Chapter 14

Drip Irrigation for Greenhouse Crops

Maintenance of Drip irrigation Systems

The use of high quality water and an adequate filtration system cannot be over emphasized. Use of poor quality irrigation water in a drip irrigation system can result in so many maintenance problems related to emitter plugging that any benefits expected relative to other irrigation methods will be eliminated. Maintaining the filtration system satisfactorily, frequent flushing of the system, and chemically treating the water if necessary will go a long way toward eliminating these problems. As mentioned earlier, taking baseline readings and monitoring flow, pressure and the condition of flush water regularly will provide guidance when maintenance should be scheduled.

Filter Maintenance

Filters are the first line of protection for your drip irrigation system, and they need regular maintenance to operate at a high level. A pressure drop across the filter is an indication of debris, so it must be monitored to maintain full flow. Pressure drops can be detected by installing gauges before and after the filter or a differential pressure gauge that compares the two sides.

Monitoring Filter Pressure Differential

Most filters incur increasingly higher friction losses between the filter inlet and outlet as the filter becomes clogged. Monitor the filter pressure differential frequently, especially as water conditions change through the season. Excessive pressure differential may lead to debris passing through the filters and/or poor irrigation system performance. Many filter systems are automated and will self-clean via an electric or hydraulic 3-way backflush valve when a pre-set filter differential is reached.

Sand Separator (centrifugal or hydrocyclone) Filters

These filters need little maintenance, but they require regular flushing. The amount of sediment in the incoming water, the volume of water used, and the capacity of the collection chamber at the bottom of the filter will determine how often and how long the flushing valve needs to operate.

Screen Filters

Regular cleaning of screen filters is very important. If they are neglected, a portion of the screening element will become caked and clogged, forcing water through a smaller area. This can push debris through the screening element and under extreme conditions rupture it. Upstream and downstream pressure gauges can help judge when a filter requires cleaning. A pressure drop of 1 to 3 psi is normal for a screen filter.

Sand Media Filters

As the media fills with particulate matter, the pressure drop across the media tank increases, forcing water through smaller and fewer channels. This will eventually disable a media filter, requiring that clean water from one tank be routed backwards through the dirty tank to clean the media. Typically media filters should be backflushed when the pressure drop across the filter reaches about 10 psi or as recommended by the manufacturer. During the backwash process, the direction of the water reverses through the sand bed. As this takes place, the dirty sand bed lifts, allowing individual particles to separate from each other. Because the organic contaminants are lighter than the actual sand media, they rise to the top of the media tank where they are flushed via the backflush manifold. This Backflushing requires exact flow rates to make the media “dance” and be thoroughly cleaned

Line Flushing

Flushing should occur as often as needed to keep lines clean and will depend on seasonal water quality, temperature and the effectiveness of the system filter. The main lines, submains, and particularly the lateral lines should be flushed periodically to ensure sediments are cleared from the system, which can cause a potential hazard by clogging emitters. Flushing is effective only when the flow rate within the main, sub-main or distribution line is sufficient to allow for proper flushing velocities in the system.

Emitter Inspection

Systematic checking is required to spot malfunctioning emitters or to use accurate flow and pressure measurements and analyze their rates of change over time. Slow clogging causing partial blockage results from sediments, precipitates, organic deposits, or mixtures of these. Physical deterioration of parts is a concern with pressure compensating emitters. The flow passage may slowly close as the compensating part wears out. Mechanical malfunction can also be a problem in flushing emitters.

Chemical Treatment Chlorination

Water with a high organic load (algae, moss, bacterial slimes) should undergo chlorination. While a high organic load may be present in water from any source, it is most likely to be present at high levels in surface water from rivers, canals, reservoirs, and ponds. Although using good filters, such as sand media filters, can cut down on organic clogging, the best way to deal with the problem is to add a biocide to the irrigation water, such as chlorine. Adding chlorine to water produces hypochlorous acid, which then undergoes an ionization reaction producing hypochlorite.

Chlorine Concentration Schemes

When the purpose of chlorination is improving filtration performance, the injection point should be close to the filtration system to assure even-distribution throughout the filters. The following are several possible chlorine concentration schemes:

Sources of Chlorine

The most common chlorine sources are sodium hypochlorite (a liquid), calcium hypochlorite (powder or granules), and chlorine gas. Common household bleach, sodium hypochlorite (NaClO), is used in many small drip irrigation systems and the easiest form of chlorine to handle.

Calculations for Liquid Chlorine

Calculate the injection rate using sodium hypochlorite with the following formula:

Chlorine Application Precautions

Chlorine material (liquid, solid or gas) is dangerous to humans. Before using chlorine, read all safety instructions provided by the chlorine manufacturer. Regard all instructions for chlorine treatment as subordinate to all legal provisions and to the instructions of the chlorine manufacturer.

Chemical Treatment Acidification

Mineral precipitates can form deposits (scale) that clog emitters. The most common deposits are calcium or magnesium carbonates and iron oxides. Water with a pH of 7.5 or higher and a bicarbonate level higher than 100 ppm has a risk of mineral precipitation, depending on the hardness of the water. Hardness of water, which is determined by the concentrations of calcium and magnesium, is classified as follows: soft (0 to 60 ppm of Ca and Mg); moderate (61 to 120); hard (121 to 180); very hard (more than 180 ppm). Moderate, hard and very hard water needs acid injection. If the water is undergoing chlorination too, acid injection may be used where the pH of the irrigation water is consistently high.

Types of Acids

Acids that can be added to the irrigation water include sulfuric acid, hydrochloric, muriatic acid, and phosphoric acid. Urea sulfuric acid, an acid with a nitrogen fertilizer value, can also be used. This product is safer to use and is marketed as N-pHURIC®.

Determining the Amount of Acid Needed

The amount of acid required to treat a system depends on: 1) the strength of the acid being used, 2) the buffering capacity of the irrigation water, and 3) the pH (of the irrigation water) needed to dissolve mineral precipitates in the lines and emitters. The required pH of the irrigation water (target pH) depends on the severity of mineral deposits. Two approaches can be used to determine the amount of acid needed.

Application

After the desired amount of acid has been injected and distributed throughout the drip irrigation system, turn the system off and let the low pH water remain in the lines for several hours, preferably overnight. This allows sufficient reaction time for the acidified water to dissolve mineral precipitates. Once completed, flush the lines to remove dislodged and solubilized materials

Chemical Treatment Organic Farming

Since many of the materials routinely applied for system maintenance in conventional greenhouses aren’t allowed in organic farming, alternative materials must be used. Likewise, since many of the alternative materials used in organic fields are prone to clog a drip irrigation system, it’s wise to install secondary filtration at each zone in case materials precipitate out of solution between the pump station filter outlet and the inlet to the zone (see Typical Drip System Layout illustration in Figure 14.1). The National Organic Program (NOP) Standards for Drip Irrigation (Simonne, et al. 2008) below provide guidance regarding what drip maintenance materials are allowable in certified organic production.

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