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Particle and debris removal

Pre-treatment or Filtration

Dr. Youbin Zheng, Siobhan Dunets and Diane Cayanan
University of Guelph, Guelph, Ontario, Canada

Pre-treatment or pre-filtration of irrigation water involves the use of various filter types to remove organic and inorganic particulate matter (debris, sediment, soil particles, algae, etc.) from the water prior to treatment for pathogens. Pre-filtration is important for 2 reasons. Firstly, larger particulate matter has the potential to clog the irrigation system (e.g. emitters). Secondly, the effectiveness of many pathogen treatments (e.g. all chemical treatments using oxidizers, UV) is decreased significantly in the presence of particulate organic matter.

There are a variety of filter types available. The most important way in which these filter types differ is in their “fineness”, or the size of the smallest particle they are able to filter out. Fineness may be expressed in micron size or, for coarser (less fine) filters, mesh size (a mesh to micron conversion table can be found here http://www.netafimusa.com/files/literature/wastewater/Mesh-vs-Micron.pdf). “Mesh” refers to the number of openings (or “pores”) present along a one inch transect (so 1 in./width of one pore= mesh number) (Netafim, n.d.a). “Micron” refers to the filter pore diameter in micrometres (µm).

Ultimately, the fineness of filter required for a particular operation will vary, and will depend on the water source used (which determines the size of particles present in the water). Multiple levels of filtration are often required as much larger particles must be filtered out before water reaches the filter intended for finer particles. Otherwise, these large particles may damage the finer filter. Whether multiple filtration levels are required or not depends on how dirty the water source is. For example, if the water source is municipal water, only a single filtration level of 200 mesh (75 micron) fineness may be required. However, if the source is a slow moving or stationary body of water such as a pond, multiple levels of filtration may be required, as well as a finer end filter around 600 mesh (25 microns; this is to remove fine particulate organic matter) (Fisher, 2011).

It is also important to choose the correct size of filter. The “size” of the filter refers to the maximum flow rate that it is able to accommodate. There are often multiple filter sizes available for each level of fineness. Water pressure of the irrigation system must also be taken into consideration. Each filter model will have a limit to the maximum pressure at which it can operate. As well, the minimum operating pressure required for your irrigation system must be noted in order to ensure this pressure will be maintained when a self-cleaning filter is backflushing (see below for an explanation of this phenomenon) (Tekleen, 2003).

Regular cleaning of filters is very important, and required frequency of cleaning will depend on how clean the original water source is. Some filters (ex. screen, disc) are available in both manual and automatic-cleaning varieties. While manual-cleaning filters are less expensive, these filters are only practical if source water is clean (Netafim, n.d.b). Automatic-cleaning, while more costly, will ensure the filter is cleaned on time (Netafim, n.d.b).

Deciding whether to add another level of filtration is ultimately a question of balancing costs. While any pre-filtration requires extra cost, the operator must take into consideration the maintenance costs or losses that would be incurred without the particular level of filtration. For example, if an operator was thinking of installing a fine membrane filter, it would be important to compare the cost of the filter to the cost of increased UV power or increased chlorine injection that would have to take place if finer organic materials were not removed. While often some level of pre-filtration is necessary or beneficial, increasingly fine filtration may not be appropriate for every operation. With a fine enough filter, anything can be filtered out of irrigation water, but the issue is ultimately cost.

The table below shows different filter types available for different levels of filtration, starting with filters for the removal or coarser particles at the top to finer particles at the bottom. Click on a particular filter type to be directed to a description.

Filtration
Activated Carbon Belt filter Cartridge Disk filter Membrane filter Rapid sand filter Screen filter Slow sand filters

Suppliers

For more information on exact costs and what filtration would be right for your system, please contact a supplier. Some filter suppliers include:

Pelmar Engineering Ltd.: Screen, disk, cartridge, media filter, sand separator

REFERENCES

Fisher, P. 2011. Water treatment: A grower’s guide for nursery and greenhouse irrigation.

Netafim. N.d.a Mesh vs. micron comparison chart.

Netafim. N.d.b Water filtration: Why it’s needed.

Tekleen. 2003. Automatic water filter streamlines drip irrigation.


This work was supported in part by the Canadian Ornamental Horticulture Research and Innovation Cluster and Canadian Ornamental Horticulture Alliance

For more infomation please contact Dr. Zheng (yzheng@uoguelph.ca)

link to CESRF website