Calcium scale, and to a much lesser extent iron scale, can be purged with acid.
For system purging, lower the pH of the water in the irrigation system to 2.0 or less to achieve maximum effectiveness of the acidification.
Determine the amount of acid required to decrease the pH to 2.0 or less by titrating a sample of the irrigation water. A titration curve (Fig. 2) is unique for each water source and type of acid.
Since water quality can change with time, re-titrate every few months. Use at least 1 gallon of irrigation water, an eyedropper, and a calibrated portable pH meter.
Stir the irrigation water to ensure complete mixing of the acid before the pH is measured.
Fig. 2. Example of an acid-irrigation water titration curve.
Caution: Low pH water can damage irrigation system hardware:
Corrosion accelerates rapidly as pH decreases below 5.5.
Injecting acid requires special filters and injection pump gaskets. Check with the manufacturer of the equipment before acid treatment to ensure compatibility.
Design chemical injection ports to protrude into the center of the pipeline to ensure adequate mixing of acid with water.
Typical acids that may be injected into irrigation water to purge a micro-irrigation system:
Sulfuric acid – In Florida tests, a 1% solution of sulfuric acid using 38% H2SO4 as the base material removed iron scale from interior tubing walls. However, removal did not necessarily mean dissolution. Sulfuric acid is effective at loosening scale without dissolving it, so line flushing following acid treatment is imperative.
Citric acid – A 1% citric acid solution removed iron scale from interior tubing walls in Florida tests. In this case, the removal was due to scale dissolution.
Hydrochloric acid (sold commercially as muratic acid) can effectively remove mineral scale. Hydrochloric acid can be purchased with an inhibitor that minimizes its corrosive effect on metal parts.
Sulfamic acid is a dry granular material that makes a strong acid when mixed with water. Although it is more expensive than hydrochloric acid and is less aggressive, sulfamic acid offers a number of advantages. In its dry form, it is relatively safe to handle. Sulfamic acid is particularly useful in treating calcium scale but is less effective on iron.
Hydroxyacetic (glycolic) acid has been reported to be effective in treating iron scale in wells. Its effectiveness in reclaiming micro-irrigation system is not known.
Use this general injection procedure:
Inject acid just long enough for the acid-water mixture to fill the entire interior volume of the irrigation system mainlines, sub-mains, manifolds, and lateral tubing.
Allow the acid-water mixture to stand inside the system for at least 24 hours.
Rinse the irrigation system using flush-out valves or by opening lateral tubing ends following acid injection. If the system is not flushed, emitter plugging could be made worse than before if small particles detached by the acid get caught in emitters.
Evaluate purge effectiveness using the guidelines given in section 5.4.
Acids require special filters and injection pump gaskets. Check with the manufacturer of the equipment before acid treatment to ensure compatibility.
CAUTION: Always add acid to water; do not add water to acid. Adding water to acid can cause a violent reaction, and may cause the acid to splash on the person pouring the water. Individuals working with acids should wear protective clothing and eyewear. Also, be sure that adequate safety devices are provided, including a shower and eyewash.
Commercial products designed to purge scale from micro-irrigation systems are now under development. As with water conditioners (scale-preventers), commercial purging products are proprietary mixtures of several chemicals. No general recommendation for commercial product use is provided in this guide because the wide variety of scaling and plugging problems found requires customized diagnosis and recommendation. If a commercial product is chosen for purging, always follow the manufacturer’s recommendations for use as listed on the product label.
5.4. Evaluating system purge treatments
Determine if a purge treatment (injection of acid or a commercial purging product) has worked by installing monitoring devices (tubing inserts) prior to chemical injection, or by evaluating the water application uniformity of the micro-irrigation system.