- Author: Ben Faber
A crucial aspect to microsprinkler and dripper performance is maintaining the size of the orifice as it was delivered from the factory. Even small changes in the size of the orifice can have significant effects on the volume of water distributed in the orchard. One of the most common causes of a decrease in orifice size or even clogging is a result of the high lime content of our waters. Calcium carbonate precipitation can readily be observed by the whitish deposits that form on emitters and microsprinklers. With the drought it's important to make that water go farther.
Reasons for carbonate precipitation include the following:
1. Change in the pH of the groundwater. This can occur when groundwater is pumped. Pumping reduces the pressure of the groundwater as it flows into the well. This reduction releases dissolved carbon dioxide gas causing the pH of the groundwater to increase. The pH increase can result in carbonate precipitation.
2. Evaporation of water in the dripper or microsprinkler. Evaporation increases the concentration of chemicals dissolved in the water remaining in the emitter. Because of its low solubility in water, calcium carbonate readily precipitates during evaporation.
3. Increase in water temperature. The solubility of calcium carbonate is reduced as water temperature increases.
4, Injection of certain chemicals, such as bleach or some fertilizers that interact with the water.
The problem of lime precipitation depends primarily on the pH of the water. At pH values less than 6, mostly dissolved carbon dioxide and a small amount of carbonic acid exist in the water. At pH values between about 6.5 and 10, bicarbonate is the dominant species. When water evaporates from the irrigation system the bicarbonate precipitates as lime if there is adequate calcium in the water. The potential for carbonate clogging is highest when bicarbonate concentration in the water exceeds 2 milliequivalents per liter (meq/L) and the pH exceeds 7.5.
This relationship of bicarbonate to water pH indicates that lowering the pH will prevent or reduce carbonate clogging of the system. Lowering the pH, dissolves any existing carbonate precipitation and prevents the formation of lime deposits.
A water's pH is lowered by injecting acids. The common acids, such as sulfuric and hydrochloric (muriatic) have been used, as well as the more expensive citric and nitric acids. An acid/fertilizer compound of a combination of urea/sulfuric acid (N-pHURIC ®) has proven to be useful and much safer than straight acids. This acidifying material is most commonly used for water treatment rather than as a source of nitrogen. If the material is used in any significant amount, its nitrogen contribution to the fertilizer program needs to be considered.
Determining frequency and amounts of acid to prevent clogging can be fairly matter of fact. Depending on the rate at which carbonate precipitation occurs, acid injection may only need to occur intermittently during the irrigation cycle to. It might only require 30-60 minutes to maintain a pH of 4. With more problematic waters, continuous acid injection to maintain a pH between 5 and 7 may be necessary.
The amount of acid needed to lower the pH to a desired level depends on the bicarbonate/carbonate concentration in the water and the target pH. The water can be sent to a lab for determining the acid amount or a trial and error approach can be used. Acid can be added to water in increments while measuring the pH until the desired pH is reached. Water pH can be measured with pH test strips or a hand held pH meter. Test kits are also readily available at swimming pool supply stores.
Other than acid for correcting lime clogging, there are several other amendments being sold on the market. Sodium hexametaphosphate has also been used and works against iron and manganese clogging. The small amount of sodium is not a problem. It is safer than an acid, but costs a bit more.