How to size an RO plant — capacity, recovery and feed flow
Short answer: Start from your daily requirement, decide how many hours a day the plant will run, and that gives the LPH rating. Then work backwards through recovery to find the feed flow, which sizes the pump, pre-treatment and pipework.
Feed LPH = plant LPH ÷ recovery
Step 1: Daily requirement
Be honest about this, and include peak days rather than the average:
| Application | Typical demand |
|---|---|
| Small office drinking water | 200 – 500 L/day |
| Restaurant | 500 – 2,000 L/day |
| Hotel, 30 rooms | 3,000 – 6,000 L/day |
| Packaged water unit | As per production plan |
| Boiler feed | Steam output plus blowdown |
| Process water | Per process requirement |
Step 2: Running hours
Fewer hours means a bigger plant. There is a genuine trade-off:
| Running hours | For 5,000 L/day | Comment |
|---|---|---|
| 4 hours | 1,250 LPH | Larger plant, higher cost |
| 8 hours | 625 LPH | Usual choice |
| 16 hours | 313 LPH | Smaller plant, needs storage |
| 24 hours | 208 LPH | Smallest plant, no downtime allowance |
Step 3: Choose the recovery
Recovery is what proportion of feed water becomes permeate. It is limited by feed TDS, because the reject stream concentrates as recovery rises.
| Feed TDS | Safe recovery | Reject per 1,000 L permeate |
|---|---|---|
| Below 500 | 70 – 75% | ~350 L |
| 500 – 1,000 | 65 – 70% | ~470 L |
| 1,000 – 2,000 | 60 – 65% | ~600 L |
| 2,000 – 4,000 | 50 – 60% | ~800 L |
| Above 4,000 | 40 – 50% | ~1,200 L |
Pushing recovery higher scales membranes — see our membrane life guide.
Worked example
Requirement: hotel needing 5,000 L/day, feed TDS 1,200 ppm, running 8 hours.
- Plant capacity: 5,000 ÷ 8 = 625 LPH, so specify 750 LPH with margin
- Recovery at 1,200 TDS: 65 percent
- Feed flow: 750 ÷ 0.65 = 1,154 LPH
- Reject flow: 1,154 − 750 = 404 LPH
- Daily feed water needed: 1,154 × 8 = 9,232 litres
Step 4: Size the pre-treatment
Everything upstream is sized on feed flow (1,154 LPH), not plant output:
- Feed pump — 1,154 LPH at the required pressure
- Multigrade or sand filter — if turbidity is present
- Softener — if hardness is high, sized on feed flow. See our softener sizing guide
- Carbon filter — if chlorine is present
- Cartridge filter 5 micron — sized for 1,154 LPH. See our micron guide
- Dosing systems — antiscalant and SMBS, dosed on feed flow
This is the most common sizing error we see — pre-treatment sized on plant output rather than feed flow, so it chokes as soon as the plant runs.
Step 5: Membranes
For 750 LPH at 1,200 TDS you would typically use 2 to 3 × 4040 elements. See our membrane sizing guide.
Step 6: Storage
Storage should cover the gap between production and consumption. As a guide:
- Raw water tank: enough for the daily feed requirement, so the plant is not dependent on borewell timing
- Treated water tank: at least half a day's requirement, so consumption can continue during CIP or maintenance
Instruments to include
These are not optional extras — without them you cannot tell what the plant is doing:
- Rotameters on permeate and reject — see our placement guide
- Pressure gauges before and after the cartridge and membrane
- TDS meters on feed and permeate
- ROMATIC controller for automatic operation and TDS diversion
What to give us for a quotation
- Daily requirement in litres
- Available running hours
- Feed water TDS and hardness
- Feed water source and turbidity
- Chlorine present or not
- Available raw water supply rate
- End use — drinking, boiler, process or packaged
Our plant calculators will work through the sizing for you.
Get your RO plant sized
Send your daily requirement, feed water analysis and running hours, and we will size the complete plant.
💬 WhatsApp par poochhein 📞 9910646957Frequently asked questions
How do I calculate RO plant capacity?
Divide your daily requirement by the running hours. For five thousand litres a day over eight hours you need about six hundred and twenty five LPH, and you would specify seven hundred and fifty LPH to allow margin.
What recovery should an RO plant run at?
It depends on feed TDS. Seventy to seventy five percent below five hundred TDS, sixty to sixty five percent between one and two thousand, and fifty percent or less above four thousand. Pushing higher scales the membranes.
How much raw water does an RO plant need?
Divide plant output by recovery. At sixty five percent recovery, producing seven hundred and fifty litres per hour needs about eleven hundred and fifty litres of feed. Check your borewell can actually supply that.
Should pre-treatment be sized on plant output or feed flow?
Always on feed flow, which is higher than plant output. Sizing the cartridge filter, softener and pumps on plant output is the most common design error and causes the pre-treatment to choke immediately.
Can I size the plant for twenty four hour running?
No, because the plant needs time for CIP cleaning, cartridge changes and breakdowns. Size on sixteen to eighteen hours maximum so there is margin for maintenance and unexpected stops.