Mechanical washing of the irrigation network:
The presence of algae and microorganisms, suspended solids and solids such as Fe, Mn and Ca that precipitate, constitute a potential problem, which must be prevented with adequate maintenance of the irrigation system.
A quick flush of the irrigation network consists of opening the flushing valves, located at the end of the sub-matrices of each block, letting the water run for one or two minutes while watering. Similarly, groups of about five laterals per block are opened until the water comes out clear.
Chemical washing of the irrigation network:
This operation must be carried out by responsible and competent personnel, with knowledge of the irrigation equipment.
Chlorine as an oxidant and biocide:
The most economical and widely used biocide is chlorine, in the form of sodium hypochlorite (10% – 12%).
Mechanism of action of chlorine:
When chlorine is added to irrigation water, part of it is absorbed by organic matter, without destroying it. This chlorine is called combined chlorine. Chlorine also reacts with ammonium ions, forming chloramines. Part of the chlorine reacts with inorganic compounds such as H2S and Fe, converting to chloride.
Excess chlorine, that is, the chlorine that has not been consumed in these reactions, is called free chlorine and acts as a biocide. Chlorine applied in the form of sodium hypochlorite (CIONa) is hydrolyzed when dissolved in water, turning into hypochlorous acid, which is a strong oxidant.
Its biocidal action is due to the fact that it stops the oxidation of glucose by cells, a phenomenon essential for life. Since ClOH is a weak acid, it is only present at pH below 7.8, although for it to be most effective the pH must be lower than 7.5. The optimum pH is 5.5-6.
For values less than 5, the proportion of ClOH decreases and that of Cl2 increases, which is volatile and is lost, requiring the application of higher doses. The death of microorganisms requires a minimum contact time of 30 minutes.
Chlorine, however, is not a definitive solution for all problems. It is preferably used as a preventative measure. When mucilages composed of bacteria and iron or sulphur precipitates have already formed, its action is of little use.
Preventive treatment with chlorine:
In preventive treatment with chlorine the following objective must be achieved: in the emitter more
away, for at least 45 minutes, the water should come out with a free chlorine concentration between
0.5 and 1.0 ppm. If the time is less than 45 minutes there is no certainty in the bactericidal effect. If the
free chlorine concentration is lower, the effect can even be counterproductive, since the quantity
Insufficient chlorine can stimulate rapid growth of bacteria.
To check that the treatment is correct, the free chlorine content must be measured at the most remote emitter. It is very important that the measuring system registers free chlorine and not total chlorine. For this purpose, orthotolidine, commonly used in swimming pools, which turns yellow in the presence of chlorine, is unsatisfactory. A good measuring system is the one using DPD (N-diethyl-p-phenyl-diamine) which allows free chlorine to be measured.
To achieve the concentrations mentioned in the emitters, doses between 3 and 10 ppm of total chlorine may be necessary at the point of application, although this figure must be tested or measured in practice. When the pH is greater than 7.5, the chlorine requirements are greater, and the level of free chlorine at the end of the emitters must be in the order of 2 to 3 ppm.
Application of much higher concentrations does not increase biocidal efficacy and may instead damage the roots of young trees. Free chlorine concentrations of 30 ppm should never be reached.
Treatments can be repeated every six (6) hours. Chlorine can be applied at any time during irrigation, but it is advisable that no chlorine comes out of the emitters during the last hour.
Injection should be done before the filters to avoid bacterial growth in the sand (sand filters). It is preferable to clean the filters before treatment. This reduces the need for chlorine.
Calculation of the amount of chlorine:
Example: to obtain a concentration of 10 ppm, or 10 gr per m3 of water, and knowing that sodium hypochlorite (ClONa) has 10% of chlorine, 0.1 liters of ClONa per m3 of water are required (100 cc/m3 of H2O).
If 20 m3 of water need to be treated, 2 litres of sodium hypochlorite are needed. The 2 litres of ClONa are dissolved in 100 litres of water and injected into the network in the required time.
It does not matter how much it is diluted (100 or 200 liters of water etc.). If 5 ppm is required -> apply the
1/2 ClONa in equal amount of water.
Chemical precipitates
Calcium Carbonate. Preventive treatment:
Calcium carbonate is a salt with very low solubility (0.031 g/l) although at pH close to 6 the solubility increases almost 100 times. The classic preventative treatment against limescale deposits is acidification.
Various acids can be used for this:
- Hydrochloric acid (HCl) 12 N
- Sulfuric Acid (SO4H2) 36 N
- Nitric Acid (NO3H) 16 N
- Phosphoric Acid (PO4H3) 45 N
IMPORTANT: Do not apply acid with irrigation equipment without consulting first.
The most frequent are the first two, but due to the danger of handling sulfuric acid, the most
used is ClH. Nitric and phosphoric acids provide some amounts of nutrients.
Calcium Carbonate. Cleaning treatment:
When limestone fillings have already occurred, cleaning is done by applying
acids. The most economical method is to inject them into the network.
Acid must be added to the water in sufficient quantity to lower the pH of the mixture to 2. To
This requires a titration of the water in the laboratory, calculating the acid factor.>, that
is the number of meq of acid needed to lower a liter of water to a pH between 3 and 5.
On the other hand, Vt is calculated, the volume of the pipes to be treated multiplied by 2.5 as a coefficient of
safety. If the normality of the acid is N, the volume of acid to be applied is:
Vt – a
Go = __
1000N
The application of water and acid is done at the lowest possible pressure.
When it is verified by means of a pH indicator paper that the acidified water comes out of the most distant emitter, the water inlet is cut off and left like this for one hour, after which a cleaning is carried out at the highest possible pressure that the installation can tolerate, first opening the ends of the highest order pipes until the water comes out clean.
This pipe is then closed and the following pipes are opened and so on up to the sides, all with the aim of preventing the incrustations released from the pipes from reaching the emitters.
After cleaning the pipes and continuing with high pressure, the emitters are cleaned for 15 minutes.
Fe, Mn and S precipitates. Preventive treatment:
These elements are soluble in a reduced state, but when oxidized they precipitate and can block the emitters. Oxidation can be caused by bacteria, by contact with the air or with oxidants contained in the water. Preventive treatment responds to the following tactic: cause oxidation and precipitation before the filters to retain the particles there.
An effective method of preventing these precipitates is the continuous application of oxidants such as sodium hypochlorite. If the pH of the water is below 6.5, chlorine can prevent Fe precipitates when the concentration of Fe is below 3.5 ppm. If the pH is above 6.5, precipitates are prevented with Fe concentrations up to 1.5 ppm. The application of acids may be necessary to improve the pH. The necessary concentration of sodium hypochlorite is calculated at a rate of 1.0 ppm ClONa per 0.7 ppm iron. The reaction is very rapid.
In the presence of manganese, care must be taken when applying hypochlorite, since the oxidation of Mn is much slower than that of Fe, and precipitates may form after passing through the sand filter.
Fe, Mn and S precipitates. Cleaning treatment:
When the emitters are partially blocked, acid can be applied in the manner previously described for the treatment of limestone fillings.
Presence of algae in wells and water reserves:
Copper Sulfate (do not use on aluminum material, it is toxic to fish). The dose is 0.05 – 2.00 ppm.
