Control of water-borne microbial problems such as plant and human pathogens, algae and biofilms is an important issue in greenhouse/nursery production. In a recent survey of growers, researchers and industry support personnel (aka. sales people), the top 5 attributes of a water treatment system were identified as;
1) The treatment and its by-products had to be crop safe
2) The treatment had to be consistently effective against a wide range of water quality problems
3) The system had to be readily scalable to be useful over a range of sizes of greenhouse/nursery operation
4) It had to be easy to set and then monitor the dosage the treatment system was providing
5) Any waste products from the treatment system had to be soft on the environment
It was interesting to note that the cost to install and operate the treatment system came in well down the list of priorities.
The survey participants were then asked to evaluate the strengths/weakness of range of systems presently used to treat greenhouse/nursery water. Treatment systems evaluated included; a) chlorination (via sodium or calcium hypochlorite, chlorine dioxide or chlorine gas), b) U.V. treatment, c) ozonation, d) filtration, e) reverse osmosis and f) treatment with copper. It was noteworthy that opinions varied greatly as to the relative merits/limitations of the various water treatment options.
Control of Microbes - ultra-filtration (>10 um pore diam) was regarded as the most effective technique for removing plant and human pathogens from the greenhouse/nursery water supply. All chlorination methods were also regarded as providing effective control of target microbes.
Control of Biofilms – all methods of water treatment methods except U.V., reverse osmosis, and filtration were considered to provide adequate control of biofilms.
Interaction with dissolved fertilizers – reverse osmosis and ozonation were not considered to be suitable for treatment of water once fertilizers had been added.
Impact of water quality on efficacy – a high pH or high levels of dissolved solids in the water source would negative impact the performance of all water treatment options – except for filtration.
Crop safety – there was a general consensus that treating the water by filtration, ozonation, U.V., or reverse osmosis represented no risk of producing by-products that could potentially harm the crop. There was however considerable disagreement about the potential for the other treatment systems to produce toxic by-products.
Worker safety –treating water with chlorine gas and sodium hypochlorite raised concerns about worker safety
Overall - Chlorine based water treatment systems controlled the widest array of potential problems in greenhouse/nursery water sources – and they were also regarded as the easiest to operate and lowest cost treatment option. Ultra-filtration, reverse osmosis, ozone and U.V. treatments were recognized as being suited to treat specific problems – but they were also considered to be more complex and/or more expensive than chlorination. Estimated treatment costs ranged from < $ 0.25 to > $1.00 per 1000 gallons of water treated.
Source : Raudales, Irani, Hall and Fisher (2014). HortTechnology 24 355-368.