Nothing to Waste: The Water Your DI System Throws Away
Updated: Sep 24
In a previous Greening the Bench post, Choose Your Level: Reduce Water Waste with the Right Purity, we looked at how matching the water grade to the task stops you generating a fivefold of reject water for a grade of purity that isn't needed. But that blog did end on a bit of a loose thread with what to do with the reject water itself. That is what this post is all about!
The DI water system in your lab almost certainly begins with a reverse osmosis membrane, which is what generates the reject water. It splits the incoming mains water into two streams. One becomes the Level 1 or Level 2 water you use for your experiments. The other, two to eight litres for every litre of purified water, goes down the drain.
One quick note. If your DI water comes without a reverse osmosis stage, there is unlikely to be a reject stream to find. Everything below applies to the RO-fed systems that produce most lab DI water.
What Reject Water Actually Is
Reject water carries the dissolved salts and minerals that the RO membrane refuses to let through. That makes it more concentrated than the mains water that went in, typically somewhere between two and five times. But just because it is concentrated doesn't necessarily mean it should be classed as contaminated. As it does not contain any solvents, reagents or biological material.
That distinction is the whole reason reuse is worth considering. You are not treating waste or cleaning anything up. You are redirecting water that is just 'harder' than the stream from your mains supply.
The Case for Capturing It
First lets focus on how much of this reject water a lab may generate. A modest DI system producing 30 litres of purified water a day is sending roughly 90 to 240 litres to drain alongside it. Over a working year that is somewhere between 20,000 and 55,000 litres from a single unit. If you multiply that by the number of systems in a research building and the figure stops being abstract very quickly.
Then there is the cost, which lands twice. You pay for that water coming in and usually on the way out as well, as it is often assessed on metered supply. So every rejected litre is billed at both ends for doing no work at all. Capturing even part of it shows up on a utility bill rather than only in a sustainability report.
And there is resilience. Water scarcity is a problem for most countries now and with lab buildings typically consuming 3-5x more water than an office building, we know that there are areas for big savings.
Where Reject Water Can Go
Not every use below is appropriate in every case, and the right answer depends on your reject quality, your building and your organisation. These are the options worth investigating, running roughly from simplest to most involved:
🪣 Glassware pre-rinse and wash-up
The first rinse of dirty glassware does not need purified water, or even especially soft water. A collection vessel beside the wash-up sink would cover this with essentially no infrastructure, which makes it the easiest place to start.
🧊 Cooling and water baths
Where equipment uses water for cooling or heating, reject water often meets spec. Check the manufacturer's conductivity and hardness limits first, because building up of limescale is the risk here rather than contamination.
🌱 Plants and grounds
Campus planting, glasshouses and green roofs can often take reject water directly. Salinity is the thing to watch, so share a conductivity reading with your grounds team before anyone commits.
🧯 Plant and back-of-house
Buildings have many unknown uses for water and your facilities teams will know which of these exist in your building, and this is exactly the sort of reuse other sectors already do routinely.
🚽 Toilet flushing and grey water
The biggest prize and the hardest to retrofit, since it needs a storage tank, a pump and a separate circuit. Worth raising early if your building is heading for refurbishment, because the cost is modest at design stage and considerable afterwards.
Making it work safely
Before jumping into anything, it is important to make sure all the right people are aligned. From lab mangers, quality assurance, facilities and EH&S members.
Three things to get right before anything is started:
Test first - Measure conductivity, hardness, and chlorine if it is relevant to the end use. Reject quality varies with your incoming mains and with the age of the RO membrane, so specific readings from your DI system on a frequent basis may be necessary.
Keep it separate and labelled - Reject water lines must be clearly marked, physically separated from mains supply and protected against backflow, as specified by plumbing regulations.
Do not let it sit - Stored water at room temperature is a legionella consideration. Any tank needs turnover, covering and a maintenance schedule, which is precisely why facilities should own the storage rather than the lab.
This means the conversation starts with your facilities team rather than with a bucket under a bench. Bring them a volume figure and they can usually tell you whether there is a safe use for it.
Action of the Week
Find out where your DI system's reject line goes, and how much is going down it.
Follow the reject tube physically. On many benchtop units it runs a short distance into the nearest sink.
Measure the amount that is wasted for your system per litre of purified water. Grabbing a measuring cylinder does the trick here.
Multiply your daily DI water volume by that ratio. That number, alongside your conductivity, hardness and chlorine measurements is what you can provide for the facilities team.
Conclusion: Nothing Should Leave Unused
The water your DI system throws away is one of those cases where it is the sustainable option and the cheap option. You may find the answer for your lab is "not this year", and that is okay. But knowing the number, and knowing where the pipe goes, is what turns a vague good intention into something an estates team can actually cost. Every small action counts.
Frequently Asked Questions
What is reverse osmosis reject water?
Reject water, is the stream carrying the dissolved salts and minerals that a reverse osmosis membrane will not let through. In a lab it is produced by the RO stage that sits at the front of most DI water systems. It is typically two to five times more concentrated than the mains water that went in, but it contains no solvents, reagents or biological material, so it sits somewhere between mains water and hard tap water rather than being lab waste.
How much water does a lab DI system send to drain?
An RO-fed DI system typically discharges around two to eight litres of reject water for every litre of purified water it produces. A unit making 30 litres a day therefore sends roughly 90 to 240 litres to drain daily, or somewhere between 20,000 and 55,000 litres over a working year from a single system. Cartridge-only DI systems with no RO stage produce no reject stream, but they consume exchangeable resin instead.
What can DI and RO reject water be reused for in a lab?
Suitable uses include the first rinse of dirty glassware, equipment cooling and water baths where the manufacturer's hardness and conductivity limits allow, campus irrigation and glasshouses, cooling tower make-up, boiler feed and floor cleaning, and, in buildings with a non-potable circuit, toilet flushing. The simplest options need almost no infrastructure; the largest-volume ones usually need facilities involvement or a refurbishment.
Is it safe to reuse reject water from a DI system?
It can be, provided three conditions are met. Test the reject stream for conductivity and hardness, and chlorine where relevant, since quality varies with your incoming mains and the age of the RO membrane. Keep any reuse line clearly labelled, physically separated and protected against backflow. Avoid long-term standing storage, because water held at room temperature raises a Legionella risk, so any stroage needs turnover, covering and a maintenance schedule typically owned by facilities.
