Governing Formulas
The hydraulic power of a pump moving a flow against a pressure is the product of flow, pressure, and a
dimensional constant. Because 1 bar × 1 m³/h equals 1/36 kW exactly, the
hydraulic demand in kilowatts is Q_f × P ÷ 36. The specific energy consumption (SEC) is that
power referred to a cubic metre of permeate — not feed — so the pump demand is divided by the
permeate flow produced at the entered recovery.
SEC = Q_f × P / (36 × η_p × Q_p) Where:
-
Q_f= Feed flow to the RO train [m³/h] -
Re= System recovery (permeate ÷ feed) [%] -
Q_p= Permeate flow (Q_f × Re) [m³/h] -
P= Average applied feed pressure (membrane feed end) [bar] -
η_p= Feed pump efficiency [%] -
P_h= Hydraulic power demand (Q_f × P ÷ 36) [kW] -
SEC= Specific energy consumption per m³ permeate [kWh/m³]
Derived Equations:
P_h = Q_f × P / 36 SEC = P_h / (η_p × Q_p) P_motor = Q_f × P / (36 × η_p) P_erd = Q_b × P × η_erd
Energy recovery devices capture the excess pressure in the brine stream and transfer it back to the feed.
The recovered power Q_b × P × η_erd offsets the pump motor demand, so SEC with an ERD is
computed on the reduced motor power.
How to Use This Calculator
- Enter the feed flow to the train (m³/h) — this is converted internally to the per-feed-m³ SEC convention used across the RO toolkit.
- Enter system recovery so permeate flow can be derived (SEC is per m³ of product water).
- Enter the average feed pressure at the membrane feed end. For sea water SWRO a 40–60 bar band is typical; brackish BWRO commonly runs 8–20 bar.
- Enter the feed pump efficiency — 70–85% is a realistic high-flow range.
- Optionally enter an ERD efficiency (isobaric chambers: 93–97%) to see the SEC with energy recovery.
- Optionally enter operating hours per year to project annual energy consumption.
Interpreting Results
SEC is the headline operating metric. Modern SWRO with ERD commonly lands near or below 3 kWh/m³; BWRO is typically 0.5–1.5 kWh/m³. The motor-power card shows the shaft demand into the pump, which feeds generator or grid sizing.
Accuracy Limits & Design Notes
- SEC is based on a single average feed pressure; real systems vary pressure across the vessel train and over membrane life (fouling raises demand).
- Intake, pre-treatment, discharge, product-boosting, and balance-of-plant loads are not included.
- ERD offset assumes brine energy recovery at the entered efficiency; verify with the ERD vendor's projection.
- Run inputs are per m³/h and per bar. If your data is m³/day, divide by 24 before entering.
Project the Full RO Cascade in the Workbench
This calculator sizes the energy demand of a single train. For a complete project — feed concentration, recovery, rejection, permeate/brine quality and mass balance — continue into the RO Engineering Workbench. Note the workbench accepts flows in m³/day.
Frequently Asked Questions
What is SEC kWh/m³ on desalination datasheets?
SEC is the electrical energy used to produce one cubic metre of permeate. It is the standard efficiency metric for RO plants; seawater plants typically report 2.5–4 kWh/m³ depending on pressure, recovery, pump efficiency, and energy recovery.
Why does feed pressure dominate energy cost?
Hydraulic power scales linearly with pressure (Q × P), and pressure is set by osmotic pressure, temperature, feed salinity, and membrane conditions. Every 10 bar of feed pressure adds roughly 0.28 kWh/m³ of permeate at 50% recovery and 80% pump efficiency.
How much energy does an ERD save in SWRO?
A well-run isobaric ERD typically saves 40–60% of the feed pump energy compared with throttling brine to atmosphere, bringing seawater SEC from the 4–5 kWh/m³ throttled range down to about 2.5–3.5 kWh/m³.
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.