Water filtration for irrigation systems

Suspended solids need to be removed from within irrigation water to prevent clogging piping, valves, nozzles and emitters. These small particles 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 or suspended iron.

Filter selection

Before selecting a filter, a water analysis should be done to identify the type and quantity of solids. Keep in mind that seasonal changes such as algae growth or spring runoff can affect 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 then determined, taking into account the maximum water usage. Also consider the needs of any proposed expansion. Filters are available from 10 to more than 1000 gallons per minute capacity.

The level of filtration should next be determined (Table 1). If you need water for an impact sprinkler system, having a 30-mesh screen filter may be adequate to remove leaves and trash. On the other hand, if the system you are supplying is a microirrigation system, a 200 mesh disk filter may be required.

The pressure loss created by the filter should be minimal. Pressure loss is related to the size of the filter opening and the water flow. Use the next filter size larger if the loss is excessive.

Screen filters

These come in several 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. These have internal 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 can be manually disassembled for cleaning. Models that are cleaned by turning on a set of brushes that wipe the screen are also available. Automatic hydraulic flushing of debris from the screen can be found on some models. Observe the difference in pressure between the inlet and outlet to determinine when to clean the screen.

Sieve bend screens are installed as a first stage to remove larger solids from return water from ebb and flow and hydroponic system water. The moving belt screen has also been developed for this purpose.

Disk filters

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 a low solids concentration. They have a low head loss and use only a small amount of water to backflush.

Media filters

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 are removed through a separate drain valve.

Centrifugal separators

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.

John W. Bartok Jr. is an agricultural engineer, an emeritus extension professor at the University of Connecticut and a regular contributor to Greenhouse Management. He is an author, consultant and a certified technical service provider doing greenhouse energy audits for USDA grant programs in New England.