Because of the importance of filtration, the different specifications available, and die range of possible problems with water quality, expert advice should be sought.
Wire mesh or gauze.These need to be of a large surface area and relatively fine mesh size to suit microirrigation applications. They are generally used as back-up filters, although self-cleaning versions are available. The mesh provides only a single surface to remove particles, and particles of irregular shape or those that are compressible might still pass through. Cleaning can be difficult, because the particles may be lodged hard into die mesh.
Centrifugal or spiral filters.These do not have a screen of any type, but cause the water to spin in passing from inlet to outlet. Particles are removed from the water by centrifugal force, collecting at a disposal outlet at die base of die filter. These can be used as initial or primary filters to reduce die load on the main filters, particularly when sandy material is present in die water.
Figure 7.7 Multiple disc-type filters, mounted in a tower. This version is self-cleaning
Disc type.These consist of many plastic discs pressed together in a cylinder. The surface of die discs is manufactured with angled grooves at opposing angles, so mat water passes through a tortuous pathway of intersecting grooves between adjacent discs, removing particles as it travels. This provides a longer pathway for filtration to occur, compared to a screen, with different disc and groove patterns corresponding to a range of equivalent mesh sizes.
Multiple cylinders are connected together to give die required flow rate. Cleaning is accomplished by loosening die discs and flushing with clean water. Self-cleaning types are available, which use only a moderate amount of water for backflushing.
Figure 7.8 Media filtration unit. Multiple tanks are necessary to accommodate the required flow rate, and to provide for automatic backflushing.
Mediafilter.Water is passed vertically through a large tank containing graded gravel or sand. Multiple tanks are used to provide die required flow rate. Although die most expensive, these are generally considered the most effective type, particularly where the water supply is stored in dams. Although routinely specified for microirrigation systems in Australia, some irrigators have experienced problems with media filters being clogged very quickly, requiring frequent backflushing, which could be a result of improper selection of filter size and capacity. Some systems could benefit from some form of pre-cleaning equipment. In some cases, the filter could be removing material which will pass through die dripper anyhow, which could overwork the filter.
Cleaning is achieved by temporarily reversing the flow through the tank (backflushing). During this operation, the media is suspended, releasing contaminants. A reasonably large amount (and velocity) of backflush water is required, normally going to waste. Automatic backflushing is available; as the filter commences to collect suspended material, there is an increase in die pressure required to push die water through. This results in a larger difference in pressure between upstream and downstream sides of me filter, which increases over time. When die pressure difference reaches a pre-set limit, die filter is automatically switched to backflush mode. Whilst being flushed, the filter cannot be in normal working mode, so installation requires at least two modules.
A back-up filter is required immediately after die media tanks to capture any media mat might escape die tank.
Aspects of water quality.In me event of a pipe repair in die field, there is a high risk of material entering die pipe network after die filter. Flushing will be essential immediately after repair, and some projects have manual clean back-up filters installed at die inlet to each block.
Unfortunately, filters are only best at removing suspended particles, and may not remove other problem material mat may be present:
• Algae present in die water supply may accumulate and grow in or downstream from die filter. Periodic chlorine treatment may be necessary, with the concentration of chlorine selected to suit the particular situation. Sometimes, continuous chlorine injection may be required, for example where wastewater is used.
Figure 7.9 In this system, a manual clean disc flier is installed with the valve at the entrance to each irrigation block as a back-up to the main filters (system under construction)
• Clay colloids are small enough to pass through the emitter, but certain chemical reactions can occur with other elements dissolved in die water, which causes the clay particles to flocculate. Some chemicals can be flocculated in storage reservoirs prior to use. Salts of iron, common from bore water, will react with oxygen, hence it may be necessary to oxygenate me water prior to use.
• Bacteria can form a slime inside pipes, which can detach from the pipe wall and cause blockages. Chlorine treatment may be necessary.
• Acid may need to be injected to lower pH and to control build up of calcium based deposits.
• Temperature fluctuations can initiate chemical reactions, although burying the pipeline will minimise hot temperatures. Fertiliser injection should be done before the end of die irrigation to enable clean water to flush the chemical through.
• Insects crawling into the emitter can be a problem. Keeping the emitter above ground level helps, as does periodic flushing with lateral ends open. Some emitters are designed with "anti-ant" closing devices.
Flushing of die laterals should be a regular practice; at the commencement of each season, and during die season as required. For manual flushing, die lateral end is folded onto itself and held by a clip. The clip is removed for flushing, men replaced. Laterals can be fitted with automatic flushing-valves mat release some water on every start-up. For larger projects, where flushing of individual laterals is inconvenient or too expensive, blocks of laterals can be connected into a common flushing sub-main, die whole block flushed by opening one or two valves. A reasonable water velocity is required during flushing, to dislodge and move die contaminating particles. Sub-main ends should also be fitted with a flushing valve.
Material may also accumulate in mainlines, either a gradual build-up of fine material, or through a pipe repair. Each irrigation block can be fitted with a back-up filter, but die low points of die mainline should be fitted with a scour valve to enable flushing of die mainline.