Draw the comparison boundary
State whether abstraction, pretreatment, recirculation, compressed air, oxygen production, CIP, controls and distribution are included. If a shared auxiliary is outside the skid meter, allocate its consumption using measurements or an explicit assumption. Its location in another room is not a reason to remove it from the assessment.
Distinguish electricity from heat and purchased gases. Oxygen has a cost even when the electricity needed to produce it is consumed off site. Electrical kWh and total cost per cubic metre can be compared in separate columns; combining them into one unexplained number makes the decision difficult to interpret.
Match accumulated energy to net output
For the assessment period, SEC = E/useful V, where E is kWh and V is cubic metres of accepted water. Deduct water used for washing or diverted for quality reasons when it does not reach the defined use. Under variable operation, sum energy and volume rather than averaging hourly ratios without weighting.
Illustrative example: 600 kWh and 300 m³ of gross permeate give 2.00 kWh/m³ gross. If only 270 m³ is delivered as useful water, the same energy equals 2.22 kWh/m³ net. Including a further 90 kWh of external auxiliaries changes the complete comparison to 2.56 kWh/m³ net.
Distinguish rated power from actual energy
A 15 kW pump does not necessarily consume 15 kWh for every calendar hour. Record absorbed power, operating time and load states, or use suitable electrical meters. Efficiency depends on the duty point and the combined motor, drive, pump and piping system rather than the motor rating alone.
Include production, standby, rinsing and cleaning over a representative window. A ten-minute clean-water test does not describe a month with fouling and outages. Report recovery, temperature, feed quality, pressure and production so that another engineer can explain why the consumption figure has changed.
Compare equivalent service and sensitivity
Require the same outlet quality and net production. An option may use less electricity per cubic metre of feed while producing less reusable water. For concentration duties, include the effect on downstream volume and treatment cost; an apparent saving in one stage may create additional consumption in another.
Present normal, minimum and adverse-load scenarios together with measurement uncertainty. Identify measured values and projections. A useful purchasing figure is a traceable estimate for the agreed system, supported by a verification method, rather than an isolated promise of low energy use.
Common questions
Can feed-based and permeate-based kWh/m³ be compared?
Only after converting both to a common basis and showing recovery. They use different denominators and can reverse the apparent result of the comparison.
Does a higher kLa automatically mean lower kWh/m³?
No. kLa characterises transfer under specified conditions. Energy, gas, treated volume and process outcome must be measured within the same boundary.