In-Depth Comparison of Reverse Osmosis and Ultrafiltration Water Purifiers: Wastewater Ratio and Mineral Retention

Which one should you choose, RO reverse osmosis or ultrafiltration? Is pure water without minerals bad for your health? What does a wastewater ratio of 2:1 mean? Why does an RO purifier need to be plugged in? This article explains everything thoroughly from the perspective of water treatment chemistry and membrane science.

1. What's in Tap Water

Common Pollutant Categories

Particulates: Sediment, rust, suspended solids
- Source: Aging pipes, secondary water supply
Microorganisms: Bacteria, viruses, parasites
- Source: Pipe damage, secondary contamination
Heavy Metals: Lead, mercury, cadmium, arsenic
- Source: Industrial pollution, aging pipes (lead pipes/copper pipe solder)
Organic Compounds: Pesticide residues, industrial solvents
- Source: Agricultural/industrial discharge
Disinfection Byproducts: Chloroform, haloacetic acids
- Source: Reaction of chlorine disinfection with organic matter
Hardness Ions: Calcium, magnesium ions
- Source: Geological dissolution
- Hard water itself is harmless but affects taste and causes scaling

The Pros and Cons of Residual Chlorine

Pros: Ensures continuous disinfection in the pipe network
Cons: Affects taste, produces disinfection byproducts
National Standard Limit: ≥0.05 mg/L (at the end of the pipe network)
Water purifiers can remove residual chlorine to improve taste

2. Core Filtration Technology Comparison

PP Cotton Filter

Filtration Precision: 1-5 μm
Filtration Target: Sediment, rust, large particles
Lifespan: 3-6 months
Function: Primary filtration, protects downstream filters

Activated Carbon Filter

Filtration Mechanism: Adsorption
Filtration Target: Residual chlorine, organic compounds, odors, some heavy metals
Lifespan: 6-12 months
Types:
- Granular Activated Carbon (GAC): High adsorption capacity, fast water flow
- Carbon Block (CTO): Higher filtration precision, less carbon shedding
- Post-activated Carbon: Improves the taste of pure water

Ultrafiltration (UF)

Filtration Precision: 0.01-0.1 μm
Working Principle: Membrane pore size screening
Can Remove:
- Bacteria (0.5-5 μm) ✅
- Most viruses (0.02-0.3 μm) ⚠️ Some small viruses can pass through
- Colloids, large molecular organic compounds ✅
- Sediment, rust ✅
Cannot Remove:
- Heavy metal ions ❌
- Dissolved salts ❌
- Small molecular organic compounds ❌
- Hardness ions ❌
Advantages:
- No electricity required
- No wastewater produced
- Retains minerals
- High water output rate
Disadvantages:
- Cannot remove heavy metals
- Not safe enough for areas with poor water quality

Reverse Osmosis (RO)

Filtration Precision: 0.0001 μm (0.1 nanometers)
Working Principle:
- High-pressure pump pressurizes → water molecules pass through the RO membrane → solutes are rejected
- Osmotic pressure must be overcome → requires pressurization → requires electricity
- Tap water osmotic pressure is approx. 0.3-0.7 bar → RO working pressure is approx. 5-8 bar
Can Remove:
- Almost all dissolved salts ✅
- Heavy metals ✅
- Bacteria and viruses ✅
- Organic compounds ✅
- Pesticide residues ✅
- Hardness ions ✅
Product Water Purity: TDS typically <10 mg/L
Advantages:
- Most thorough filtration, safest output water
- Suitable for all water qualities
- TDS value can be monitored directly
Disadvantages:
- Produces wastewater (concentrate)
- Requires electricity
- Relatively slow output rate (requires a pressure tank or high-flow membrane)
- Removes all minerals

3. The Truth About "Pure Water Has No Minerals"

Sources of Minerals for the Human Body

Contribution from Drinking Water: Approx. 1-5% of mineral intake
Contribution from Food: Approx. 95-99%
The calcium content of one glass of milk ≈ the calcium content of 300 glasses of pure water
The iron content of one bite of spinach > the iron content of a day's intake of pure water

WHO Perspective

The contribution of minerals in drinking water to health is limited
A balanced diet can fully compensate
Mineral deficiencies should first be addressed by adjusting diet, not relying on water
However, long-term consumption of very low TDS water may pose extremely minor health uncertainties

Practical Advice

People with a normal diet → Pure water is perfectly fine
People with insufficient mineral intake → Should improve their diet, not rely on water
If concerned about water safety (heavy metals/contamination) → Prioritize RO
In areas with excellent water quality → UF is also acceptable

4. Detailed Explanation of Wastewater Ratio

What is the Wastewater Ratio?

Wastewater Ratio = Pure Water : Concentrate
Example: Wastewater ratio 2:1 → Produces 2 parts pure water, discharges 1 part concentrate
Wastewater ratio 1:1 → Produces 1 part pure water, discharges 1 part concentrate

Physical Reason for Wastewater Production

The solutes rejected by the RO membrane need to be flushed away by the concentrate
Otherwise, solutes accumulate on the membrane surface → membrane clogs → lifespan shortens
Concentrate discharge is a necessary design to protect the RO membrane

Relationship Between Wastewater Ratio and RO Membrane Lifespan

High Wastewater Ratio (3:1, 4:1):
- More pure water, less wastewater → saves water bills
- But the solute concentration on the RO membrane surface is higher → more prone to scaling
- Membrane lifespan may be shortened
- Requires stricter water quality
Low Wastewater Ratio (1:1, 1.5:1):
- More wastewater → more thorough flushing
- Longer membrane lifespan
- Suitable for areas with poor water quality
Adjustable Wastewater Ratio:
- Supported by some high-end models
- Can increase the ratio when water quality is good
- Decrease the ratio to protect the membrane when water quality is poor

Utilizing Wastewater

Although the concentrate is not drinkable, it can be used for:
- Mopping floors, flushing toilets
- Washing clothes (not suitable for delicate fabrics like silk)
- Watering plants (some plants are okay, be mindful of salt content)
- Collecting and using it can significantly reduce waste

5. RO Flow Rate and Output Speed

Flow Rate Unit

GPD (Gallons Per Day): Daily water production of the RO membrane
50 GPD ≈ 7.9 L/h ≈ 130 mL/min
400 GPD ≈ 63 L/h ≈ 1.05 L/min
600 GPD ≈ 95 L/h ≈ 1.58 L/min
800 GPD ≈ 126 L/h ≈ 2.1 L/min
1000 GPD ≈ 158 L/h ≈ 2.63 L/min
1200 GPD ≈ 189 L/h ≈ 3.15 L/min

Flow Rate Selection Guide

Flow Rate	Water Dispensing Experience	Suitable Household	Pressure Tank Required?
50-100G	Very slow, needs tank	Not recommended for new purchase	Yes
400G	Slow but acceptable	1-2 people	No
600G	Moderate	3-4 people	No
800-1000G	Fast	4+ people	No
1200G+	Very fast	Large family/commercial	No

Pros and Cons of a Pressure Tank

Advantages: Stores water in advance, provides instant flow
Disadvantages:
- Takes up space
- Stored water may be subject to secondary contamination (requires periodic cleaning)
- Risk of aging rubber bladder inside the tank
- Current trend is towards high-flow, tankless designs

6. Filter System and Replacement Costs

Typical RO Purifier Filter Structure

PP Cotton (3-6 months) → Pre-filtration
Pre-activated Carbon (6-12 months) → Removes chlorine/organics
RO Membrane (18-36 months) → Core filtration
Post-activated Carbon (6-12 months) → Improves taste

Composite Filters vs. Individual Filters

Composite Filters:
- PP cotton + activated carbon combined
- Easy to replace (pull out and plug in)
- But PP cotton has a shorter lifespan → the whole unit is replaced early → higher cost
Individual Filters:
- Each filter is replaced independently
- Replaced according to actual lifespan → more economical
- But the replacement process is slightly more complex

Estimated Annual Usage Cost

Solution	Annual Filter Cost	Notes
UF	200-400 RMB	No RO membrane, low cost
RO 400G	300-600 RMB	Includes RO membrane amortization
RO 800G+	400-800 RMB	High-flow membranes are slightly more expensive
Some Brands	600-1200 RMB	High premium on proprietary filters

What to Look For When Buying

✅ Whether filters are universal (reduces long-term cost)
✅ RO membrane lifespan and replacement price
✅ Whether there is a filter life reminder
❌ Only looking at the machine price, ignoring ongoing costs
❌ Proprietary filters with non-transparent pricing

7. Installation and Maintenance

Installation Requirements

Under-sink space: Height >40 cm, Depth >40 cm
Water inlet: 2-point / 3-point inlet tee
Drain: Concentrate drains into the sewer
Power: RO requires a 220V outlet
Winter water temperature: RO membrane efficiency drops significantly below 5°C

Routine Maintenance

Replace filters on time (most important)
Flush for 15 minutes after first use or filter replacement
If not used for a long time (>3 days), flush again before use
Periodically check for leaks
Monitor TDS value changes (for RO units)

Water Quality Monitoring

TDS Meter:
- Measures total dissolved solids
- Tap water is typically 100-300 mg/L
- RO output should be <50 mg/L
- Sudden TDS increase → RO membrane may be damaged
Residual Chlorine Test Strips:
- Tests the effectiveness of the activated carbon filter
- Presence of chlorine → activated carbon is exhausted

8. Buying Decision Guide

Choose RO Reverse Osmosis When

Water quality is poor (TDS >300)
Living in an old neighborhood / using secondary water supply
Concerned about heavy metal contamination
Preparing infant formula
Pursuing the safest drinking water

Choose Ultrafiltration When

Living in an area with excellent water quality (TDS <150)
Don't want to plug in / produce wastewater
Want to retain minerals
Budget is limited

Pitfall Checklist

❌ "8-Stage Filtration" → Number of stages doesn't equal effectiveness
❌ "Mineral Water" → Might just mean it wasn't filtered properly
❌ "Wastewater Ratio 5:1" → Excessively high ratio affects membrane lifespan
❌ "Zero Wastewater" → Violates physical laws, or uses other methods to handle it
❌ Only look at the machine price → The annual filter cost is the bigger expense
✅ Look for NSF certification / health permit approval
✅ Compare long-term usage costs
✅ Consider after-sales service and filter availability

Summary: RO provides the most thorough filtration but produces wastewater and consumes electricity. UF retains minerals but cannot remove heavy metals. The issue of mineral deficiency from pure water is far more reliably addressed by diet than by water. When choosing a water purifier, first assess your water quality, then select the technology, and focus on the annual filter cost rather than the machine price.