Water treatment and hygiene applications for better profits in the greenhouse

Hygiene is a profession, just like farming is a profession. But not all farmers and growers know what kind of water treatment and biocide is best suitable for the application they have in mind. With so many choices like Chlorine dioxide, Standard hydrogen peroxide, Hydrogen peroxide with peracitic acid, an ECA unit , UV-C, it is hard to choose from. We asked Mike J. de Jong of De Jong ECOservices in The Netherlands to share his knowledge about Hygiene with the readers of Hortidaily.com. Enjoy!

A few weeks ago whilst visiting the Greentech – which proved to be really boring – I got myself roped in to writing another article about hygiene… so for the past 2 weeks I have been thinking what and how to write this. Do I want to write a reference paper again with lots of big words or do I want to write from my own 20+ years’ experience. I thought about this and decided that this time round it would probably be more useful to write from experience and using normal everyday words!

Hygiene is a profession, just like farming is a profession. “Doing” hygiene generally means that you are doing it wrong and wasting time and money. Getting advice from a technician who generally gives you crop growing advice or pump installation is about as useful as an ashtray on a motorbike. Nothing personal, but they have no in-depth knowledge about hygiene and how cleaning product or biocides work except for general knowledge, what they have been told to say or what has been the general fashion.

Hygiene is a lot more difficult to get right as besides identifying the situation, you also have to identify the right chemical for the job. And each situation generally requires a different solution, so that “just applying acid or chlorine” will not do the job.

At home when you do the dishes the old fashioned way – ie by hand - you start by rinsing the dishes to get rid of most of the rubbish, then you wash them preferably in hot water using a dishwashing soap. The only thing you don’t do is disinfect your dishes. The reason is that they are clean and that by rinsing and soap cleaning your dishes you have removed all food rests and around 98% of the bacterial pollution. 

In a greenhouse situation it’s pretty much the same with regards to surface cleaning. If you rinse, then use a good soap solution, you will remove around 98% of the microbial pollution, leaving around 2%. Now 2% may not seem a lot, but it can still be millions of micro-organisms. Using an appropriate biocides should then be used to eliminate the remaining 2%. But this is much harder than you may think as the remaining 2% is the toughest to get rid of.

Disinfection is actually measured in logarithm; you never get a 100% kill. So you might have a log3 kill which is equivalent to 99.9 or a log4 which is 99.99. The minimum you need to achieve is a log4.

Nowadays growers are offered a range of biocides, some “suitable” for surface disinfection only, others “suitable” water disinfection, a few for both surface and water disinfection (incl. biofilm control). So let’s have a look at the most common one: 

Chlorine, or bleach as it is also know has good biocidal properties, is effective for about 20 minutes but is generally used in the wrong way. First and most important chlorine is prone to resistance, so you cannot use it in a continually. It is also very pH dependant and should ideally be used at pH 6 -7.5. Above or below that the chlorine becomes less effective. Chlorine makes a good surface disinfectant or when used as shock treatment in water (ie – return water before storage tank). Chlorine however should not be used for continual water treatment as it is useless and has absolutely no effect on biofilm – in fact it helps biofilm grow. Unfortunately it is still advised for use at the end of the growing season to clean and disinfect irrigation systems from biofilm and micro-organisms. It doesn’t work as chlorine has no effect on biofilm, does not remove it, does not kill any of the micro-organisms in the biofilm. Basically do not use it to clean and disinfect you water distribution system as it is money thrown out the window. 

I my opinion chlorine dioxide should never be used in greenhouses or in open water distribution system. It is an accident waiting to happen – and it won’t be long before someone gets seriously hurt or an entire crops gets destroyed – I have already come across some very near misses...

Chlorine dioxide is a mixture of chlorine and acid which when mixed in a water distribution system form chlorine gas. It is extremely dangerous and highly corrosive. In fact it should always be dosed with an anti-corrosive, but generally isn’t to shave some of the costs off its already high price. Although chlorine dioxide is effective against micro-organisms and biofilm (the slime layer in a water distribution system), the dosage is generally too low (crops don’t like it so you are very limited in dosage rates) to be effective over time. Besides, there are better and safer products!

Great product, just very limited in its effectiveness as once it starts to react there is no way of stopping the reaction and in about 20 minutes it will be used up. To achieve a reasonable surface disinfection you need a minimal dosage rate of 20%. For water treatment the dosage needs to be really high and even then it will run short and still not provide an adequate clean and disinfection. The one great thing about hydrogen peroxide is that micro-organisms can never become resistant to it.

Although often sold as stabilised hydrogen peroxide it is actually activated hydrogen peroxide and last about as long as standard hydrogen peroxide. Because of the combination with the peracitic acid, it has a much better disinfection than standard hydrogen peroxide and can be dosed at a much lower rate. Very suitable for surface disinfection. I have often come across this product when used as a continual water treatment biocide and it is in my opinion not suitable for this purpose. The longer this product is used the less well it works. But even worse is that its by-products go on to form a biofilm which is incredible difficult to remove, and therefore causes continual problems within the irrigation system.

Silver peroxide as it is know is a combination of hydrogen peroxide with a whole bunch of ingredients to stabilise the peroxide and silver nitrate to activate the peroxide. It’s a fairly slow, long lasting disinfectant but quite effective. Downside is that it contains silver nitrate which is a poison. It cannot be used for continuous water treatment in irrigation systems as the silver gets absorbed by plants/fruits/vegetables and there is a zero tolerance for silver in food. As a surface disinfectant it works quite well if you are prepared to take the tainting of surfaces for granted as they will get discoloured (black) due to the silver. Other downside is that in water with elevated chlorite levels (mineral levels) it rapidly loses its stability as the silver falls out and the peroxide breaks down.

This product is known under various names depending on where you live. It’s a stabilised and activated hydrogen peroxide. It’s actually quite a clever product that fully breaks down into water and oxygen and makes an excellent surface disinfectant that can last up to 48 hours. As a water treatment biocide it can last up to 120 hours and has excellent biofilm properties. For water irrigation systems its used in two ways: choc treatment (2%) and continual treatment - average dosing rate is 30 ml/m3 water, and it will remove/prevent biofilm and keep water bacterial levels very low. Most problems with this product occur when growers start using it mid-season, thinking that their water distribution system is clean. They often find that what they think is clean and what is reality is quite different and end up with blocked drippers due to biofilm being released into the system.

I don’t recommend acids as biocide for surface disinfection with the exception of peracitic acid. With regards to water disinfection I do not find them suitable either. In my opinion, acids are best used to regulate water pH and for removing mineral deposits at the end of a culture. Water should then be dosed to a pH of 1-2 and left in the system for 24 hours. This should be followed by for example a 2% treatment with Oxyl-Pro/Loxyde/Oxibon.

This is a system that basically makes chlorine through electrolysis. In my opinion just a glorified way of producing chlorine. Besides the cost and maintenance of the machine, you also have the continual high cost of the chemicals to keep the system going. It is also extremely corrosive (watch you greenhouse rust away). The effectiveness is still affected by water pH – which in greenhouses is always too low to be optimum and the bacterial tests I have seen on this product were all done at neutral pH…. Simply put, there are simpler, cheaper and more effective ways to treat your water.

Works well in clean water, not recommended for water that is discoloured. Best this can archive is a log3 disinfection if you maintain a clean system and regularly change the lamps. You will get reinfection straight after the UV. Generally a high investment that is not really suitable for greenhouse use. Besides the costs of the lamps you also have the continual cost of electricity.

Roughly this is what the choices are. From this you have to select which biocide is best suitable for the application you have in mind.

Next time how to properly clean your irrigation system and how to treat return water.