Governing Formula & Cost Model
RO operating cost is dominated by pumping energy, expressed here as a specific energy consumption (kWh per m³ of permeate). Costs for chemicals, membrane replacement, and maintenance are supplied by the operator so the result reflects their real prices — no market cost data is embedded.
SEC = (Qf × Pf) / (ηp × Qp) Where:
-
Qf= Feed flow to the high-pressure pump [m³/h] -
Y= System recovery [%] -
Pf= Feed pressure at the pump outlet [bar] -
ηp= High-pressure pump (motor + pump) efficiency [%] -
ηERD= Energy recovery device efficiency (optional) [%] -
SEC= Specific energy consumption per m³ of permeate [kWh/m³] -
T= Energy tariff (user-supplied) [per kWh]
Derived Equations:
Phyd = Qf × ΔP SEC = Phyd / (ηp × Qp) SEC_ERD = SEC × (1 - ηERD × (1 - Y)) OPEX = SEC × T + chemical + membrane + maintenance An energy recovery device (pressure exchanger, turbocharger, or Pelton wheel) recovers energy from the concentrate flow. The credit is proportional to (1 − Y), so trains running at higher recovery capture less from an ERD.
How to Use This Calculator
- Enter feed flow, recovery, and feed pressure (use the RO feed pressure calculator for the pressure).
- Enter pump efficiency and, if fitted, the ERD efficiency.
- Enter your energy tariff and any chemical, membrane replacement, and maintenance unit costs (optional).
- Read the OPEX per m³ of permeate and the rolling daily operating cost.
Interpreting Results
Typical SEC figures are about 2–3 kWh/m³ for brackish RO and 3.5–4.5 kWh/m³ for seawater without ERD, falling below 2.5 kWh/m³ with a pressure exchanger. Multiply by your tariff for the energy line item.
Accuracy Limits & Design Notes
- Capital cost, labour, plant utilization, and financing are outside this OPEX model.
- SEC is based on feed and permeate flows only — intake, pretreatment, and product transfer pumping are not included.
- Unit costs are user inputs; benchmark them against your actual procurement and membrane change-out programs.
Frequently Asked Questions
What is a realistic RO operating cost?
For brackish water, operating costs of 0.10–0.30 ¥/m³ are typical; seawater with ERD runs about 0.30–0.60 ¥/m³ depending on tariff. Energy is usually 50–70% of OPEX.
How much membrane replacement should I budget?
Membrane lifetimes of 5–7 years are typical. As a rule of thumb, replacement costs work out to roughly 0.02–0.06 per m³ of permeate, scaled by the number and size of elements.
Why does recovery affect SEC?
Higher recovery means more permeate produced from the same feed and pump power, lowering kWh per m³ — but it also raises brine concentration and required feed pressure, so the optimum is a balance.
Engineering Disclaimer
Engineering Note: This calculator provides simplified engineering estimates for preliminary analysis and educational use. Actual RO system performance depends on membrane type, feedwater chemistry, temperature, pressure, recovery, concentration polarization, pretreatment, and operating conditions. Final system design should be verified using manufacturer data, validated design software, applicable standards, and qualified engineering review.
Technical References
- DuPont FilmTec™ Reverse Osmosis and Nanofiltration Technical Manual (Form No. 45-D01504-en).
- AWWA Manual M46: Reverse Osmosis and Nanofiltration, American Water Works Association.