Suspended solids need to be removed to prevent clogging of piping, valves, nozzles and emitters in an irrigation system. This becomes more important the smaller the nozzle or emitter opening.
Suspended solids may come from many sources. Surface and recirculated water may contain sand, soil, leaves, organic matter, algae and weeds. Ground water, although usually clean, may contain fine particles of sand. All to these can be removed through filtration.
Before selecting a filter, a water analysis should be done. The type and quantity of solids should be determined. Keep in mind that seasonal changes such as algae growth or spring runoff can change the loading. Chemicals and minerals in the water can also cause plugging. These may have to be addressed with chemical treatment methods.
The flow rate needed to supply the irrigation system should be determined, taking into account the maximum water usage. This usually occurs during the summer and may amount to 0.3 to 0.4 gallons/sq ft of growing area/day. Also consider the needs of any proposed expansion. Filters are available from 10 to over 1000 gpm capacity.
The level of filtration should be determined (Table 1). If you need water for an impact sprinkler system a screen filter having a 30 mesh may be adequate to remove leaves and trash. On the other hand, if the system you are supplying is a microirigation system, a 200 mesh disk filter may be required.
The pressure loss created by the filter should be a minimum. Pressure loss is related to the size of the filter opening and the water flow. Use the next size larger filter if the loss is excessive.
These come in all sizes and shapes. Intake screens may be placed on the suction end of the pipe supplying water from a pond or stream to remove leaves or algae. Self-cleaning intake screens are available that have high pressure nozzles that clean the screen surface as it rotates.
In-line screen filters can be used as a final filtration if the water is fairly clean. They are low cost and available for a wide range of flows. The water passes through one or two cylindrical screen elements. Suspended particles are deposited on the outside. The most common models are manually disassembled to be cleaned. There are also models that are cleaned by turning a set of brushes that wipe the screen. Automatic hydraulic flushing of debris from the screen is available on some models. Determining when to clean the screen can be done by observing the difference in pressure between the inlet and outlet.
The filtering element of a disk filter is made of a large number of flat, grooved rings that are stacked tightly together. The degree of filtration is determined by the number and size of the grooves. Intake water surrounds the filter element and is forced through the grooves trapping the particles.
Cleaning is accomplished by reversing water flow. This expands the disk stack and a high pressure water-air spray spins the disks throwing off the trash. This is dumped to a drain.
Disk filters are best suited for water sources with low solids concentration. They have a low head loss and use only a small amount of water to backflush.
Units containing several filters can be assembled with controls so that backflushing of one filter can take place without affecting water flow. These units are now popular for greenhouse operations.
Media filters are best for removing organic matter, such as algae and slime and fine inorganic material, such as silt and clay. The filter is a steel or plastic tank containing sand, quartz or other inert material sized to provide the level of filtration desired. Incoming contaminated water flows through the bed depositing the solids. When the pressure loss reaches a predetermined level backflushing begins. Clean water is forced up from the bottom causing a turbulent expansion of the media. Entrapped contaminates are loosened and flow through a separate drain valve.
These filters are good for removing sand or other heavy matter from well water. They operate by introducing inlet water in a spinning motion inside a steel cone. Heavier particles as small as 200 mesh are forced by centrifugal action to the outside and slide down to a collection chamber at the bottom.
Centrifugal separators are low cost, create very little pressure loss and have high efficiency. They have no moving parts and can be arranged in parallel to increase capacity.
Good, clean water is important to efficient operation of irrigation systems. The wide range of sizes and types of filtration equipment available today fits all the needs of the greenhouse industry.
|*human hair is about 150 mesh = 0.004in.|
Extension Professor Emeritus and Agricultural Engineer
NRME Department, University of Connecticut, Storrs CT 06269-4087