Water recirculation: Why growers discharge greenhouse drain water - but don’t have to

By: Jerre de Blok

There are several reasons why growers still choose to discharge their drain water and not reuse it. But no matter how legitimate those arguments may seem, Ridder has a solution for these problems. In this blog, we'll tell you why these challenges don't have to be an obstacle to getting started with water recirculation.

 

Water recirculation in your greenhouse

Growers' main argument is to keep discharging their drain water because of phytosanitary problems. When a disease or a pest is found on the crop, like recently with Tomato Brown Rugose Fruit Virus (ToBRFV), many discard their water immediately out of fear of further contamination. Even though viruses can indeed spread through water, they can also easily be prevented without having to discharge the drain water. UV disinfection is considered a very reliable method for removing harmful micro-organisms from the water. This method has been widely used in European horticulture for many years now. In addition, renowned research institutes mapped out exactly which UV dose is required for which micro-organism.

Eliminate pathogens

The Ridder VitaLite uses ultraviolet (UV) light to eliminate pathogens (harmful bacteria, fungi, and viruses) in the water. The UV dose is determined by the intensity of the UV radiation in the water and the residence time of the water in the UV reactor. While the disinfection process is active, the required UV dose is always guaranteed. This is because the water flow through the UV reactor is automatically corrected based on the UV intensity measured by the UV sensor. The relationship between UV intensity and water flow is illustrated with a calibration curve in the VitaLite software. The calibration curve has been validated extensively to ensure that every combination of flow and intensity will result in the expected elimination percentage of micro-organisms. 

First, a collimated beam test with reference bacteria (Bacillus pumilus) determines the effect of the UV dose on bacteria elimination. Then, a test with reference bacteria (B. pumilus) takes place in the VitaLite to determine the effect of the water flow and the UV transmittance (T10) on bacteria elimination and, therefore, the effectiveness of the UV dose. Finally, there are field tests with the VitaLite to verify the effect on native flora (bacteria, fungi, yeasts) in greenhouse drain water. Inside each UV reactor, you will also find a flow plate present. This flow plate affects the water flow and ensures that all micro-organisms receive the same UV dose. This makes the UV more effective and increases the efficiency of the VitaLite: the same UV dose can also be guaranteed at a higher flow. For those who want even more security, there are two taps on the system. This allows you to take samples yourself and send them to a laboratory for confirmation.

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Break down growth inhibitors

Another reason why growers prefer to discharge their drain water over water recirculation is the presence of growth inhibitors. These include organic molecules that are secreted by plant roots and inhibit the growth of other plants. It goes without saying that you do not want these growth inhibitors in your water. But discharging the water, for this reason, is not necessary either. You can break down or remove these substances from the water. Organic matter can be broken down using a process called advanced oxidation. Under the influence of UV light, hydrogen peroxide is converted into highly reactive hydroxyl radicals that can safely break down organic substances in the water. Scientific studies show advanced oxidation as a proven method of purifying water from organic substances. By adding hydrogen peroxide dosing on the VitaLite-, Ridder offers a suitable and recognized solution for this problem. It is the same process with a higher UV dose, that the Ridder CleanLite uses for breaking down pesticide residues.

Removal of sodium from drain water

A different type of growth inhibitor is sodium. Sodium builds up in drain water during recirculation. The accumulation of sodium in drain water forms a problem because it affects the EC value significantly. The higher the EC, the more difficult it is for the plant to take up water. This finally causes the plant to wilt. Sodium also restricts the uptake of calcium, which is necessary for the plant's cell walls. If the plant receives too little calcium due to this, it becomes more vulnerable to physiological abnormalities such as tip burn and abiotic abnormalities of the fruit, such as blossom end rot. For many growers, this is an important reason to discharge drain water. However, with the NoNa+, Ridder has developed an effective solution for the removal of sodium from drain water based on CED technology. Membrane technology and electrodialysis are successfully combined in this stand-alone water treatment unit. NoNa+ is available in a number of configurations and capacities. It is also an energy-saving solution that does not require powerful pumps. The water flows along the membranes, not through them, as with reverse osmosis and nanofiltration.

Thanks to this, you do not need powerful pumps to pressurize the water; you can save a lot of energy by using smaller pumps. The principle of this water recirculation system is to keep the sodium content constantly under control at a safe level, so that drain water does not have to be discharged unnecessarily and can be reused.

Water recirculation in a greenhouse

Would you like to know how you can take full advantage of water recirculation in your greenhouse? In our guide: 'The business opportunities of water recirculation in a greenhouse', we explain the principles of water recirculation, outline the main advantages, and explain why it could make sense for you to choose recirculation.
Download and read the guide here.

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