Iron Fouling in Membrane Water Treatment Systems

Iron fouling poses a persistent challenge in membrane water treatment systems, impacting efficiency and requiring robust mitigation strategies. Here we delve into the mechanisms of iron fouling,  and effective techniques to combat its effects on membrane performance.

Understanding Iron Fouling:

Iron fouling occurs when either soluble or precipitated iron in the feedwater enters the membrane system and accumulates on the membrane surface blocking the surface and reducing permeability. This phenomenon is particularly prevalent in ground (bore) water sources but can also occur in surface water (rivers and creeks).

Impact on Membrane Systems:

There are several impacts on membrane systems including:

• • Reduced Permeability: Iron fouling restricts water flow through the membrane, leading to decreased permeability and a decline in overall system efficiency.
  • Increased Operating Costs: The need for more frequent cleaning and maintenance to address iron fouling contributes to higher operating costs for membrane water treatment systems.
  • Increased oxidation: Iron acts as a catalyst for oxidation and breakdown of different compounds into oxidizing species (SMBS for example). This can lead to an increased risk of membrane damage and loss of salt rejection’.
  • Antiscalant deactivation: Iron precipitates can also react with antiscalants and deactivate them, increasing the required dose rates (and costs) as well as putting the system at risk from other scales.

Causes of Iron Fouling:

Iron fouling tends to occur due to three main reasons:

• • Natural Presence in Water Sources: High iron concentrations in natural water sources, such as groundwater, can lead to iron fouling. Understanding the source water composition is crucial for anticipating and managing this challenge.
  • Corrosion and Oxidation: Corrosion of pipes and equipment can introduce iron into the water.
  • Biological Activity: Microbial activity in the water can contribute to the release of iron, exacerbating fouling issues. Iron-oxidizing bacteria, in particular, can accelerate the formation of iron deposits on membrane surfaces.

Despite the different sources, the mechanism is largely the same; soluble ferrous iron (Fe²⁺) enters a system then air oxidizes it to the insoluble ferric form (Fe³⁺), as can be seen in the below Pourbaix diagram. This diagram shows what species will be present if you look at the Oxidation Reduction Potention (ORP vertical axis) vs pH (horizontal axis) and also shows that this process can easily occur even at mild pH.

2Fe²⁺ + 1.5O₂ => Fe³⁺ => Fe₂O_{3 (s)}

We will generally see iron in either colloidal form (if it’s entering the system already precipitated) or forming a cake (if it is entering in soluble form and then precipitation in place).

Most membrane manufacturers recommend that iron be removed to below 0.5 mg/L, but problems can occur at even lower concentrations if precipitation is occurring.

Detection:

Detecting iron fouling is generally fairly easy. Water analysis from a NATA accredited lab can show soluble and insoluble iron as well as ferrous iron. Be aware to exclude all air from the sample bottle before sending it in to prevent oxidation during transportation. Additionally, the characteristic orange rust colour can be seen on the front of membranes as well as on other process equipment like valves.

Mitigation Strategies:

Implementing robust pretreatment processes, such as oxidation and filtration, can significantly reduce iron concentrations in the feedwater, minimizing the risk of fouling.

• • Oxidation and solids removal: Oxidation and settling is the cheapest method for removing iron. This can be accomplished as easy as cascading the water down aseries of steps, air sparging or by using oxidizing agents such as hypochlorite or potassium permanganate. The precipitated solids can then be removed by settling or filtration.
  • DMI-65 filters: DMI-65 is a specialized filtration media designed for the removal of iron and manganese from water sources. The media operates through a catalytic oxidation process, where soluble iron and manganese are oxidized and then filtered out of the water.
  • Operating anaerobically (Keeping the iron reduced): There are times when high(er) iron water can be effectively processed by excluding oxygen the water and keeping the iron in a reduced state. While this is known to work, making sure the system remains anaerobic can be difficult as even small air leaks can start the precipitation process. Reducing agents such as SMBS can help. Its been reported that iron concentrations up to 3mg/L can be processed with this approach.
  • Chemical Cleaning Protocols: Regular and targeted chemical cleaning procedures to remove iron fouling are essential. Acid cleaners (such as citric acid or hydrochloric acid) can be effectively in removing light iron fouling, but for heavily fouled systems, mixed fouling or aged deposits then a speciality cleaner is often required.
  • Monitoring and Control: Monitoring of water quality, including iron concentrations, enables proactive management. Adjustments to pH levels and the use of anti-scalants can help control iron fouling.
  • Biocide Treatment: Incorporating biocides into the water treatment process can control microbial activity, reducing the release of iron from biological sources and mitigating fouling risks.

Chemical Treatments

Antiscalant A-119

The A-119 is a unique antiscalant that even prevents iron precipitation after oxidation by air. The active chemistry disperses these metal oxides, and silica particles to prevent their deposition in the membrane system. This unique chemistry is very valuable if the iron is not able to be removed in pretreatment or if the pre-treatment system isn’t able to be easily upgraded. The formulation is designed to continue inhibiting scales even in the presence of iron that would deactivate many other scale inhibitors.

Cleaner C-227

C-227 is a great chemistry that works to remove heavy iron fouling much better than citric acid or hydrochloric acid. As discussed above, iron can be a complex foulant and often mixed with alumino-silicates and biological material. While repeated citric acid cleans can work on mild iron fouling, for heavier iron fouling C-227 will effectively dissolve and remove the fouling very effectively. This cleaner is also very effective at removing aluminium fouling.

Conclusion:

Iron fouling presents a formidable challenge in membrane water treatment systems, but with a comprehensive understanding of its causes and effective mitigation strategies, operators can maintain optimal system performance. By incorporating advanced pretreatment methods, implementing tailored cleaning protocols, and embracing proactive monitoring, the water treatment industry can continue to provide reliable and efficient solutions, even in the face of iron fouling challenges.

Do you have a technology you need to bring to market? or need support piloting a new treatment process. Contact us today to discuss how we can help make this a reality.

Our services you may be interested in:

Process Auditing