Another column on water hygiene by Mike De Jong:

"Chlorine dioxide is not suitable for horticulture and a serious accident is waiting to happen"

A couple of weeks ago, at the request of one of the editors, I wrote an article on disinfection. To my amazement, it was actually published after the chief redactor changed a bit of the introduction to make me look even better than I really am!
Problem is that apparently that article was – so I am told – quite well read and they then asked me to write a follow up. That means that once again I have to sit down and put fingers to keyboards and write a new piece. So do me a favour and don’t read this as otherwise they will ask me to write again!

By Mike J. de Jong, De Jong ECOservices in The Netherlands

One of the things that really get to me is people selling useless equipment to farmers, especially nowadays with the poor prices they are getting. Yes farmers have become very suspicious about anything new, but if someone comes along with a shiny new piece of equipment farmers somehow have to buy it – especially if it is expensive!

To this point, a couple of weeks ago I was in Bretagne, France. It’s their version of the “Westland in Holland”, and is the main area for greenhouse growing. New French regulations are coming in force requiring growers to re-use irrigation water, which means that they have to spend some money to get this done, few new pipes, filter and storage tank(s).

Now it’s a good idea to do shock disinfection on return water before it’s stored for re-use. You do this after the sand filter but before the storage tank. This disinfection doesn’t need to be 100%, its purpose is just to knock down bacterial levels a bit before the water is stored.

This water is then re-used and mixed with new water and put through the irrigation system – ie sand filter, nutrient tanks, (optional) filter (not sand), disinfectant (to prevent biofilm) and then into the greenhouse.

Some smart aleck went into all these greenhouses and convinced the growers to buy for €50,000.00 worth of chlorine dioxide dosing equipment to be used to treat the return water. He went in and probably scared them by using the word “biofilm” and “micro-organism” and then showed pictured of shiny dosing pumps. A lot of them bought and a lot of them are regretting it…

It’s absolutely useless to dose return water with 1.5 ppm of chlorine dioxide. Yes it will kill the bacteria and remove the biofilm in the storage tank. But you need special tanks to store water with chlorine dioxide or they will rust away. You need to seal the tanks so the gas doesn’t escape, you will get significant oxidation of metals (watch the roof beams rust away), etc. As I said before, chlorine dioxide is not suitable for horticulture and is a serious accident waiting to happen.

This all adds to the cost. Believe me, there is no point in having near sterile return water at this stage, and as for biofilm control, who cares – it just cost loads of money. Besides, as soon as the water leaves the storage tank, it goes through a sand filter and gets re-infected, then mixes with new water and gets re-infected, then there is the pipework it goes through where there is biofilm were it gets re-infected, nutrient tank also re-infection.

It is a good idea to do a shock treatment of return water, but just use a simple dosing system and hit the water with standard chlorine at around 4-5 ppm (Do not use hydrogen peroxide or any other biofilm removing biocide as we do not want to disturb the biofilm). Total cost of equipment is around €1200.00 and that includes injection pump and installation. Chlorine is good enough for this, it’s very cheap and after 20 minutes it will be worn out which is perfect. As for the biofilm in the storage tank, leave it for what it is and clean it out once a year (high pressure hose, disinfection with a stabilised peroxide like Loxyde – no rinse, ready to go).

By all means re-use the water, but you do not need to spend a fortune on hygiene at the storage stage. Just remember, it only needs to be properly treated with a biocide that is effective against micro-organisms and biofilm just before it enters into plant contact, and dosed in such a way that there is no residue by the time it goes through the sand filter on its way to the re-use tank.

By Mike J. de Jong, De Jong ECOservices in The Netherlands

De Jong ECOservices
Mahlerstraat 17
2162 AM Lisse 
Tel: +31 (0)252 418125
gsm: +31 (0)65428 2322
Website: (under re-construction)

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