Hardness
Hardness is the most common membrane foulant and, if ignored, will scale a membrane over time. Pretreatment with a water softener is strongly recommended when hardness exceeds the RO system’s limits. Once the system is in operation, a decrease in the water output will indicate scaling of the membrane from hardness minerals. You can also check the weight of a membrane after it has been in service for a period of time. A fouled membrane (dried) will weigh significantly more than a new membrane because of precipitated hardness inside the membrane.

Pressure
While minimum, maximum, and ideal pressure values vary by manufacturer, 40 psi (minimum), 60 to 80 psi (ideal), and 100 psi (maximum) are common. Insufficient water pressure can cause slow water production and premature membrane fouling as contaminants build-up rather than being flushed away. Sustained water pressure above the manufacturer’s maximum psi can result in leaks or damage to the membrane and components.

A booster pump should be considered if the home’s water pressure is below the recommendation, if you are installing the RO system in a basement or running a service line from the RO storage tank to an appliance. Increasing the overall pressure in the system will allow more volume to be drawn from the tank before the pressure dips below a shutoff value. If connecting the RO system to a fridge water dispenser or ice maker, read the fridge manufacturer’s manual for minimum pressure requirements. It may also recommend bypassing the fridge filter.

Iron
While an RO system will remove iron, levels above the manufacturer’s maximum guideline can lead to slow water production, premature membrane fouling, and clogging of the RO’s carbon and sediment filters. Clear water (ferrous) iron can be removed effectively by a water softener or iron filter. Particulate (ferric) iron can be removed effectively by a five-micron filter or an iron filter.

If the iron level exceeds drinking water standards and the iron is soluble, but softening the water is not an option, then an iron filter is appropriate. If none of these are an option, then regular replacement of the RO membrane will have to be accepted.

Manganese (Mn) and Hydrogen Sulphide (H2S)
If the feed water doesn’t meet the manufacturer’s guidelines for manganese (Mn) and hydrogen sulphide (H2S), prefiltration is recommended. The ideal filtration solution will depend on several factors including the water’s hardness and iron levels in addition to Mn and H2S. For homes with low iron and dissolved manganese, an ion exchange water softener can act as both a softener and filter. Air and chemical oxidizing filters are effective if there is H2S and Mn levels are higher.

Chlorine & Chloramine
Thin-film composite (TFC) RO membranes are highly sensitive to chlorine and chloramine, both of which can attack the polyamide layer and permanently reduce salt rejection performance. The most effective protection is activated carbon prefiltration, which must be properly sized to the disinfectant levels in the feed water and household water use. While most RO systems include a carbon cartridge ahead of the RO membrane, these filters provide limited contact time and may not adequately reduce chlorine or chloramine levels when concentrations exceed the manufacturer’s guidelines.

TDS & other factors
TDS levels above manufacturer limits (2,000 mg/l for quick change reverse osmosis) place excessive osmotic pressure on the membrane, which in turn reduces permeate flow and accelerates scaling. Turbidity (suspended solids or cloudiness) in the feed water can clog prefilters and cause membrane fouling. For this reason, most RO systems are equipped with a sediment cartridge filter (typically five microns). In cases such as surface water wells or certain municipal systems during spring runoff, additional turbidity filtration may be required.

RO membranes are rated for specific temperature ranges, typically 4 C to 38 C (40 F to 100 F). Colder water reduces permeate flow, while hot water can damage the membrane structure and seals. In Canadian homes, feed water temperature often drops during winter months, which should be accounted for when sizing an RO system. A unit rated for 50 gallons per day (GPD) at 25 C may produce only 30 to 35 GPD at 5 C.

Finally, water with a low or high pH should be corrected before the RO system. Low pH can corrode housings, fittings, and downstream components, while high pH promotes scaling.

Maintenance considerations
Even when feed water meets manufacturer specifications, regular maintenance is essential to keeping an RO system operating at peak performance. You should always educate your customers on these best practices.

Sediment and carbon prefilter replacement should typically occur every six to 12 months, depending on water quality and usage. Neglecting prefilters allows contaminants to reach the RO membrane, reducing its lifespan.
RO membrane replacement should generally happen every two to five years, though this depends heavily on feed water quality and system design. Testing permeate TDS with a handheld meter can help you identify when a membrane is approaching the end of its life.

Annual sanitization of the storage tank and housings helps control microbial growth and is an opportunity to identify leaks, degraded O-rings, or fouled components for your customers.

Always provide homeowners with clear guidance on maintenance requirements and emphasize the importance of professional servicing for long-term reliability.

Reverse osmosis remains one of the most effective technologies for producing safe, great-tasting drinking water in Canadian homes. But like any precision system, its success depends on proper application. By understanding the water composition factors that impact membrane performance, tailoring pretreatment strategies to the water source, and emphasizing ongoing maintenance, you can ensure that every RO installation delivers long-lasting value.