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WATER QUALITY
One can seldom choose the quality of available irrigation water , however Irrigation Engineers have a wide range of systems that can be used to achieve the correct quality of water for the type of irrigation system being designed . Specifically designed inlet structures combined with pre-filtration , the appropriate choice of filter system at the primary stage , being either sand , mesh / screen or disc and the use of “ block filters ” just before the emitters , represent some of the options available to the designer .
Poor quality of water may contain organic and inorganic solids as well as dissolved solids that adhere to the filter discs as inorganic residue . These particles need to be removed through a combination of backwashing and in manual disc filters , through the removal of the disc element and sluicing with water and if necessary , the use of acidic solutions .
The following table represents one of the water grading definitions used in the RSA . Note that the clogging time in minutes is measured by a Clogging Capacity Meter .
Water quality
Levels of totally dissolved solids ppm ( parts per million )
Clogging in minutes
good |
0 < TSS < 50 ppm |
T > 15 |
average |
50 < TSS < 100 ppm |
5.0 < T < 15 |
bad |
100 < TSS < 150 ppm |
2.5 < T < 5.0 |
very bad |
150 < TSS < 200 ppm |
1.5 < T < 2.5 |
Needless to say , it is essential for the Irrigation Engineer to determine the quality of water prior to the selection and design of the filtration system . The clogging meter is used to determine the amount of organic matter in the water , however the chemical composition of the water also has to be analysed in order to determine the amount of iron or manganese which leads to a build up of inorganic residue in the filter . The choice of the filter system , the choice of the grade of the filter elements and the backwashing / maintenance regime are all dependent on the water quality and the type of irrigation system being designed .
• Degree of filtration quickly changed through colour coded ring replacement
• Range of sizes ( 3 / 4 ”– 6 ”) and flow capacities ( 1 – 120m ³/ h ) available
• May be used as stand-alone units or in modular filter batteries
• May be installed with or without backwashing cycles
• Disc filters have an extremely small footprint , as opposed to say a sand filter
Colour Mesh Micron Blue 40 400
Yellow 80 200
Red 120 130 Black 140 100 Green 200 55 Grey 600 20
Discs are colour coded according their equivalent mesh size or the more commonly used , sizing in microns . ( A micron is equivalent to 0.001mm or 1 / 1000th of a millimetre ). The major advantage of the disc system is that the grade of filtration can be easily changed by inserting a finer or coarser disc stack into the filter body , with the colour coding of the discs making the system extremely user friendly . Disc filters are generally used for Primary Filtration in drip irrigation , sprinkler and micro sprinkler systems with most water qualities .
SERVICE / MAINTENANCE PROCEDURES FOR DISC FILTERS
The service procedures for disc filters may differ , dependent on the size and features of the filter , as well as the various brands available in the RSA market . Also , the procedures will be far more complex for battery installations and even more so if backwash cycles are included . Specific maintenance manuals and training will be provided by the Irrigation Engineer , whose responsibility it is to ensure that the user of the filtration system is fully able to maintain the system for optimum efficiency .
DISC FILTERS new discs ( note colour coding )
Disc filters contain thin colour coded polypropylene discs which are diagonally grooved on both sides to a specific size in microns . A set of these discs is stacked on a hollow spine and enclosed in a corrosion resistant , polyamide housing . The grooves run in opposite directions and when compressed , form a series of intersecting grooves which trap the solids when raw water is passed through the filter under pressure . Water passes from the outer to the inner edges of the discs . The particles to be filtered out are trapped in the voids between the discs and the filtered water passes through the centre of the filter body to continue to the irrigation system .
Disc filters have the following advantages :
• High efficiency and low maintenance
• Simple and quick cleaning procedures
• Greater holding capacity of solids across the entire ring surface
• Compression of the rings in use increases efficiency examples of clogged discs
The following procedures are based on a single , manual disc filter .
1 . Shut off the water supply to the filter . If backflow from an elevated tank is possible , ensure that the discharge side of the filter is also isolated from the tank
2 . Remove the plug or open the valve at the base of the filter body , if fitted , in order to drain the filter
3 . Remove the clamp or unscrew the filter housing from the head of the filter . Ensure that the O-ring is not nicked or damaged in any way during this operation .
SABI | FEBRUARY / MARCH 2017 31