The Grower's Toolbox: Recirculation with or without disinfection

April 03, 2008
Written by Albert Grimm
There is a broad spectrum of options available, each with its own advantages and issues


Whether you need to reduce your runoff so that you comply with regulations, or whether you want to save on water and fertilizer, you have a very broad spectrum of options for recycling and disinfecting irrigation water, each with its individual advantages and issues. In this feature I will summarize some of the most important considerations.

If you are just vaguely familiar with the subject, recycling irrigation water may be synonymous to you with concepts such as zero-runoff, or ebb-and-flood irrigation, but theavailable possibilities are far broader in scope than that. We can choose between three basic principal systems for the recycling of irrigation water.

1. Closed loop systems, usually used with subirrigation techniques (trough benches, hanging troughs, concrete floors, etc.), where the water is cycled repeatedly through the same crops. This involves:

• Collecting and storing excess irrigation water as it drains after irrigation.

• Filtering and disinfecting the water.

• Blending with fresh water to make up irrigation water for the next cycle.

2. Systems where we re-use nutrient solution from disease sensitive crops onto disease insensitive crops, in order to avoid the spread of pathogens.

3. Open recirculation, where part of the runoff water and the subfloor drainage water is contained and directed into ponds or other holding facilities. This water is then re-used as one source of primary irrigation water. This involves:

a. Catching all runoff from the greenhouse (e.g., subfloor lined with polymer film below the drainage system).

b. Collecting the water at a central location (pond).

c. If necessary, nutrients are removed from the water.

d. Filtering and disinfecting the water.

e. Water is blended with fresh water sources for irrigation of all greenhouse crops.

growers1
FLOOR FLOOR
Flood floors do not
necessarily involve large
volumes of concrete.
PHOTOS COURTESY ALBERT GRIMM


spotlight2
AUTOMATION
Flood floors also facilitate greater use of large automation equipment.

growers3
INEXPENSIVE
These floors are constructed with relatively inexpensive
materials and are quite
effective.

growers4
BENCHES
Recirculation is even
possible in facilities with hoses and booms over open expanded-metal benches. 


SOME PRACTICAL OPTIONS

There is not one perfect solution to irrigating greenhouse crops in closed systems. All options are developed as a compromise between economics, crop management needs, and production efficiency. All closed irrigation systems have in common that the grower has to gradually adapt his production methods and his crop control management to the needs and possibilities that are inherent to each system. The learning curve is often very steep, and attention to detail becomes the key to success.

Top-Down Irrigation gives growers easier control over crops compared to subirrigation. When you try to recirculate your water, however, there are more issues with nutrient balances. Some examples for top-down irrigation used in recirculation systems:

• Drip irrigation with artificial growing media in bags. Troughs under the media, or plastic lined trenches on the greenhouse floor, drain the leachate into collection pits. It is used primarily in cutflower and vegetable crops.

• Boom irrigation over benches with expanded metal sheets as a growing surface. Sloped floors lined with concrete or plastic film collect the runoff and leachate. It is used in commercial propagation, and in large ornamental facilities.
 
• Hand-watering or sprinkler watering over floors designed to collect run-off water.

Subirrigation requires the grower to develop a totally different mindset compared to traditional methods. Most subirrigation systems are very easy to operate, but growers have to acquire new skills and broad knowledge if they want to achieve the same level of crop control as was possible with top-down irrigation. Most subirrigation systems require at least some top watering, if only to establish the plants in the substrate. Some examples for subirrigation used in closed systems:

• Continuous flow systems for vege-tables and cutflowers (NFT).

• Floating hydroponics for vegetables.

• Flood benches and flood floors. These are used primarily in the production of container grown ornamentals.

• Trough benches for container-grown ornamentals.

SOME SUBIRRIGATION OPTIONS FOR CONTAINER-GROWN PRODUCTS

Flood Floors


• Flood floors do not necessarily require concrete. Solid concrete floors, however, are a prerequisite for the use of automated, heavy transport equipment, and so, the most important motive for the construction of flood floors is mechanization, not water recirculation.

First the Pros:

• Intensive automation of production becomes a possibility. The concrete surface allows heavy machinery to operate in the production area, and allows for sophisticated transport, spacing, and crop management systems.

• Very uniform watering.

• Higher crop densities possible, because space for walkways is not necessary.

Now the Cons:

• Very expensive to install.

• High installation costs dictate year-round use of production area.

• Water management is difficult. Crops are either wet or dry. Flood irrigation is not suitable for crops where precise water management is critical for the quality of the product.

• Flood irrigation demands very uniform crops, in order to take advantage of the automation options.

• Very difficult and costly to retrofit older greenhouses.

Trough Benches

First the Pros:

• Easier to retrofit older greenhouses.

• Cheaper to install than concrete flood floors.

• Can be implemented gradually, because existing irrigation equipment can be adapted to be used with these benches.

• Easier to control crops than any other subirrigation system. From my experience, troughs are the ‘grower’s choice’ of subirrigation systems.

• Less possibility for widespread contamination with virulent pathogens, because fewer plants are exposed to each other’s root zone in each trough.

• Trough surface easier to disinfect and clean than the concrete of the floors.

Now the Cons:

• Does not lend itself to easy auto-mation and internal transport.

• The metal of the troughs conducts and reflects heat, and can lead to overheating of the root zones in container crops. Needs careful management in summer.

No matter what system we use to recirculate irrigation water, we have to be prepared to deal with key issues that could have impact on our crops:

Plant pathogens can be carried with the water from infected plants. If the water is not treated, serious plant diseases spread easily throughout the crops.

Balancing nutrients in the mix of recirculated water and fresh make-up water can be challenging, especially with top-down irrigation systems.

Allelochemicals are natural plant toxins that are produced by many species. In nature, these substances act like selective herbicides, suppressing other species in the fight for survival.

In the monocultures of greenhouse vegetable production, these chemicals are not a general concern. In ornamental production, where return water from different species is mixed and recirculated, we don’t know enough about these allelochemicals to conclude the potential for crop problems.

DO WE NEED TO DISINFECT IRRIGATION WATER?
In an anecdote from my own career, I was faced with severe, repeated and unexplainable crop losses in a particular greenhouse installation. It took me a few crop cycles to understand that the plants were killed by Erwinia, an omnipresent bacterial disease that does not normally affect healthy plants.

The primary irrigation water source was rainwater from a large underground cistern and the bacteria were apparently feeding on the organic matter in the dust that was washed into the cistern with rain from the greenhouse roof. Harmless in low concentrations, the bacteria proved to be a formidable pathogen after it was allowed to breed in our water system for a few years.

We cleaned the cistern very thoroughly, removed about a half-inch of deposits, and disinfected the walls and floors. And we began to circulate the water in the cistern through the sandfilter of the irrigation system, whenever we did not irrigate crops. This removed all the particles and organic dust that may have been introduced with the rainwater. This, along with some water disinfection systems, completely eliminated the problem with Erwinia.

Apart from diseases like Erwinia and common water molds like Pythium or Rhizoctonia, our industry faces much more serious diseases that potentially distribute with recirculated irrigation water. Such catastrophic diseases include Ralstonia solanacearum, a bacterial disease on CFIA’s quarantine list, and certain critical waterborne pathogens for which effective chemical controls are not available, like Fusarium in cyclamen, or bacterial canker in tomatoes.

CRITERIA FOR THE EFFECTIVENESS OF DISINFECTION METHODS:

• Some methods kill all potential pathogens.

• Some systems are designed to kill only a percentage of all pathogens present.

• Some methods kill only certain types of pathogens.

• Some methods have a residual effect that travels with the water into the crop zone.

Disinfection with Chemical Treatment H2O2 (Hydrogen peroxide)

First the Pros:

• Relatively inexpensive water disinfectant.

• No long-term or accumulative residues.

• Ideal environmental fate – breaks down into water and oxygen.

And the Cons:

• Relatively poor biocide.

• Sensitive to physical impurities.

• Heavy metals instantly catalyze H2O2  – cannot come in contact with metal parts, chelated micronutrients, etc.

• Interferes with micronutrients.

• Phytotoxicity issues at higher doses.

Chlorine or Bromine

First the Pros:

• Relatively inexpensive.

• Good biocides – effective decontaminants.

• Residuals are carried into the crop zone.

And the Cons:

• Many severe phytotoxicity issues.

• Interferes with nutrients.

• Concentrations are difficult to monitor and raw material is difficult to handle.

Ozone

First the Pros:

• No long-term or accumulative residues.

• Ideal environmental fate.

And the Cons:

• Does not control all pathogens.

• Very high investment and operating costs.

• Interference with nutrients.

Chlorine Dioxide

Chlorine dioxide (ClO2) is a relatively new treatment option that was adapted from the food industry. ClO2 must not be confused with gaseous chlorine or bleach.

First the Pros:

• Used in controlled concentrations and by itself it is less phytotoxic than traditional chlorine treatment.

• Relatively inexpensive.

• Has good residual effect into the crop zone.

And the Cons:

• Relatively poor biocide – only effective against certain fungal spores and bacteria.

• May produce phytotoxic metabolites under certain circumstances.

• Application, monitoring, and sensor technology not yet adequate for greenhouse use.
 
DISINFECTION WITH PHYSICAL TREATMENT

UV Light

First the Pros:

• Very compact installations – do not require much physical space.

• Systems designed for low flow rates are less costly to install than other physical methods.

Now the Cons:

• Higher flow rates expensive to achieve.

• Physical impurities in the water reduce the effectiveness.

• Interferes with chelated micronutrients.

Heat – Water Pasteurization

First the Pros:

• Most effective and reliable method for disinfecting irrigation water.

• Very high flow rates possible.

And the Cons:

• High cost of installation.

• Significant operating costs – with current energy prices about 50 cents per cubic metre of water.

• Units are relatively large – need installation space.

Slow (Sand) Filtration – Root Zone Filtration (Biofilters, Artificial Wetlands)

First the Pros:

• Very low operating cost.

• Effective also at removing nutrients and contaminants other than pathogens.

And the Cons:

• Requires much space for installation. Filter surfaces of 100 square feet to filter 1 cubic metre of water per hour.

• Cost effective only for very large scale applications.

• The filter substrate needs continuous maintenance.

Albert Grimm is the head grower at Jeffery’s Greenhouses in St. Catharines, Ontario. •  e-mail to: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

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