Troubleshooting Reverse Osmosis Systems
Introduction
Reverse Osmosis systems are the Best Available Technology for removing TDS or dissolved salts/organic molecules such as PFAS from water. However, without a properly designed pre-treatment system or proper operations/maintenance, they can develop some common problems.
Trouble with the performance of an RO system typically expresses itself as one of several symptoms:
Symptoms
Loss of Normalized permeate flow:
Normalised permeate flow is flow that has been corrected by a water temperature factor.
In practice this is normally seen as a feed pressure increase in order to maintain the permeate output. This could be due to:
- First stage problem: deposition of particulate/colloidal matter or start of biofouling
- Last stage problem: scaling – Scaling is a water chemistry problem originating from the precipitation and deposition of sparingly soluble salts. The typical scenario is a brackish water system operated at high recovery without proper pretreatment. Scaling usually starts in the last stage and then moves gradually to the upstream stages. Waters containing high concentrations of calcium, bicarbonate and/or sulfate can scale a membrane system within hours. Scaling with barium or with fluoride is typically very slow because of the low concentrations involved.
- Problem in all stages: advanced bio-fouling
- Metal oxide fouling occurs predominantly in the first stage. The problem can more easily be localised when permeate flow meters have been installed in each array separately.
- Organic Fouling: The adsorption of organic matter present in the feedwater on the membrane surface causes flux loss, especially in the first stage. In many cases, the adsorption layer acts as an additional barrier for dissolved salts, or plugs pinholes of the membrane, resulting in a lower salt passage.
Increase in Normalized TDS passage (Salt Passage):
In RO this is typically associated and detected by witnessing an increase in permeate conductivity. This is normally due to a leaky O-ring or destruction of the membrane by oxidizing biocides such as Chlorine.
Another root cause could be crystalline or sharp-edged metallic particles in the feed water that may have entered into the feed channels and scratched the membrane surface.
An Increase in Pressure Drop:
The difference between feed pressure and concentrate pressure (Delta P) at constant flowrate becomes larger. As the tiny pores in the membrane become clogged, it takes a higher pressure to push the water through resulting in an increased demand for a higher feed pressure.
WARNING: Ignoring increased Delta P or decreased permeate flow might result in Permanent Fouling which CIP cannot clean.
How do we Troubleshoot/Identify Symptoms?
Tips for Troubleshooting Biofilm:
A scientific way to identify what is wrong with an RO membrane is to perform a forensic autopsy. But before that, it is a good idea to check the design of the RO system using an RO projection. Download our presentation on RO Software Projection. Then you can get down and dirty and autopsy an RO element.
Biofilms feel slippery to the touch, often have a bad smell.
A quick test for biofouling is the burn test: a sample of biofilm is collected with a spatula or the point of a knife and incinerated over the flame of a lighter. The smell of a burnt biofilm is like the smell of burnt hair (This is really just a quick test for an indication but not for a proof). Download our RO Membrane Autopsy presentation here.
Tips for Troubleshooting Scale on an RO Membrane:
To identify scaling:
- Check the feed water chemical analysis for the scaling potential at prevailing system recovery.
- Analyze the concentrate for levels of calcium, barium, strontium, sulfate, fluoride, silicate, pH and Langelier Saturation Index (Stiff & Davis Saturation Index for seawater).
- Try to calculate the mass balance for those salts, analysing also feedwater and permeate.
- Inspect concentrate side of system for scaling.
- Weigh a tail element: scaled elements are heavy.
- Autopsy tail element and analyse the membrane for scaling: the crystalline structure of the deposits can be observed under the microscope. A foaming reaction with acid indicates carbonate scaling. The type of scaling is identified by a chemical analysis, EDXRF or ICP analysis.
- Tip: Scaling is hard and rough to the touch, like sand paper, and cannot be wiped off.
Corrective Action
After confirming the cause of the problem with your RO system, correction action such as a CIP, modifying pre-treatment, increasing the reject flow, or dosing an Anti-scalant can be undertaken. You must always start with reviewing the design by looking at the Software Projection and Water Analysis.
For a general introduction to Reverse Osmosis, please download my presentation. For my prezzy on CIP click here.
I hope you have found this page useful.
Your water treatment expert:
RamiRO
Rami Elias Kremesti M.Sc., CSci, CEnv, CWEM
