Irrigation-filtration systems

Filtration is a vital component of any irrigation system. Think of it as an insurance policy designed to protect all of the downstream components of your irrigation system. First and foremost, filters prevent sprinkler plugging. In addition, filtration prevents solids from plugging valves or accumulating in your water-distribution piping system. Little debate exists over the need for filtration. The debate is over what type of filter should be used for your particular application.

Generally speaking, irrigation-filtration equipment includes four classifications. They are:

* Screen filters * Centrifugal separators * Disc filters * Sand-media filters

Water quality (specific to your irrigation system) is the determining factor of which filter technology you choose. Some filters work well on inorganic particulate, such as sand and sediment. Others function better when dealing with organic contaminants such as algae. Knowing your water quality, coupled with a basic understanding of the filter types available, makes your filter-selection process easier.

Screen filters Screen filters are the most widely used filters and are available in a variety of sizes and configurations. Most of them employ a stainless-steel screen as the actual filter media, but some screen filters use a fabric sock as an alternative to a stainless-steel screen.

Screen filters, typically used for relatively clean water such as city water or well-water applications, filter contaminants such as sand, sediment or other solid particulate. They are not recommended for use on surface-water applications where algae are present. Algae tend to stick to the screen and may rapidly plug the filter screen.

Even in a well-water application, a screen filter is susceptible to being overwhelmed, and therefore plugged, if the solids loading is relatively high. In the event that a screen filter clogs, you must remove it from the filter housing for manual cleaning.

Manufacturers of screen filters offer a variety of screen options referred to as "mesh options." The orifice sizes of the sprinklers used in your irrigation system determine the screen-size selection. As a rule of thumb, the hole opening in the filter screen must be roughly one-fifth the size of the smallest sprinkler orifice used in your system. For example, if your system uses micro-sprinklers with an orifice size of 0.050, you must consider a 50-mesh screen which has a hole opening of 0.01 (0.050 / 5 = 0.01).

Several automatic screen filters also are available. These filters have an internal mechanism that vacuums debris from the screen when it becomes dirty. Automatic screen filters contain moving parts and can be costly. However, they handle more challenging water-quality applications.

Centrifugal separators Centrifugal separators, in theory, are not actually filters. In some cases, they offer alternatives to a filter or are pre-treatment devices for other types of filters. Centrifugal separators also are called sand separators.

As water enters the sand separator, it spins at a relatively high velocity. This centrifugal-spinning action forces heavier particles to the outside walls of the separator housing. The clean water, free from many of the particles, flows up through the center of the separator and exits the outlet. As spinning particles lose their velocity, they sink into the collection chamber at the base of the separator. From there, particles are periodically purged from the collection chamber.

Sand separators are quite simple. They rely solely on the centrifugal-spinning action of water and, therefore, require no filter screens or other media that need to be cleaned or backflushed.

You must use caution when installing a sand separator on an irrigation system. Consider the following important factors:

* Sand separators are for use only on irrigation systems where the primary contaminant is sand. They are ineffective in removing organic contaminants such as algae.

* A sand separator operates most effectively within a relatively small flow range. If your system has varying flow rates, do not use a sand separator. For example, if you size a separator for an irrigation system that is designed to run at 100 gpm and, at times your flow is only 50 gpm, the separator will have insufficient velocity to separate the solids.

* Sand separators are effective in removing particulate; however, they tend to introduce significant pressure in your system. Depending on the size of the sand separator, they operate with a minimum of a 5-psi pressure loss and as high as an 11-psi pressure loss.

Sand separators do not remove all of the sand from the water. For this reason, it is common to install a screen filter (or some other type of filter) downstream of the separator to remove any of the particles that pass through the separator.

Disc filters The filter medium, in any disc filter, consists of multiple-injection, molded-plastic discs. The multiple discs stack on top of each other, forming a three-dimension filter cartridge. Each individual disc contains grooves, molded into its surface. These molded grooves provide for the mesh (or micron) rating of the filter. Typical options include 80-mesh, 100-mesh, 120-mesh and 200-mesh ratings.

Dirty water passes from the outside of the discs across the center. As the water passes through the stack of discs, non-organic and organic contaminants are removed. Clean water then exits the outlet of the disc-filter housing and out into the irrigation system.

Manual and automatic disc filters A manual-disc filter consists of an injection-molded plastic filter pod as well as the internal-disc filter cartridge. Manual-disc-filter models typically are used on irrigation systems where automatic, self-cleaning ability is not necessary or is cost prohibitive. As with a sand separator, our Turbo-Disc filter operates with a spinning action that draws heavier sand away from the disc-filter cartridge, to the outside walls of the filter housing. The sand particles eventually fall down to the bottom of the tapered lid where they flush through a port at the base of the filter. Over time, lighter contaminants, such as algae, accumulate on the outer surface of the disc stack and penetrate into the depth of the discs. On manual disc-filter models, the disc cartridge must be periodically removed from the filter housing for manual cleaning.

Automatic disc-filter models are equipped with manifolds, backwash valves, solenoids and a backwash controller. The backwash controller monitors the pressure differential across the filter. When the filter becomes dirty, an automatic backwash routine initiates, sequentially backflushing one disc-filter pod at a time, using filtered water from the other pods to backwash the dirty pod. Like other automatic filters, automatic disc filters require approximately 40 to 45 psi during the backwash routine. If this pressure is not available, you must install a pressure-sustaining valve to boost the pressure only during the backwash cycle.

Sand-media filters In use for many years, sand-media filters function well in applications where the primary contaminant is organic. Sand-media filters are pressurized, vertical tanks filled with some type of sand media. The media tank retains the sand by a system referred to as the "under-drain assembly." Small holes in the under-drain assembly prevent sand from escaping the media tank, yet the filtered water passes through the under-drain into the irrigation system.

Like automatic-disc filters, sand-media filters have automatic-backflushing capability. Only one media tank is backflushed at a time, and it uses filtered water from the other tank(s) for the backflush. Each system requires at least two media tanks.

As the dirty water passes from the top of the sand bed down through the depth of the sand, the sand traps contaminants. After the water passes through the entire sand bed, it exits through the under-drain assembly and through the outlet manifold. Sand filters provide excellent filtered-water quality and they handle relatively high solids loading.

Typically, you program backwash controllers on sand-media filters to backflush at time intervals or based on pressure differential. During the backwash process, the direction of the water reverses through the sand bed. As this takes place, the dirty sand bed lifts, allowing individual particles to separate from each other. Because the organic contaminants are lighter than the actual sand media, they rise to the top of the media tank where they are flushed via the backflush manifold.

Like other filters, the sand-media in the sand filter is available in a variety of sizes. You must choose based on the orifice size of the smallest sprinkler in the irrigation system.

Purchase decisions When in the market for an irrigation filter, you need to compile some facts. When armed with these facts, you should contact various filter manufacturers or irrigation dealers, and ask for their recommendations. The following is a list of facts that manufacturers and irrigation dealers will want to know. Be prepared to give them this information when contacting them.

* What is the maximum flow rate of your irrigation system in gpm?

* What mesh or micron level do you need for your particular irrigation system? If you do not know this, be prepared to tell them the orifice size of your sprinklers.

* Does your irrigation system use sprinklers, micro-sprinklers or drip irrigation?

* What is the maximum pressure of your irrigation system in psi?

* Does your irrigation system currently use a filter? If yes, what type and size?

* What is the water source (well water, surface water or reclaimed water)?

* What is the water condition (sand or algae)?

An irrigation filter is designed to protect your entire irrigation system. Take the time to evaluate the different options available. After you choose and install the filter, make sure you follow appropriate maintenance procedures. Whenmaintained properly, the filter should function efficiently for many years.

Christopher Shuster is vice president of sales and marketing for Miller-Leaman Inc. (Daytona Beach, Fla.).

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