Disinfection Methods . . . an Australian Perspective

Fully-automated Priva Vialux HD/UV disinfection unit, which is suitable for treating both large and small quantities of water.

Fully-automated Priva Vialux HD/UV disinfection unit, which is suitable for treating both large and small quantities of water.

Disinfection systems are essential insurance policies as environmental pressures force growers to recirculate their nutrient solution. PH&G profiles the various disinfection methods, from an Australian perspective.

Compiled by STEVEN CARRUTHERS

Water disinfection first came to prominence in the late 1970s when the soil fumigant methyl bromide was discovered in drinking water in Holland. It was the catalyst that stimulated the horticulture industry to move from soil to soilless growing methods. Initially, the systems developed were open or ‘free drainage’ systems, but with the introduction of closed (recirculating) hydroponic systems, the risk of root pathogens spreading throughout the system was high. To avoid dramatic losses in yield and quality, a range of methods to disinfect the nutrient solution were developed to kill waterborne pathogens in recirculating hydroponics systems, both high-tech and low-tech, and at varying cost. The degree of effectiveness against target pathogens also varied.

With post-harvest sanitation, it is desirable for the rinsing water to contain sufficient active disinfection chemical to kill off pathogens quickly. Because plant produce is not growing, relatively high doses can be used without damaging produce.

Similar strengths are needed to kill off pathogens in supply water and recycled nutrient solutions. However, while active, these oxidising agents also attack any organic matter and hence can damage or even kill growing plant roots. Therefore, chemical disinfection needs to be carefully managed by deactivating solutions before use to avoid harming plants. The only disinfection methods that don’t utilise chemical agents are heat and UV treatments.

Deactivation is not possible in continuous flow systems such as NFT, so if chemicals are used the dosage must be kept very low.

No method of disinfection can cure infected plants, it is used to try to minimise the spread of disease.

A commercial heat treatment installation on a Dutch greenhouse nursery.

A commercial heat treatment installation on a Dutch greenhouse nursery.

Heat treatment
Heat treatment is a widely used disinfection method used in the Netherlands. The technique itself is used for the pasteurisation of milk. The Institute of Agricultural and Environmental Engineering (IMAG-DLO), which belongs to the Agricultural Research Department of The Netherlands, developed the heat treatment method to be technically suitable for recirculating nutrient solution.

In practice, excess nutrient solution returns from the plants and is collected in a catchment tank. From this tank the solution is pumped into a heat exchanger, where it is preheated to a temperature of about 800C.  In a second heat exchanger, the solution is heated to the disinfection temperature using an external heat source. The disinfected solution flows back to the first heat exchanger, to be cooled down and subsequently stored in a so called ‘clean water tank’.

There has been much discussion about the disinfection temperature. In early trials, a 100% killing of pathogens was achieved at 95C, at an exposure time of 10 seconds. For safety, an exposure time of 30 seconds was recommended, at which point nearly everything is killed: bacteria, fungi, viruses and nematodes. Research (Runia, 1998) proved that similar results could be obtained by decreasing the disinfection temperature and increasing the exposure time: 90C for two minutes, or 85C for three minutes.

The big advantage of the latter disinfection temperatures, is that no extra, external heater is needed, which makes the unit cheaper. The amount of gas to heat the water (1 cubic metre of gas per cubic metre of water) is a disadvantage. In the Dutch situation, (natural gas) heat treatment is economic for nurseries larger than 1ha.

In Australia, heat treatment is mainly used at post-harvest, to kill pests such as fruit fly. Heat ‘fumigation’ technology includes vapour heat treatment (VHT), high temperature forced air (HTFA) and microwave technology. CSIRO research has been completed on VHT of tomatoes, rock melon, honeydew melon and zucchini with positive results recorded. Extensive research has been undertaken on capsicum and cucumber, but treatments required to control fruit fly resulted in severe damage.

Microwave irradiation uses high frequency electric fields to heat any material to kill pathogens and pests. It can be extended to many crops, treatment times are short, and it uses less energy than VHT and HTFA.1 To date, there is no research available on microwave technology for nutrient solution or supply water disinfection. However, there is a general interest within the Australian cut flower export industry for de-infestation by microwave irradiation to kill insects, which could have benefits for greenhouse vegetable production. Tomato has been targeted for future work.

Ozone treatment
Ozone or O3 is one of the most effective bactericides known to science, but for many people it is still a mystery. Put simply, ozone is a gaseous oxygen molecule with boosted oxidising strength. It can be naturally observed as the fresh smell during and after a thunderstorm.

Table 1. Oxidation Chart

Table 1. Oxidation Chart

On the oxidation chart ozone lists above chlorine, iodine and bromine. However, unlike these chemical options, ozone dissolved in water will simply break down into oxygen within about 15 to 20 minutes, leaving no chemical residues. This also makes ozone the ideal sanitising agent for washing produce to greatly reduce bacteria, fungi, mould and viruses.

Ozonation extends the shelf life of produce. Produce rinsing systems using chilled water and ozone treatment have been proven to exceed the requirements of SQF2000, which assures buyers and customers that food has been produced, processed and handled according to the highest standards.

According to Gary Davies, the principal of Adelaide-based Black Max Ozone Generators, the use of ozone for hydroponic applications is expanding as farmers seek to increase crop yields, reduce operating costs and reduce environmental degradation by minimising the use of water and chemicals on the farm.

There are two types of ozone generators—UV (ultra violet) lamp units and CD (corona discharge) units.

Produce rinsing system using chilled water and ozone treatment generated by the Black Max ozone generator.

Produce rinsing system using chilled water and ozone treatment generated by the Black Max ozone generator.

UV ozone units
With UV units, the lamp emits UV light at 185 nanometers (nm). Air (usually ambient) is passed over the lamp, which splits oxygen (O2) molecules in the gas. The resulting oxygen atoms (O-), seeking stability, attach to other oxygen molecules (O2), forming ozone (O3). The ozone is injected into the water, where it inactivates contaminants by rupturing the organisms’ cell wall.

Disadvantages of UV systems are:

•   Maximum ozone production rate is 2 grams/hr per UV bulb, depending on size

•   Highest concentration of ozone that can be produced by 185-nm UV lamp is 0.2% by weight, approximately 10% of the average concentration available by corona discharge

•   Considerable more electrical energy is required to produce a given quantity of ozone by UV radiation than by corona discharge

•   Lower gas phase concentrations of ozone generated by UV radiation translate into the handling of much higher gas volumes than with CD-generated ozone

•   UV lamps solarise over time, requiring periodic replacement.

The advantages of UV systems are:•   Less cleaning required

•   It’s easier to clean

•   There are no nitric oxide by-products

•   UV ozone systems are not affected by humidity.

Two Black Max installation diagram with megazone injector manifold.

Two Black Max installation diagram with megazone injector manifold.

A popular UV ozone system used by small and medium-size hydroponic farms in Australia, is the Black Max Ozone Generator (www.blackmaxozone.com.au). It is reliable, requires virtually no maintenance, comes with a long-life lamp, and it has a fairly constant output even in higher humidity locations. The unit introduces ozone into a water tank via an injector, or injector manifold, creating a vacuum (Venturi effect). In a typical recirculating system, the water is pumped out of the holding tank, through a filter, through the injector and then back into the tank.

Corona discharge ozone units
The technologies involved in corona discharge ozone generation are varied, but all operate fundamentally by passing dried, oxygen-containing gas through an electrical field. The electrical current causes the oxygen molecules to split, as described for UV ozone systems. Past this common feature the variations are many, but the generally accepted technologies can be divided into three types—low frequency (50 to 100 Hz), medium frequency (100 to 1,000 Hz), and high frequency (1,000 + Hz). Since 85% to 95% of the electrical energy supplied to a corona discharge ozone generator produces heat, some method for heat removal is required. Also, proper cooling significantly affects the energy efficiency of the ozone generator, so most corona discharge systems utilise one or more cooling methods using air or water.

Critical to CD ozone systems is proper air preparation. The gas feeding the ozone generator must be very dry, because the presence of moisture affects ozone production and leads to the formation of nitric acid. Nitric acid is very corrosive to critical internal parts of a CD ozone generator, which can cause premature failure and will significantly increase the frequency of maintenance.

The disadvantages of CD systems are:

•   Periodic cleaning unless pure oxygen or an air dryer is used

•   Nitric oxides can form unless pure oxygen or an air dryer is used

•   Humidity affects ambient air-fed CD systems.

The advantages of CD systems are:

•   Doubling the ozone output per given volume vs dry air

•   Small construction allowing generator to be installed in virtually any area

•   A more pure form of ozone without creating other harmful or irritating gases if using dry air or oxygen as a feed gas

•   Corona cell life can exceed 10 years

•   Can create high quantities of ozone

•   Can be more cost-effective than UV-ozone generation.

The T4600 is an air cooled, 30gm/hr, six-cell corona discharge ozone generator for larger ozone requirements.

The T4600 is an air cooled, 30gm/hr, six-cell corona discharge
ozone generator for larger ozone requirements.

A literature review shows neither ozone generation system has a clear advantage. The choice depends on the amount of ozone required.
Corona discharge systems available on the Australian market include Oxyzone’s Tempest 4000 Series (www.oxyzone.com.au), which is just as effective processing water in storage systems as it is when treating the water directly in-line. At the other end of the market is the Lenntech BV range of ozone systems (www.lenntech.com), which uses patented Advanced Technology dielectric segments together with a state-of-the-art IGBT power supply and all necessary components for the safe and continuous operation at full-load in an industrial environment (see News & Products).

A cautionary note. From an OH&S perspective, excessive levels of ozone in the air is poisonous. People who inhale ozone during repeated exposure may permanently damage their lungs or suffer from respiratory infections. Breathing in ozone-charged air may also cause chest pain, coughing, throat irritation or congestion. Spaces where ozone gas can acccumulate should be well ventilated, such as post-harvest rinsing sheds, and when cleaning water storage tanks.

* * * * *

According to PH&G columnist Rick Donnan, there is a compromise involved in using ozone to disinfect nutrient solutions.2

“It works most effectively at higher pH, say 7.0, but its impact on micronutrients (trace elements) is minimised at much lower pH, say 4.0. The compromise is usually to operate at about 6.5 and accept and compensate for the loss of micronutrients. The major impact is on iron (Fe) and possibly on manganese (Mn),” says Rick.

“[The same as with all methods of chemical disinfection] it is the oxidising power of ozone, which kills the pathogens and attacks any other organic or readily oxidisable matter present [including healthy plant roots]. The combination of the oxidising power, plus the relatively high pH, results in the breakdown of the iron chelate, leading to the precipitation of the iron, which hence becomes unavailable. The degree of breakdown depends upon the type of chelate and the concentration and exposure time of the ozone, but it is usually quite high. FeEDTA is the least stable iron chelate.

“It is the breakdown products of the iron chelate, which in turn can react with, and precipitate, the manganese ions. This would be the case when using manganese sulphate, however, it is uncertain whether it also applies with chelated manganese,” adds Rick.

“In practice, if you change to recirculation, growers need to do regular full chemical analysis of the feed and run-off and this will indicate what is needed in the feed to compensate for any loss in ozone treatment,” he advises.

“Another aspect of this is that once recirculating, you will no longer be able to continue using a fixed composition micronutrient mix. Growers will need to add the different micronutrients individually.”

Fully-automated Priva Vialux HD/UV disinfection unit.

Fully-automated Priva Vialux HD/UV disinfection unit.

Ultraviolet Radiation (UV)
UV water treatments have been popular with growers for several decades. In the 1980s, organic material in the water caused unreliable results and the required investment was high. Now, however, the system is comparable with heat and ozone treatment.

UV is an electromagnetic radiation, and the wavelength between 200 and     300 nm (UV-C), with an optimum at 254 nm, has a strong killing effect on micro-organisms. These micro-organisms die because the system alters their DNA, destroying them in the process.

There are three types of lamps, with high, medium and low pressure gas inside the bulbs. All appear to have the same performance in eliminating pathogens, but in general, the high-pressure lamp is less energy efficient.

The recommended dose of UV varies from 80 mJ/cm2 for eliminating bacteria and fungi, up to 250 mJ/cm2 for eliminating viruses. These values are only valid when the transmission of light through the water is adequate, which can be achieved by using a sand filter; otherwise, too many debris particles protect the pathogens by shading, enabling some to survive the treatment. IA cleaning cycle is essential when transmission drops too low. It is also possible to increase the transmission properties of the drain water by mixing it with supply water. Less filtering is needed, but the disinfection capacity increases and, consequently, the price can be high.

As for ozone, UV disinfection of the nutrient solution knocks out iron, which needs to be added post-treatment.

At the upper end of the market is the fully-automated Priva Vialux HD/UV disinfection unit, which is suitable for both large and small quantities of water. Different doses can be selected, depending on the crop and the types of pathogen being targeted. The temperature of the water remains virtually unchanged during the process. You can add the HD/UV oxidation module to the Priva Vialux HD/UV disinfection unit, which combines high pressure UV-disinfection and hydrogen peroxide during the disinfection process. The oxidising action of hydrogen peroxide eliminates growth inhibitors and crop protection agents in the drain water, basin water or surface water.In Australia, the Priva Vialux HD/UV disinfection unit and oxidation module is available from Victorian-based Powerplants (www.powerplants.com.au).

Hanovia PureLine UV System

Hanovia PureLine UV System

Also popular is the Dutch-manufactured Lenntech medium pressure [lamp] UV system for hydroponic water treatment (www.lenntech.com). Features include UV monitor, automatic wiper, transmission sensor, and power variation (see News & Products).

At the lower end of the market is the Berson and Hanovia PureLine UV System (www.fluidquip.com.au) (see News & Products).

Slow sand filtration / Slow biological filtration
Slow sand filtration, also known as slow biological filtration, involves allowing irrigation water to slowly pass through a bed of sand or other porous material such as pumice, rockwool, etc., for treatment. It uses both physical and biological pathogen control mechanisms. As water passes through the fine-grained medium, pathogen propagules are ‘strained’ out as they encounter small pore spaces (Fisher, 2011). However, in addition to this physical filtration mechanism, pathogens are also controlled by a biofilm  of beneficial micro-organisms (known as schmutzdeke) that, over time, forms on the surfaces of sand grains and removes pathogens through antagonistic interactions and competition. That is, these micro-organisms either directly attack potential pathogens or compete with them for resources, ultimately helping to decrease the pathogen population. Beneficial  bacteria formulations, such as Companion® Microbial Inoculant (see News & Products) and products containing the beneficial fungi Trichoderma, are sometimes added to the media mix.

It is generally agreed by researchers that slow sand filtration provides an effective and relatively inexpensive means of controlling pathogens in recirculating irrigation water (Stewart Wade, 2011). 3 However, slow sand filtration does not eliminate viruses.

Sand grain texture will affect porosity and ultimately, biofilm creation. It is recommended sand grains be round to avoid packing and about 0.3 mm in diameter to maximise surface area (Fisher, 2011). Media depth should be a minimum of 80 cm (Stewart -Wade, 2011).

Figure 1. Key components of a slow sand filter (University of Guelph).

Figure 1. Key components of a slow sand filter (University of Guelph).

The supporting gravel layer allows water to drain freely from the sand bed while preventing sand from escaping to the outlet tank. The layer may be around 0.15 m and should be composed of gravel, or some other material that will allow free drainage. Gravel may range from fine (2-5 mm) to coarse (7-15 mm) in separate layers (van Os, 1999).

IMAG-DLO experiments proved that a flow rate of 100 L/m2/hr increases the performance of the filter, as does the selection of finer sand (0.15-0.35 mm; < 0.4 mm).4 Unlike ozone and UV disinfection, slow sand filtration doesn’t take out viruses.

Key advantages of sand filtration over other disinfection methods include that it does not affect the nutrient profile, it is not hindered by low light transmission and it does not require storage and use of chemicals. Filtration is also a required precursor to all other disinfection options.

Slow sand filters are simple to construct. Commercial slow sand filters suitable for hydroponic applications are available from US-based Blue Future Filters (www.bluefuturefilters.com).

HyperLogic RO System.

HyperLogic RO System.

Membrane filtration
Membrane filtration can be divided into reverse osmosis (RO), hyper-, nano-, ultra- and micro-filtration depending on the size of the membrane. Reverse osmosis is able to remove all ions from a solution. It is mostly used for filtering sources of supply water, which contain high levels of potentially toxic ions such as sodium.

The operating cost of RO is dependent on the membrane, which are manufactured with different pore sizes and support structures. They are expensive  and prone to choking over time. Correct pre-treatment of supply water is essential to increase the longevity of membranes.

The cost-effectiveness of micro-filtration was tested in Australia in 2010.5  Fungi and bacteria are the primary pathogens of concern for most growers and can be retained from nutrient solution using microfiltration. Currently, the removal of viruses from nutrient water is not a critical requirement for Australian growers. If this changes, viruses can be removed by adding ultra-filtration (or another appropriate disinfection option) to the water treatment system.

The potential of microfiltration as a disinfection option was assessed by NSW DPI using a complete package system, comprising a Tekleen automatic back-washable 200 micron filter followed by a two-module Siemens Dizzer Series 420, multibore UF system, which was purchased from Siemens Water Technologies.

The specified requirements were that the unit needed to be able to treat at least 20,000L/day and to screen to a pore size of 0.2 microns. (The Siemens package processes up to 35,000L/day. Systems with larger treatment capacities are available.)

The assessment was based on three criteria—it must work, it must be cost-effective and it must be practical. The first criterion was that microfiltration must reliably remove the target pathogens from the water being treated. A key fungal pathogen (Fusarium oxysporum) and a key bacterial pathogen (Clavibacter michiganensis—bacterial canker) were used to test the technology with a series of five preliminary trials. In all trials, the pathogens were isolated from the pre-filtered, inoculated water as expected.

Microfiltration is comparable in costs to chemical treatment options but without the potential hazards and without impacting on the nutrient solution. However, microfiltration appears uneconomical for smaller enterprises.

Reverse osmosis units suitable for hydroponic applications are available from Hydrologic Purification Systems (www.hydrologicsystems.com), and microfiltration equipment from Siemens Water Technologies (www.siemens.com.au/water) and Veolia Water Solutions & Technologies Australia (www.veoliawaterst.com.au), which was awarded the contract for the design, supply, construction and commissioning of the Water Treatment Plant for the d’Vine Ripe Two Wells (SA) hydroponic tomato project.

Hydrogen peroxide
Hydrogen peroxide treatment is a much cheaper alternative to ozone, UV and membrane filtration disinfection, however, it is a weak oxidator. While the performance is improved by adding a weak acid, it is still not good enough to eliminate all pathogens. High dosages (400 ppm) are needed to eliminate viruses, but it was found that a small percentage of nematodes survived treatment (0.3%), and these were capable of infecting plants.

There are several hydrogen peroxide products on the market including Intra Hydrocare (www.intracare.nl), which has been registered for the Dutch horticulture industry with approvals in four biocide classes—PT02, PT03, PT04 and PT05. During tests as a disinfectant by the Dutch CTGB (Board for the Authorisation of Plant Protection Products and Biocides), the product was found to be effective for specific pathogens including Pythium and Fusarium. The manufacturer reports that a 1% Hydrocare solution is fully capable of achieving the required lethality coefficient.
Intra Hydrocare is biodegradable. The key ingredients are hydrogen peroxide and a silver salt, with additional stabilisation.

Chlorination
Chlorination is the most common form of disinfection used by hydroponic growers. It is inexpensive and readily available. In Australia, chlorine is the only biocide that can be legally added to nutrient solution.

Calcium hypochlorite (Ca(OCl)2), commonly known as ‘pool chlorine’, is the most common disinfectant used by growers. Without going into the chemistry, chlorine’s ability to kill micro-organisms results from its strong oxidising power and the disruption of the functioning of the micro-organisms. In common with other disinfection chemicals, if strong enough, chlorine will kill all pathogens with which it comes into direct contact. This is good for disinfecting water and recovered nutrient solution, however, if used at this strength it will also attack and kill plant roots. But not only that, it is not systemic so it won’t kill pathogens it cannot contact, including those which have invaded plant roots causing disease.

To disinfect a potentially pathogen contaminated water supply, columnist Rick Donnan says it is generally accepted that 10ppm of active chlorine should be sufficient to kill most pathogens in otherwise clean water. 6

“At this concentration, only a short time of about 15 minutes is required to be effective against fungal diseases such as Pythium and Fusarium. Lower concentrations are usually effective, but require a longer time,” says Rick.

“Probably a concentration of about 5ppm held for hours would be effective for clean water,” he advises.

“If growers are taking untreated water direct, such as dam or irrigation channel water, then a higher level is advisable, such as 15ppm. When the load could be increased, such as following rain, go higher again to about 20ppm.

“By far the most effective method of dosing is to inject concentrated calcium hypochlorite solution continuously at your planned rate into the raw water as you fill the holding tank,” he adds.

“The least effective is to put the total calculated amount of concentrate as a single batch into the full tank and recirculate to try to distribute it. Slightly more effective is to add the concentrate batch early in the filling process.

The problem with both these approaches is that, firstly, mixing will be erratic and, secondly, the active chlorine will be deactivating over the time taken to fill the tank. Consequently, you will need to dose higher than with direct injection, and still some of the water will be incompletely treated.”
Because chlorine can damage plant roots, lower rates of chlorine are added to recirculating nutrient solutions than to stored water. Active chlorine is deactivated by time, sunlight exposure and aeration, finishing up as chloride ion.

It’s obvious chlorination is here to stay and is unlikely to be replaced by any superior disinfectant in the foreseeable future. Calcium hypochlorite is a cheap and an effective chemical for sterilising hydroponic channels, pipes, pumps, tanks and pots, in the absence of plants. Similar to all disinfection chemicals, chlorine should be used as a preventive measure rather than a cure for diseases in hydroponic systems.

There are various commercial kits available to measure chlorine in pools, and these kits are suitable for hydroponics. Similar but more accurate colourmetric kits are also available from Merck Pty Ltd (www.merck.com.au). At the next level are microsensor electrodes that can detect concentrations of chlorine as low as 3 ppb (parts per billion), available from Hanna Instruments Australia (www.hannainst.com.au),  Milwaukee Instruments (www.milwaukeeinstruments.com), and US-based Agriculture Solutions LLC, among others. Chlorine injectors and monitors are available from Dosing Systems Australia  (dosingsystems.com.au) and TWC Distributers, Inc. (www.twcdistributors.com).

The AIS Iodine Dosing System from Australian Iodine Solutions.

The AIS Iodine Dosing System from Australian Iodine Solutions.

Developing technologies

Iodine
The Australian-designed ISAN iodine disinfection system, developed by Ioteq Systems made its appearance in 2004. In 2007, the system was redeveloped by Australian Iodine Solutions (www.australianiodinesolutions.com) and renamed the AIS Iodine Dosing System. The newer system uses much less iodine to treat the same job, and claimed to be far more efficient than its predecessor. The AIS Iodine Dosing System can be programmed for tank or direct injection dosing.

Iodine is a powerful oxidising agent with the ability to effectively kill micro-organisms. It is used across a wide variety of industries for a number of different applications, including post-harvest fruit and vegetable sanitation, food processing and waste water recycling.

For hydroponic applications, the system uses a patented form of pure iodine, which is supplied in a sealed canister, thus eliminating handling and mixing of chemicals. The system constantly monitors the active iodine biocide available and quickly self adjusts dosage levels to meet any increase in the organic load. Anecdotal evidence suggests the system process does not react with nutrients, or require pH monitoring. According to the manufacturer, the system uses iodine at a much slower rate than chlorine or other options. It is safe for the environment and easy to handle.

The forerunner to the AIS dosing system was trialled in a commercial NFT lettuce system, and as a sanitation agent for washing fresh-cut salads. The following points were noted:

•   Initial investment was expensive, but it lasted a long time. The grower’s experience suggested between $10 – $15 per application in a 10,000 litre tank.

•   To prevent damage to the plants, the grower only used Iodine at low rates for 30 minutes in the cool of the evening. It then needed to be totally purged from the system followed by a fresh water rinse prior to refilling the main tank with nutrient.

•   The by-products of Iodine when left in the tank appeared to be harmful to plants during the heat of the day, even at very low rates. However, if used correctly, the grower never saw any adverse effects and saved tens of thousands of dollars over many years in potential lost plants.

•   The grower used the waste Iodine water on gardens on and off for some time. It proved detrimental to some high water use plants like bamboo.
For disinfecting nutrient solution, iodine is like chlorine and other oxidising agents in that organic matter will chew it up very quickly. This means the dirtier the water, the more iodine is needed.

“The grower reported that when the Iodine was gone, there was a residual by-product that was harmful to plants; hence, why he flushed out the root systems after use. However, he found Iodine very effective for fresh-cut post-harvest washing, at approximately 7-8ppm.

“It cost no more than a few dollars per day,” he said.

“Yes, we did noticeably increase the shelf-life of our salads with the use of Iodine in the wash water,” he added.

However, there were some trade-offs using iodine.

“When used at too high levels, it did become irritating to the eyes when working in the salad processing room. We had a large well-ventilated salad room.

Australian Iodine Solutions say when iodine is dosed at the recommended levels, there should not be a problem with eye irritation, unless the end user is using a fine mist spray that allows it to be air-borne, or overdosing the treated water.

Aquaox system

Aquaox system

Electrolysed water
Electrochemical disinfection and disease control with electrolysed water is a fairly recent development, where an electrical current running through salty water creates a powerful mixture of chemicals that kills microbes such as bacteria, fungi and viruses. The mixture is called ‘electrolysed water’.

The official term for this process is Anodic Oxidation (AO), and the machine that makes electrolysed water is an AO-unit. Other names for electrolysed water include EW, hydrolysed water, electrically activated water, electrochemically produced water, activated water, active water (and more). Suppliers of AO-units have chosen brand names such as Envirolyte (www.envirolyte.co.nz), ECA-water (www.radicalwaters.com) and Aqua-ox (www.aquaox.net).

Electrochemical disinfection is a promising technology for disinfection and disease control. It is already used successfully in many industries. In medical applications, electrolysed water is used to assist wound healing by killing bacteria and damaged tissue.

In horticulture, the main application is as a biocide, meaning ‘life-killing’ agent. It is used to break down the slimy build-up in hydroponic lines that often contains bacteria, fungal spores and other microbes. It is also used successfully for disinfection of seeds, flowers, fruits, vegetables, equipment and packing materials. In the near future it may be used for disease control in greenhouses.

Electrolysed water is produced in an AO-unit also known as Electrochemical Disinfection Device. This is basically a tank with a salt solution, with two electrodes placed in the tank on opposite sides. One electrode (called the anode) is positively charged, while the other (the cathode) is negatively charged. A voltage put on these electrodes sends a current through the water. This current splits molecules into smaller particles and creates new chemicals. Using table salt, (sodium chloride, NaCl), the process releases chlorine, which then forms new chlorine-containing chemicals, in particular chlorine gas, hypochlorous acid and hypochlorite. Simple table salt works well and has advantages, but it adds a small amount of sodium to the nutrient solution, which is sometimes undesirable in hydroponics. Other salts can be used such as potassium chloride (KCl) and magnesium chloride (MgCl2). In a test, all three salts produced electrolysed water of similar strength and with similar effects against a fungal disease. Magnesium salt gave slightly more damage to plants in one test.

Greenhouse Technical Consultant, Elly Nederhoff, reports some of the particles produced by Anodic Oxidation are electrically charged and are very aggressive. 7

“Some other newly formed compounds are ‘oxidants’ that can ‘burn’ germs. (Also some innocent gases such as oxygen and hydrogen are formed and they simply escape),” reports Elly.

“The remaining mixture contains water, charged particles and active chemicals, especially free chlorine. This mixture is the actual ‘electrolysed water’ that destroys organic matter. It also destroys living creatures including bacteria, fungi, viruses, viroids, algae, protozoa and nematodes.”
Electrolysed disinfection of the nutrient solution is simiar to  chlorination, and particularly good for post-harvest rinsing.

Conclusions
Disinfection systems are an essential component of closed hydroponic systems, acting as a form of insurance for growers to prevent disease outbreaks occurring.

All disinfection methods—chemical and non-chemical—work very similar; that is, they kill exposed pathogens (and organic plant matter), with the choice of disinfection method coming down to cost and ease of use. However, no disinfection method will kill all pathogens. Hidden away in tiny pits and crevices in irrigation lines and the growing system, some pathogens escape exposure to treatments. However, disinfection treatments generally keep pathogens in check.

Chlorination, ozone and UV are the most popular disinfection methods used by small, medium and large hydroponic installations in Australia. Also popular is slow sand filtration, which partly eliminates pathogens, and keeps some beneficial microflora alive; however, to be effective, it needs to be carefully managed, including the flow rate.

It’s imperative to note that all oxidising agents attack healthy plant roots as well as pathogens. If you overdose on any, it will kill plants.

Nor will any of the disinfection methods described here cure a disease. The purpose of disinfection is to prevent disease outbreaks, and to minimise plant damage by reducing pathogen spread from a disease outbreak.

Most disinfection methods are based on eliminating pathogens completely and sterilising the nutrient solution. However, there is another theory that perhaps it is not right to kill all life in the solution, since certain microflora also play a role in the suppression of diseases. For example, Pythium can spread rapidly in a sterilised environment, while its growth in a non-sterilised environment is much slower. Similar phenomena can be seen with Phytophthora. The conclusion is that certain micro-organisms are able to suppress diseases. If they are killed, the nutrient solution loses its ‘suppressiveness’ and a fast outbreak of a disease may be a consequence.

Slow sand filtration represents one method where the existing microflora are not killed, but only part of the micro-organisms are eliminated. Therefore, this method could play an important role in the suppression of diseases by micro-organisms.

(Special thanks to Rick Donnan for his insights and advice on the various disinfection methods.)  

References
1 Leach, Peter—Heat Treatments and Methyl Bromide Fumigation, Horticulture Australia, Dept of, October 2011

2 Donnan, Rick—Reader Inquiries, Practical Hydroponics & Greenhouses, May/June 2003, Issue 70

3 Dr Zheng, Youbin and Dunets, Siobhan – University of Guelph, www.ces.uoguelph.ca/water/PATHOGEN/SlowSand.pdf

4 Os, Erik van, Nutrient Cleaning—the Dutch Approach, Practical Hydroponics & Greenhouses, July/Aug 1999, Issue 47

5 Badgery-Parker, Jeremy—Filtering the Options, Practical Hydroponics & Greenhouses, Nov/Dec 2010, Issue 115

6 Donnan, Rick—Reader Inquiries, Practical Hydroponics & Greenhouses,  Sept/Oct 2004, Issue 78

7 Nederhoff, Elly—Electrolised Water, Electrochemical disinfection and disease control with electrolysed water, Practical Hydroponics & Greenhouses,  March/April 2012, Issue 123.  Ω

PH&G May 2014 / Issue 143


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