RO Membranes

Evoqua Advantage carries industrial and commercial membrane elements from the most well-known and respected manufacturers in the world including Hydranautics, Filmtec™, and Toray.

Most Hydranautics elements feature LD Technology™, reducing element fouling, and feed pressure requirements which allows longer element life and lower energy consumption. Filmtec™ manufactures the industry's most reliable and durable membranes using automated equipment that ensures maximum consistency of membrane surface area possible.

Considerations for Choosing an NF or RO Membrane

For new RO and NF systems, typically the required quantity and quality of the product water determines the size of the membranes used, the total size of the system and the type of membrane used. Whether for new or replacment, Evoqua always recommends that customers work backwards from water quality to determine the best membrane for any application.

High Rejection vs. Low Energy

Generally customers should choose the lowest energy membrane that allows them to meet their desired effluent water quality. The energy required for producing purified water with an RO or NF system is in the form of pressure that is required to overcome the osmotic pressure between the permeate (product water) and the concentrate (waste water). Accordingly the biggest factor in how much energy is consumed in an RO or NF system is the total dissolved solids (referred to as TDS) of the feedwater and the quality required for the permate. For seawater membranes with 99.5%+ salt rejection, the feed water is typically in the 25,000-35,000 TDS range and the requirement for drinking water is generally 500 PPM TDS Or less. Osmotic pressure along to overcome this difference in TDS is in the 800-1000PSI range regardless of the membrane used. All that being said, many different membranes are available for customers to be able to select a membrane that meets treatment goals without consuming excess energy to improve the permeate quality more than required. Other factors that affect feed pressure:

  • Water temperature - As water gets colder, the viscosity increases greatly which results in higher pressures to move the same amount of water through the RO system and the membranes
  • Flux through membrane - Flux is described as the amount of water that passes through the active membrane area for a unit of time (so a flux in GFD measures gallons through each square foot of membrane area per day). The higher the flux, the higher the energy required. Flux also affects the permeate quality and membrane fouling - higher flux results in better permeate quality but in addition to consuming more energy, can increase the risk of fouling.
  • Specific system piping and backpressure.

Lastly on the topic of water pressure and RO systems - any time a replacement membrane of a different manufacturer and/or model is being considered, it is highly recommended that the user or application engineer run a system projection to confirm not only the projected water quality by ion (where specific ion concentrations are known) but also to confirm that the existing system pump can meet the required flow rates and pressures shown. All major membrane manufacturers offer projection software to help ensure that customers can choose the best membrane for their application.

Application Specific Membrane Considerations

    Seawater Desalination for Drinking Water - Along with large amounts of sodium chloride, seawater also contains smaller amounts of other ionic contaminants including boron. Boron is damaging to the health of plants and animals for the concentrations it which it is found in most seawater feedwaters. Although most seawater RO membranes reject boron to some degree, the porosity and surface charge of the membrane can make a difference in meeting specific treatment goals. Most seawater membranes list boron rejection under test conditions that can help narrow down specific membrane models to be used in system projections. The species of boron present in water is pH dependent and so pH is an important factor in Boron rejection.
  • Brackish Water for Industrial Commercial Applications - Brackish water is typically defined as those waters with TDS in the range of 5,000 to 15,000 PPM TDS. Brackish water RO systems typically operate in the 200 to 600PSI range depending on exact feed water TDS values. Along with general TDS rejection, there are specific ions of interest for industrial applications depending on downstream equipment. Most important for industrial applications is silica. Silica is weakly charged in water and so is more difficult to reject. Also because silica is weekly charged, it is weakly heldy by any downstream ion exchange systems and is the first ion to breakthrough (come off of) an anion resin bed. Breakthrough of silica past the anion bed of an ion exchange system typically triggers ion exchange regeneration so the less silica to pass through an RO membrane, the less often the ion change system needs to be regenerated. Regeneration of ion exchange systems requires system down time and chemical expense so significant savings can be made by maximizing upstream membrane silica rejection.
  • Commercial/Residential Applications - With some exceptions, most commercial and residential RO applications such as car washes and municipal drinking water point of use (inside a home) applications are bulk dissolved solid removal type applications where price and energy consumption are the most important factors. For these types of applications, the TDS of the feedwater is typically in the 250-1000PPM range and so extra low energy membranes with pressures as low as 100PSI are typically used.

RO and NF Membrane Fouling Considerations

In addition to dissolved solid rejection, another area of interest is membrane durability and fouling. Fouling of membranes usually occurs in three different ways- biological fouling of the membrane surface and/or feed spacer, inorganic fouling of the membrane surface and plugging of the feed channel with particulate matter. Feed water from surface water sources or other biologically active waters can cause biofouling (biological buildup or growth on the membrane surface or feed channel spacer). Biofouling is most effectively removed by high pH chemical treatments however high pH can damage some membranes causing reduction of rejection. Refer to manufacturer's datasheets for the allowable cleaning range for any membrane and remember that pH is a logarithmic scale and a pH of 12 is 10 times more basic than a pH of 11. Fouling in the feed spacer layer of RO and NF membranes can be reduced or prevented. Inorganic membrane surface fouling is ususally caused by exceeding the soluability of one or more ions in solution. Running accurate system projections can help identify feed waters that antiscalants are recommended for reducing scaling or membrane surface fouling by inorganics. Also, running an RO system at higher fluxes can cause scaling at the membrane surface through a process called concentration polarization. Concentration polarization describes a temporary buildup of ionic concentration at the membrane surface that is typically caused by operating at too high of a flux rate for influent water conditions and/or insufficient cross flow (waste water flow) across the membrane surface. Reducing recovery percentage (percent of product water vs feed water) is the easiest way to prevent or reduce concentration polarization. Reject recirculation can also be used to limit concentration polarization without sacrificing recovery rate (but does reduce product water quality and decrease overall solubility limits). Finally particulate matter plugging of the feed channel is common where particulate pretreament is insufficient. RO systems should have a minimum of high efficiency 5 micron filtration for the influent (1 micron preferred). A silt density index test can help identify if the pretreatment is sufficient. Also, choosing a membrane element with a wide feed spacer (34mil or larger) can help to reduce particulate plugging.