Flushing - Mains, Submains and laterals
Flushing constitutes an important maintenance routine. In micro-irrigation systems provisions must be made to flush mains, submains and lateral lines to remove settled sediments.
Mainline should be flushed at scour valve locations, whilst submains are flushed at slushing points usually located at the ends of the lines while the system is running and allow water to run into a container until it runs clear. Collect some of the dirty water in a glass or clear plastic container and examine the contaminants. Take note of the nature of the impurities in the water. If there be significant amount of contaminant in the flush water, find out what it is. Does it appear to be bacterial slime? Are large aggregated particles present? Is there evidence of iron precipitation? Is there any material that could be sand from the media filter?
If unsure, have water examined and take corrective action to cure the problem. If chlorine or acid treatment is required follow the steps below for treatment.
Please note whenever a repair has been carried out on any pipe work in the system, flushing and cleaning of filtration will have to be carried out.
At the end of the season empty mainlines, field filters and valves and ensure all ball valves are opened to drain the water from within the ball and then shut again to prevent vermin and contaminants entering the system. This draining of components will help in preventing frost damage.
Chlorinating the system
Chlorination is recommended to reduce blockages due to organic matter. Chlorine is an oxidising agent that kills bacteria, algae and other organic matter and prevents new growth. Certain bacteria cause iron to precipitate and form a red filamentous sludge that attaches to pipes and may block emitters (sprinkler nozzles as well as drippers). Continuous injection of chlorine prior to the filter, at the rate of 0.5 ppm to 1 ppm, can also be used to precipitate the iron and prevent it moving past the filter.
The most common chlorine compounds are sodium hypochlorite (liquid) and calcium hypochlorite (solid). Sodium hypochlorite (10% chlorine) is easier to use and relatively safe.
There are four main chlorine application methods:-
The procedures below should be used as a guide to developing a procedure that suits your system, water conditions and management.
Intermittent treatment involves periodic sterilisation by chlorination and can also be used to prevent build-up of organic matter in the system. This is the most commonly used preventative treatment. 30 ppm chlorine is injected 3 - 4 times during the season.
Continuous Treatment uses a constant injection rate, usually of 5 to10 ppm, adjusted so that 1 ppm of chlorine is detectable at the end of the furthest lateral from the pump. A swimming pool tests kit can be used to detect the chlorine and its level.
Superchlorination involves injecting a higher concentration of chlorine, about 500 ppm, into a system in which organic matter has accumulated. The system is then shut down and the chlorine left in it for 24 hours. The system is then flushed step by step: mainline, submain and finally through the ends of the laterals. A direct flush through the laterals can cause mainline sediments to clog the emitters.
End of Season Chlorination - initiate a normal back-flush cycle. Before adding cleaners, chlorine or additives to media systems consult the chemical suppliers. Time the sequencing of the back flush valves’ opening and closing speeds with a stopwatch and make note of the time.
Employ a chlorination program over the entire system to ensure all bacteria and algae are killed before the system is shut down. Chlorinate at about 10-20 ppm for 30 - 60 minutes (furthest emitter) then irrigate for a further 60 minutes. After this time, open submain flush points and laterals to flush the system until the water runs clean.
Disconnect power to or remove batter from controller(s).
Rinse and drain all filters, manifolds, control filters, ball valves, back-flush valve actuators, back-flush lines, fertigation valves, booster pump volutes...all above ground components that may freeze. It is imperative that this process is followed.
Brush any rusted spots on the cast iron filter components, screen filters or polyester coated tanks with a wire brush to clean metal and apply a minimum of two coats of paint.
Flush all laterals and carry out required flushing (as listed above).
Procedure
Flush the system (depending on when the system was last flushed, you may choose to just run the system prior to chlorinating).
Inject chlorine until it reaches last emitter (usually 20-30 minutes).
Run the pump/inject for a further 10-15 minutes.
Leave chlorine in the system for 12-24 hours*.
Flush the system*.
* Depending on the amount of organic matter build-up, you may flush the system straight after chlorinating, or chlorinate at the end of an irrigation and flush at the beginning of the next irrigation.
Chlorination Hints
Warning! Chlorine solutions are dangerous to human beings and animals - wear proper protection for your eyes, hands and body. Always read the label before opening the container.
Direct contact between chlorine and fertiliser can be explosive.
Chlorine can cause the corrosion of some pump construction materials if left to sit in them.
Although more expensive, chlorine in liquid form, such as sodium hypochlorite, is much safer to use.
Water travels about 1 metre every 3 seconds in low volume irrigation (mini-sprinkler or drip) systems. To estimate how long it will take chlorine to reach the furthest point in the system divide the distance from the injection point by 20 to give time in minutes .
To check the concentration of chlorine at the end of the lateral, use a swimming pool test kit. You should be able to smell the chlorine at the end.
When using chemical solutions in an irrigation system, a suitable back-flow prevention device is required to prevent reversed supply to the water supply.
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