Filtering Through Filtration Choices
When landscape professionals think about designing an irrigation system, most minds wander immediately to spray patterns, drip hoses, timers and installation. But success is shortlived with most irrigation systems without good filtration to safeguard the components and help ensure reliability.
Fortunately, there are plenty of great options on the market today for landscape and commercial filtration, so with a little forethought, it’s easy to find a system that suits the water, the space and the objectives of any irrigation project. The key to good filtration is to think about it early in the process and view it as an integral link in a long chain that extends from the water supply to the landscape— and right through to your bottom line. The right choice yields an effective system, a satisfied client, and good recommendations. The wrong choice leads to hassles, callbacks and frustration.
Specifying a filter is a lot like a doctor writing a prescription for a patient. You need to know the problem and the symptoms. You also have to know how serious the challenge is, and how much treatment is appropriate.
Like a doctor, you’ll need to do an examination. Not all particles in water are created equal. The size of the particles, their shape and their hardness, can influence your choice of filter.
Sand tends to be crystalline and abrasive, wearing out spray nozzles and valves and blocking drip emitters. Clay is made up of smaller, plate-shaped particles that lock together to form clumps. Bacteria and algae are deformable, or flexible, so they can often squeeze through filters that would easily trap soil particles of the same size. Mollusks can be a big problem that start as tiny larvae that quickly grow to clog pipes and sprinkler heads. Some solids are dissolved in the water, precipitating or crystallizing out when conditions are right to form slime or scale.
Start by considering the source of the problem. Surface water is likely to contain algae, bacteria and mollusks such as snails or mussels. Well water is more apt to carry suspended sand or clay, depending on the soil that surrounds the intake. Those contaminants can change seasonally, too. Algae may be more of a problem in the heat of summer, when fertilizer washes into streams and ponds and causes algal blooms. Sediment levels may increase when water tables drop beyond the easy reach of wells, or when creeks run high after gullywasher storms.
Some close observation and careful consideration can give you a fair idea of what kinds of particles are likely to be in the water. So can pulling a sample and letting it settle in a jar. But for a thorough understanding of what’s in the water, it’s worthwhile to send a sample to a water testing lab. Ask for an analysis of what’s in the water, and be sure to request a particle size distribution (PSD) by volume and count. That will identify how many particles are in a particular volume of water and how large they are.
Obviously, particle size is the vital factor in choosing the degree of filtration you need. The best 100-micron filter in the world will not be very effective against particles that are 40 microns in size.
The bottom line: don’t treat until you diagnose.
A variety of technologies exist to filter particles of various sizes.
Here are the most popular ones in landscape and commercial irrigation applications:
Media filters pass water through canisters of fine particles, or media, such as sand or diatomaceous earth. Water finds its way through the microscopic spaces between the particles, while solids get trapped in contact with the media. Sand media filters are quite literally an ancient technology: writings in Sanskrit and ancient Greek indicate that the technology dates back thousands of years. The forebears of today’s sand media filters were designed in the early 1800s. Media filters that contain activated charcoal are often used to filter drinking water.
Sock or cartridge filters are also a time-honored tradition—in fact, a fine fabric bag developed around 500 BC was called the Hippocratic Sleeve, a tribute to its inventor, Hippocrates, the father of modern medicine.
Settling tanks use a combination of chemical attraction and gravity to collect particles into heavy masses, then let them sink in still water.
Disk filters are stacks of finely grooved plastic disks stacked tightly on top of each other. As water passes from the outside to the inside of the stack, suspended particles adhere to the grooves.
Because they have a great deal of surface area and capture particles along a relatively long path, disk filters are especially good at picking up deformable, organic particles such as algae.
Screen filters work like superfine strainers, capturing particles too large to fit through their mesh. Screen filters are excellent at removing inorganic material such as suspended sand or clay.
Sometimes, it pays to employ more than one filtration technology. For instance, if the goal is protecting sensitive plantings by removing chlorine from the source water, an activated charcoal filter would be the best choice. However, it would be important to remove suspended solids from the water before they reach the charcoal filter—otherwise, the charcoal could quickly plug up with sand or clay, resulting in high labor and material costs from frequent replacement of the activated charcoal.
In short, put the right filter in the right place, doing what it’s designed to do. Don’t use a super-fine filter for gross filtration.
Pros and cons
Just as not all particles are created equal, not all filtration technologies are created equal.
Sand media systems are big and bulky, and can be difficult to clean. Settling ponds are also very large and slow. Cartridge or bag filters require manual replacement. And screens vary widely. Wedge-wire screens feature rectangular gaps that are significantly taller than they are wide. When a particle floats by vertically, a wedge-wire screen may let it through even though the particle is larger than the filter’s nominal size. By contrast, woven screens have uniformsized mesh, though they are limited because they can’t be woven finely enough to capture soluble contaminants or particles below about 10 microns in size.
It’s also important to get the specs on the amount of pressure needed to operate the filtration system optimally, and how much head loss occurs between the inlet and outlet side of the system.
Those strengths and weaknesses highlight the value of prescription filtration.
Most filtration systems must be washed manually or employ some sort of backflush system to remove trapped contaminants. The frequency of cleaning cycles and the level of automation have significant bearing on the labor demands required to operate and maintain the system.
Cleaning can take many forms.
The simplest is opening the filter housing, removing the screen, and washing it with a hose. Manual cleaning of sand media or some screen systems could involve shutting off outlet valves and engaging a back wash process that stirs up the media and flushes out trapped particles.
Some filter sare equipped with cranks that allow operators to b r u s h o r siphon out debris without opening t h e f i l t e r housing or interrupting the filtering process. Some use a solenoid-governed valve to initiate a flushing sequence at a specified time—for instance, the first couple of minutes of an irrigation cycle. And the most sophisticated filters feature automatic self-cleaning technology, which engages at a specified time interval or when a target pressure differential is reached on either side of the filter.
In cartridge or bag filters, consumables can be a concern. Disposable filter cartridges can be costly to replace, both in terms of the purchase price of the cartridges and in the labor it takes to make the switch. Replacing many cartridge filters also opens the system to the outside, increasing the chance that contaminants will enter the system downstream of the filter, or that the system could be inadequately sealed once the new cartridge is in place.
The need for cleaning depends in part on the type of particles in the water—even if they’re the same size. Compare 10-micron sand particles and 10-micron clay particles.
They are the same size and roughly the same deformability, but for a given filter area, you will probably need to flush more often where clay is present, because the particles tend to stack up like tiles and blind screens or media more quickly.
Increasingly, irrigation professionals must look downstream at the fate of their backflush water. Some communities don’t allow backflush water to be directed into the sewage system, or they charge high treatment costs for wastewater. Others don’t allow the discharge of backflush water into streams or lakes. An increasing number are requiring discharge into the soil via sumps or injection wells. And residents and neighbors are more sensitive than ever about water efficiency.
As a result, the amount of backflush water a system produces can be a very significant variable in making a good choice. The difference can be significantly variable, too: for instance, Amiad data shows that automatic self-cleaning screen filters can generate 75 percent less backflush water than comparablysized sand media filters.
Backflushing can be a noisy process in some systems—not a big deal in an installation hidden in a well house on a large corporate park or golf course, but a significant concern in a compact housing development whose sprinklers run at night. If noise is a potential issue, consider the sound, frequency and duration of the backflush process.
Aesthetics can play a significant role in choosing a filtration system, too. Out of sight in the back of a sprawling park or campus, an array of large sand media filter tanks can be an acceptable use of space, but it’s doubtful that many residents would enjoy seeing a large piece of their acreage devoted to the same tanks if they were closer to home. Sleeker, more streamlined technologies could suit the look of a project better, and because they often have a much trimmer footprint, they tie up less budget on concrete pads, piping, manifolds and other infrastructure costs.
What the clients see—and how often they see your staff on-site managing high-maintenance systems or unplugging nozzles—are significant predictors of how happy they will be with their irrigation system. And the first step in enjoying the arc of a well-directed spray of water or the glistening of a lush, newly watered landscape is making sure the water gets the proper filtration before it reaches the irrigation system.
That’s the prescription for a healthy, profitable irrigation job.
EDITOR’S NOTE: Jim Lauria is vice president for sales & marketing for Amiad Filtration Systems in Oxnard, California. A chemical engineer by training, he has more than 25 years of experience in liquid/solid separation processes and water treatment.
In addition to filtration degree and flow, goals such as minimizing noise and footprint can be key factors in selecting filtration systems.
This automatic self-cleaning system uses the pressure inside the filter to push solids out through a nozzle that concentrates the effect on less than one square inch of screen at a time.
Weave-wire screens have uniform-sized openings that can provide filtration down to the 10-micron level, strengthened by heavy-gauge steel mesh.