Water Conditioning & Purification Magazine

What You Need to Know About RO Feedwater Characteristics and How They Affect RO Performance

By Robert Slovak

Knowledge of the source, the treatment and the physical and chemical quality (i.e., the characteristics) of the water supply is essential for the long-term, trouble-free operation of small commercial/industrial RO systems.

The market we are referring to includes small line pressure RO systems, 50-100 gpd, that may serve the food service and office or custom home drinking water market and small pump pressurized systems that include <1,000 gpd systems that might serve food service, office drinking water, car detail, produce mister, humidification and other applications and that operate at low recover (< 33 percent) and require no special pretreatment other than filtration and chlorine removal; and 1,000-3,000 gpd systems that might serve the hospitality, small carwash, ingredient water, small manufacturing, etc. and do not operate over 50 percent recovery and the pretreatment is limited to filtration, chlorine removal and softening.

Water supply conditions should be obtained from a combination of the water supplier’s test reports, past dealer experience and in-house or field testing. It is strongly recommended that dealers create a special form to record the following information and keep it with the customer file for future reference.

What is the source of the water supply?
This information may be useful when diagnosing system performance problems and discussing them with the supplier over the service life of the RO system.

  • Deep well, municipal
  • Surface supply, municipal
  • Deep well, small community
  • Surface supply, small community
  • Deep well, private
  • Shallow well, private
  • Surface supply, private
  • Is the water supply quality subject to significant variation depending on the season or the selection of sources by the supplier?

What primary treatment methods are used on the water supply?
This information may be useful when diagnosing system performance problems, especially those related to membrane scaling, fouling and deterioration.

Is the water supply potable?
This needs to be answered if the RO system permeate is going to be used for human consumption in any way – including ingredient water for beverages or contact with food. Does it meet public health standards for microbiologically safe water? Important! RO systems for potable water applications should only be used on microbiologically safe water supplies and never be used as a primary means of removing harmful microorganisms. Always check the regulations on such applications with your state and/or local public health authority.

Feedwater Total Dissolved Solids (TDS) level
Measure this parameter using a TDS/conductivity meter, calibrated in ppm (mg/L) or microSiemens (uS) with temperature compensation or obtain a complete laboratory test report. The relationship (ratio) between TDS in ppm and conductivity in uS depends on the ionic composition of the water but generally falls between 0.5 (for NaCl solutions) and 0.7 (for natural ground waters). The TDS/conductivity level is necessary because:

1. It is an important factor in determining the minimum feed pressure required. The dissolved ions have the effect of ‘reducing’ the effective feedwater pressure by generating approximately one psi of osmotic back pressure (i.e., reverse pressure) for every 100 ppm of TDS.

Example:

  • TDS of the water supply: 1,000 ppm (mg/L)
  • Feed pressure: 150 psi
  • Osmotic pressure: (1,000/100) = 10 psi
  • Net pressure on membrane (not including hydropneumatic tank back pressure, if included in system) = 150 psi – 10 psi = 140 psi

2. It is a factor in determining the expected product quality since RO membranes reject a percentage of the feedwater TDS.

3. It should be considered when choosing a membrane type since some have better rejection than others. TFC membranes typically have higher rejection than CA (now rarely used) membranes over a wide range of TDS levels.

Feedwater pressure
If the system is operating at line pressure, measure with a standard pressure gauge anywhere in or outside the location, provided water is not flowing. Important! For well systems you need to determine both the high and low pump pressure settings. If the system is operating on pump pressure, a permanently installed, liquid-filled pressure gauge is recommended between the pump and the first membrane.

1. RO production rate is directly proportional to the net pressure, which is the feed pressure minus the osmotic pressure minus any backpressure from a hydropneumatic storage tank. For example, doubling the net pressure doubles the production rate and halving the net pressure halves the production rate.

Example:

  • Assume: The RO supplier claims that the TFC RO membrane production rate is 75 gpd at 60 psi and 77°F.
  • Determine: What is the initial (when the tank is empty) production rate for the same membrane at 100 psi feed pressure, 600 ppm TDS and tank air precharge pressure of 10 psi?
  • Net pressure = (feed pressure) – (osmotic pressure) – (tank back pressure)
  • Calculate: Net pressure = 100 – 6 – 10 = 84 psi
  • Calculate: New production rate = 75 gpd x (84 psi/60 psi) = 105 gpd @77°F

2. Feed pressure affects the percent rejection of TDS. As the net pressure falls below a certain, level the percent rejection starts to drop off, too. TFC membranes typically have high rejection (>95 percent) over a wide pressure range. As a general rule, the initial calculated corrected net pressure is 30 psi (when the tank is starting to fill).

Example:

  • Conditions: You want to install a small line pressure commercial RO with a TFC membrane at a location with 40 psi feed pressure and 350 ppm TDS. The tank air precharge pressure is 10 psi.
  • Question: Do the conditions meet the minimum initial net pressure recommendation?
  • Net pressure = (feed pressure) – (osmotic pressure) – (tank back pressure)
  • Calculate: Initial net pressure = 40 – 3.5 – 10 = 26.5 psi
  • Installation is not recommended since 26.5 psi falls below the recommended 30 psi and not sufficient for acceptable results. To improve, you could lower the tank air precharge pressure to five psi (compromising delivery flow) but the best solution is to add a small booster pump or a permeate pump.

Feedwater temperature
Measure with a standard dial or electronic thermometer after allowing the source to flow for at least one minute, or determine from historical seasonal records.

1. Water temperature is a major factor in determining the RO production rate because it increases or decreases the viscosity of water. Colder feedwater has higher viscosity (thicker water), which slows down production rate, while warmer water has lower viscosity (thinner water), which increases production rate. For predicting a more exact production rate at higher or lower temperatures, refer to a temperature conversion chart from the manufacturer of the particular membrane being used (see Chart 1).

Example:

  • Assume: Most RO membrane suppliers measure membrane production rate at 77°F (25°C). The RO you are installing claims a TFC membrane production rate of 750 gpd at 150 psi and 77°F.
  • Conditions: You want to install the commercial RO at a location with 50°F (10°C) feedwater.
  • Question: What is the expected decrease in production rate of the RO membrane just due to temperature effects?
  • Look up temperature correction factors for TFC membranes. At 50°F find the correction factor of 0.57.
  • Calculate: Multiply this factor times the production at 77°F: 0.57 x 750 gpd = 428 gpd.

2. Do not exceed the maximum operating temperature or membrane degradation will occur within a short period of time. The typical recommended maximum operating temperature for TFC membranes is 100°F (38°C). (Geographic note: In the south west, it is not all that uncommon for water out of the well to hit 120 °F – where not really recommended, these RO units do continue to work well.  In such cases, try to either install a cooling pre tank or change out membranes more frequently.)

3. Membranes should never be allowed to freeze or permanent damage is likely to occur. (Survival stories – cases where small elements have been subjected to freezing temperatures for extended periods and survived – appear to be due to new unwetted membranes and/or dry assembly.)

4. Warmer feedwater may promote hardness scaling and bio-fouling on the membrane surface, resulting in loss of performance and shortened life.

Feedwater pH
Measure pH only at the installation location (with time, the original pH of water samples is likely to change). Use a high-quality reliable and calibrated pH meter.

1. Feedwater pH is less of a factor today because TFC membranes are almost exclusively used. TFC membranes can be installed on virtually any water supply pH (up to pH 10+) without loss of performance.

2. Feedwater pH is a significant factor in determining whether some dissolved solids such as iron, manganese and hardness minerals will become less soluble and cause a potential scaling/fouling problem. Generally, the higher the pH, the greater the problem these dissolved solids present in precipitating and scaling and, thereby, in fouling the membrane.

How is the feedwater disinfected?
Public water supplies may be disinfected with chlorine, chloramines, chlorine dioxide or ozone. Private and well supplies can be disinfected with chlorine, UV or ozone – or not disinfected at all. It is best to contact the local supplier or private water supply operator and ask “how often” and “with what” do they disinfect the water supply. If this information is not available, use a test kit for free and total chlorine to determine if any form of chlorine is present in the water. It is very important to understand the different forms of chlorine and how they are measured. The very basic relationship is expressed as:

Total Chlorine = Free Chlorine + Combined Chlorine (e.g., chloramines)

In the application of RO systems, a public water supply is considered ‘disinfected’ if it is disinfected at a central source, either continually or on some regular schedule. A residual of disinfectant does not necessarily have to be measurable at the installation location (i.e. it can dissipate in the distribution system). A private water supply is considered disinfected if it is continuously or periodically disinfected with an approved method and/or regular tests confirm that it is free of harmful microorganisms.

CA membranes should only be used on disinfected water supplies due to the potential for bacterial degradation. Consult the membrane supplier for their recommendation.

TFC membranes, which dominate this market, must be protected with a dechlorinating media or filter whenever there is a chance that measurable free chlorine is in the feedwater. Chloramines have much less oxidizing effect on TFC membranes, but there is generally some free chlorine present in water supplies disinfected with chloramines.

The deterioration of TFC membranes due to disinfecting/oxidizing agents is expressed in ppm-hours (e.g., ppm of free chlorine x hours of exposure). It specifies, approximately, the number of hours of continuous exposure at a given ppm concentration of the oxidant that the membrane can tolerate before a detectable loss of performance is detected. Modern TFC membrane formulations typically have a 1,000 ppm-hour tolerance to free chlorine and more than 5,000-ppm hours to chloramines. Always contact the membrane manufacturer for their specific guidelines.

Does the RO feedwater contain any impurities that are likely to scale or foul the membrane in normal use?

Excessive hardness, iron, manganese, alum, silt, silica, tannins, iron bacteria and algae could potentially foul or scale membranes. In general, these impurities should be minimized using pretreatment prior to the RO. Consult with the RO system supplier for their recommendations on the limitations of these impurities.

The recovery of the RO system is the primary factor in determining the likelihood of scaling and fouling because specifies to what degree the impurities are concentrated in the reject stream. The definition of the percent recovery and its relationship to the concentration factor of feedwater impurities is as follows:

For line pressure commercial RO systems, which typically operate at 25 percent recovery into an open container, the following guidelines are generally applicable:

  1. If hardness exceeds 15-20 grains per gallon (257-342 mg/L), a softener is normally required, assuming less than 0.3 ppm of iron is present. If iron is present, a softener or iron removal filter may be required at lower hardness levels. Tech tip: Be aware that demand water softener flow meters may not register the flow of small RO systems, with less than 100 gpd permeate flow. Compensation may be required by making appropriate changes to the softener control valve settings.
  2. If dissolved iron exceeds 0.3 mg/L, an iron filter or softener is required.
  3. If iron bacteria are present, they must be eliminated with industry-recommended treatment.
  4. If manganese exceeds 0.05 mg/L, an oxidizing filter is normally recommended.
  5. If tannins exceed one mg/L, they must be removed with industry-recommended treatment.
  6. If the Silt Density Index (SDI) of the water supply exceeds ‘five’ (most likely only in surface water supplies), particulate fouling of the membrane and loss of production performance may occur in a short time. Special backwashing media and/or filter cartridges are recommended for these applications. Consult your supplier for their recommendation.

For larger pump pressurized commercial RO systems that operate at higher recoveries (33 to 50+ percent) there are no hard and fast rules. At hardness levels over 10 grains per gallon (171 mg/L), a softener can be excellent insurance against scaling. Also, the prefiltration capability in terms of particle size removal should be upgraded. Under circumstances involving high dirt loads and oxidizeable metals or biological loads, periodic fast-forward flushing of the membrane may also extend membrane life. (An antiscalent can be injected prior to the commercial/industrial RO unit to assist with some of the mitigating factors that can lead to fouling.  First contact your RO manufacturer for information regarding these products, depending on system size and particular application.)

Potable water applications: Does the feedwater contain any health-related contaminants such as nitrate, heavy metals, radionuclides, Cryptosporidium etc., which may exceed EPA Maximum Contaminant Level (MCL) Standards?

Determine the levels of these contaminants from a current and reliable water analysis. The dealer has the responsibility for making sure health-related contaminants, which can be controlled by RO technology, are reduced to safe levels over the system’s service life.

Important! RO membranes do not reject all individual dissolved solid contaminants to the same percent rejection as the TDS. Some contaminants such as nitrate and arsenic III have unique chemical characteristics that allow them to permeate the RO membrane more easily. The rejection level of these types of contaminants is further dependent on the membrane type (CTA vs. TFC) and water chemistry and operating parameters (i.e., pressure, temperature, pH, cations and anions, oxidation-reduction potential [ORP], etc.). Extensive research has been conducted on RO’s ability to reduce arsenic.  It is important to note that RO membranes will reduce initially on average 98+ percent of arsenic V and 65-75 percent of arsenic III. It is beyond the scope of this article to discuss the details of these special RO performance characteristics. Contact your supplier for additional advice in dealing with individual health-related contaminants.

Important! If the RO system is being used to control health-related contaminants, it is essential that the local Health Department be consulted for their requirements. Third-party testing is also recommended. A reliable RO water quality monitor must be installed and, if applicable, a specific contaminant (e.g., nitrate) test kit supplied to the customer. Periodic lab testing is also recommended to provide verification that the RO system is performing as expected.

About the author
Robert Slovak, cofounder of Water Factory Systems and a principal of Next RO, is the author of a definitive text on installing POU RO devices. Contact him at RobtSlovak@aol.com or at Next-RO, 217 South Pacific Coast Highway,Redondo Beach, CA 90277. Tel: (310) 379-0610; Fax: (310) 634-1841; Email: info@nextro.com; Website: www.next-ro.com.

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