RO Systems and Water Meters: The Hidden Cost Nobody Talks About
If you live in a flat, have a shared water supply, or live in Scotland, I have good news: you've got a secret advantage when it comes to reverse osmosis (RO) water filters. But if you're on a water meter—or about to be—there's a hidden cost that could add £59 to £117 per year to your water bill. And that's before we talk about water pressure.
Let me explain why around 40% of UK households can run RO systems without worrying about waste water costs, while the rest need to do the math carefully. And why your top-floor flat might give you free waste water but cost you £600 in other ways.
The RO Waste Water Reality
Here's what most RO system sellers won't tell you upfront: for every 1 litre of filtered water an RO system produces, it sends 3-5 litres down the drain. This isn't a flaw—it's how the technology works.
How RO Waste Water Works:
Reverse osmosis forces water through a membrane with pores 0.0001 microns wide (1,000 times smaller than a human hair). Contaminants can't pass through, but they accumulate on the membrane surface. To prevent clogging, the system continuously flushes this concentrated "reject water" down the drain.
Typical ratio: 4:1 (4 litres wasted for every 1 litre filtered). Some systems claim 3:1, older systems can be 5:1 or worse.
For a family filtering 10 litres per day (drinking water, cooking, tea/coffee), that's 40 litres per day going down the drain. Over a year, that's 14,600 litres—or 14.6 cubic meters—of wasted water.
If you're on a water meter, that waste costs money. If you're not on a meter, it's completely free.
The 40% Advantage: Who's Unmetered in the UK?
According to UK Government data from December 2024:
- Around 40% of households in England and Wales do NOT have a water meter (60% are metered)
- 2.63 million homes in Scotland remain unmetered (Scottish Water operates differently)
- Total: Over 10 million UK households pay a fixed annual water bill regardless of usage
For these households, RO waste water costs absolutely nothing extra. Your water bill stays the same whether you filter 10 litres or 100 litres per day.
Who's Most Likely to Stay Unmetered:
- Multi-story flats and apartments - Shared supply pipes make metering too expensive
- Terraced houses with shared supplies - Would need to separate the supply first
- Older properties with joint supplies - Common in Victorian and Edwardian buildings
- Scotland - No plans for universal metering
- Northern England - Yorkshire, Manchester, Newcastle areas not doing compulsory metering
Who's Getting Metered (And Should Worry About RO Costs)
If you live in a "water-stressed" area, your water company can install compulsory meters under the Water Industry Act 1991. This is happening right now across South East England:
Companies Actively Installing Compulsory Meters (2024-2025):
- Thames Water - London, Swindon, Guildford, Waverley, Oxford, Reading
- • Installing 78,000 meters in 2024-2025
- • Goal: Most customers metered by 2030
- Affinity Water - 160,000 meters installed, "3 in 4 customers already have one"
- Southern Water - Hampshire, Sussex, Kent
- South East Water - Kent, Sussex, Surrey, Berkshire
- South Staffs Water - Selective compulsory metering
Important: Check If Metering Is Planned
If you're currently unmetered in South East England, check with your water company whether compulsory metering is planned for your area. The "free RO waste water" advantage might disappear in the next few years. However, flats and shared supply properties often cannot be metered even in compulsory metering areas—it's too expensive to separate the supplies.
The Math: What RO Waste Water Actually Costs
UK water costs vary by region, with Thames Water charging £4.02 per cubic meter (£2.47 clean water + £1.55 wastewater) as of April 2025. Other companies range from £3.00-£5.00/m³.
| Daily Filtered Water | Daily Waste (4:1 ratio) | Annual Waste | Cost @ £4.02/m³ |
|---|---|---|---|
| 5 litres | 20 litres | 7.3 m³ | £29/year |
| 10 litres | 40 litres | 14.6 m³ | £59/year |
| 20 litres | 80 litres | 29.2 m³ | £117/year |
| Unmetered household | Any amount | Any amount | £0/year |
For a typical family filtering 10 litres per day (drinking, cooking, tea/coffee), that's £54 per year in wasted water if you're metered. Over the 10-year lifespan of an RO system, that's £540 in waste water costs.
The Water Pressure Problem (And Why Flats Get Hit Twice)
Here's where things get complicated. RO systems don't just waste water—they also need high water pressure to work properly.
RO System Pressure Requirements:
- Minimum: 40 PSI (2.76 bar) - System will work but slowly
- Ideal: 60 PSI (4.14 bar) - Optimal performance
- Below 40 PSI: System won't function properly, very slow flow, poor filtration
UK mains water pressure averages 3 bar (43.5 PSI)—just barely enough for RO systems. But pressure varies significantly:
- Ground floor homes: Typically 2-4 bar (29-58 PSI) - usually fine
- Top floor flats (3rd floor+): Often 1-2 bar (14-29 PSI) - too low for RO
- Rural areas: End of supply network, often low pressure
- Old buildings: Corroded pipes reduce pressure
Here's the irony: multi-story flats are the most likely to stay unmetered (free RO waste water), but they're also the most likely to have low pressure (need expensive booster pump).
How to Test Your Water Pressure (Before Buying RO)
Don't buy an RO system without testing your pressure first. Here are two methods:
Method 1: Simple Flow Test
(No equipment needed)
- 1. Place 1-litre measuring jug under tap
- 2. Turn tap on FULL
- 3. Time how long to fill 1 litre
- 4. Calculate litres per minute
Results:
• Under 10 L/min = LOW (likely under 40 PSI)
• 10-15 L/min = MODERATE
• Over 15 L/min = GOOD
Method 2: Pressure Gauge
(Accurate measurement)
- 1. Buy water pressure gauge (£15-25)
- 2. Attach to outside tap or washing machine connection
- 3. Turn on water, read gauge
- 4. Look for 40+ PSI (2.76+ bar)
Where to buy:
Monument Tools gauge at Screwfix, Toolstation, B&Q (£15-25)
Booster Pumps: When You Need One and What They Cost
If your pressure is below 40 PSI, you'll need a booster pump. This adds significant cost:
Booster Pump Costs:
- Equipment: £80-250 (basic to mid-range)
- Installation: £100-200 (professional plumber, 1-2 hours)
- Total upfront: £180-600
- Running costs: £10/year electricity (24-36W, runs only when filtering)
Popular UK booster pumps include the Hommix Easy Install Booster Pump (£120) and generic 50 GPD boosters (£80-100). Professional installation is recommended unless you're confident with plumbing.
The Complete Cost Breakdown: 4 Scenarios
Here's what RO systems actually cost in different situations:
Best Case: Unmetered + Good Pressure
- • RO system: £250-500
- • Installation: £100-200
- • Waste water: £0
First year: £350-700
Annual ongoing: £0
Unmetered + Low Pressure
- • RO system: £250-500
- • Booster pump: £80-250
- • Installation: £200-400
- • Waste water: £0
- • Pump electricity: £10/year
First year: £530-1,150
Annual ongoing: £10
Metered + Good Pressure
- • RO system: £250-500
- • Installation: £100-200
- • Waste water: £54/year
First year: £404-754
Annual ongoing: £54
Worst Case: Metered + Low Pressure
- • RO system: £250-500
- • Booster pump: £80-250
- • Installation: £200-400
- • Waste water: £54/year
- • Pump electricity: £10/year
First year: £594-1,204
Annual ongoing: £64
Over 10 years, the difference between best and worst case is £640-1,790 in ongoing costs alone.
Should You Get an RO System? A Decision Framework
Use this framework to decide if RO makes sense for your situation:
RO Makes Sense If:
- ✓ You're unmetered (flats, shared supply, Scotland, Northern England)
- ✓ You have good water pressure (40+ PSI / 2.76+ bar)
- ✓ You have specific concerns: lead pipes, PFAS, microplastics, very hard water
- ✓ You want the most comprehensive filtration (95-99% contaminant removal)
- ✓ You're willing to invest £350-700 upfront for long-term peace of mind
Consider Alternatives If:
- ⚠ You're metered and filtering large volumes (20+ litres/day = £108/year waste)
- ⚠ You have low pressure and don't want to spend £180-600 on a booster pump
- ⚠ You're in South East England and compulsory metering is coming soon
- ⚠ Your main concerns are chlorine taste or moderate hardness (carbon filters work fine)
- ⚠ You're on a tight budget and ongoing costs matter
RO Probably Isn't Worth It If:
- ✗ You're metered + have low pressure (worst case: £594-1,204 first year, £64/year ongoing)
- ✗ You don't have specific contaminant concerns (UK water is 99.98% compliant when it leaves treatment works)
- ✗ You're renting short-term (can't justify £350-700 investment)
- ✗ You only want to improve taste (activated carbon is much cheaper and doesn't waste water)
Alternatives to RO for Metered Households
If you're metered and concerned about ongoing costs, consider these alternatives:
- Activated carbon filters - Remove chlorine, taste, odor, some VOCs. No waste water. £50-200 upfront, £30-60/year filters.
- Carbon block + KDF filters - Also remove lead, mercury, some heavy metals. No waste water. £100-300 upfront.
- UV filters - Kill bacteria/viruses but don't remove chemicals. No waste water. £150-400 upfront.
- Water softeners - Solve hard water problems. Some waste during regeneration but much less than RO. £400-800 upfront.
For most people concerned about taste, chlorine, or moderate hardness, activated carbon filters are the most cost-effective solution. They don't waste water, don't need high pressure, and cost far less to run.
Reserve RO for situations where you genuinely need comprehensive contaminant removal: lead pipes (pre-1970 homes), PFAS concerns(not tested in UK), microplastics, or very hard water causing appliance damage.
Learn more about filter types and which one is right for your situation in our Water Filtration 101 guide.
The Bottom Line
If you're one of the 37.3% of UK households without a water meter, RO systems are significantly more economical. You get comprehensive filtration without worrying about waste water costs. Just make sure you have adequate pressure (40+ PSI) before buying.
If you're metered, do the math carefully. At £54-108 per year in waste water costs, RO systems add up over time. Factor in booster pump costs if your pressure is low. For many metered households, activated carbon filters offer better value.
And if you're in South East England on an unmetered supply, check whether compulsory metering is planned for your area. Your "free waste water" advantage might not last forever—though flats and shared supply properties will likely stay unmetered permanently.
Key Takeaways:
- ✓ RO systems waste 4 litres for every 1 litre filtered (4:1 ratio)
- ✓ 37.3% of UK households are unmetered = free RO waste water (save £54-108/year)
- ✓ Multi-story flats most likely to stay unmetered but often have low pressure
- ✓ South East England getting compulsory meters (Thames, Affinity, Southern, South East Water)
- ✓ RO systems need 40+ PSI (2.76+ bar) - test pressure before buying
- ✓ Booster pumps cost £180-600 upfront + £10/year electricity
- ✓ Best case: Unmetered + good pressure = £0/year ongoing
- ✓ Worst case: Metered + low pressure = £64/year ongoing + high upfront costs
- ✓ Consider activated carbon filters if metered and concerned about costs