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Key concepts in informed filter selection

The filtration system is a key part of any successful irrigation system. With so many factors affecting the workings of a filter and how it should be implemented, filter solutions will never be ‘one size fits all’ or ‘plug and play’. Each irrigation system and the circumstances under which it operates will require a different filter setup, as well as various operation and maintenance activities.

Filters have an important function

A filter prevents solid particles of a predetermined size and larger, from entering the irrigation system. The goal is to protect the emitter and the efficiency of the entire system. As water passes through the filter, unwanted solid particles are caught in the selected filter medium, be it disc, screen or media (sand or gravel).

To make an informed decision and finally select the correct filter for an irrigation system, it is important to consider all the factors that will affect the type of filter, filtration grade and filter capacity.

These factors include:

  • Type of irrigation emitter.
  • Water quality.
  • Required flow rate.
  • Type of water source.
  • Available flow rate.
  • Available pressure for head loss.
  • Minimum and maximum system pressure.
  • Available energy source.
  • Preferred maintenance practices on site.

From this list, the three most important factors to consider are the type of irrigation emitter in use, water quality and the required flow rate.

System capacity

Water quality and required flow rate will determine the capacity or size of the filtration system. When embarking on developing an irrigation system for a new development, one of the first questions asked will relate to the quality of the available water. No filtration decisions, and by extent no irrigation decisions, can be made without knowledge of the water source and quality. Water quality is influenced by various factors, including solid particles (inorganic or organic) in the water, chemical composition, biological factors, temperature and oxygen levels.

It is crucial to continuously monitor water quality as it may vary during the production period as seasons and other variables change.

Although it is not the only factor to consider, the amount of total suspended solids (TSS) in the water indicates estimated water quality. The quality of reservoir water may change frequently as water comes from different sources. Furthermore, the quality of water available for irrigation can be influenced by various environmental and operational factors.

Factors that may have an impact on water quality include:

  • Type of water source.
  • Seasonal changes.
  • Rainfall, temperature and other climate conditions.
  • Placement of suction point.
  • Holding time.

Filter types

There are three main types of filtration systems. These are media filters (sand or gravel), disc filters and screen filters. Each type has different features, advantages and shortcomings that make them suitable for various irrigation scenarios. Let’s consider how each filter works to evaluate the value of each of these filter types.

Screen filters

A screen filter consists of a fine stainless-steel mesh or screen that is formed into a cylindrical cartridge. Screen filters offer surface filtration, which means there is only one layer of protection or only one chance for the filter to catch the particles of a predetermined size and prevent clogging in the irrigation system. If this layer fails to catch a particle, there is no second or third chance to catch it in subsequent layers.

In a screen filter the unfiltered water enters through the inlet towards the outside of the coarse screen or strainer. Water then passes through the coarse screen or strainer from the outside to the inside of the coarse screen. The outer coarse screen or strainer has relatively big cylindrical holes to prevent the occasional larger objects, such as rocks or debris, from entering the fine screen section of the filter. The coarse screen or strainer reduces the possibility of large objects damaging the fine screen.

When water enters through the coarse outer layer it flows towards the filtering chamber. It then passes through from the inside to the outside of the fine screen where particles will be caught. The filtered water exits through the filter outlet to the irrigation system.

The pressure differential between the inlet and outlet increases as the fine screen accumulates more dirt. When the pressure differential reaches a preset level, the controller initiates a backflush sequence which opens the flush valve. When the flush valve is opened, it creates a loss of pressure in the hydraulic flushing chamber which causes the suction assembly to start vacuuming the inside of the fine screen, removing the particles through the backflush manifold.

Disc filters

Disc filters are comprised of a cylinder of compressed discs. Each disc has grooves that run in alternate directions that criss-cross when two discs are placed on top of each other, forming a series of meshes. The grooves form passages of varying dimensions through which the water passes from the outside to the inside of the discs. Disc filtration is multi-layered and offers depth filtration. If the first layer does not catch a particle, there are consecutive chances for it to be caught in subsequent layers.

Water is filtered as it enters the cylinder of compressed discs from the outside to the inside. The number of layers depends on the selected micron rating of the discs. In the case of Netafim’s unique disc design, there can be between 18 (for 400-micron discs) to 32 (for 20-micron discs) layers or stopping points in each track, resulting in unique depth filtration.

Furthermore, the design allows the stopping points to have the same micron rating throughout the entire disc. As the particles collect on the discs, the pressure differential will increase until it reaches a preset level, where the controller will then activate the backflush mode. The inlet valve will close, preventing the unfiltered water from passing through, and the outlet towards the backflush manifold will open. The downstream pressure is now higher than the upstream pressure, which is opened to the drain.

This causes the compression spring inside the spine to release and the backflush process starts. The spine piston rises, releasing the pressure on the discs. Downstream clean water flows from the spine and out of the jets towards the discs. This jet of water spins the discs free and clear, loosening the trapped solids and ejecting them out of the system through the backflush manifold. After the backflush cycle is complete, the inlet valve opens, causing the spring to compress the discs back into filtration mode.

Media filters

A media filter consists of a pressurised tank containing the filtration medium, which is generally a crushed basalt or a graded silica sand. Media filtration, like disc filtration, is multi-layered and offers depth filtration. If the first layer does not catch a particle, there are second, third and consecutive chances for it to be caught in subsequent layers.

The media is held inside the tank by slotted devices that allow the filtered water to pass through to the irrigation system but retain the media. This media must be angular with sharp edges to provide the best environment for trapping debris. The media must be replaced when the gravel or silica sand wear and become rounded.

The slotted device can collapse, and the filtration media can enter the irrigation system causing blockage of the microsystem. Therefore, media filters should always be combined with a secondary check filter of the same filtration grade to catch the loosened media and protect the irrigation system.

The unfiltered water enters the pressurised tank from the top where a plate disperses it onto the media. This plate prevents the unfiltered water from hitting the media directly when entering the tank and reducing the compaction of the media. The water is filtered as it passes through the media from the top of the media to the bottom, where it then enters the slotted devices towards the irrigation system.

As the particles accumulate in the media and reach the set pressure differential, the controller activates the backflush mode. The controller opens the valve to the backflush manifold. The water enters the tank from the bottom, pushing the media apart while flushing all the debris towards the top and out of the tank through the backflush manifold.

Filter selection

Although it is very simplified and each choice will be determined by the unique circumstances of an irrigation project, the table below indicates filtration type selection based on physical water quality and the type of irrigation used. Filtration selection is not straightforward, as it involves a long list of variables.

Quality and knowledge

Filtration technology has greatly improved over the past few years, which means that clogged emitters pose less of a challenge in modern irrigation systems. However, we have to deal with deteriorating water quality, which once again increases the risk.

Finally, remember the importance of using quality filtration systems. Do not cut corners when planning the filtration section of an irrigation system. The success of filtration will determine the efficiency and sustainability of the entire system. Do not allow filters to cause a weak link in a well-designed system.