Before reusing your drain water, read this…

Why Reusing Water is Smart. And When it Isn’t.

Drain water is what’s left after irrigation or fertigation. It holds nutrients like nitrates and potassium, which is why many growers recycle it. By reusing it right, you can spend less on fertiliser, use less new water, and progress nicely on your sustainability and compliance goals.

However, if the recycled water isn’t filtered or treated, it could land up being detrimental to your crops. Some plants carry fungi, bacteria, and viruses, and those pathogens settle in the drain water. If it flows back into your system, without treatment, it can make the entire crop sick.

So, filtration is a must, and choosing the right system will depend on your crop, climate and how you grow.

That raises a bigger question: how do you treat it properly?

There are four common methods for filtering greenhouse drain water: ozone, reverse osmosis, UV, and ultrafiltration. Each solves the problem differently, and in understanding how they purify water, you’ll be able to choose the method that works best for your crops and greenhouse.

Comparing the Four Most Common Filtration Methods

1. Ozone Treatment

Ozone treatment works by injecting ozone gas directly into the water, where it reacts with contaminants through a process called oxidation. This reaction breaks down harmful pathogens like bacteria, viruses, and fungi at a molecular level.  

Best for niche setups that need ultra-clean water. Not common for everyday greenhouse use.

Pros:

• Highly effective at killing bacteria, viruses, and fungi
• Powerful disinfection method to meet strict cleanliness standards

Cons:

• High energy use
• Can damage beneficial elements if not dialled in
• Requires careful handling and precise calibration to operate safely and effectively

2. Reverse Osmosis (RO)

Reverse osmosis works by pushing water through a tightly woven membrane that filters out nearly everything except pure water molecules. This includes minerals, salts, and microbes — basically wiping the water clean. It's one of the most effective filtration methods out there, but also one of the most aggressive. 

Best for situations where you need pure water for cooling systems or complete control over exactly what nutrients and minerals are in the water like in medical cannabis or R&D labs.

Pros:

• Extremely thorough filtration resulting in pure H20
• Great for precision mineral control

Cons:

• Removes ALL nutrients — not ideal if you want to reuse fertigation water
• Expensive to install and run
• Produces salty wastewater that needs to be dealt with

3. Ultraviolet (UV) Filtration

UV treatment works by hitting the water with a strong dose of ultraviolet light. This disrupts the DNA of bacteria, viruses, and other microorganisms so they can’t reproduce or infect your crops. It’s a chemical-free method, which means it won’t affect the nutrient content of your water. 

Best for general greenhouse use. A solid option for most growers. We often work with the Priva Vialux UV system. It’s well-tested and used in a wide range of greenhouses.

Pros:

• Doesn’t affect nutrient levels
• Low energy use and easy to maintain
• Reliable for day-to-day operations

Cons:

• Water needs to be clear and flow at a steady rate, or some pathogens might slip through.
• Doesn’t remove suspended solids in the water

4. Ultrafiltration (UF)

Ultrafiltration pushes water through a semipermeable membrane with tiny pores that physically block out bacteria, viruses, and other harmful microorganisms while allowing most of the beneficial nutrients and salts to pass through.

Best for growers who want to reuse both water and nutrients efficiently with a mechanical process. In a recent greenhouse build for our client in the Seychelles, we helped set up a UF system powered by the NUF startup, who repurpose dialysis cartridges as their filters! It’s a smart, low-cost way to get high-quality filtration without nutrient loss.

Pros:

• Keeps around 98% of salts and nutrients
• Filters out a wide range of pathogens
• It can be cheaper to run long-term

Cons:

• Filters need regular cleaning or replacement
• Needs basic maintenance to keep performance up

What to Think About Before You Choose Your Filtration Method

All four systems work. But not everyone makes sense for every farm. Reusing drain water can help you cut costs, reduce waste, and stay compliant. But only if it’s safe.

Before choosing a system, think about:

• What you want to keep, and what you want to remove
• How sensitive your crops are to disease
• Maintenance and energy costs
• Whether your goal is pure water, nutrient reuse, or both

For most greenhouses, UV and Ultrafiltration hit the sweet spot. They’re reliable, efficient, and keep the good stuff in your water while removing the risk. That’s a big win for growers who want to reuse water without compromising crop health.

You don’t have to figure this out on your own. We help growers across Africa with needs-based filtration systems that make economic and agronomic sense.

There are no one-size-fits-all solutions. And we’re here to help you make the right call for your farm and your crops.

Want to talk filtration? Get in touch on +27 21 987 6980 or info@vegtech.co.za.