# Post-Treatment > Part of: [Irrigation](/irrigation.md) · Provider: [DutchGreenhouses](/) Post-treatment disinfects collected drain water before it re-enters the supply. Ultraviolet and heat dominate Dutch practice; heat is the only method effective against every pathogen class regardless of water clarity, while ultraviolet performance depends on transmission through the water rather than on lamp power. ## Facts | Property | Value | |---|---| | Selective ultraviolet dose | 100 mJ/cm² | | Heat disinfection regime | 95 °C | | Pre-filtration required before ultraviolet | 50 µm | | Ultraviolet transmission of drain water | 20–40 % | | Iron chelate destroyed at the complete ultraviolet dose | 20–40 % | Irrigation # Post-treatment. The return flow gets cleaned before it returns. ## Disinfecting the return flow before nutrients re-enter the loop In a recirculating greenhouse, the water that drains from the crop is too valuable to discard — but it is also the most dangerous water in the system. Post-treatment is the step that makes reuse safe: it disinfects the returning drainwater, removing pathogens before the reclaimed nutrients are allowed back into the mixing chain. Skip it, and a single infected root zone can seed a disease that travels through the entire irrigation loop and every plant on it. Drainwater carries fungi, bacteria and viruses alongside the unused fertiliser it collects, so the goal of post-treatment is a clean, pathogen-free return flow without throwing away the nutrients worth reclaiming. Two proven technologies dominate. UV disinfection passes the drain under high- or medium-pressure ultraviolet lamps, where the dose delivered inactivates micro-organisms as the water flows through. Heat disinfection instead raises the water to a set temperature for a defined holding time — typically recovering that energy through a heat exchanger so the treated water is cooled back down and the incoming water pre-warmed, keeping running costs in check. Both approaches are well established; the choice depends on drain volume, energy availability and the pathogen risk profile of the crop. Sizing is where post-treatment succeeds or fails. Disinfection capacity has to be dimensioned on the greenhouse's actual drain volumes and peak return rates: an undersized unit cannot keep pace with irrigation, buffers overflow, and the disinfection stage becomes the bottleneck that throttles the whole recirculation loop. Because post-treatment sits directly upstream of re-dosing, its reliability also underpins the fertiliser savings and discharge compliance that recycling is meant to deliver. During engineering we match the disinfection method and capacity to your drain flows, crop sensitivity and energy strategy, so the return flow is always clean, on time and ready to re-enter the loop. Get in touch to specify the right post-treatment for your irrigation system. From the knowledge base ## Reads you might find interesting ceacultivationbusiness-case ### Grow Like a Pro: Starting Smart, Growing Strong How Grow Like a Pro combines smart farming, CEA technology and local partnerships for eco-friendly, profitable, pesticide-free greenhouse pr Read article education ### The Case for a College Greenhouse Why a college greenhouse belongs in the curriculum: teaching science, craftsmanship, business logic and sustainability, and preparing studen Read article education ### The Case for a High School Greenhouse Why every high school greenhouse matters: how it makes STEM tangible, builds responsibility and teamwork, and helps students discover their Read article education ### Greenhouses in Education: The Case for Schools Greenhouses in education reconnect students to reality — the full case for why every school, high school, college and trade program should h Read article historyconstruction ### The Origins of Greenhouse Horticulture: My Backyard A tour through greenhouse horticulture history: from 19th-century Westland fruit walls to the “City of Glass” and today’s international high Read article company ### DutchGreenhouses Rebranded: A New Corporate Identity Announcing our new corporate identity — a visual style reflecting our dedication to advanced, high-quality greenhouse projects and The Futur Read article ## FAQ **Does 250 millijoules per square centimetre kill everything?** No, despite being described as the complete dose. Tobacco mosaic virus needs 440, Verticillium microsclerotia over 600, Aspergillus conidia 550 and Thielaviopsis chlamydospores over 3,000. Slow sand filtration leaves Fusarium, viruses and nematodes through, and ozone does not kill nematodes at any practical dose. Only heat is effective against every class. **What limits ultraviolet performance in practice?** Transmission through the water, not lamp power. Drain from rose or tomato can transmit as little as 7 percent of germicidal ultraviolet through a 10 millimetre layer, against 94 percent for basin water. Organic matter and iron chelates are the main absorbers, so pre-filtration and blending matter more than adding lamps. **Does ultraviolet treatment affect the nutrient solution?** Yes. At the complete dose a low-pressure unit destroys 20 to 40 percent of the iron chelate, and a medium-pressure unit up to 77 percent. Iron therefore has to be re-dosed after disinfection rather than before it. Ozone and hydrogen peroxide degrade chelates the same way; heat and slow sand filtration do not. **Is disinfecting the water enough to stop disease spreading?** No. Dutch research is explicit that disinfecting the water while the delivery system stays contaminated means reinfection at every watering. Ebb-and-flood is the highest-risk topology, because large volumes pass many plant batches before reaching the disinfector. Cleaning also has to precede disinfection, since organic residue consumes the oxidant first. ## Related [Irrigation](/irrigation.md) · [Drainwater Recycling](/irrigation/drainwater-recycling.md) · [Hydroponics](/irrigation/hydroponics.md) · [UV Disinfection](/glossary/uv-disinfection.md) · [Drain Water](/glossary/drain-water.md) · [Substrate](/glossary/substrate.md)