Operating cost per cubic metre: the items that actually move the number

A useful OPEX benchmark does not begin with the invoice total. It begins with a system boundary. Decide first what “treated” means: raw influent to final effluent, equalised flow to discharge, or a specific treatment step such as tertiary filtration, dewatering, or flotation. Then assign every recurring cost to the same denominator, usually cubic metres treated over a month or a year.

The standard method is to separate costs into five groups. Energy includes pumps, mixers, blowers, drives, and control systems. Chemicals cover coagulants, flocculants, pH correction, cleaning agents, and antifoam where relevant. Maintenance includes wear parts, service contracts, lubrication, planned shutdown work, and corrective repairs. Labour covers operators, supervision, sampling, and routine cleaning. Sludge disposal includes transport, landfill or thermal treatment fees, water content penalties, and container handling.

To build the benchmark from first principles, use the following formula:

OPEX per m³ = (energy + chemicals + maintenance + labour + sludge disposal + consumables) / m³ treated

The next step is to split cost drivers into controllable and fixed. Controllable items respond to operating choices: polymer dose, chemical setpoint, wash-water use, runtime strategy, solids loading, cake dryness, and preventive maintenance discipline. Fixed or semi-fixed items include depreciation, minimum staffing, contract availability fees, and some inspection costs. The benchmark becomes meaningful when management can see which part can be improved within one budget cycle and which part requires a capital project or contract renegotiation.

For tertiary filtration, a realistic OPEX range is often about 0.03 to 0.20 EUR per m³ treated, depending on solids load, cloth or media type, backwash demand, and energy needs. Cloth-based tertiary filtration tends to sit in the lower to middle part of that range when the influent is stable and solids capture is efficient. The main cost drivers are pressure drop, solids loading, cleaning frequency, and the cost of downtime caused by fouling.

For dewatering, the benchmark is usually much more sensitive to sludge characteristics. A sensible planning range is about 2 to 12 EUR per tonne of dry solids processed, or roughly 0.10 to 0.80 EUR per m³ of feed sludge depending on solids content. Here, the decisive variable is not only the press itself, but the dryness achieved at discharge. Every additional percentage point of cake solids can reduce disposal cost materially. A screw-press often lowers labour intensity and power demand compared with more complex mechanical systems, but its advantage disappears if polymer selection or feed consistency is poor.

For flotation steps, especially when used for oil, grease, or light suspended solids, an indicative operating range is often 0.08 to 0.40 EUR per m³ treated. Energy for recirculation, air saturation, chemical conditioning, and skimming all matter, but the biggest swing factor is usually chemical efficiency. A polymer-preparation system that produces consistent dilution and mature solution can reduce overdosing and improve floc structure, which lowers both operating spend and downstream sludge volume.