Filters in the Landscape: Maintaining Irrigation Design Integrity
filtrationThe continued emphasis on water control and conservation is contributing to the downsizing of irrigation emission devices, like drip or microspray irrigation, which provide pinpoint applications at very low rates. Logically, a device that drips half a gallon per hour just doesn't have the flow or pressure to force debris through the micro-orifice.
Proper filtration will keep these inorganic and organic pollutants in check. For property managers of large or multiple projects, the time saved preventing troubleshooting and repairing irrigation components and replacing plant material make the minimal expense and easy installation and maintenance of filtration devices absolutely mandatory.
As with the human body, the health of an irrigation system greatly balances on the quality of elements passed through it. And, like the human body, if such elements aren't carefully "screened" they will ultimately clog important conduits affecting flow and pressure of sustaining fluids.
Even the "cleanest" municipal water transports silt and sediment to an irrigation system, where they can hinder diaphragm valve seats and plug solenoid ports and low-volume emitters. Peek into your toilet tank and you'll see the deposits left by potable city water. If you're still unconvinced, run some water from a hose-bib through an old nylon stocking to see what is captured or into a jar to settle overnight.
Repairs or additions to water delivery networks tend to increase sediment loads in irrigation water. From city mains to residential irrigation lateral lines, backsiphoning can pull dirt and debris back into the piping network. Always flush debris from sprinkler lines after repairs.
When using well or surface water for irrigation, organic slimes add to the concern and can require specific filtration devices. Over time, the accumulation of algae in the piping and emitters will affect the uniformity of irrigation applications, resulting in over-watering or dying plant material.
Screen filters are an affordable way protect the irrigation system from inorganic material and are simple to install and maintain. Essentially, they are partitions of porous material that trap solids carried by the water as it passes. Some products stage more than one screen in the assembly to polish water to the micron you desire.
Generally, a 200-mesh screen filter is adequate, however some manufacturers recommend 150-mesh. For screen filters, the screen aperture size should be about one-tenth the size of the emitters openings. A 200-mesh screen has 200 openings in the screen per linear inch.
To size the screen, divide the flow of the system by the amount of area available for filtration to determine how many GPM are being filtered per square foot of area. Select a system that provides a larger surface area for filtration, as it will have a lower operating-pressure loss and provide longer operating times between cycles. Generally, if sized correctly, these screen filters wont take more than 5 or 6 psi of your working pressure.
Screen filters work well with a low to moderate load of sand, silt or clay. However, they have a limited capacity to store large amounts of contaminants before cleaning. When the surface of a screen filter becomes blinded, it must be flushed or cleaned.
Disc filters, stacks of plastic, grooved, ring-shaped discs, trap organic and inorganic contaminants. As water moves through the discs, material is trapped in the grooves and walls of the device.
As with screen filters, disc filters can be sized to fit the irrigation system to the micron of protection necessary. Disc filters are recommended for even trace amount of organic material in the irrigation water. Organics stick to screen filters and create maintenance problems.
You can filter the entire irrigation system, groups of laterals or individual laterals. For maintenance and repair purposes, filters should be installed above ground or in a valve box. Install the filters upstream of the control valve to protect as much of the irrigation equipment as you can. If the irrigation system is using an injector unit for fertigation, place the filter downstream of that unit. Install the filter upstream of any pressure regulators to ensure the device is handling unrestricted water flow. Be sure to position these filters at installation for top or bottom accessibility.
Pressure gauges upstream and downstream of the filter unit will measure pressure differential in the line before and after the filter. When your pressure drops to unacceptable levels (4 or 5 psi, depending on the system), clean the filter.
Preserving Designed Operation
After installing filters on a new or existing system, check them after the first week to ensure that any debris that might have entered during installation isn't absorbing flow downstream. Maintenance is the key to manual-flush filters. If it plugs, you've lost any semblance of uniformity at the emitters, and the filter will likely rupture. Remember that the more water you run through the system, (June, July and August) the more often the filter(s) will need maintenance.
Even if the filter units are equipped with simple ball valves for flushing without removing them from the irrigation system, periodically detach the filter unit from the piping network and inspect it. If algae is spotted on a steel screen filter, take an old toothbrush and gently scrub it until removed.
There is no prescribed maintenance schedule for cleaning filters, but some material is obviously harder to clean than others. Therefor, cleaning the filtration unit too often is better than not often enough.
Irrigation systems are designed within very specific flow and pressure parameters. The simplest way to ensure that water is delivered according to design, is to provide and maintain basic water-polishing filtration capability.