# Drainwater Recycling > Part of: [Irrigation](/irrigation.md) · Provider: [DutchGreenhouses](/) Drainwater recycling returns disinfected drain to the supply instead of discharging it. The limit is sodium: the crop takes up almost none, so it concentrates with every pass until discharge becomes unavoidable. Disinfection does not address this, which is why a recirculating greenhouse is rarely fully closed. ## Facts | Property | Value | |---|---| | Sodium concentration factor per cycle | 3 × | | Revised sodium tolerance for tomato | 15 mmol/L | | Drain volume needed to refresh a slab | 1–1.5 L/m² | | Discharge as share of water supplied | 0–10 % | | Nitrogen emission limit for tomato | 21 kg N/ha per year | Irrigation # Drainwater recycling. The loop closes: near-zero discharge growing. ## Closing the loop for near-zero discharge growing Substrate cultivation does not absorb all the water and nutrients it receives — a meaningful share, often 20–40% of what is dosed, runs off as drain. Left to waste, that stream is a double loss: water and expensive fertiliser poured away, and, increasingly, a regulatory problem. Drainwater recycling turns that loss into a resource by collecting the drain, cleaning it and blending it back into the irrigation supply, so the greenhouse approaches a closed, near-zero-discharge loop. The recycled stream cannot simply be pumped straight back. Drainwater carries unused nutrients that are valuable to reclaim, but it also carries pathogens — fungi, bacteria and viruses — that could spread through the whole crop if returned untreated. Recycling therefore always runs hand in hand with disinfection: the collected drain is captured in a buffer, disinfected (by UV or heat), and only then re-dosed and mixed with fresh water and fertiliser to the correct EC and pH before going back to the plants. Because the reclaimed water already contains residual nutrients, the dosing system continuously measures and corrects the recipe so nothing is over- or under-applied. The economic and environmental case is compelling. Reusing drain water sharply cuts both water consumption and fertiliser spend, which in water-scarce regions is often the single biggest operational saving a greenhouse can make. Just as importantly, many jurisdictions now cap or prohibit the discharge of nutrient-rich water to the environment; in those places recycling is not an efficiency option but the licence to operate at all. Designing the loop well means sizing the collection, buffering and disinfection capacity to the greenhouse's real drain volumes — an undersized disinfection unit quickly becomes the bottleneck that limits the entire system. During engineering we dimension the recycling loop to the crop, the substrate strategy and the local water regulations, so your facility captures the savings without ever compromising crop health. Get in touch to design a closed-loop irrigation system for your project. 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 **Why does a closed loop still have to discharge?** Because sodium is not a nutrient and the crop takes up almost none of it. Transpiration removes pure water and leaves the sodium behind, concentrating it roughly threefold per cycle. Since EC is held at a setpoint, rising sodium displaces nutrient cations within it, and the risk is cation antagonism rather than osmotic stress. **Is the sodium limit for tomato 8 millimoles per litre?** Not any more, on either count. That figure came from a table in the Besluit glastuinbouw that was repealed on 1 January 2013, so there is no per-crop sodium limit in current Dutch law. Wageningen trials since then put tomato at 15 with peaks of 20, cucumber at 7.5 to 10 and gerbera at 10 to 12. **Does a higher drain percentage flush the slab better?** No. What refreshes a slab is drain volume, roughly 1 to 1.5 litres per square metre, not the percentage. A large share of a very small irrigation shot still amounts to almost nothing, and many small shots hold a steadier water status with less salt accumulation than few large ones. **Is discharging drain water the main route by which nutrients reach surface water?** No. Wageningen ranked the residual water streams and deliberate drain discharge came fourth. Ahead of it sit drainage from soil-grown crops at around 2,500 cubic metres per hectare per year, plain leakage, and crop-changeover cleaning water. Leakage in particular cannot be prevented by water treatment, only by finding and repairing leaks. ## Related [Irrigation](/irrigation.md) · [Post-Treatment](/irrigation/post-treatment.md) · [Pre-treatment](/irrigation/pre-treatment.md) · [Drain Water](/glossary/drain-water.md) · [Reverse Osmosis (RO)](/glossary/reverse-osmosis.md) · [Rainwater Basin](/glossary/rainwater-basin.md) · [EC (Electrical Conductivity)](/glossary/ec-electrical-conductivity.md)