Water treatment solutions –about filter bag and filter cartridge

In many filtering applications, a choice between the use cartridge filter or a bag filter has to be made. Both are sediment filters, that is to say they reduce the amount of sediments transported by the fluid trough filtration.
There are some differences between these two filter systems:

- The choice of cartridge filter depends on the application. Cartridge filters are preferable for systems with contaminations lower than 100 ppm, that is to say with contamination levels lower than 0.01% in weight.

Cartridge filter can be surface or depth-type filter: depth-type filters capture particles and contaminant through the total thickness of the medium, while in surface filters (that are usually made of thin materials like papers, woven wire, cloths) particles are blocked on the surface of the filter.
Surface filters are best if you are filtering sediment of similar-sized particles. If all particles are i.e. five micron, a pleated 5-micron filter works best because it has more surface area than other filters. Compared with pleated surface filters, depth filters have a limited surface area, but they have the advantage of depth.

It can be generally stated that if the size of filter surface is increased, higher flows are possible, the filter last longer, and the dirt holding capacity increases. Cartridge filters are normally designed disposable: this means that they have to be replaced when the filter is clogged.

- Bag filters are in general frequently used for dust removal in industrial applications. The flow can be from the outside to the inside of the filter (that means, the separation of particles happens on the external surface of the filter) or the other way around, depending on the application. The particles are normally captured on the internal surface of the bag filter.

Bag filters are in general not designed for replacement when they are clogged, but some bag filters for gaseous applications like dust removal can be cleaned, for example by mechanical shaking or by backwashing with compressed air (so called reverse-flow bag filters).

Bag filters are mostly surface-type filters.

- A rule of thumb is that for concentrations higher than 5 mg/m³ a surface filter is favored, while for concentrations lower than 0.5 mg/m³ a depth-type filter is preferred. In general surface filters can by backwashed and cleaned more easily, while depth-type filters normally have to be disposed when clocked.

Bag filters for water treatment overview

HOW BAG FILTERS WORK

Like its name suggests, a bag filter is a small bag made of a filtering material that attaches to a sturdy ring at the opening. The bag sits inside a filter housing and as the bag fills with water, it seeps through the filter's tiny holes, leaving behind unwanted contaminants. When you want high flow rate and basic purification, a bag filtration is a great solution.

WHAT IT FILTERS

A point of entry water system, the primary job of bag filters is to reduce the amount of silt, sand, dirt and other types of sediment in your water. Some bags have a micron rating small enough to filter out some particulates, but they will not clean out bacteria like e. coli or chemicals like chloramines. To remove contaminants like those, additional filter systems will be needed.

KEY USES

Bag filters are unique in that their primary use is in commercial, agricultural and industrial settings. Rarely, if ever, is a bag filter recommended for residential filtration. What makes a bag filter best for these industries is its ability to capture sediment without slowing down the flow rate. If a large piece of sediment is trapped in the bag, water can just flow around it because there is still a plenty of open surface area. This differs from filters that have small openings - if the filter surface is blocked by sediment, the water gets slowed or even stopped. Not only is maintaining flow rate beneficial, but you can remove the filter from its housing and manually remove the trapped material. The reusability is another key benefit of a bag filter.

CHOOSING A BAG FILTER

If removing sediment is your priority, a bag filter system is a great choice. With so many options available, it can feel overwhelming to decide which is right for you. To help narrow the selection process, consider these key questions:

·         What type of sediment are you filtering out? By knowing the size of the contaminant, you can determine the micron rating (size of holes in the filter) and media type (polyester, polypropylene or nylon) that will perform best. Additionally, the type of sediment will influence the kind of filter housing you should use.

·         What is the pH level of the water being filtered? It's important to know the pH of your water because the housing material can be impacted by it. If the water needing filtration has a higher or lower pH, choose a different housing material.

·         How much water is being used on a daily basis? If you anticipate needing to filter a high volume of water, you'll need to select a system that can support it.

·         What is the maximum and minimum water temperature? Filters perform differently at different temperatures, so selecting a filter that is best at high temps will not be useful if your water temp is low.

·         What is the ideal flow rate? Flow rate and micron rating are tied to one another - a higher micron rating has a greater flow - so it's important to balance the sediment size and desired flow rate to create a filtering system that meets your needs.