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Growing crops in a soilless substrate appears wasteful in terms of water and fertilizer because in order to properly flush the substrate, it is necessary to apply 30%-50% more water than the evapo-transpiration rate (plant requirements).
However, it is possible to recycle drainage water, but in order to recycle this water, it must first be possible to collect and store it.
Recycling technologies offer several advantages: significant water and fertilizer savings (50%-70%), enhanced quality yields, (as the result of increasing water applications), improved availability of nutrients and more efficient flushing, which reduces ground water contamination and ecological damage in agronomic regions.
Key Challenges
Chemical Aspects
The water source contains chloride and other elements, very little of which is absorbed by the plant. Thus in a closed system, chloride will accumulate over a period of time.
When it exceeds a certain concentration, depending on the sensitivity of the crop, it can adversely affect the crop. Different plant types react differently: tomatoes are considered to be relatively resistant, while roses are considered sensitive.
Phyto-Pathological Aspects
Localized infection from fungi, bacteria, nematodes, and viruses can spread in the system and potentially cause swift contamination of the plant population.
Recycling Methods
- Collecting drainage water from a soilless substrate, and using it to irrigate an adjacent crop grown in soil (open system).
- Recycling drainage water in a closed system; with or without disinfecting the drainage water.
- Recycling drainage water diluted with rain water or with desalinated water.
- 2/3 : 1/3 method: the drainage water discharged from 2/3 of the greenhouse can be used to irrigate the remaining third (a common method in roses).
Common Disinfection Methods
1. Biological Filter
A biological filter composed of a corrugated tank filled with sand or volcanic ash (2mm) enables drainage water solution to trickle through the filter at a given rate. A population of beneficial microorganisms which develops in the biological filter destroys pathogens. The main problem is clogging, which will result in changes in the flow rate.
2. Chlorination
By injecting chlorine gas into the drainage water, disinfection effectiveness depends on water turbidity, the concentration of active chlorine, duration of contact and the acidity of the treated water (optimal pH: 5.5-6.5).
A concentration of 3 ppm for 30 minutes has been found to be effective in exterminating a wide range of pathogens, including viruses. Chlorine residue control is required, because a concentration of 1 ppm chlorine remaining in the water after chlorination can be harmful to the plants. It is necessary to wait until the residual chloride decomposes after this treatment.
3. UV Irradiation Disinfection
When drainage water is irradiated with UV radiation, the intensity of the treatment depends on the lamp voltage selected according to the type of pathogen. The effectiveness of the treatment depends on the degree of water turbidity that may necessitate initial filtration of suspended particles.
UV radiation also breaks down various chelates, so after disinfection, it is necessary to add microelements, such as iron, to prevent shortages. It is necessary to constantly clean the UV lamps as dirt tends to reduce disinfection efficiency.
4. Heat Disinfection
Drainage water can be heated to high temperatures via a heat exchanger and heated to 90°C for 30 seconds or to 85°C for three minutes in order to exterminate all pathogens. A temperature of 60°C for a period of two minutes is effective only against bacteria, fungi and nematodes. Before using heated water for irrigation, it must be once again put through a heat exchanger to cool it down. This method is considered highly efficient and poses no danger to plants; however it requires a considerable investment in energy.
 Download PDF version: Drainage Water Recycling
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