Set a common comparison boundary
A cartridge filter, coagulation stage, UF and equalisation tank do not provide the same service. Compare complete trains from intake to the quality needed at point of use, including residual streams. Fix average and peak flow, operating hours, feed variability and required availability. Without this boundary, a skid quote can look cheap because it leaves infrastructure or waste management to the buyer.
The US EPA's RO/NF cost model separates equipment, pretreatment, cleaning, membrane replacement, operation and indirect costs. Its value here is the breakdown of cost items; its drinking-water prices and assumptions should not be copied directly to an industrial effluent.
| Cost item | Question for a quotation |
|---|---|
| Installed capital | Are civils, tanks, pumps, instruments, controls and commissioning included? |
| Operation | What energy, chemicals, cleaning water and operator hours are required? |
| Reliability | How do cleaning, replacement and production change during fouling events? |
| Residual outlets | What does sludge, blowdown, cartridges and concentrate management cost? |
Build the cost of usable water
For each scenario, add annualised investment to operating cost and divide by the cubic metres meeting specification and reaching use. Apply the same horizon, discount rate, price schedule and availability assumptions. Lower membrane consumption may justify more pretreatment, but only with credible fouling and replacement evidence.
Run sensitivity on variables likely to reverse the decision: peak solids and fats, cleaning interval, membrane life, energy price, chemicals, waste disposal and annual demand. Show a range and the threshold where the preferred option changes; one figure hiding many assumptions implies false precision.
- Separate measured costs, binding quotes and assumptions.
- Specify who bears downtime and contingency risk.
- Update the model with pilot and first-month operating data.
What the pilot should prove
A trial should compare representative feed and permeate, pressure and flux stability, cleaning response and residual production. If an option removes solids but passes the constituent defining final quality, its lower cost does not solve the project. The decision matrix may include operational risk, footprint and flexibility alongside money.
Common questions
Is comparing skid prices enough?
No. Installation, omitted pretreatment, cleaning, replacement, downtime and residual-stream destination can outweigh the initial equipment-price difference.