To purify residual water from crop protection products, various techniques are used, such as ozone (O3), ultraviolet radiation (UV) with hydrogen peroxide (H2O2) and a combination of ozone, hydrogen peroxide and ceramic membrane. UV light and ozone are also disinfection techniques for killing bacteria, fungi and viruses. Disinfection methods such as heating and ultra-filtration do nothing for the purification of crop protection products in the water.
Disinfect and purify
Installations that disinfect the drain water with UV or ozone, do not, without a doubt, meet the requirements for the purification of crop protection products, according to Dick Marck of DLV Glas en Energie. UV light and ozone can achieve 95% purification if the preconditions are adjusted. With UV, a higher dose is required in combination with hydrogen peroxide for the breaking down of crop protection products. With ozone, a longer period of exposure (batch or contact time) in the reactor vessel is required for purification.
Heating
The principle of heating as a disinfection method is based on the application of the correct combination of temperature and period of exposure, for killing the harmful microorganisms. The standard for a 99.9% effective disinfection of (drain) water is either a period of exposure of at least 30 seconds at 95°C, or a period of exposure of 180 seconds at a temperature of 85°C.
Ultrafiltration
Ultrafiltration is a filtering method whereby microorganisms are filtered out of the water by means of membrane technology (straws). This method continuously has the same fineness of filtration, with which fungi, bacteria and viruses are captured but not the nutrients and possibly crop protection products. As a result, drain water has been converted to disinfected nutritional water. Pre-filtration up to 5 microns is required.
Micro filtration, ultra filtration and nano filtration are similar methods; They differ only in the size of the filter openings. The particles that are retained by ultra filtration (up to 0.011 micron) are smaller than by micro filtration (up to 0.1 micron).
UV light
With UV disinfection of the drain water, the UV-C light (short-wave radiation) with the wavelength of 254 nanometer is the most effective. This UV-C radiation damages the cell structures, thereby destroying fungi, bacteria, viruses and nematodes. The dose to be administered, expressed in millijoule per square centimeter, is dependent on the pathogens that are present. For a selective disinfection (fungi, bacteria and nematodes) a radiation dose of 100 mJ/cm2 is needed, for a complete disinfection, this is 250 mJ/ cm2.
T10 transmission value
The effectiveness of UV disinfection depends on several factors: radiation dose, light transmittance, flow and turbulence of the water. The light transmittance of the water is expressed in the T10 transmission value, the percentage of UV-C light remaining after going through a 10 mm water layer. For drain water, it is around 25%. This transmission can be increased by dilution with, for example, rainwater. In case of insufficient light transmittance, the effect of UV disinfection is too low. A good pre-filtration through say a sand or sieve filter is therefore required. Some growers use an oxidation module with hydrogen peroxide to make the water clearer.
Ozone
Ozone can be used as disinfection against fungi, yeasts, algae, viruses and bacteria. Because of a significantly faster action than chemical products, the effectiveness of ozone is high. By oxidation (combustion) the microorganisms are rapidly killed, root exudates and phenols are broken down, leaving behind only O2, CO2 and H2O. The benefits of ozonization are the increase in oxygen content in the water and the removal of organic pollution, causing an increase of the T10 transmission value.