14.5.1: Filtration

Most irrigation water requires filtration for microirrigation. Normally, filtration equipment is located just downstream of the pump at the control station. Domestic water, particularly municipal supplies, are already filtered so the homeowner or proprietor does not normally have to filter the water supply for microirrigation. In rural settings, filtration is almost always required. The filter system commonly used in microirrigation is a media filter followed by a screen filter. Examples of filtration systems for a large and a small field are shown in Figure 14.10. If the irrigation water has a heavy sand load, the water should pass through a sand separator or a settling basin before passing through media and screen filters.

Suspended particles that might plug a system can be either inorganic or organic. Algae, bacteria, diatoms, larvae, fish, snails, seeds and other plant parts are the major organic solids while sand and soil particles are the primary inorganic solids. Because a consistently clean water supply is vital, filtration and chemical treatment must be furnished for the worst possible conditions. In a few cases, chemical coagulants are required to control silt, clay, or suspended colloids. Chlorine may be required sometimes to control algae and other organic materials.

Well water is usually low in organic materials, but it can contain sand. Therefore, a screen filter is frequently adequate. Irrigation water may be saline or be chemically unstable thereby producing chemical precipitates. In some cases, water supplies contain chemical constituents that provide nutrients for bacterial growth. For these waters, chemical treatment is required.

The size of particle that can be tolerated by a water applicator should be indicated by the manufacturer because it depends on applicator construction. Typically, the recommendation is to remove all particles larger than one-tenth the diameter of the orifice or flow passage of the emitter. This is necessary because particles may become grouped and bridge the passageway. Many manufacturers recommend removing particles larger than 0.003 to 0.006 inches in diameter.

In addition to the main filtration system, small screen filters should be installed at the inlet to each lateral or manifold as a precaution against plugging. These auxiliary screens prevent debris from entering the system when the main filters are cleaned or if breaks or openings occur in the distribution system.

Fine particles settle out when flow is slow or stops. The clogging that results may not be rapid, but it is inevitable. As a safeguard, either manual or automatic flushing devices should be installed at the end of each lateral. These protective devices are particularly important to clean the system after installation and repair.

Settling basins, ponds, or reservoirs can remove large quantities of sand and silt. They should be long and narrow with water discharged into the basin at one end and removed from the opposite end to provide settling time for the suspended materials. If water remains in the basin for at least 15 minutes, most inorganic particles larger than about 0.003 inches will settle out.

Figure 14.10. Typical control station and filtration system of media filters for (a) a subsurface drip system for a large field in Nebraska (photo courtesy of Laszlo Hayde, IHE Delft Institute for Water Education), and (b) a microirrigation system for a small field in India (image from Indiamart, https://www.indiamart.com/ proddetail/drip-sprinkler-irrigation-system-20348028048.html).

a)

b)

About 98% of the sand particles intercepted by a screen with 0.003-inch openings can be removed by a vortex separator. Centrifugal force is the principal employed by a vortex separator to remove high-density particles from the water. Organic materials, however, cannot be removed by this method because they have low density.

Media filters are used frequently in microirrigation systems. The filter consists of fine gravel and sand of selected sizes placed in graded layers inside a cylindrical tank (Figure 14.10). These filters are very effective in filtering inorganic and organic materials because they can be trapped throughout the depth of the media bed. Long, narrow particles, such as algae and diatoms, are more likely to be caught in the multilayered media bed than on the surface of a screen.

A drop in pressure of 2 to 3 psi occurs from the inlet to the outlet of a clean media filter. As the pores of the media become plugged with contaminants, the pressure drop increases. It is normally recommended that the media filter be flushed to remove the accumulated contaminants when the pressure drop reaches 10 psi. If the water is relatively clean and flushing is not needed frequently, manual flushing may be suitable. Where frequent cleaning is required, automatic flushing can be actuated by a timer or by sensing the pressure differential across the media filter.

Where suitable, screen mesh filters provide a simple and efficient means for filtering. Hole size and the total amount of open area in the screen determine a screen filter's efficiency and operational limits. Screen filters are used to remove fine sand or small amounts of algae. They are commonly used where the water is expected to be clean, i.e., pumped groundwater, municipal supplies, and following other filter systems.

Screen filters differ by their configuration for cleaning. The need for cleaning, as with media filters, is determined by the rate at which the filter clogs. This rate of plugging is normally monitored by the drop in water pressure across the filter. It is customary to clean screen filters whenever the pressure difference between the inlet and outlet to the filter is between 3 and 5 psi. Manual cleaning by opening the filter, removing the screen, and washing it is satisfactory when cleaning is not required frequently. If frequent cleaning is required, an automatic flushing system is normally installed. Back flushing, blow down, and gravity flow are examples of configurations for automatic cleaning. The flow of water is reversed in a backflushing filter to remove the collected materials. A high velocity jet of water is run over the screen to sweep away collected particles without opening the filter for blow down filters. A gravity flow filter functions by discharging the water supply onto and through a large screen before pumping it into the irrigation network. Some gravity flow filters have jets under the screen to lift particles and move them off the screen.

The screening material can be constructed of stainless steel, nylon, polyester, or other noncorrosive materials. A stainless steel screen offers strength. Nylon mesh in some blow down filters flutters during flushing which aids to dislodge collected particles.

The flow rate through a screen filter should not exceed 200 gallons per minute (gpm) per square foot of screen open area. The wire or plastic mesh itself obstructs much of the open area. For example, a screen constructed of stainless steel with 0.003-inch openings has 58% open area. An equivalent nylon mesh with the same size openings has only 24% open area. Thus, it is important to consider the actual open area of a screen when sizing a filter.

The total area of screen (As) needed for a screen filter can be calculated from:

\(A_s=\dfrac{Q/Q_m}{O_a}\)

where: Q = flow rate through the filter,

Qm = minimum flow rate permissible per unit area, and

Oa = fraction of open area within the screen.

Example 14.5

A golf course manager has access to a municipal water supply and needs only a screen filter to protect a microirrigation system. If the system is designed for a maximum of 300 gpm and a stainless steel screen with 0.003-inch openings is to be used, what area of screen will be required?

Given: Q = 300 gal/min

Screen opening = 0.003 i

Find: Total filter screen area (As)

Solution

Maximum permissible flow rate (Q_m) is 200 gal/min per ft² of screen open area The fraction of open area of a stainless stell screen (O_a) with 0.003 in openings is 58% of the total screen area (A_s) is:

\(A_s=\dfrac{Q/Q_m}{O_a}=\dfrac{\left(300 \dfrac{\text{gal}}{\text{min}} /200 \dfrac{\text{gal}}{\text{min ft}^2} \right)}{0.58} \)

\(A_s = 2.6 \text{ft}^2 \)