Solutions for Premature Filter Fouling
Filter fouling can damage a filtration system, leading to manufacturing disruptions. Properly designed systems are the best way of reducing the probability of fouling the membrane filters, but problems can and do occur. The design parameters to pay attention to include the type of membrane material, pore size, operating pressure, flow rates, temperature, and pH.
Premature fouling is caused by excess particles in the fluid that weren’t expected for the employed filter. For example, bacterial or organic slimes can form extracellular polymeric substances (EPS) called biofilm. Other types are scaling (silica, calcium carbonate) and suspended solids (clay, iron, manganese) which slow fluid flow through the filter, requiring additional filter changeouts, which translate into stopped processes, potential product loss, added costs, and an overall reduction in process efficiency.
The fouling reduction should start by considering simple steps of feed pretreatment rather than cleaning membranes. These approaches are based on prefiltration using a high pore size membrane filter, pH adjustment of the feed, adsorption onto activated carbon, and softening by ion exchange.
This article provides insight and solutions for when one or more filters in a system are experiencing the fouling issue.
Let’s look at a few different scenarios for premature fouling: a single-stage, multi-stage with prefilter issues, and multi-stage with final filter issues. We will presume that the flow rate and desired pressure drop were within the required range upon installation; the volume of fluid being filtered is within the original design limits of the system; and that the filters are chemically compatible with the fluid.
Causes of Premature Fouling
The culprit for premature fouling can be found in the filter, the fluid, a process component, or process activity. Fouling can be detected in your filtration system by a reduction of permeate flux, increase of differential pressure, and poor permeate quality. When a filter has been performing well for its expected life cycle and suddenly underperforms, chances are that the filter fouling is responsible. Inspection of the filter is always the standard operating procedure to follow. The root causes to consider are:
- Fluid hasn’t been tested or has large variations
- Changes in upstream processes have created more particles
- Aging equipment (pumps, valves, piping) shed particles into the fluid
- Changes in fluid sources (vendors, other plants, etc.) that have different kinds/distributions of particles
Single Filter System Fouling
Purified water and other very low particle loads are usually assigned to single filter systems. The source fluid, like municipal tap water, will foul the filters before their time if it contains more particles than anticipated. One of the following options can be used to correct the situation:
- Increasing the filtration area will enhance the dirt-holding capacity
- Change or add filtration media to increase capacity
- Add filter stages to protect costly final/sterilizing filters
The graph below shows Flow vs Time for a filter without prefiltration (rapid clogging) and how that same filter can perform with an adequate prefilter installed.
Flow vs Time Without Prefiltration
Prefilter Fouling in A Multi-Filter System
The graphs below show two scenarios tracking DP vs Time for two filters in a series. One where the prefilter is protecting the final filter but is fouling faster than desired. The other shows the prefilter not fouling, but not providing adequate protection.
Prefilter Fouling
Final Filter Fouling
When the prefilter is the one fouling and the final filter is experiencing minimal pressure drop rise, then the prefilter is being asked to capture too much of the particle load. Prefilters' primary job is to protect the more expensive final filter from fouling; nonetheless, the final filter should still be required to perform some of the system's work. To balance the particle removal load, consider one of the following:
- Use a prefilter with a larger micron rating to allow more of the smaller particles to flow downstream to be removed by the final filter
- Change the media to one with a higher dirt-holding capacity
If the process requires that the prefilter removes all target particles and the final filter is not intended to remove any considerable load, treat the prefilter as a single filter system and follow the options above.
Read how a pharmaceutical company solved its fouling issue.
Final Filter Fouling in a Multi-Filter System
When the final filter fouls prematurely and the prefilter does not experience a significant differential pressure increase, then the prefilter is not performing its function of protecting the final filter. The solution is to use a prefilter with a smaller pore size rating to capture more particles. If the new prefilter also fouls prematurely and the final filter fouls earlier than intended, replace the prefilter with a smaller pore size and follow the options for the single filter system above to improve the capacity of the new prefilter. Furthermore, filtration systems designed for high-fouling fluids frequently feature 3 or even 5 filtration stages.
Each filtration requires its own method to reduce fouling, but the family of methods can be generally described by some approaches that can improve your product quality while reducing process costs. Here are some methods that can be put in place to keep your filtration system running at high performance for a longer time.
- Membrane properties such as the pore size distribution, ionic charge on the membrane, pH tolerance, and hydrophobic capacity need to be suitable for the application to reduce fouling. The bottom line is to select the membrane that is best for your process.
- Cleaning: Use this approach when your process requires it. Several methods should be considered based on chemical compatibility, type of membrane, module configuration, and application.
- Hydraulic cleaning: These are the back-flushing, alternate pressuring, de-pressuring, back-shocking, and reversing the flow direction approaches. The back-shock method is very helpful because the interval of back flushing applied in a few seconds does not allow fouling to occur and the flux remains high.
- Chemical cleaning: This important method is based on the chemical resistance of the membrane. The agents used for cleaning may include bases (NaOH), weak and strong acids (citric acid, H3PO4), disinfectants (H2O2), gases (ethylene oxide), and steam. Please refer to our Chemical Compatibility Chart or contact CPF if you have any questions about your filter chemical compatibility.
Evaluating Premature Fouling & Avoiding the Problem
Searching for the root cause of premature fouling takes time and effort. CPF can provide the services needed on your site or in our lab to help you investigate. The first steps are to determine the nature and source of the foulants, the size, and number of particles, as well as their materials. Inspection of the filter media and components will determine if the fluid and/or contaminants had any unexpected effects on the filters themselves. Reviewing the filter specifications will confirm if its intended use is in line with the source fluid and task. All results will guide further actions and filter choices.
The key to avoiding premature fouling is to evaluate fluids before system design to make sure that contaminant size and quantity are appropriate for the selected filters and the fluid.
Testing & Analysis Services
CPF’s Technical Team is experienced, equipped, and available to help you design systems that avoid premature fouling either in response to one of the scenarios above or as an up-front system design. As each facility, fluid, and filter system design is different; our experts tailor filtration solutions that work for your operation. Learn more about Critical Process Technical Services.
