Dealing with silt in a micro-irrigation system
by Lizanne E. Wheeler, M.Sc. & Patrick D. Brown,
CID Vineyard Water Systems

How do you deal with silt in a drip system?" is a frequent query that we receive from colleagues and clients. Since it’s a common problem, just how do we "deal with silt"? Silt, by definition, has a soil particle size falling between clay and sand. The U.S. Department of Agriculture classifies silt as having a particle size between 0.002 and 0.05 mm. We generally think of silt as being of natural origin, but frequently irrigation systems are loaded with "self-inflicted silt," whose origins are fertilizer insoluble fractions, solution grade gypsum, and mineral precipitates.

Like sand, naturally occurring silt is typically composed of irregularly shape and sized particles, predominately of quartz. Because of its small size, silt is able to sustain an adhering film of clay to its surface, giving it many clay-like attributes, including plasticity, stickiness, and adsorption. The finest filter used in agriculture is typically 200-mesh equivalent, which will capture particles as small as 0.08 mm. So silt, which is smaller than 0.05 mm, should easily pass through agriculture filters as well as drip emitters or micro-sprinklers. If this is the case, why do we have to deal with silt at all?

When the irrigation source water contains heavy loads of suspended particles, the first challenge is filtration. Sand and larger silt particles can be effectively removed with hydrocyclonic sand separators, which spin the water and remove the heavier particles through centrifugal and gravitational forces. Larger, less dense particles can be removed by a variety of filtration methods including screens, disks, and sand media filters. That leaves the finer silt and clay particles, which theoretically should pass through the filter, but they don’t.

Unfortunately, due to its sticky nature, silt can aggregate and become an irrigation challenge, forcing us to "deal with silt" after all. Silt commonly builds up on the surface of a filter and "blinds it," reducing the flow to nearly zero. Silt that does pass through filters can settle out in pipelines and hose laterals where the water velocity is slow enough to allow for its deposition. Silt has also been shown to build up on screen-filters and extrude through the mesh like toothpaste, creating emitter-plugging size particles downstream of the filters.

Media filter plugging

Depending on its nature and the type of filter, silt may or may not pass through the filter. For example, the silt may be sticky enough to adhere to the media sand and begin to accumulate. Over a period of time, the silt can form a thin layer or cap across the top of the media bed, greatly restricting the flow through the filter. If the silt is sticky enough, it may be very difficult to rinse out of the media bed during a normal backflush sequence. This can be remedied by maintaining a fine-grade media like #20 crushed silica in the tank and increasing the backflush frequency and flow rate to discharge the contaminated top layer of media sand on a regular basis.

The resulting accelerated loss of media sand requires careful monitoring, and topping off sand levels becomes a frequent necessity, typically on a monthly basis. If the silt is being successfully captured on the surface of the media, the duration (actual flushing time per tank) can be shortened to reduce the amount of media discharged during each sequence.

A second media filter challenge occurs when silt passes through the media bed and accumulates around the filter’s underdrain, which is quite common with epoxy-cake underdrains. This occurs gradually and without a noticeable increase in the pressure differential across the filters. It can only be detected by digging through the media bed with your hand to inspect the cleanliness of the sand as you pass deeper into the tank. All of the sand below the top few inches should be clean. If you find silt, mud, or clay deep in the media bed, you will soon be forced to "deal with silt."

In the first example, silt was captured on the surface of the media, requiring frequent, quick, high-velocity backflushes to toss out the media sand that had become contaminated with the sticky silt. In the case of silt penetrating deep into the media bed, less vigorous but longer duration backflushing is required to lift the silt out of the media matrix and discharge it without a significant loss of filtration sand. Once the filters have been thoroughly rinsed of silt, the backflushing sequences should be performed on an appropriate time-clock basis, not on a pressure-differential basis. The media sand must be inspected regularly (monthly) to insure that the backflushes are adequate to maintain clean media beds.

It may be that the silt is of such a size that it is captured in the media, though it would be small enough to pass through emitters or micro-jet or pulsating (frost control) sprinklers. An example of this would be plugging a fine grade media sand such as #20 crushed silica, (~200-mesh equivalent) with silt in the range of 180 mesh, when the emitters or micro-sprinklers require only 120-mesh filtration. Replacing the media with a courser grade (#16 crushed silica, ~160-mesh equivalent) may allow the silt to pass through the filters and emitters without any plugging.

Screen filter plugging

Screen filter plugging with silt is generally a result of sticky silt particles adhering to the mesh fabric or a trapped particle (sand) and accumulating to such a level that they bridge across the screen’s openings. As in the case of media filters, it may be that the problem silt is of a small enough size that it would easily pass through the emission devices without plugging them. If this is determined to be the case, changing the mesh size to a coarser-grade filtration may solve the problem.

Another potential screen plugging solution is to increase the velocity of the water across the screen’s face. If the velocity is high enough, the energy of the water "pushing" the silt overcomes the ability of the silt particles to stick to one another and bridge across the screen’s openings. There are several agricultural screens designed to take advantage of this principle. The most common utilizes "spin-plate" nozzles that force the water into a centrifugal spinning action across the face of the screen. It is possible to place rubber stoppers in one or more of the nozzles to increase the velocity of the water across the screen, depending on the situation. Another design uses a "torpedo" on the inside of a tubular screen to decrease the filter’s volume without decreasing the surface area of the screen, resulting in a dramatic increase in the velocity of the water past the face of the screen.

Because silt accumulation on a mesh screen gradually creates a finer mesh size until bridging and blinding occurs, the phenomenon of screen plugging appears to be exponential in that very little pressure-differential is observed until the blinding occurs. Because of this, the backflush cycles should be scheduled regularly, rather than triggered by a pressure-differential, to prevent reaching the "critical mass" of accumulated silt which is capable of blinding the screen.

Silt downstream of the filters

It is also quite common to find silt accumulated in the mainlines or hose laterals of a micro-irrigation system. This occurs when silt that has passed through the filter (or has been injected below the filter!) and reaches an area of the irrigation system where the velocity of the water is low enough to allow the particles to settle out. Similar to "silting out" in a river delta, in this phenomenon, particles aggregate to form layers or lenses of sticky mud. Because of the adsorptive properties of the clay fraction, these silt layers contain the minerals and nutrients necessary for biological growth. Therefore they serve as a food source as well as a suitable place of attachment, for the bacteria and algae that comprise the first link in the food chain of the ecosystem that can potentially develop in any micro-irrigation system.

It is important to frequently flush this silt from the buried lines and hose laterals to keep the biology of the drip system under control. A good flush requires increasing the water velocity in the lines to at least two feet per second. It is important to keep in mind that as the silt layers age and go through several wet/dry cycles they harden and become more difficult to remove. In some cases, they will actually become cemented together in a limestone matrix, (discussed in PWV, "Gypsum in the drip" March/April 1999).

To effectively flush buried mainlines, it is necessary to open the flush valves at the ends of the lines and allow the water to run for several minutes. Depending upon the system design and pumping capacity, it may require closing off a portion of the field to generate enough velocity in the mainline to scour the silt out. Some systems must be retrofitted with flush valves of appropriate size to allow for sufficient flow.

In a similar fashion, frequent flushing of the polyethylene hose laterals will minimize the biological contamination associated with silt accumulation. To achieve an adequate flush velocity of two feet per second through the end of a typical hose lateral (0.620 inch inside diameter) requires a flow of approximately two gallons per minute flushing out the end. This increased flow rate causes a much greater pressure loss down the hose length, requiring greater than normal hose inlet pressures to sustain the flow. It is usually necessary to increase the field pressure and only flush a small percentage of the hose laterals at a time. The field pressure can be increased by adjusting the pressure-reducing pilot on the block valve or by closing a portion of the blocks normally irrigated with the set.

Summary

Here are the ways to deal with silt:

• Minimize the entry of silt into the system with appropriate pump intake suction designs (do not pull water off the bottom of a reservoir).
• Use a hydrocyclone pre-filter if the sediment load is heavy (above 2 parts per million).
• Adjust the duration and frequency of the filter backflushing sequences to an appropriate level (as described earlier in this article).
• When using sand media filters, frequently inspect the media bed down to the underdrains for evidence of silt accumulation.
• Frequently flush all buried pipelines to prevent the accumulated silt from hardening.
• Insure that the time-consuming hose-end flushes are effective by increasing field pressure and only flushing a few laterals at a time.
• Silty systems generally require more frequent sanitizing. Monitor the biological growth in the system by capturing hose-end flush water into a clear jar and holding it up to the light. Adjust the chlorine program accordingly.

Lizanne E. Wheeler and Patrick D. Brown are independent irrigation design consultants, specializing in bilingual training and developing management programs for micro irrigation systems. They may be contacted at Vineyard Water Systems, 521 Lucerne Road, Cayucos, CA 93430, tel; 805/995-0587. Reprints of their recent publications are available at www.vineyardwater.com.