to 18 gallons per square foot of membrane area per day (gfd). Most of today’s 4 x 40 membranes have 80 to 85 square feet (sq. ft.) of membrane total (1 or 2 have 90 sq. ft.). With those conditions, if you multiply 17 (gfd) x 85 (sq. ft.), the result is 1,445 gallons per day (gpd).
As you increase beyond 1,445, you are increasing the fouling potential of the membrane. Even with good pretreatment, this is not recommended, unless you want to replace the membranes every year.
The flux rate of the membrane should drop as the water quality goes down: 15 gfd for surface wells, 12-13 gfd for brackish water (5,000-10,000 ppm), and 8-10 gfd for seawater. If you consider a good, conservative design of 1,500 gpd per membrane, by adding them up you will be able to determine the size RO required to produce the volume of water needed.
Flow and recovery
When considering RO recovery, remember that flow rates can be your friend. If the water is extremely high in TDS and/or has high fouling potential from organic loading, the more concentrate flow you can get away with and the lower the recovery, the better.
Standard design practices are 10 gpm feed flow maximum per 4-inch pressure vessel, and 5 gpm minimum concentrate flow exiting the last pressure vessel. Systems that have 1 gpm permeate and 3 gpm concentrate (without recycle) are asking for trouble. The cross-flow over the membrane is not enough to keep the insoluble salts from scaling and blocking the membrane.
If the water is pretreated with a softener or antiscalant, recycling is an option to lower the amount of water going to drain, but only if the calcium hardness is being controlled. When higher recovery is required because the end user does not want to waste water (as in applications where water is metered), longer membrane arrays will allow for the recovery to be increased, as well as for upfront investment to provide pretreatment that will allow for concentrate recycling.
Next month, the author will look at optional features for an RO system, as well as storage-tank considerations.
Lawrence Jessup CWS-VI, CI, is president of operations for Watertech, Inc. of Lake Alfred, FL, and was formerly the president of Inaqua International and senior applications engineer for Crane Environmental, an RO manufacturer. Watertech, Inc. is a utilities construction and service company serving Florida, Georgia and the Carolinas, and is the sole contractor for the Florida Department of Environmental Protection for groundwater contamination clean-up. He can be reached at [email protected] or 941-374-1902.
