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 its 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
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 dont.
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 filters 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 screens
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 screens 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 screens
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 filters 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
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.
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
- 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
- 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