By Stephen Hamilton

Where does water come from? Does water ever get used up? What’s in my water? Is my water safe to drink? How is water made pure to drink? These are common questions that are asked to water treatment professionals because people are becoming more aware of their water quality.

This article addresses the general overview of reverse osmosis (RO) and membrane technology. Before one can understand ROs, a basic education in water contaminants is important. The US Environmental Protection Agency (EPA) categorizes impurities in water by primary and secondary contaminants. Primary contam­inants are standards that protect the public health by limiting the levels of certain contaminants in drinking water. These contam­inants are things in the water defined as health hazards such as microbials, lead, arsenic, nitrates, DBVs, pesticides, PFAS, etc. Secondary contaminants are non-enforceable guidelines that produce aesthetic and cosmetic effects. These impurities are things in the water that are generally considered “safe” but different states may choose to adopt these as enforceable, so it is important to check your local state codes. Examples of these contaminants include things like color, iron, total dissolved solids, sulfates, chlorides, odor, etc.[2]

This article is going to focus on total dissolved solids (TDS). TDS is the total weight of solids that are dissolved in the water, given in PPM per unit volume of water. When measuring TDS by electrical conductivity, two probes are placed into a premeasured sample and low voltage is passed between the probes. A higher conduc­tivity reading in PPM is directly related to how many dissolved solids are in the water. When measuring TDS by conductivity, substances that are non-conductive will not show in the results. The water discussed in this article is defined as fresh water, brackish water, and highly brackish water (see Picture 1). An RO membrane removes many different cations, anions, ionized salts, colloids, but they cannot remove low molecular weighted compounds like alcohols and phenols. Also, membranes cannot remove dissolved gasses like carbon dioxide, methane, or hydrogen sulfide.[2,3]Reversing the Osmosis Process
If one could achieve water without any contaminants this would be classified as “pure” water, but since pure water only exists in a theoretical sense, the term “pure” will be substituted for treated or product water for the remainder of the article. The three main ways to treat TDS in water is through distillation, deionization, and reverse osmosis. There are other potential desalination techniques such as zeolitic imidazolate framework membranes or the use of low energy hydrogel super polymers, but these are currently being evaluated and debated. When treating water in a whole house POE application, it is more practical to treat with RO technology because of the return on investment and operation and maintenance costs, such as the amount of energy used for distillation or the maintenance and regenerants used with capacitive deionization or ion exchange deionization.[4,5]

Osmosis is the natural phenomenon that provides water to plant and animal cells. This natural process is where water moves from a lower TDS medium to a higher TDS medium across a semi-permeable membrane (ex. roots in plants, cells in animals). Reverse osmosis is the process of applying pressure on the higher TDS liquid to push water back through the semi-permeable membrane onto the side that has a lower TDS liquid.

There are three typical membranes used in water treatment: cellulose acetate (CA), cellulose triacetate (CTA), and thin-film composite (TFC). TFC are the membranes described in this article because these membranes are the most ubiquitously used today. This membrane is comprised of a layer of polyester spacer webbing, a microporous polysulfone interlayer, and an ultra-thin polyamide barrier layer on the top surface. These membranes are typically crossflow membranes, which means water flows across them. The water flows across the membrane and straight down the drain because water takes the path of least resistance. As the drain water is restricted, the water that wants to flow to the drain is forced across the membrane and then the contaminants build up on the surface of the membrane as they are removed. The more restriction applied to the drain water, the more permeate water is created but the quicker membrane fouling occurs. The less restriction applied to the drain water, the less permeate water created, but the membrane is more resistant to fouling. Also, the less restriction the better the membrane is getting cleaned by the cross-water flow.[1]

RO Performance, Ratings, and Requirements
There are many factors that affect RO performance and ratings. Membranes are rated at 500ppm of TDS at 77oF (25⁰C) with a pH of 8.0 at 50 psi. A 50 gpd (gallons per day) rated membrane may not produce 50 gpd because of varying factors. Factors that affect performance include pressure, temperature of the water, recovery, and feedwater TDS. The following are examples of how each of these affect performances.

Graph 4 shows performance versus feed water TDS. Higher TDS coming into the membrane means there will be less product water and less TDS rejection. Table 2 shows recommended feed water concentrations for membranes, pre-treatment solutions, and the potential problems if feed water is out of the specifications.

POE and POU RO System Anatomy
POU RO systems are rated by stages, which is defined by how many filters are installed on the flow path. Picture 3 shows a typical 50 gpd, 5-stage POU RO system. The first three stages in the system are pre-membrane treatment to reduce suspended solids, chlorine, and chloramines in the feed water. This helps protect the membrane and reduce contaminants like low molecular weight molecules and hydrocarbons. Water flows through the auto drain shut off (ADS) to the fourth stage, which is the membrane, and the water is split into concentrate and product water.

From there the product water flows through a check valve and the ADS that shuts off the incoming water once the storage tank fills. The concentrate water flows to the drain via a capillary drain flow restrictor, or other forms of concentrate restrictors like a needle valve. The final product water flows from the storage tank through the fifth stage polishing filter to remove any unwanted taste and odor from the system.

Also, it is important to note that POU RO system performance changes with feed water TDS, pressure, temperature, and differ­ential pressure. Differential pressure is the pressures at two points in a water system. In a POU RO, this is the pressure difference between the incoming water and the water stored in the pres­surized bladder tank. Storing water in an atmospheric tank will reduce the backpressure and differential pressure. This reduced back pressure will increase permeate production and quality of the product water.[3]

POE RO systems are customizable, meaning there are different ways to flow membranes, drains, and different installed features like gauges, meters, controllers, pumps, and filters depending on applications and markets served.

Picture 4 is an example of a POE RO system setup. Most system setups will start with pretreatment to remove some contaminants out of the water, then the water will pass through a pre-filter and then the RO membrane(s) and to a holding tank. Out of the holding tank the water is pumped into the plumbing system. If this water is used for potable applications, then a UV system is typically used after the storage system to keep down microorganisms. If this water is used in a plumbing system that contains metals like copper or brass, then the water is neutralized by a neutralizing system that contain calcium carbonate, magnesium oxide, or through soda ash injection.

Customized Stages of POE ROs
POE ROs can be defined by different stages of membranes. A single-stage system is where feed water is passed through a single membrane. A single-stage system can also include a concentrate recycle, in which some of the concentrate water is fed back to the feed water side of the membrane. This helps reduce waste from the system.

A two-stage system is where water is passed through a single membrane or membranes in parallel and the concentrate water is sent through a second membrane with the product water tied together. The two-stage system can also have a concentrate recycle where the second stage membrane’s concentrate is sent back to the feed water.

A single-pass system is where water is passed through a single membrane only. A double-pass system is where the product water is passed from a series of membrane to another series of membranes. The system can also have a concentrate recycle from the final membrane to keep down on waste. POE ROs can also have membrane flushes where water is bypassed around the flow restrictor to flow water at a higher gallons per minute to help clean contaminants of the membrane boundary layer. This water can be a simple feed water flush or a more complicated permeate flush.

Conclusion
Membrane technology is the core to an RO system. But POU and POE RO performance can vary between manufactures. It is important, when comparing RO systems, that one looks at how many stages, passes, features, membrane housing material, ease of maintenance and filter change, pump and motor quality, and controller features. These subtle changes in design can swing RO performance, pricing, and environmental waste.

References

  1. Dupont. (2021, December 1). FilmTec™ Reverse Osmosis Mem­branes Technical Manual. Water-Solutions. Retrieved April 1, 2022, from https://www.dupont.com/content/dam/dupont/amer/us/en/water-solutions/public/documents/en/RO-NF-FilmTec-Manual-45-D01504-en.pdf
  1. Environmental Protection Agency. (n.d.). How EPA Regulates Drinking Water Contaminants. EPA. Retrieved April 17, 2022, from https://www.epa.gov/sdwa/how-epa-regulates-drinking-water-contaminants
  2. Harrison, J. F., & McGowan, W. (1993). Wqa Glossary of terms. Water Quality Association.
  3. Karamchedu, C. (2016, October 10). Addressing global water scarcity with a novel hydrogel-based desalination technique using saponified starch grafted polyacrylamide’s hydrophilic properties to harvest fresh water with a low energy and chemical footprint. IEEE SusTech. Retrieved April 17, 2022, from https://ieee-sustech.org/wp-content/uploads/sites/261/2016/10/CK_desalination-abstract.pdf
  1. Li, J., Zhao, Z., Yuan, S., Zhu, J., & Bruggen, B. V. (2018). High-perform­ance thin-film-nanocomposite cation exchange membranes containing hydrophobic zeolitic imidazolate framework for monovalent selectivity. Applied Sciences, 8(5), 759. https://doi.org/10.3390/app8050759
  1. Louisiana’s oil. Wetlands and Coastal Erosion. (n.d.). Retrieved April 17, 2022, from http://www2.southeastern.edu/orgs/oilspill/wetlands.html

About the author
Stephen Hamilton currently serves as the national sales manager – wholesale and distribution at Franklin Water Treatment, LLC. He has a bachelor’s degree in pre-medicine from Olivet Nazarene University with a background in biology and a minor in chemistry. He has over a decade of experience in the water treatment field. He previously served as a factory worker, technical advisor, system design, and commercial water treatment specialist for Franklin Water Treatment, LLC. In addition to his current role, he is also one of the managers of the Churubusco facility for Franklin Electric Co. He can be reached at Stephen.Hamilton@fele.com.

About the Company
Franklin Electric is a global leader in the manufacturing and distribution of products and systems focused on the movement and management of water and fuel. It offers pumps, motors, drives, and controls for use in a wide variety of residential, commercial, agricultural, industrial, and municipal applications. Learn more at fele.com.

 

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