1. Introduction: membrane fouling in reverse osmosis systems
One of the main concerns for any Reverse Osmosis operator is membrane
fouling, as it is the primary reason for excessive downtime, high operating costs, and poor membrane performance. Ultimately, membrane fouling can lead to the undesired need for replacing all or a number of membranes.
Membrane fouling depends on a number of RO operational factors. The most important are:
- Feedwater quality
- System recovery rate
- Effectiveness of the water pre-treatment
RO membrane foulants are inorganic and organic particles and bacteria found in the system feed water, that cannot pass through the RO membranes. These particles get trapped and accumulate on the membrane surface creating a barrier that blocks the water permeability and reduces the RO membrane performance.
A significant membrane fouling can be flagged when there is a 10-15% decrease in permeate flow, a 10-15% decrease in permeate quality, or a 10-15% increase in pressure drop across the membrane train.
When membrane fouling happens more than once a month after cleaning, an improved RO pretreatment or an RO plant re-design must be considered.
2. Membrane foulants in reverse osmosis systems
In most cases, membrane fouling is the result of inadequate pretreatment or a combination of it with variable water quality:
2.1 Suspended Solids
Suspended solids (SS) are measured using turbidity and Silt Density index (SDI). The lower the turbidity and SDI, the lower the risk of membranes being fouled with suspended solids.
Best practices recommend a reverse osmosis feed water turbidity below 0.5 NTU and SDI below 4.
Reverse osmosis membranes fouled with suspended solids will typically present a lower permeate flow and an increase in pressure drop.
|Feed Water Quality Parameter||Parameter Considerations|
|Turbidity (NTU)||Maximum NTU < 1, recommended NTU < 0.5|
|Silt Density index (SDI)||An SDI greater than 4 means that an improved pre-treatment is required|
|Total Suspended Solids (mg/L)||Does not correlate well with turbidity, however if TSS > 1 mg/L, pretreatment needs improvement|
2.2 Metals: Iron, Manganese and Aluminum
RO membranes will be fouled with precipitated metals such as Iron, Manganese, and Aluminum, present in many underground waters. These metals can also catalyse the effect of any oxidant in water, damaging the membranes permanently.
Other reasons why Iron and Aluminum can be present in the RO feed are overdosing of coagulant (Iron or Aluminum salt) or corrosion of pipes and tanks upstream of the reverse osmosis plant.
When Iron and Manganese are dissolved in reduced form (Fe2+ or Mn 2+) and their concentration is below 1.0 and 0.1 mg/L respectively, they can be removed by RO membranes without any fouling risk. However, when Iron and Manganese are oxidized (Fe3+ or Mn 3+) in precipitate or colloidal form, their levels should be reduced below 0.05 and 0.02 mg/L respectively, to prevent mineral fouling of the reverse osmosis membranes.
Aluminium’s minimum solubility is at pH 5.8. If the RO runs at pH 7–9, which is quite common, this should keep the aluminum in solution preventing any problems to the reverse osmosis plant.
Iron and Manganese can be removed from water as part of the RO pre-treatment by means of specific Iron removal filters.
Metal fouling will increase pressure drop and decrease permeate flow. However, when metals oxidize the reverse osmosis membranes, the result will be a lower salt rejection and higher permeate flow.
2.3 Hydrogen Sulfide (H2S)
Hydrogen sulfide is a gas typically found in well water as a result of sulfate-reducing bacteria oxidizing organic matter.
As any other gas, Hydrogen Sulfide is not rejected by reverse osmosis membranes, an will be in the same proportion in all flows. However in the presence of metals such as Iron and Manganese, the oxidation and formation of elemental Sulfur (S) and insoluble metal sulfides can occur.
Sulfur (S) is a yellow-white substance that precipitates in the membrane surface and sealing rings. Metal sulfides can also precipitate in the membranes.
Similar to other scales, Sulfur and metal Sulfides will increase pressure drop and decrease permeate flow.
Silica (SiO2) is quite common in underground. Depending on the PH it can be present as “reactive silica”, which is quite soluble or “unreactive silica”, present in colloidal form.
Dissolved silica is not a problem for RO membranes and an be rejected as any other dissolved salt. However, colloidal silica which is a a soft gel-like substance, can cause significant membrane fouling.
Metalssuch as Iron, Calcium and Magnesium can also react with Silica to form silicate compounds like Iron Silicate or Calcium Silicate that also present a significant risk to membrane fouling.
Silica fouling will increase pressure drop and decrease permeate flow.
2.5 Microbial and Biofilm
Microbial foulants are microorganisms, typically bacteria, and organic compounds segregated by them. These foulants form a biofilm on the membrane surface responsible for higher pressure needs and lower permeate productions.
Sometimes bacteria growth affects the whole membrane. However it is also common to find bacteria colonies populating only certain areas what can lead to increased concentration polarization and salt passage.
Bacteria can be deactivated before the RO process by means of water disinfection systems, however when oxidants are used they must also be removed before the RO process to protect the membrane from oxidation damages.
Polymers used as flocculants for water conditioning before filtration are another common RO membrane foulants.
When dosed in excess or poorly mixed, those polymers don’t form bigger conglomerates and are not removed during filtration. When they reach RO membranes they create a fouling film that prevents the water flux, increasing the pressure drop and decreasing the permeate flow.
Hydrocarbons are contaminants not found in water unless they have been leaked from human activities such as wastewater discharge or ship spills.
Similar to polymers, they have a very significant fouling effect even in very low concentration and must be prevented from entering the RO plants.
2.8 Calcium Carbonate (CaCO3) and Calcium Sulfate
Although scaling and fouling are different principles, we will include the scaling of Calcium salts in this discussion.
Salcium carbonate is the most common mineral scaling in brackish water RO desalination plants while Calcium Sulfate is the most frequent foulant in SW RO systems.
The solubility limit of Calcium salts tend to increase with the water salinity, what means that the higher the salinity of the feed seawater, the lower the Carbonate scale risk. At the same time the solubility limit is also PH dependent, with a higher solubility at lower PH.
For the above reasons, brackish water reverse osmosis plants where higher recovery rates are expected are more likely to face Calcium scale issues.
Calcium Sulfate scale is a much harder than Calcium Carbonate and is more difficult to remove
When the feedwater temperature exceeds 35°C, Calcium Carbonate precipitation and fouling takes place at an accelerated rate, so feedwater temperature is an important parameter to be monitored when the risk exists.
Calcium Carbonate and Calcium Sulfate scaling can be prevented by means of industrial water softeners.
Calcium Carbonate and Sulfate scaling is relatively easy to detect and manage. The main symptoms are low permeate production and low salt rejection. The low salt rejection is a consequence of the concentration polarization, in which the membrane surface is exposed to a higher salinity concentration.
3. How membrane fouling impacts reverse osmosis plants
As detailed in the previous paragraphs, membrane fouling can be responsible for one or more of the following effects:
- Increased pressure loss
- Increased salt passage
- Loss of flow (this is often the parameter that flags the warning)
In order to detect the membrane fouling, react in time, and decide whether a chemical cleaning is necessary, monitoring the main plant parameters and performance is key to recognizing when membrane elements are becoming fouled.
4. Conclusions: membrane fouling reduces reverse osmosis performance
Reverse osmosis foulants are small particles and bacteria found in the system feed water, that get trapped and accumulate on the membrane surface, preventing water permeability and reducing the membrane performance.
Watercore can help preventing RO membrane fouling and reducing its impact on the overall RO desalination system with the following procedures: