Separator Filter vs Bag Filter: Which Pre-Stage, When

The water coming from the well looks clear; and yet the cartridge filters block within weeks, the pump is under constant strain and the membrane loses its performance far earlier than expected. The maintenance items pile on top of one another and the cost becomes unpredictable. The shared cause of that picture is usually one thing: coarse particles are entering the system without being separated out at all. A separator filter is a pre-separation unit used to close exactly that gap.

In this article we take up how a separator filter works, what it separates and what it does not. Then we compare it with the bag filter heading by heading, and explain which is preferred when and how the two are used in sequence.

What Is a Separator Filter and How Does It Work?

A separator filter is a mechanical pre-separation unit that spins the water inside its body and separates the particles heavier than water by centrifugal effect. The sand and sediment separated out settle into the chamber at the base of the body, while the cleaned water carries on into the system through the top outlet. The particles collected are discharged through the drain valve.

The working principle is not a straining process; that is the point most often confused. There is no screen or porous material inside the unit holding the water back.

The water enters the body tangentially and starts to spin inside it. That spinning motion applies an outward force to the particles heavier than water. The particles move towards the wall of the body and travel downwards into the collection chamber at the base.

At the centre, the water now cleared moves upwards and carries on into the system through the outlet. Two streams are formed in that way: the water going into the system and the particles collecting at the base.

A A separator filter therefore contains no consumable. The particles held collect not in a cartridge but at the base of the body and are discharged regularly.

Why Is a Pre-Separation Stage Needed?

When sand, gravel and coarse sediment reach the main systems, they fill the cartridges quickly, put a load on the pump and wear the membrane and resin surfaces mechanically. A pre-separation stage removes those particles from the system at the very start and so protects every stage behind it.

The basic logic of a treatment chain is to move from coarse to fine. Every stage protects the one after it; when that order is broken, the most expensive components wear first.

What happens where no pre-separation is done is this:

The third card is the most decisive in terms of cost. A relatively low-cost pre-separation unit protects a far more valuable component for years. We took up why the membrane is so sensitive and how its surface wears in our membrane filter article .

On plants using well water that stage is all but essential. We examined which problem in well water is handled by which stage in our treatment stages for well water .

The general framework of filtration stages and the coarse-to-fine chain logic are taken up in the technical literature too; the Pennsylvania state environmental agency conventional filtration training module explains that approach in a general framework.

What Does a Separator Filter Separate?

This unit separates only the solid particles heavier than water: sand, silt, coarse sediment and particles of rust. Because the separation rests entirely on a difference in density, it has no effect on components in solution in the water. That distinction is decisive for setting the right expectation.

The principle itself already defines that limit. A centrifugal effect only works on a substance denser than water. A component in solution has no density of its own; it moves along with the water.

The types of particle it separates are these:

• Sand and gravel: The most common and heaviest group of particles in well water.
• Silt and coarse sediment: Granular material from underground and surface sources.
• Particles of rust: Metal particles breaking away from old plumbing.
• Pieces of scale: Hard residue breaking away from deposits inside the pipe.
• Heavy sediment: Solids markedly denser than water.

That list draws the field of the unit clearly. A A separator filter separates that group effectively; the headings outside it are the work of different stages.

Two factors affect the efficiency of separation: the density and the size of the particle. Heavy and coarse particles are separated easily, while a different stage is needed for light and very fine suspended matter.

Figure 1: The centrifugal separation principle in a separator filter

How Does a Bag Filter Work?

In a bag filter the water is passed through a bag placed inside the body. The particles are held on the surface and in the weave of the bag while the cleaned water leaves the body. When the bag saturates it is replaced; so it is a stage that contains a consumable.

The principle is direct straining. The weave of the bag the water passes through forms a barrier; the particles that cannot pass through the weave collect inside.

That is the basic point where it differs from the separator: here the separation rests not on a difference in density but on a physical barrier. And that lets it cover a different range of particles.

The working character of a bag filter is defined by these headings:

Changing the bag is a simple operation in operating terms; but when it is not done regularly the flow is restricted and the system pressure falls. So the monitoring is carried out through the pressure difference.

The Difference Between a Separator Filter and a Bag Filter

The two units work on different principles. The separator separates the particles heavier than water by centrifugal effect and contains no consumable; the solids collected are discharged. The bag filter strains, and the saturated bag is replaced periodically. That difference settles the operating load and the field of use.

The table below compares the two units heading by heading:

The “consumable” row in that table is the most decisive in terms of operating cost. When a bag filter is used on a well with a high sand load, the bag is changed very often; that creates both spending and labour.

The “behaviour of flow” row matters operationally. In a separator the solids held collect at the base and do not narrow the path of flow directly; in a bag filter the resistance rises as it fills and the system pressure is affected.

That comparison is not a ranking of superiority. The two units address different problems; which one suits is established by the type of particle, the load and the flow.

Which Is Used When?

On wells where the sand load is high and continuous, the separator is preferred; because it runs without a consumable its operating load is low. On lines where the particle load is variable or finer, a bag filter is used. Where the load is both high and varied, the two units are built in sequence.

The decision is made with three questions: how coarse are the particles, how heavy is the load, and how continuous is it?

The typical scenarios separate out like this:

The fourth row is the build met with most often in practice. The separator takes the coarse load and markedly reduces the amount of particles reaching the bag; and that lowers how often the bag is changed.

That pair is common on irrigation and greenhouse sites; the particle load has to be reduced in stages to protect the drip lines. We took the subject up in our irrigation water treatment article; for greenhouse applications you can look at the greenhouse water treatment systems can be consulted.

On production plants, pre-separation is assessed for protecting the process line. We took up the general framework in our process water article .

Figure 2: The filtration chain built from coarse to fine

Its Relationship With the Sand Filter

The separator is the coarsest stage; a bed-type sand filter is the next step, holding finer particles. The right order runs from coarse to fine: first centrifugal separation, then bed filtration, then the cartridge stages, and advanced treatment last.

Those two units are not alternatives to one another; they are different links in the same chain. When they are confused, either the coarse load reaches the bed or an unnecessary stage is added.

A typical chain is built like this:

How bed filtration works, the logic of backwashing and when it is preferred we took up in our sand filter article article; we do not go into the detail of the bed here.

On sources with a high particle load, automatic backwash units are preferred; the industrial automatic sand systemsused for that need make the step after the separator.

Breaking the order produces this result in practice: when coarse sand reaches the bed directly, the surface of the bed blocks quickly and the backwash grows more frequent. With a A separator filter positioned in front, that load largely disappears.

What Does a Separator Filter Not Remove?

This unit removes none of the components in solution in the water. It has no effect on hardness, chlorine, odour, the dissolved solids load or metal ions. Those headings call for separate stages such as softening, activated carbon or reverse osmosis.

This section was written to prevent buying with the wrong expectation. The field of the unit is clear and its limits are just as clear.

The right-hand column is the list to know before buying. A plant with a limescale problem that fits a A separator filter and expects the problem to be solved will get no result; removing hardness is the work of an entirely different stage.

In the same way activated carbon is needed for chlorine-related taste and odour, and reverse osmosis for the dissolved solids load. Those stages do not take the place of pre-separation, and pre-separation does not take theirs.

Maintenance and Operation

On a separator, maintenance consists of discharging the collection chamber at the base regularly. On a bag filter the bag has to be changed periodically. Both units are followed through the inlet-to-outlet pressure difference; the frequency is set by the particle load.

The maintenance character of the two units differs markedly, and that difference bears directly on planning the operation.

1. Empty the Drain Valve Regularly

   The solids collecting in the separator are discharged through the base valve. The frequency is set by the particle load; on sources with a high sand load it is done more often.

2. Watch How Full the Chamber Is

   When the chamber fills, the efficiency of separation can fall. So discharging should be tied to a routine, with a calendar used rather than observation.

3. Follow the Pressure Difference

   The difference between inlet and outlet is the most practical data showing the state of the system. A rise in the difference points to saturation on a bag filter.

4. Renew the Bag on Time

   A saturated bag restricts the flow and lowers the system pressure. When the change is put off, the stages behind are affected too.

5. Check the Body and the Gaskets

   The cover gasket is reviewed while the bag is being changed. A tired gasket leads to leaks.

6. Keep Records

   When the discharge and bag change dates are recorded, the period becomes predictable and planning gets easier.

It is not possible to give an exact period; the frequency varies with the particle load of the inlet water and the working flow. The same unit calls for an entirely different maintenance rhythm on a different well.

The drain connection is part of the planning too. The discharge line of the separator should be led to a point where emptying can be done practically; otherwise the operation becomes difficult and gets put off.

How Is the Right Unit Chosen?

Six headings are assessed in the choice: the working flow, the particle load and size range, the connection size, the body material, the discharge and drain access, and the installation space. The exact sizing is done from the water analysis and the working conditions of the line.

A unit sized wrongly produces two different problems: chosen too small, the separation efficiency falls; chosen too large, the flow speed inside the body may not create enough centrifugal effect.

The headings to assess are these:

• The working flow: The unit should be chosen to suit the real flow of the line.
• The particle profile: Whether the load is coarse or fine determines which unit suits.
• The connection size: The inlet and outlet connections must be compatible with the existing line.
• The body material: It should suit the working conditions and the environment of use.
• Access for discharging: Reaching the emptying point and the drain connection should be planned.
• The installation space: Enough room should be left for the unit and for working on it.

In practice the build takes shape around the need. For a coarse and continuous sand load, separator filter systemsare preferred, and for a variable and finer particle load, bag filter units .

Water Point Su Arıtma Teknolojileri operates from Esenyurt, İstanbul under its founder Ali Genç, with more than 20 years of experience in the sector.

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