Selecting the correct filter for the job

Selecting the correct filter for the job

The purpose of filtration in an irrigation system, is to protect the emitter, and to an extent, the efficiency of the entire system. With precision irrigation systems, such as drip and micro-sprinkler irrigation, the risk of emitter clogging increases as emitters are smaller. It becomes increasingly important to make the correct choices in filter selection. Filtration must be designed meticulously, as it is a very expensive component of the irrigation system and will determine the efficiency and sustainability of the entire system.

A host of factors must be considered when making decisions around: the type of filtration system, the filter grade as well as the capacity of the system.

The three most important factors are:

  • The type of irrigation emitter selected – drippers or micro-sprinklers
  • Water quality
  • Flow rate required.

The type of emitter selected will determine the grade of filtration required and is often the most important factor to consider. The grade of filtration must be equal to or smaller than a fifth of the emitter outlet size. The water quality and the flow rate required will determine the capacity (size) of the filtration system.

Factors to consider in filtration system selection:

  • Minimum and maximum flow rates required
  • Minimum and maximum system pressure
  • Outlet size of the emitter and type of irrigation system
  • Availability of power (Is AC power available or must we provide for DC power)
  • Water quality and water source
  • Maximum allowable pressure loss (pressure available for head loss)
  • Maintenance capabilities

Water quality wisdom

As we all know, water quality is one of the main determining factors in filtration selection. Understanding water quality classification, can allow you to make a better decision around filtration selection. The quality of water is determined by solids (organic or inorganic) in the water, the chemical composition of the water and other factors. No filtration decisions, and by extent no irrigation decisions, can be made without knowledge of the water source and water quality.

Types of filtration

There are three main types of filtration systems, namely media filters (sand/gravel); disc filters and screen filters. Each of these types have different advantages and shortcomings that make them suitable for use in different situations. Furthermore, the filtration process is divided into pre-filtration, primary filtration and secondary filtration.

Most importantly, it is important to understand the difference between surface filtration and depth filtration when the selection of filter type is made. The greatest difference being that depth filtration offers several layers of filtration, while surface filtration offers only one level.

Media filters offer depth filtration and consist of a pressurised tank containing the filtration media.  Water enters through the filter inlet and percolates through a thick layer of graded particles. 

Disc filtration also offers depth filtration. There are second, third and successive chances to catch the contaminant in the subsequent layers.  A disc filter comprises of a cylinder of compressed discs enclosed in a corrosion resistant housing.  Each disc has grooves that run in alternate directions which 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.  The particles to be filtered are trapped between the discs and filtered water continues through to the irrigation system

A screen filter offers surface filtration. It consists of a fine stainless-steel mesh or screen that is formed into a cylindrical cartridge.  Water passes through from the inside to the outside of the screen and then to the irrigation system.  The particles to be filtered are caught on the inside surface of the screen. 

Filter selection

Although it is very simplified and each choice will be determined by the unique circumstances of an irrigation project, the table below gives an indication of filtration type selection based on physical water quality and the type of irrigation used.

Common mistakes

It is clear that filtration selection is not a straightforward matter, as we are working with a very long list of variables. Given the intricacy of the equipment and subject matter, there are several common mistakes that many a designer makes, even after the error of selecting the wrong filtration type has been avoided.

A few tips to remember:

  • Do not underestimate the importance of regular maintenance in filter success
  • Do not omit the extra water required for a backflush cycle in total flow calculations
  • Remember that a minimum working pressure will be required during the backflush cycle of a filter
  • Do not install the water meter before the filter, if you do not want to measure your backflush water
  • Do not release backflush water back into the irrigation water source near the intake
  • Make sure that each filter bank is equipped with the correct type and size air valve
  • Calculate the diameter and length of the backflush pipe correctly
  • Assure that PD switches are set up correctly and that backflush intervals and durations are set up correctly on the flush controller
  • Drain filters and switch off the power to the back-flush unit in off seasons
  • Keep in mind that a filter bank with two large filters might be less efficient than a bank with three or more smaller filters
  • We often must deal with metals, carbonates and salts in water that cannot be removed by a normal filtration system. This often implies that settling dams or floating suction pump need to be considered and often makes continuous water treatment necessary.

Target efficiency

Most importantly, remember to not cut corners when planning the filtration section of your irrigation system. As mentioned, the success of filtration will determine the efficiency and sustainability of the entire system. Do not allow your filters to cause a weak link in your system.