If you are buying RO membranes for a water treatment system, the first question is always: 4040 or 8040?
The short answer: 4040 is for smaller commercial systems, 8040 is for larger industrial systems. But the right choice depends on your flow requirements, space, and budget.
Quick Comparison Table
| Specification | RO 4040 (4") | RO 8040 (8") |
|---|---|---|
| Dimensions | 4" × 40" | 8" × 40" |
| Flow Rate | 200-300 GPD (0.3-0.5 TPH) | 6,000-9,000 GPD (1.0-1.5 TPH) ✅ |
| Salt Rejection | 99.5% | 99.5% |
| Operating Pressure | 150-250 psi | 150-300 psi |
| Membrane Area | ~80 ft² | ~365 ft² ✅ |
| Price Range | $80 - $150 ✅ | $300 - $600 |
| Best For | Commercial RO, drinking water | Industrial, desalination ✅ |
1. Size and Dimensions
The names tell you the size: 4040 is 4 inches in diameter and 40 inches long, while 8040 is 8 inches in diameter and 40 inches long. Both fit standard pressure vessels of the same diameter.
The 8040 has about 4.5 times more membrane surface area than the 4040, which is why it can produce much more water.
2. Flow Rate and Capacity
This is the biggest difference. A single 4040 element produces roughly 200-300 gallons per day. An 8040 element produces 6,000-9,000 gallons per day — about 30 times more water.
For a small bottled water plant doing 1,000 GPD, one 4040 is enough. For a municipal plant treating 100,000 GPD, you need multiple 8040s in series.
3. Price Comparison
| Quantity | RO 4040 | RO 8040 |
|---|---|---|
| 1-10 pcs | $80 - $150 ea | $300 - $600 ea |
| 10-50 pcs | $65 - $120 ea | $250 - $500 ea |
| 50+ pcs | $50 - $100 ea | $200 - $450 ea |
4. Applications
RO 4040 is best for:
- Bottled water plants (small to medium)
- Drinking water kiosks
- Commercial RO systems
- Small-scale brackish water treatment
- Lab and pharmaceutical (small volume)
RO 8040 is best for:
- Municipal water treatment plants
- Industrial process water
- Seawater desalination
- Large-scale bottled water production
- Wastewater recycling systems
How These Elements Are Written (and Searched)
The 4040 and the 8040 are both 40 inches long — only the diameter differs. That is why the two are so often confused in search and in quotations, and why they turn up in half a dozen written forms:
| Written as | Where you see it |
|---|---|
| 4040 / 8040 | The standard industry shorthand, and how we list them |
| 4" x 40" / 8" x 40" | Specification sheets and datasheets |
| 4x40 / 8x40 | Handwritten notes and search terms |
| 80 40 / 80-40 | A frequent typo for 8" x 40" — the quotation mark is dropped |
| RO 40 | Shorthand for the 40-inch element length |
If you are searching for an 80 40 membrane, an 8x40, a 4x40 membrane or an RO 40 element, you are looking at one of these two products. The 4040 is the 4-inch diameter element at 200–300 GPD; the 8040 is the 8-inch diameter element at 1.0–1.5 m³/h. If your system was built for 8-inch pressure vessels, the 4040 will not fit — and vice versa.
5. Which One Should You Buy?
Choose 4040 if: Your system needs under 5,000 GPD. You have limited space. You want a lower upfront cost.
Choose 8040 if: Your system needs over 5,000 GPD. You want lowest cost per gallon. You have space for larger pressure vessels.
Rule of thumb: If you are starting a small bottled water business, go with 4040. If you are setting up an industrial treatment plant, go with 8040.
Need Help Sizing Your RO System?
Tell us your daily water demand and feed water quality. We will recommend the right membrane configuration.
📦 Free samples available — just pay shipping
Request a Quote →FAQ
Can I use 4040 and 8040 in the same system?
Technically yes, but not recommended because they need different pressure vessels and flow configurations. Stick to one size per system.
How long do RO membranes last?
With proper pretreatment and maintenance, RO membranes typically last 2-3 years. Feed water quality is the biggest factor affecting lifespan.
Do you offer custom RO membrane configurations?
Yes, we can customize membrane elements for specific feed water types including brackish water, seawater, and wastewater applications.