Understanding sand

Learn the basics of working with sand media filters to get the most out of irrigation applications.
By Charles Burt, PhD, PE, PhD, CID, CAIS
A sand filtration system with the words "understanding sand" overlaid on top of the image.
(Photo: Charles Burt)

Sand media filters can be used to clean water used in agricultural irrigation, but it’s important to be able to use them correctly to get the greatest benefit from them. This article will be a quick overview of the basics of using sand media filters, which are the most common drip/micro filtration method in the United States.

Understanding sand media

Sand media filters should be considered “polishing” filters. They are not intended to remove massive quantities of organic matter, trash, fish and sand. Those items must be removed by prefiltration. There are numerous ways to prefilter the water, such as reservoirs, which are sometimes necessary to remove large amounts of sand from well or canal water. Reservoirs are also often necessary for wells that have large changes in flow rate throughout the growing season, acting as a buffer to enable an irrigation system pump to deliver a consistent flow rate.

Sand media filter tanks need to be selected for the flow rate, where the dirtier the water, the more tanks will be needed. The ports need to be large enough to allow for inspection and maintenance. (Photo: Charles Burt)

Coarse screens, whether stationary or rotating, can also remove large quantities of organic matter at the intake from canals. Another option is installing sand walls in front of irrigation district canal turnout gates so that sand tends to pass by the turnout instead of entering it.

Avoiding common issues

There are a few major problems that irrigators typically encounter with sand media tanks, including the fact that the backflush flow rate must be adjusted properly, or the sand tanks may either become plugged with dirt or be completely empty without the farmer knowing it has happened. Note that all sand media tanks are not created equal. Some backflush very well and lose almost no sand; others lose large amounts of sand at even medium backflush flow rates (see table 1).

Table 1. Backflush sand collected by ITRC with five different commercial 48-inch sand media tanks

The backflush flow rate pipeline must not be too small or too long or go uphill, which could cause the backflush flow rate to be too low. Additionally, the differential pressure switch and associated hardware can freeze in the winter and be ruined. Those need to be drained or removed before freezing sets in. The tanks must be selected for the flow rate. The dirtier the water, the more tanks are needed (see table 2).

Table 2. Recommended vertical media tank sizes for emitter and row crop drip systems

Corrosion is a concern for 304 stainless steel if water salinities are above 750 parts per million. Type 304 stainless steel will exhibit crevice corrosion with combinations of chloride at greater than 300 ppm, together with sulphate concentrations of less than 300 ppm. Type 316 stainless is more resistant. Sacrificial anodes can be used with stainless steel as well as with mild steel. Part of the problem may be due to the thin plates typically used in stainless steel tank construction; when one compares various stainless steel products, the wall thicknesses should be scrutinized.

The ports on tanks must be large enough to allow irrigators to easily inspect, remove and replace media.

(Photo: Charles Burt)

Backflush controller settings are important. This is different from the flow rate setting, which is adjusted with a valve on the backflush line. The backflush may be set too infrequently and for too short a duration. The only way to tell if the combination of timing and flow rate are correct is if there is access to the downstream end of the backflush line. Place a nylon sock over the end of the line and see what comes out and for how long. Many installations have backflush pipe discharges that are inaccessible and the backflush is never adjusted properly.

Table 3. Typical media sizes and types for agricultural high flow rate media tanks

The sand (media) must match the filtration requirements, such as shown in table 3. The 0.47 mm crushed silica and 0.78 mm crushed granite are two widely used media types. Some manufacturers show #11 crushed granite providing finer filtration than #16 crushed silica. If very fine media is used, a base layer of garnet media (effective media size greater than 1.0 mm) should first be placed at the base of the tanks, filling above the underdrain or gravel pack to a depth of about 6 inches (15 cm). This will keep the fine media on the top of the bed and help prevent the fine media from washing out through or plugging the underdrain.
A very detailed description of sand media filtration and other filtration methods can be found in the free download of “Drip and Micro Irrigation Design and Management for Trees, Vines and Field Crops”, 5th edition, on the Irrigation Training and Research Center website at itrc.org.

Charles Burt, PhD, PE, PhD, CID, CAIS, was the original chairman of the designer certification program and was the IA Person of the Year in 1997. He is professor emeritus of California State Polytechnic University’s irrigation program and founder and chairman of the Irrigation Training and Research Center. He is active in irrigation district modernization, pumping/energy issues and on-farm irrigation, with work in over 30 countries.
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