Antiscalant dosing calculation — step by step
Short answer: The usual RO antiscalant dose is 3 to 5 ppm on feed water. For 1,000 LPH of feed that works out to roughly 4 grams per hour of neat antiscalant. It is never dosed neat — you make up a 5 or 10 percent solution first, then meter that in with the pump.
Below is the complete calculation, how to make up the solution, and how to set the pump.
How much dose — by feed TDS
| Feed TDS | Hardness | Dose (ppm) |
|---|---|---|
| Below 500 | Low | 2 – 3 ppm |
| 500 – 1,500 | Moderate | 3 – 4 ppm |
| 1,500 – 3,000 | High | 4 – 5 ppm |
| Above 3,000 | Very high | 5 – 6 ppm (confirm by lab test) |
Step 1: Neat chemical required
Example: feed 770 LPH, dose 4 ppm → (770 × 4) ÷ 1,000 = 3.08 grams per hour
Over 24 hours: 3.08 × 24 ≈ 74 grams a day. So a 25 kg drum lasts around 11 months.
Step 2: Make up the solution
Dosing 3 grams an hour directly is not practical — no pump can meter such a small quantity accurately. That is why a dilute solution is prepared.
| Solution | In a 10 L tank | Duration at 3 g/hr |
|---|---|---|
| 5% | 500 g antiscalant plus water | ~7 days |
| 10% | 1 kg antiscalant plus water | ~14 days |
| 20% | 2 kg antiscalant plus water | ~28 days |
Which to choose: 10 percent is the most common. Too dilute (2 to 3 percent) and you are refilling the tank daily; too concentrated (30 percent and above) and the pump runs at a very low stroke where accuracy falls away.
Step 3: Work out the pump setting
Example: 3.08 g/hr with a 10 percent solution → (3.08 ÷ 10) × 100 = 30.8 ml per hour
Now compare with pump capacity. An Edose NEO 6 LPH delivers 6,000 ml per hour at full stroke, and you need 31 ml:
Stroke = (31 ÷ 6,000) × 100 = 0.5 percent
A better approach — working backwards
Start from the pump instead. A pump should run at 30 to 70 percent stroke, which for a 6 LPH pump means 1,800 to 4,200 ml per hour.
If you need 3.08 g/hr and the pump delivers 2,000 ml/hr, the solution strength works out to (3.08 ÷ 2,000) × 100 = 0.15 percent — only 15 grams in a 10 litre tank.
The practical compromise: making up such a dilute solution means refilling daily. So in real plants people use a 5 or 10 percent solution and accept a low stroke setting, or fit a timer so the pump runs in short bursts.
Worked example — 1,000 LPH RO plant
| Parameter | Value |
|---|---|
| Plant output | 1,000 LPH |
| Recovery | 65% |
| Feed flow | 1,538 LPH |
| Dose | 4 ppm |
| Neat chemical | 6.15 g/hr (~148 g/day) |
| Solution | 5% (750 g in 15 L) |
| Solution flow | 123 ml/hr |
| Pump | Edose NEO 6 LPH at ~2% stroke |
| Tank | 15 litre LLDPE, lasts ~5 days |
| Drum consumption | 25 kg lasts about 5.5 months |
Where to inject
Antiscalant should always go in after the cartridge filter and before the HP pump. The reasons:
- Injected before the cartridge, the chemical is simply trapped by the filter
- Injected after the HP pump, you need an expensive high pressure dosing pump
- Injected in between, it mixes properly and reaches the membrane
Four common mistakes
- Calculating on permeate — the dose always applies to feed
- Making up solution and forgetting it — antiscalant solution should not be kept beyond 15 to 20 days, as bacteria can grow in it
- Assuming the pump is running — check the tank level weekly. If the level is not dropping, the pump is not dosing
- Thinking more dose means more protection — excess antiscalant can itself foul the membrane, and it wastes money
For chemicals see our RO antiscalant range, and for the ZeroScale line see the RO chemicals page.
Get your dosing calculated
Send your plant capacity, recovery and feed TDS — we will work out the exact dose, solution strength and pump setting.
💬 WhatsApp par poochhein 📞 9910646957Frequently asked questions
What is the correct RO antiscalant dose?
Usually three to five ppm on feed water. For low TDS water two to three ppm is adequate, while above three thousand TDS you may need five to six ppm. A water analysis is the reliable way to fix the dose.
How concentrated should the antiscalant solution be?
Five to ten percent is the most common. A very dilute solution means refilling the tank daily, while a very concentrated one forces the pump to run at a very low stroke where accuracy is poor. Five percent suits most plants.
Where should antiscalant be injected?
After the cartridge filter and before the high pressure pump. Injected before the cartridge it is simply trapped by the filter, and injected after the high pressure pump you would need a much more expensive high pressure dosing pump.
How long can a made-up solution be used?
Not more than fifteen to twenty days. After that bacteria can grow in the solution, which is itself harmful to the membrane. Choose a tank size so that the solution is consumed within two to three weeks.
What happens if I dose too much antiscalant?
There is no additional benefit, and the excess can itself deposit on the membrane and act as a foulant. It also wastes chemical. Staying at the correct dose is the right approach.