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Irrigation Water Filtration
& Filter Recommendations

 

Why a Filter?

Water filtration is important for all irrigation systems. Now before someone argues with me, yes, some sprinkler systems are used to spread solids, such as treated sewage, for disposal. But even those in my experience have incorporated some form of filtration upstream of the system to prevent solids which are too large from entering the system.

Filters can help extend the life of, and lower the maintenance on, your sprinkler system. For drip systems they are a necessity to prevent emitters from becoming plugged. Even if small sand particles can pass through your system without clogging it, they cause wear on the equipment. Automatic valves contain very small water passageways in them which can become plugged resulting in the valve failing to either open or close. A small grain of sand caught in a spray nozzle can result in a dry, dead spot in a lawn.

While sand is probably the first thing most people think needs to be filtered out of the water, organic materials can be just as important to remove. Algae can grow inside the system, especially in drip tubes. Another situation occurs when a small piece of organic matter snags somewhere in a valve, fitting, emitter, or sprinkler. The organic matter by itself may not be large enough to be a problem. But soon another piece comes along and gets caught in the first. Then a very small grain of sand that would normally have passed through the system without problems becomes caught in the organic matter. Soon a large build-up of crud forms and the flow is blocked. Have you ever had the hose on your vacuum cleaner clog up with a wad of hair, small objects, and dirt? Each one of those objects went into the hose, so they should have made it through to the canister. But they didn't because they all got caught together. The same thing happens in your irrigation system. How about a small fish or a clam? They go into your system when they are small (often as eggs) and once in there they grow! Laugh if you want, but I've seen it many times! Freshwater clams are very common in city water systems. That's right, there's a very good chance that every time you get a drink from the faucet you're drinking clam water! Yuck... (but be realistic, has it killed you yet? Or perhaps you've never eaten clam chowder? Or maybe you should take a look in your cat or dog's bowl at what THEY drink without getting sick. The truth is that your body processes dirt much better than your irrigation system does!)

Types of Filters

Filters are broken down into different categories dependent upon the method used to filter the water. A brief description of the most common types follows.

Screen filters:

Screen filters are probably the most common filters and in most cases the least expensive. Screen filters are excellent for removing hard particulates from water, such as sand. They are not so great at removing organic materials such as algae, mold, slime, and other unmentionables! These non-solid materials tend to embed themselves into the screen material where they are very difficult to remove. In other cases they simply slide through the holes in the screen by temporarily deforming their shape.

Screen filters are cleaned by flushing them with a stream of water or removing the screen and cleaning it by hand. Depending on the flush method used you will probably have to periodically hand clean the screen to remove garbage not removed by flushing. Several methods of flushing are common. The simplest is a flush outlet. The outlet is opened and it is hoped that the debris washes out of the flush outlet with the water! An improved variation on this is the directed-flow flush. Again a flush outlet is opened, but in this case the structure of the filter is designed so that the flush flow rushes over the face of the screen sweeping the debris along with it. Somewhat like hosing off a sidewalk with a strong stream of water. This is the most common method found in inexpensive filters. The most effective method of flushing is the backwash method, but these filters are typically more expensive. In this method the flush water is forced backwards through the screen for a very effective cleaning. This is accomplished by either using two filters side-by-side (the clean water from one is used to flush the other) or by "vacuuming" the screen with a small nozzle which is moved over the screen by a mechanism in the filter, "sucking" the debris off of it. (Although it is referred to as vacuuming it is really a form of backflush. The water is forced backwards through the screen by the water pressure in the system, not by a true vacuum.)

Cartridge Filters:

Cartridge filters are essentially a variation of the other types listed here, depending on what the cartridge is made of. Most cartridges contain a paper filter which works just like a screen filter. Most also remove organics well because the paper texture is rough enough to snag the organic matter. While some cartridges can be washed, most of them you simply replace when dirty.

Media Filters:

Media filters clean the water by forcing it through a container filled with a small, sharp edged, "media". In most cases the media material is uniform sized, crushed sand. The water passes through the small spaces between the media grains and the debris is stopped when it can't fit through these spaces. Media filters are best for removing organic material from the water. This is where the importance of the sharp edged media comes into play. These sharp edges snag the organics which would otherwise slime and slither their way through the small spaces. This is why it is important to use sharp media. Whenever someone tells me their media filter doesn't work my first question is always "where did you get your media material for the filter?" Their answer is almost always something to the effect of "uh..., I just used some sand from the creek up the road, why?" River, beach, and creek sand tend to have rounded, soft edges and are not suitable at all for media filters! Media filters are the type of filters most commonly used for high volume cleaning of water from rivers and lakes. They are used by both large farms and municipal water systems. They most often are 3 to 6 foot diameter rounded tanks sitting on short legs, and are almost always in groups of two or more. I've seen municipal water systems with media filters that are over 12 feet tall and 10 feet in diameter! They tend to be a wee bit large and heavy for the average homeowner! Media filters are cleaned by backflushing. The force of the water going backwards through the filter lifts and separates the media which frees the debris and washes it out through a flush valve. Because a small amount of media is often washed out too, it is necessary to periodically add some more to the filters. Because sand is not easily flushed out of them, media filters are not good for situations where the water contains a lot of sand. The sand will not flush out and soon the filter will be completely filled with sand which you will have to remove by hand. Media filters must be carefully matched to the system flow rate for proper operation. Always consult the media filter manufacturer's literature for proper sizing procedures!

Disk Filters:

Disk filters are a cross between a screen filter and a media filter, with many of the advantages of both. Disk filters are good at removing both particulates, like sand, and organic matter. A disk filter consists of a stack of round disks. The face of each disk is covered with various sized small bumps. A close up view of the bumps reveals that each has a sharp point on the top of it, somewhat like a tiny pyramid. These bumps are very small, thus a typical disk looks a lot like the old vinyl 45 RPM records! Because of the bumps, the disks have tiny spaces between them when stacked together. The water is forced between the disks, and the particulates are filtered out because they won't fit through these gaps. The organics are snagged by the sharp points on the bumps. For automatic cleaning of the filter the disks are separated from each other which frees the debris to be flushed out through a flush outlet. For less expensive disk filters you must remove the disks and hose them off.

Centrifugal Filters:

Also known as "sand separators", centrifugal filters are primarily for removing particulates, such as sand, from the water. They are great for situations where a lot of sand is present in the water as they don't clog up nearly as quickly as other types of filters. The dirty water enters the filter where it is swirled around the inside of a cylinder. The centrifugal force causes the sand particles to move to the outside edge of the cylinder where they gradually slide down the side to a holding tank at the bottom. Centrifugal filters are reasonably inexpensive, very simple, and are very effective for removing sand from water. Because many wells pump sand up along with the water you will often see a centrifugal filter installed on a large well. Some centrifugal filters are designed to be installed inside the well. These typically are attached to the bottom of a submersible pump. It is not unusual for a very small amount of sand to pass through a centrifugal filter. For drip irrigation systems I always add a "backup" screen filter when using a centrifugal filter as a safety precaution. A centrifugal filter used in combination with a media filter after it is an excellent combination. The centrifugal pulls out the sand, the media filter then removes the organics. This combination is very often used in municipal water treatment, where a third activated charcoal filter may be added to remove chemicals. Note that the centrifugal filter selection must be carefully matched against the system GPM or the filter will not work correctly. Always consult the manufacturer's sizing guidelines when designing a centrifugal filtration system for your irrigation system.

Tutorial continues below...


 

Filter Recommendations

So which filter to use? There is no fixed answer. Your budget, water quality, and availability of the filter and parts must all be considered. Screen filters are generally the least expensive. If you have a city water supply with nothing more than a periodic grain of sand or flake of rust in it, a screen filter will be fine in most cases. Sometimes a combination of more than one type of filter will be needed. For a smaller size system a disk filter would remove both sand and organics, but you might need to frequently clean it! Here are some suggestions based on the source of your irrigation water:

Water Source Suggested Filter Types
Municipal Water System Screen Filter, Centrifugal Filter, or Disk Filter.
Well Screen Filter, Centrifugal Filter, or Disk Filter.
River or Creek Disk Filter, Media Filter and Screen Filter, Centrifugal and Media Filter.
Pond or Lake Disk Filter, Media Filter and Screen Filter, Centrifugal and Media Filter.
Spring or Artesian Well Screen Filter, Centrifugal Filter, or Disk Filter.
Organic material in water Disk Filter, Media Filter and Screen Filter, Centrifugal and Media Filter.
Sand in water Screen Filter, Centrifugal Filter, or Disk Filter.

How much Filtration do you need?

What we are asking here is what's the smallest size of particle that needs to be removed from the water by the filter? The amount of filtration you need is dependent to a large degree on what type of irrigation you have. For example, drip irrigation systems need a much higher degree of filtration in order to protect the emitters from plugging. For most applications the amount of filtration is measured by the "mesh size" of the screen or maximum size in "microns" of an object that can pass through the filter. There is a mesh vs. micron conversion table at the bottom of this page.

You always want to use the highest level of filtration that is practical. Even if your sprinkler system can easily handle a fairly good size grain of sand without clogging, removing that sand grain is still advantageous as it will eliminate the wear on the system caused by the sand grain as it passes through. The balancing factor is that the more particles removed the more often the filter clogs up and needs to be flushed. Excessive flushing can waste water and energy, so a trade off is necessary.

Rule of Thumb Guidelines

Drip Systems:

The drip emitter manufacturer will specify in their literature the level of filtration required. I almost always take it one level greater. (That is, I remove even smaller particles than they recommend.) I generally never use anything less than 100 mesh or 150 micron.

Sprinkler Systems:

A 70 mesh filter will remove most particles capable of plugging a sprinkler nozzle, however, I like to use a 100 mesh (150 micron) and often use a 150 mesh (100 micron) filter in order to also remove the particles that can cause wear on the system and damage the valves.

All Irrigation Systems:

Bottom line- I would suggest using at least a 100 mesh (150 micron) screen or equivalent in the filter. I suggest you run the water through a filter before it reaches the control valves. Small grains of sand are one of the most common causes of control valve failure, especially when using the standard electric solenoid valves used on most irrigation systems. My experience is that the savings in valve repairs will pay for the cost of the filter over the next 5 years. With a cost benefit like that it is pretty hard to argue against installing a filter!


 

Approximate Filter Size Equivalents

Micron mm Mesh
800 0.8 20
500 0.5 30
300 0.3 50
250 0.25 60
200 0.2 75
180 0.18 80
150 0.15 100
130 0.13 120
100 0.1 150
100 0.1 155
80 0.08 200
50 0.05 300
40 0.04 350
30 0.03 500
25 0.025 600
15 0.015 1000

The values in the table above are rough equivalents. While the Micron is a standard metric measurement, the term Mesh is rather subjective, especially for values over 300. For example, I've seen both 500 mesh and 700 mesh screens that have equal filtration, that is, they both filter out particles down to 30 microns in size. The problem is related to the size of wire from which the screen is manufactured. The mesh designation is based on the number of wires in one linear inch of the screen. So two screens can have the same mesh, but if one is made with thicker wire than the other, the one with thicker wire will have smaller openings between the wires. Therefore, micron is a much more accurate measurement for use in determining the size of particle that can (or can't!) pass through a filter.


 

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