The production in European greenhouse horticulture is not yet as water efficient as it could be. Soil-less growing systems are becoming common in horticultural practice in most of the European countries, although not in each country on a large scale yet. The advantages of soil-less growing systems compared to soil grown crops are as follows: ^[1]

• Growth and yield are independent of the soil type of the cultivated area;

• Better control of growth by use of improved water quality and a better fertilization;

• Increased quality of products;

• Pathogen-free start by use of substrates other than soil and/or easier control of soil-borne pathogens.

The disadvantages of these systems are:

• The required high quality of water;

• High investments and high costs for fertilizers;

• Low quantity of the water.

In most cases open or run-to-waste systems are adopted. In such open systems, superfluous nutrient solution freely leaches to ground and surface water. Because of economical motives and environmental concerns closed soil less systems can be applied. These closed systems are more efficient with the use of water and fertilizers, and cause less damage to the environment. The disadvantage of the closed systems is the risk of a rapid dispersal of soil-borne pathogens by the recirculating nutrient solution. To eliminate these pathogens, several disinfection methods can be used:

Ozone treatment

Ozone treatment can be used to disinfect the drain water ^[2]. Ozone is the second most powerful sterilant in the world and its function is to destroy bacteria, viruses and odors. An ozone supply of 10 g/h/m3 water with an exposure time of 1 h is sufficient to kill all pathogens ^[3].

UV disinfection

Another way to disinfect the drain water is the use of UV-radiation ^[2]. Ultra-violet radiation (or UV) is a proven process for disinfecting water, air or solid surfaces that are microbiologically contaminated. For eliminating bacteria and fungi an energy dose is recommended of 100mJ/cm². For viruses a dose of 250 mJ/cm² is recommended.

Heat treatment

When heat treatment is applied, a solution is heated for about 30 seconds to a temperature of 95˚C. At this temperature all pathogens are killed^[7]. A disadvantage of heat treatment is the consumption of gas. Also warm drain water contains less oxygen.

Slow sand filtration For several years commercial growers have used a slow sand filtration installation to eliminate pathogens. [4] [5] [6] Sand filtration is frequently used and a very robust method to remove suspended solids from water. The filtration medium consists of a multiple layer of sand with a variety in size and specific gravity. Sand filters can be supplied in different sizes and materials both hand operated or fully automatically.

[1] Van Os, E.A. Closed soilless systems for more efficient and environmental friendly production. Acta horticulturae, Wageningen, The Netherlands (1994)

[2] Van Os, E.A., Stanghellini C. Water reuse in greenhouse horticulture, Water recycling and resource recovery ISBN: 1 84339 005 1, IWA publishing (2002)

[3] Runia W.Th., Disinfection of recirculation water from closed production systems. Proceedings of the seminar on closed production systems, (ed. E.A. Van Os), IMAG-DLO report 96-01, 20-24 (1996)

[4] Wohanka, W. Disinfection of recirculation nutrient solutions by slow sand filtration. Acta Horticulturae 382, 246-255 (1995)

[5] Van Os, E.A., Bruins, M.A., Van Buuren, J., Van der Veer, D.J. and Willers, H. Physical and chemical measurements in slow sand filters to disinfect recirculating nutrient solutions. Proceedings 9th of international congress on Soilless culture, Jersey, 313-328 (1997)

[6] Runia, W.Th., Michielsen, J.M.G.P., Van Kuik, A.J. and Van Os, E.A., Elimination of root infecting pathogens in recirculation water by slow sand filtration. Proceedings 9th of international congress on Soilless culture, Jersey 395 – 408 (1997)

[7] Runia, W.Th, Van Os, E.A. and Bollen, G.J. Disinfection of drainwater from soilless cultures by heat treatment. Neth. J. Agri. Sci. 36, 231-238. (1988)

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