4040 vs 8040 RO membranes — sizing and output
Short answer: The four digit code is the diameter and length in inches. A 4040 is 4 inches across and 40 inches long; an 8040 is 8 by 40. An 8040 has roughly four times the membrane area of a 4040, so it produces around four times the water.
The choice is driven by plant capacity — 4040 elements for smaller plants, 8040 for larger ones where fewer, bigger elements are more economical.
Reading the code
| Code | Diameter | Length | Typical use |
|---|---|---|---|
| 1812 | 1.8" | 12" | Domestic RO |
| 2012 / 2812 | 2"–2.8" | 12" | Small commercial |
| 2540 | 2.5" | 40" | Small commercial |
| 4021 | 4" | 21" | Compact commercial |
| 4040 | 4" | 40" | Commercial, the most common |
| 8040 | 8" | 40" | Industrial |
Approximate output per element
These are indicative figures at typical brackish water conditions. Actual output depends on feed TDS, temperature and pressure.
| Element | Approximate permeate |
|---|---|
| 1812 | 10 – 15 LPH |
| 2540 | 60 – 100 LPH |
| 4021 | 150 – 200 LPH |
| 4040 | 300 – 400 LPH |
| 8040 | 1,200 – 1,600 LPH |
How many elements for your plant
| Plant capacity | Typical arrangement |
|---|---|
| 250 LPH | 1 × 4040 |
| 500 LPH | 2 × 4040 |
| 1,000 LPH | 3 × 4040 |
| 2,000 LPH | 6 × 4040, or 2 × 8040 |
| 5,000 LPH | 4 × 8040 |
| 10,000 LPH | 8 × 8040 |
When to move from 4040 to 8040
The crossover is usually around 2,000 to 3,000 LPH. Above that, 8040 elements are generally better because:
- Fewer elements to handle and replace
- Fewer housings, connections and O-rings — so fewer leak points
- Lower cost per litre of capacity
- Less space for the same output
But 4040 has advantages too
- Easier to handle — an 8040 element is heavy and awkward
- Lower cost per element — replacement is less of a single hit
- More flexible — capacity can be adjusted in smaller steps
- Widely available — easier to source quickly
Membrane types
| Type | Suited to | Operating pressure |
|---|---|---|
| Brackish water (BW) | Borewell, up to ~5,000 TDS | 10 – 15 kg/cm² |
| Low pressure (LP/ULP) | Lower TDS, saves energy | 7 – 10 kg/cm² |
| Seawater (SW) | Very high TDS, seawater | 50 – 70 kg/cm² |
| Nanofiltration (NF) | Softening, partial removal | 5 – 10 kg/cm² |
Most Indian borewell applications use brackish water elements. Low pressure types are worth considering where feed TDS is genuinely low, because they save pump energy.
Housings must match
An element needs a pressure vessel of matching diameter and length, rated for your operating pressure. Housings are available in various connection sizes — 25NB, 40NB and 50NB.
Also check:
- Number of elements per housing — housings hold 1, 2, 3 or more elements in series
- Pressure rating — must exceed your operating pressure with margin
- End port or side port — depending on your piping layout
Elements in series
In a multi-element housing, water passes from one element to the next. The first element sees the freshest water and produces most; the last sees the most concentrated water and produces least.
This is why:
- All elements should be replaced together — mixing old and new gives uneven performance
- The last element is most vulnerable to scaling, since it sees the highest concentration
- Reject flow matters most at the last element — see our flow monitoring guide
What to specify when ordering
- Plant capacity in LPH
- Feed TDS
- Feed water source — borewell, municipal or surface
- Existing housing size, if replacing
- Available feed pressure
With those we can confirm the element type, size and quantity.
Need membranes or housings?
Send your plant capacity, feed TDS and existing housing details, and we will confirm the right element and quantity.
💬 WhatsApp par poochhein 📞 9910646957Frequently asked questions
What does 4040 mean on an RO membrane?
It is the diameter and length in inches. A 4040 element is four inches in diameter and forty inches long, while an 8040 is eight by forty inches and has about four times the membrane area.
How much water does a 4040 membrane produce?
Typically three hundred to four hundred litres per hour under normal brackish water conditions, though actual output depends on feed TDS, temperature and pressure. High TDS and cold water both reduce it significantly.
When should I move from 4040 to 8040 elements?
Usually around two to three thousand LPH. Above that, 8040 elements mean fewer housings and connections, lower cost per litre and less space, though individual elements are heavier and more expensive to replace.
Which membrane type do I need?
For most Indian borewell water a brackish water element is correct. Low pressure types suit genuinely low TDS feed and save pump energy, while seawater elements are only for very high TDS applications.
Should all elements in a housing be replaced together?
Yes. Water passes from one element to the next, so mixing old and new gives uneven performance. Replace the whole set together, along with the O-rings and brine seals.