Clean Water Supplies for HydroponicsThe importance of clean water supplies for NFT, DFT and bag growing systems cannot be over emphasised. Water from ponds, dams and other surface catchments, and from shallow bores are quite likely to be contaminated by Pythium, Phytophthora or other disease organisms from time to time, although not always continuously. Roof water dams may be contaminated by dust borne fungal spores. Public potable water supplies are not always free from plant pathogens. Treatment of water supplies to prevent introduction of disease into hydroponic systems will thus often be essential and always desirable. Treatment by ultra-violet or ozone sterilisation is not expensive to operate, but does require some capital investment. Which ever treatment method is used must be effective, but there are many ineffective installations.
Rain water collectedfrom greenhouse roofs has a high chemical purity. In most parts of New Zealand annual rainfall exceeds annual water use by greenhouse vegetable crops. Dams large enough to hold sufficient winter rainfall for greenhouse use in summer are a good investment, but stored water is easily contaminated by pathogens.
Water treatment optionsUV -ultra violet treatment This is probably the most popular water treatment for hydroponic greenhouse vegetables. This treatment is simple but must be used carefully to be effective. -->">Ultra violet radiation (light with a wavelength of 254 nanometers) damages cellular nucleic acids in all living organisms, and organisms receiving a large enough dose of UV radiation are killed. Water treatment by UV is achieved by shining light from either low or high pressure UV lamps through a thin layer of flowing solution. The solution must be clean and clear or light penetration through the solution is limited. The effectiveness in killing micro-organisms depends on entirely on the dose given, and the dose required to kill varies for each species. In general bacterial are killed readily at relatively low doses, pathogenic fungi require higher doses, and different parts of any fungus may have different lethal doses, so that spores might be killed more easily than pieces of mycellium; the highest doses are required to kill viruses. The UV dose to control human pahtogens is very mmuch lower than the dose required to control plant pathogens. A wide range of equipment for UV water treatment is available ranging from simple systems with no automatic controls for example Ultraviolet Technology of Australasia Pty Ltd through to very sophisticated and expensive systems with many built in automatic safety features (Priva for example). Obviously the UV dose received by water flowing through any particular UV treatment system depends on the water flow rate. Simple UV systems typically specify the flow rate requried for control of human pathogens, but these systems can be used at a lower flow rate to control plant pathogens, and the manufacturers can specify flow rates to give specific UV dose rates. UV water treatment equipment is usually specified in term of light intensity (mW/cm2) and the dose is then the intensity multiplied by the exposure time (expressed either as mW.seconds/cm2 or as mJ/cm2). Recent practical recommendations are that 100 mJ/cm2 should be used to treat water for bacterial and fungal pathogen control or 250 mJ/cm2 for virus control. Practical requirements are that the water is filtered (through an irrigation filter with 200 mesh element) for maximum light transmission prior to UV treatment, and that UV lamps are replaced as scheduled by the manufacturer (typically 7,500 hours or about a year). Sophisticated automatic systems can regulate the water flow according to the measured lamp output and clarity of the water to provide a set UV dose.
UV treatment system
The vertical white cabinet is the UV treatment system for this NFT system. When needed, water flows through the UV treatment and into the NFT sump. No control system is fitted and the lamp is on all the time, whether or not water is flowing. Care must be taken to limit the water flow rate so that the 100 mJ/cm2 dose is given and that the lamp is changed yearly.
Ozone (O3) is a very powerful oxidising agent, and treatment of water or nutrient solutions with ozone can result in the elimination of bacterial and fungal pathogens and viruses. Disease control is complete if the redox potential of the treated solution is increased to 750 mV. The reliability of redox potential as a guide to zone concentration may be questionable, but it is a simple and effective measurement using relatively low cost redox meters. Effective ozone treatment is not easy. Ozone treatment systems available in NZ use a venturi installed in pipe through which there is a steady flow of solution to draw air through an ozone generator. Some of the oxygen in the air flowing through the generator is converted to ozone , and the ozone enriched air is discharged as a stream of bubbles into the solution flowing through the venturi. The ozone has to dissolve into the solution from these bubbles, and the contact time for solution is critical. The discharge needs to pass into a deep solution tank or an absorption tower ( a simple 3 m tall pipe) for maximum efficiency. Much of the oxidising effect of the ozone is spent on organic matter and other materials in the solution, and when solutions contain many reducing agents it is difficult to achieve a high redox potential. The efficiency of ozonation can be increased very considerably by lowering the treated solution to pH 4.0 by adding nitric acid before treatment. Ozone treatment systems must be installed in a way that avoids ozone air pollution as ozone is dangerous to human and plant health. Ozone treatment is often applied to filtered water held in large tank, as high ozone generation rates would be required for useful water flow rates. Ozone can be quite damaging to some plastic components of pipe work. Iron and manganese and some calcium may be precipitated from water containing these elements during ozonation and may form a sludge in water treatment tanks
Disinfection by chemical dosing. Chlorination is a very old method of disinfecting water and has been recommended as a treatment for fresh water in Australia. Hydrogen peroxide has also been suggested for treatment of raw water and nutrient solutions. Hydrogen peroxide is a much weaker oxidising agent than ozone, and relatively large amounts of hydrogen peroxide have to be used (100 ppm for 5 minutes to kill condia of Fusarium oxysporum f.sp lycopersici). Both of these methods involve batch treatment of filtered water held in a tank. A recent addition to the range of water treatment methods is Iodine disinfection using new fully automatic equipment with iodine concentration sensors and computerised concentration control. The system can treat high volume water flows. (more information at Iotec Australia Pty.Ltd)
Revised June 2007 İR.A.J.White June 27, 2007