KPI's for Monitoring Reverse Osmosis Units
- H2 Global Solutions
- 3 days ago
- 6 min read
Reverse osmosis is often essential in water treatment systems. Recognizing early warning signs of operational issues is critical, as neglect can lead to fouling, scaling, costly cleanings, replacements, and downtime.
When operating and maintaining reverse osmosis units in a pretreatment system, focus on these three key performance indicators (KPIs):
· Normalized permeate flow
· Pressure change (ΔP)
· Salt passage
Some operators assume RO units require cleaning every 3–4 months, but manufacturers provide different recommendations. Cleaning parameters by manufacturer are outlined below.
· Permeate flow rate drops 10%
· Normalized flow passage increases 5–10%
· Normalized pressure drop increases 10–15%
· Permeate flow decreases by 10%
· Permeate quality decreases of 10%
· Normalized pressure drops 15%
· High fouling levels occur at double the above rates
· Permeate flow decreases by 10% since startup or last cleaning
· Salt passage increases by 10% since startup or last cleaning
· Normalized pressure drops 15% since startup or last cleaning
· Normalized differential pressure increases by more than 20%
· Permeate flow decreases by more than 10%
· Salt passage increases by more than 20%
All manufacturers listed above provide free manuals and technical bulletins on their websites. For further details, refer to their official resources. This list is not exhaustive.
Failure to maintain upstream pretreatment and filtration systems can lead to frequent RO cleanings, sometimes as often as weekly.
The Importance of Trending Data
If RO parameters are not regularly recorded and trended, identifying performance issues is challenging. Without data, changes in permeate flow or conductivity may go unnoticed until problems arise. Some operators delay cleaning until a ΔP increase of 40–50 psid occurs across a stage. This significant rise can cause channeling, as suspended solids, colloidals, and bacteria accumulate, restricting flow and reducing cleaning effectiveness.
RO Trending and Maintenance Best Practices
Use manufacturers’ guidelines to set targets for normalized permeate flow, ΔP, and permeate conductivity increases. Schedule maintenance and cleaning when ΔP increases.
Key Parameters to Monitor
Monitor ΔP across each stage rather than the entire RO unit.
If using surface water as makeup, closely monitor the first stage. Colloidals passing into the cartridge filter can increase ΔP in the RO. If buildup occurs in the first stage, run an SDI pad until it plugs off. Send the SDI pad, along with a few drops of permeate water, to your water treatment provider for scanning electron microscopy and foulant identification to diagnose pretreatment issues.
Temperature
· Membranes tighten up as the water gets colder, reducing permeate conductivity.
· Conversely, RO membranes loosen up as the water gets warmer. Warmer temperatures increase permeate conductivity. Organics levels will also rise, subsequently increasing the presence of bacteria in the system.
Free Chlorine
· Free chlorine levels should be maintained at 0.0 ppm for proper operation. When adding a DPD powder pillow, the resulting sample should always be clear with no pink color.
· If using an ORP monitor, DuPont’s FilmTec manual recommends less than 200 millivolts of free chlorine. If using wastewater or gray water, the recommended levels are less than 300 millivolts. However, whenever using ORP, the probe should be calibrated either weekly or biweekly.
Microbiological Fouling
· Microbiological fouling can increase RO pressure. Limits are 100 CFUs/mL in the makeup or 1,000 CFUs/mL in the reject. If these limits are exceeded, a biocide program is recommended.
Feedwater Conductivity and Turbidity
Many RO systems have multiple streams with varying influent conductivity. Use an online conductivity meter to monitor incoming conductivity and predict its impact on permeate. Trending RO conductivity is a recommended best practice.
Monitoring an RO System
Proper monitoring of an RO system enables operators to know where fouling will occur and what cleaner to use. RO manufacturers provide free trending software, or you can use a spreadsheet to record and analyze data.
Installing a flow meter on the permeate line between the first and second stage enables individual trending and targeted cleaning for each stage.
A first stage ΔP is indicative of biofouling, colloidals, suspended solids, or organics. A second stage ΔP increase mainly indicates scaling.
Use high-pH cleaners for elevated first stage ΔP. Material not removed in the first stage will move to the second stage. With the correct antiscalant and stable recovery rate, scaling should not occur.
The Importance of Inspecting Cartridge Filters
Regularly monitor and replace cartridge filters. If sand, particles, or grit are found on the filter elements, investigate and correct upstream breaches, such as issues with bottom laterals or vessel flanges.
When opening RO membranes, contaminants from the softener, anthracite, or multimedia may be found in the lead element. These cannot be removed with cleaners and must be flushed out through the bottom drains of the cartridge filter housing.
Open both cartridge filter drain valves when replacing cartridge filters.
Inspecting the Cartridge Filter Housing
Cartridge filters provide insight into RO operations and serve as the final defense against filtration issues. If you notice visual abnormalities, odors, slime, or grit, report the issue and investigate upstream sources for correction.
Monitoring Guidelines for RO Systems
Monitor conductivity in the feed, permeate, concentrate, or reject.
Frequently sample all ions. Input ions must either exit or be rejected by the system. For example, at 75% recovery, incoming concentrations should increase fourfold in the reject stream; 100 ppm calcium in should result in 400 ppm out.
Measure and record the flow of feed, permeate, and concentrate or reject.
Monitor pressure in the feed, interstage, concentrate, and permeate.
If an interstage pressure gauge is not present, consider installing one. It serves as an indicator for the first stage, where most RO fouling occurs.
Measure the pressure in the concentrate.
Monitor feed temperature, pH, and ORP regularly.
Understanding the conditions of all RO stage flows and ΔP allows for proper cleaner selection and targeted cleaning.
RO Readings
Record all RO readings in a spreadsheet for trending, or use the manufacturer’s trending software.
First Stage
The first stage has the highest flow rate and lowest incoming conductivity, making it susceptible to bacteria, colloidals, suspended solids, organics, and coagulant fouling. Use high-pH cleaners to address these issues.
Second Stage
The second stage has the highest conductivity and is prone to carbonate, sulfate, hydroxide, and phosphate scaling. Use low-pH cleaners with the appropriate antiscalant, except for silica scale, which requires a high-pH cleaner.
The Importance of Data Normalization
Check if the normalized flow remains within standard limits. If so, cleaning is not required. Normalized trending also compensates for temperature fluctuations.
Six Key Takeaways
The following best practices are recommended for maintaining RO efficiency.
Monitor the first stage ΔP closely, especially when using surface water. Regularly check both ΔP and temperature in the first stage. If temperature decreases, ΔP and pump pressure will rise as membrane pores tighten. This does not necessarily require cleaning; instead, review the normalized data.
Adhere to target set points for RO cleaning. Most of the time, fouling will occur in the first stage, particularly if using surface water.Monitoring and trending water chemistry will reveal RO scaling. With proper antiscalant use and stable recovery rates, last stage scaling is typically not an issue.
Monitor for microbiological activity in the feed, reject, and cartridge filter housing. Even with the targeted chemistry levels, 3 ppm total organic carbon (TOC) or higher will provide food for bacteria.
Inspect the cartridge filter housing. Check for slime by smell and touch, and examine internal surfaces for chips.
Monitor permeate conductivity. If it increases, assess whether temperature or incoming conductivity has changed.
Watch for any free chlorine entering the RO membrane. 0.0 ppm and a clear color on the DPD test indicates the absence of free chlorine.
Collecting and trending more data simplifies RO troubleshooting.
Data collection and normalization help operators schedule cleanings and ensure resources are available.Monitoring these six key areas helps operators understand KPI trends, identify conditions affecting the RO, and prevent unnecessary RO autopsies.
As with any technology, exercise due diligence when evaluating these methods.
