Cartridge filter autopsies

Why would you do an autopsy on something that costs $100?

This is usually the first question that is asked. Why would you spend money on testing something so simple? There are a number of reasons, but put simply it’s because cartridge filters are very easy to remove and provide a lot of insight into performance problems.

Cartridge filters are the canary in the coal mine

We see cartridge filters as the canary in the coal mine, as these little devices can help to detect problems that are occurring in your pre-treatment, which then flow on to your RO/NF system.

Join us now as we talk through some of the different aspects of conducting autopsies on cartridge filters.

Types of cartridge filter

A cartridge filter is a piece of tubular filtration equipment used across various industries and household applications. A cartridge is encased within a housing or a casing and used to remove unwanted particles, pollutants, and chemicals from liquids. The water flows inside the housing and passes through the filter element, removing suspended particles.

There are two broad categories of cartridge filters –

Surface filters –that filter solids out on the surface and do not let contaminants enter through the medium. This is similar to a coffee filter or filter paper
Depth filters– that use a maze of material to trap the contaminants within the media

In broad terms, surface filters are generally cheaper and have a shorter shelf life compared to depth filters. However, over time, both media will become filled with contaminants. Once this occurs, surface media are more easily cleaned than depth filters. In reality, cleaning of filters is difficult and replacement is the most common solution.

Some of the more commonly used filter media include:

Melt blown cartridge filters – Melt-blown filters have a unique construction formed by thermally moulded polypropylene fibre layers that offer excellent depth filtration, higher filtration efficiency, long service life and thermal stability. They are designed to prevent contaminant unloading and ensure maximum contaminant removal.

String wound cartridge filters – String wound cartridge filters are engineered to offer excellent depth filtration and strong chemical resistance. A yarn is spun over a core in a honeycomb weave on CNC machines that produce precise micron rating. The winding gets tighter towards the core to offer gradient depth for better filtration.

Pleated cartridge filters – Pleated cartridge filters are made from polymeric media that is pleated in a vertical direction and have a combination of higher surface area, higher dirt holding capacity, and excellent micron ratings. These cartridges are suitable for use in critical applications, in food and beverage, biotech, pharmaceuticals, breweries, and water treatment sectors.

Metallic filter elements – These can be either surface filters or sintered metal for depth filtration. One of the biggest advantages of metallic filter elements is that they can be easily backwashed and re-used for hundreds of cycles without impacting performance. However, they are rarely used in normal water treatment, where cheaper materials exist.

Radial cartridge filters – Radial cartridge filters are unique in that they have a moulded one-piece structure, and despite having a pleated media they do not need a protective cage. Owing to the radial shape, these filters offer more surface area than pleated filters. Since they are built from a single layer and have a robust construction, they can be backwashed without harming the pore structure.

Vertical Pleated Filter (Pacific Water Technology)

Spiral Wound Filter (3M)

Stainless Filter (Sanyfilter.com)

Radial pleated filter (environmental xprt)

Blown Cartridge Filter (Pall)

Key problems seen in cartridge filters:

Bacterial fouling or issues with bacterial control in upstream processes like multi-media filters
Colloidal or inorganic silts fouling from high turbidity water getting through the pre-treatment system
Organic fouling from Natural Organic Matter (NOM) or other man-made or natural sources
Fouling from flocculants or coagulants used in up-stream equipment

Limitations of conducting autopsies of cartridge filters?

Unfortunately, not all RO problems can be identified by looking at the cartridge filter system. In particular, issues that are driven by the concentration process on the membrane surface, such as scaling, cannot always be seen. However, there are times when scaling is occurring upstream and depositing on the membrane, which may also cause fouling of the cartridge filters.

Other problems that are specific to the membrane system include mechanical damage driven by pressure changes or operational issues, and damage to the membrane surface from oxidation. In this case a RO Membrane Autopsy is the best path forward.

What are the benefits of conducting an autopsy on a cartridge filter?

One of the biggest benefits to testing a cartridge filter is that they can be easily removed and tested without needing to open and remove a selection of membranes. Even looking at the replacement frequency of the cartridge filters can help you understand that there is a problem, and where to start looking to improve. You are also effectively isolating the pre-treatment system for analysis.

Another key thing that can be done, that is harder to do on RO membranes, is to look at the impact of cleaning chemicals on the deposit. Evaluating the impact of mild caustic solutions, acids, even solvents or oxidizing agents like hypochlorite can give a great indication of what fouling is occurring.

What testing can be done?

At Membrane Works, we look at a number of different tests on cartridge filters. We often adjust the testing programme based on what we see when we receive the filters. Generally, a fairly common testing regime would be:

Visual inspection –we look at the fouling layer, its texture, distribution and smell which all give us some indication of what could be happening
Electron Microscopy (SEM-EDX) –electron microscopy can be difficult due to the way that the filter media develops an electronic charge in the microscope. However, using different techniques we can usually get some good images and more importantly use the EDX to analyse the deposit. This is critical for identifying clay and silt deposits that are not able to be extracted for other analysis techniques.
Bacterial activity –we will conduct a bacterial activity test on the filter, to see if this is an issue. While bacteria are always present on filters, if there is a particularly high level, or if the bacteria are clearly stressed and producing EPS protective layers, then the site can look at mitigating this.
FTIR-ATR –FTIR is a good tool for identifying contaminants. We use our extensive library of known contaminants to help us narrow down what is fouling the membrane.
Loss on Ignition (LOI) –LOI involves heating a deposit through a series of temperatures to burn off organic and inorganic components. This is very useful but relies on being able to isolate a reasonable amount of contamination, therefore it is not usually possible with depth filters.
Fouling extraction study:
- Hot water extraction – looks for extractable materials like flocculants and coagulants. The extract is dried and analysed by FTIR to identify the contaminants
- Caustic (base) extraction – caustic removes most organic foulants, including those that are from natural sources and those that are man-made. We will analyse this using either Dissolved Organic Carbon (DOC) or our unique Organic Fouling analysis tool
- Acid extraction – acids will extract many inorganic foulants (but not clays or silts) and this can give an indication of inorganic fouling. The extracted solution is analysed for composition using ICP-OES or ICP-MS
- Oxidative extraction (hypochlorite)– hypochlorite is a stronger cleaning agent and will readily remove oxidizable deposits, especially EPS and other biological contaminants.