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Water Source Influences Water Treatment Options

A complete water quality test includes BOD, a microbial scan and an herbicide scan Growers should also test for the presence of pythium and fusarium

July 13, 2015  By Dr. Mohyuddin Mirza

A dugout used by a commercial greenhouse. PHOTOS BY MOHYUDDIN

Back when I first started writing about water quality and water treatment options, I made a presentation on greenhousing to participants of “WE ARE THE LAND: Energy and Food Sustainability Conference.”

In the background of my talk there was a poster that read:



That thought about sacredness of water was always in my mind but it was further reinforced in my belief system that good quality water is the foremost important input in greenhouse farming and outdoor production.

Of course we know that without good quality water, there would be no life.

Understanding water quality before even starting a greenhouse business is important.

I have seen so many cases where a greenhouse was built based on water analysis reports from past years, and those reports were done to determine the suitability of the water for drinking purposes.

I have also seen growers who thought that if 300 mg/L of sodium is present in water and is suitable for human health, then it should be good for growing plants as well. The reality is that 300 mg/L of sodium in water renders it marginal and even unsuitable for greenhouse irrigation.  

The following is data from a 2010 Alberta and Saskatchewan greenhouse industry profile. It is evident from the table on page 24 that the major source of water is from municipal supplies, followed by dugouts and ponds. Use of well water is highest in Saskatchewan.

The point is made that water quality will vary based on the source. For example, municipal water is considered of good quality because it is treated with some type of disinfectant and may have gone through a filtration system.

Quality of well sources vary from hard water to soft water. Hard water contains significant amounts of calcium, carbonates and bicarbonates. Soft water on the other hand contains high levels of sodium, which renders it unsuitable for greenhouse irrigation.

Water from dugouts can have issues with organic matter and microbial contamination.

Electrical Conductivity (EC) and pH are the two measures with which growers are most familiar or should know about.

Many growers confuse the pH with the alkalinity of water. However, pH as we know is a measure of total Hydrogen Ions H+ present in water. The more H+ ions present, the more acidic the pH will be.

On the other hand, if OH- ions are present, the water will be more alkaline. However, alkalinity is a reflection of the presence of carbonate and bicarbonate compounds in water. It’s important we understand that pH and alkalinity are two different terms.

So to determine water quality, one should know about pH, EC, carbonates and bicarbonates, sodium, potassium, calcium, magnesium, nitrate and nitrite nitrogen, sulphur, iron, manganese, copper, zinc, boron, chloride and molybdenum. The above elements and minerals are inorganic chemicals.

Next, we need to know the biochemical oxygen demand (BOD). This is the amount of dissolved oxygen needed by aerobic biological organisms in a body of water to break down organic material present in a given water sample at a certain temperature over a specific time period. The term also refers to a chemical procedure for determining this amount.

The point is that water quality is based on both inorganic elements and also organic materials, which include humic acid, dirt particles, microbes of different types, and many others. Some microbes like coliform, including E. coli, can become a food safety issue. This happens when dugout water catchment/runoff area comes from the land where cattle manure has been applied or actively used for grazing.

A complete water quality test includes BOD, a microbial scan and an herbicide scan. Growers should also get a test done for the presence of pythium and fusarium. Once the test is done, the water treatment we select will depend on what is needed to grow the crops. With so many choices of products and machines available, we must know the end result we want.

The first thing to consider is using filters. I like “cloth filters,” even if your water is from municipal sources. With these types of filters you can see what particulate matter is in your water. This is especially meaningful when using dugout / pond water. Many growers use in-line filters and sand filters for this purpose.

In-line filters need to be checked frequently for blockage. If not cleaned frequently, the water pressure at drip lines may be reduced.

1. Reverse Osmosis: If water quality is not suitable for irrigation use, and most of the time the problem is the presence of high levels of sodium and boron, then using reverse osmosis is the best option (besides finding suitable alternative sources and collecting rain water).

The biggest challenge is the cost factor and that depends on the volume of water needed for irrigation. My recommendation is to have a water storage capacity of three to five days. The wastewater generated by the RO systems needs to be disposed of properly.

Once in a while I see the promotion of machines that will get rid of sodium from water. Growers should conduct a due diligence process before investing in such machines.

2.0: Chemicals used in water treatment for greenhouse use: There are many chemicals used for disinfection purposes. These chemicals disinfect the water supply and thus make it more useable from a hygiene viewpoint.

2.1: Chlorine: I call it a general purpose disinfectant and it has been used for decades. It is used as a biocide for surface disinfection.

I knew a grower who used bleach to disinfect his water for a post-harvest washing and it did not show the desired effect. Upon investigation it was found that the water pH was 8.2. Bleach in bottles or in drums is stabilized at a pH of 14, a very alkaline solution. For chlorine to be released and to interact with organic materials, the pH should be brought down to between 6 and 7.

A pH level below 6.0 can release chlorine very fast and can become a health hazard. Shock chlorination of well water is a fairly well-established practice. Generally 100 ppm of chlorine is used for this purpose. The proper masks should be used while dealing with chlorine solution.

I have seen damage to plants if chlorine gas leaks into the greenhouse environment.

Also remember that chlorine is ineffective in removing biofilms from irrigation lines.

2.2 Chlorine dioxide: It is a mixture of chlorine and acid which, when mixed in a water distribution system, forms chlorine gas, which is corrosive and could be dangerous if inhaled. It is effective against micro-organisms in water and biofilms, but growers must pay attention to using it safely with proper precautions. It is a very reactive chlorine.

2.3 Hydrogen peroxide: I have conducted research with this product. Many growers are using it successfully.

The key to its effectiveness is that a detectable residue should be there at the drip line in the case of irrigation systems and in water in floating hydroponics systems.

I recommend to have a residue of 5-10 ppm in floating systems and 10-20 ppm at a drip line where organic media like coir is used. This means that initial water injection may vary from 50 to 100 ppm. Hydrogen peroxide is available as 35 per cent food grade or 50 per cent technical grade.

The release of active oxygen depends on bringing the pH of the water to over 5.0.

2.4 Hydrogen peroxide with peracetic acid or acid alone: There are many products on the market where hydrogen peroxide is mixed with peracetic acid. These products are promoted to control water-borne pathogens and their dormant spores caused by algae, bacteria and biofilms. I found a product called Sanidate 12 that claims it does not off gas, has no disinfection by-products, adds vital oxygen to water, is effective at any pH level, and is safe for use in hydroponic systems.

2.5 Ozone: Ozone has been used in agriculture cultivation for a long time. I know many growers in Alberta who have been using ozone to disinfect water supplies. Ozone is O3 that is created by breaking up regular diatomic molecules (O2) with an electrical charge. Note that O3 is up to 13 times more soluble in water than pure O2. It is very unstable so it quickly converts back into O2.

The most effective way of introducing the highest possible levels of dissolved oxygen (DO) into water, ozonation, is becoming more and more common in the greenhouse.

Ozone is produced onsite from ambient air and is increasingly used for irrigation water disinfection. A key reason for ozone use in horticulture is that once the ozone (O3) has oxidized (destroyed) the pathogen, biofilm or bad bacteria, it converts to DO. The ozonation process allows about 12.5 times more DO to be put into solution than using pure oxygen.

To sum up, there are many water treatment options and strategies available for greenhouse application. I have mentioned only a few. Thoroughly discuss with suppliers the advantages and disadvantages of the system you want to use.

Their judicious use can help to resolve many production problems.

Dr. Mohyuddin Mirza is an industry consultant.

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