You can recycle drain water from coco coir slabs and grow bags, and most commercial coir greenhouses in water-regulated regions already do. The water has to be collected cleanly, filtered, disinfected with UV, heat or a slow sand filter, then blended with fresh water and corrected for EC before it goes back to the plants. Coir drain behaves a little differently from rockwool drain, mainly because it's darker and carries more fine organic matter, and that affects how you size the disinfection step.
The water and fertiliser savings, and the sodium limit that eventually forces you to bleed some water off, are covered in water use in coco coir vs soil. This article is about the hardware and the routine.
The recycling loop, step by step
Every working system follows the same order. Skip a step and the one after it stops working properly.
- Collect. Slabs sit in closed gutters that run to a sump. Drain should reach the tank without picking up soil, leaf litter or floor water.
- Store dirty. A drain tank holds untreated water. Size it for your biggest day of drain plus a margin, because irrigation peaks at midday and the disinfection unit runs at a steady rate.
- Pre-filter. Remove particles before disinfection. Sand, disc or screen filters are normal.
- Disinfect. UV, heat or slow sand filtration, sometimes ozone or a chemical oxidiser.
- Store clean. A separate, covered tank. Treated water must never mix back with untreated drain.
- Analyse and blend. Measure EC and send samples for a full analysis on a schedule. Mix clean drain with fresh water and top up the nutrients the drain is short of.
- Bleed off. When sodium or another non-absorbed ion climbs past what the crop tolerates, discharge part of the drain and replace it with fresh water.
Why coir drain needs a bit more care
Coir drain is perfectly reusable, but three things set it apart.
Colour. New coir leaches brown phenolic compounds, and drain from young slabs can stay tea-coloured for weeks. Coloured water absorbs UV, so less of the lamp's output reaches the pathogens. Growers on coir report having to slow the flow through their UV unit to hold the same dose they'd reach with clearer rockwool drain.
Fines. Small pith particles wash out of the slab with each irrigation, especially from cut drainage slits. They clog fine filters faster and shade pathogens from UV light.
Early drain chemistry. In the first weeks a new slab can release potassium, sodium and chloride into the drain, even after the coir has been washed. Unbuffered coir also takes up calcium and magnesium from the feed at the start. So the first drains don't look like your feed. Many growers send drain from a fresh crop to waste, or at least analyse it, for the first couple of weeks before it joins the loop. The details on sodium sit in sodium and chloride in coco coir.
Comparing the main disinfection methods
| Method | How it kills | What it handles well | Weak points | Running cost |
|---|---|---|---|---|
| UV | Damages pathogen DNA as water passes a lamp | Fungi, bacteria and, at high dose, viruses | Needs clear water. Breaks down iron chelates | Electricity and lamp replacement |
| Heat | Holds water at high temperature for a short time | Everything, including viruses | Energy use. Hot water holds little oxygen | Gas or other fuel, partly recovered by heat exchange |
| Slow sand filtration | A biological layer on the sand traps and outcompetes pathogens | Pythium and Phytophthora | Slow, needs space, viruses and Fusarium only partly removed | Very low |
| Ozone or oxidisers | Chemical oxidation | Broad range | Reacts with chelates and organic matter, so dosing varies with water colour | Equipment plus monitoring |
Small growers with low disease pressure often run slow sand. Large vegetable houses with virus risk tend toward high-dose UV or heat.
UV: dose and water clarity
UV dose is measured in millijoules per square centimetre (mJ/cm²). Two figures come up again and again:
- About 100 mJ/cm² is enough for fungi and bacteria. This is often called selective disinfection.
- About 250 mJ/cm² is the figure used for full disinfection including viruses. Some viruses need less, and you'll see 150 mJ/cm² quoted for pepino mosaic virus.
Laboratory work in distilled water has shown full kill of some bacterial and fungal pathogens at 40 to 60 mJ/cm². Real drain water is murkier, which is why commercial units are set well above that.
The dose a unit actually delivers depends on lamp power, flow rate and how much UV the water lets through. That last number, UV transmittance, is where coir drain bites. To keep it up:
- Pre-filter down to around 30 microns with a sand or depth filter.
- Clean or replace quartz sleeves on schedule. Iron and manganese precipitate on the sleeve and block the light.
- Expect to run slower when drain is darkest, early in a crop, and plan unit capacity on the dark-water rate.
Watch the iron. UV breaks down iron chelates, and freed iron drops out as a precipitate. Long exposure in recirculating systems has also cut boron and manganese in solution by more than a fifth. After UV, test iron and manganese in the clean tank and correct them. EDDHA chelates are the most stable but colour the water deep red, which cuts UV transmittance further. DTPA and EDTA are lighter in colour but break down more easily. Most growers compromise and top up iron after disinfection.
Heat treatment
Heating drain to about 95 °C for around 30 seconds kills all plant pathogens, viruses included. It's the most certain method and the least affected by water colour.
The costs are energy and oxygen. A heat exchanger recovers most of the heat by warming incoming drain with outgoing treated water, but you still burn fuel. And water that has been heated and cooled holds less dissolved oxygen until it has time to re-aerate, so let treated water stand or aerate it in the clean tank before it reaches the roots.
Slow sand filtration
A slow sand filter is a tank of fine sand that drain trickles through slowly. After a few weeks a biologically active layer forms at the top, and that layer does most of the work. It traps zoospores and spores and supports microbes that suppress them.
Typical design figures:
| Parameter | Common range |
|---|---|
| Filtration rate | 0.1 to 0.3 m/h (100 to 300 litres per m² of sand surface per hour) |
| Sand grain size | 0.2 to 0.6 mm |
| Best pathogen removal | At the slow end, around 0.1 m/h with fine or medium sand |
At 0.1 m/h with fine or medium sand, Phytophthora has been filtered out completely, while coarse sand or the faster 0.3 m/h rate let it through. Pythium is also well controlled. Fusarium and tomato mosaic virus are only slowed down, not removed, so slow sand alone isn't enough if you're fighting those.
Coir fines and organic matter feed the biological layer, which is fine, but a lot of suspended solids will clog the top faster. Expect to scrape the surface more often than you would with rockwool drain. A settling tank or coarse screen ahead of the filter helps.
Slow sand filters don't heat the water or strip chelates, and they keep a living microbial population in the treated water. For growers who use biological products in the root zone, that is an advantage. Biocontrol in the slab is covered in beneficial microbes in coco coir.
Blending and correcting the recycled water
Treated drain is not a finished feed. Its nutrient balance has drifted because the plants took some ions and left others.
- Measure EC every batch and set the blend ratio of clean drain to fresh water so the final mix hits your target.
- Send a full analysis regularly, more often in the first weeks of a new crop and after a change of source water. Look at sodium, chloride, sulphate, boron and iron.
- Correct the recipe, not just the EC. Drain tends to run high in sulphate and sodium and lower in what the crop takes up fast. Match the top-up stock to the analysis.
- Keep pH in range after blending. Recycled water usually needs less acid than fresh well water because it's already been acidified once.
The base feed numbers are in nutrient solution for coco coir.
Hygiene that keeps the loop clean
Disinfection fails quietly if the rest of the system reinfects the water.
- Cover the clean tank so light and dust stay out. Algae grow in uncovered tanks, and algae clog drippers.
- Clean drippers and lines between crops, because biofilm survives inside them.
- Test disinfection now and then by sending samples from before and after the unit for a pathogen count.
- Don't let floor or puddle water into the drain sump.
Is recycling worth it on a small site?
On small sites the kit can cost more than it saves. A slow sand filter feeding a small share of drain back into the main feed is often the sensible middle ground.
Coir works well in closed systems because it holds water and air evenly through the slab. Our grow bags and slabs are made from coco peat washed with water to reduce EC. We don't add calcium nitrate or other chemicals, so the coir is not buffered, and you should expect the early drain behaviour described above.
If you're planning a switch to recirculating coir and want to talk through slab and bag options, contact us.
FAQ
Can I reuse drain water from coco coir without disinfecting it?
You can, but every slab in the loop then shares whatever root disease any one slab has. Pythium and Phytophthora spread easily as swimming zoospores in recirculated water. Undisinfected recycling is only reasonable for short crops with low disease pressure, and even then a slow sand filter is cheap insurance.
What UV dose do I need for coco coir drain water?
About 100 mJ/cm² is the usual setting for fungi and bacteria, and about 250 mJ/cm² for full disinfection including viruses. Coir drain is often darker than rockwool drain, so the unit has to run at a lower flow to deliver the same dose. Size the unit on your darkest water, not your clearest.
Why is my coir drain water brown?
New coir releases phenolic compounds and tannins that colour the water. It fades as the slab ages through the crop. The colour doesn't hurt plants, but it absorbs UV light and lowers the effective dose of a UV unit.
Does UV treatment remove nutrients from the drain?
It breaks down iron chelates, and the released iron precipitates. Long exposure can also reduce manganese and boron. Test the treated water and top up micronutrients, iron in particular, before it goes back to the crop.
How fast should a slow sand filter run?
Design rates sit between 0.1 and 0.3 metres per hour, or 100 to 300 litres per square metre of sand per hour. Pathogen removal is best at the slow end with fine or medium sand. Running faster saves space but lets more through.
Should I recycle drain from brand-new coir slabs?
Check it first. New slabs can release potassium, sodium and chloride in the first weeks, and unbuffered coir draws calcium and magnesium from the feed at the start. Many growers send early drain to waste or analyse it before it joins the loop.
How often should I analyse recycled water?
Check EC on every batch and send a full analysis at least every few weeks, more often at the start of a crop or after changing water source. Sodium and chloride tell you when to bleed off. Iron and manganese tell you how much the disinfection step is stripping.