Understanding filtration › 6. Operating and purchasing
CHAPTER 6.3
Cost per tonne, not the price of the cloth
Comparing two cloths on their unit price is like comparing two cars on the price of a full tank. What counts is the cost of filtering each tonne of product, including cloths, shutdowns and filter cake.
Nicolas Ract · 8 min read · October 2026
In short
- The price of the cloths is often a small part of the total cost of filtration.
- Cake dry solids content weighs heavily when the cake is transported, dried or disposed of by the tonne.
- Calculate over a year and per dry tonne, using the site's own figures.
What to include in the calculation
Over one year, add up:
- the cloths: unit price × number of cloths × number of replacements in the year;
- the changeovers: labour, and production lost during the stoppage;
- the washes: time, water, chemicals, effluent treatment;
- the filter cake: transport, drying or disposal, paid on the wet weight;
- energy: pumping, membrane squeezing, any drying;
- quality: product downgraded or reprocessed if the filtrate comes out cloudy, where applicable.
Then divide by the tonnes of dry solids filtered in the year.
Why dry solids content changes everything
The cake removed contains the dry solids and the water left in it. Its wet mass equals the dry mass divided by the dry solids content (the fraction of dry solids):
At 1,000 dry tonnes per year, a cake at 60 % dry solids weighs about 1,667 tonnes; at 65 %, about 1,538 tonnes. Those 129 tonnes of difference are water that you transport and pay for.
A worked example
Fictitious example. The figures below are round numbers invented to illustrate the method; they come from no site and no supplier. Redo the calculation with your own values.
A 100-plate filter processes 1,000 dry tonnes per year. The cake is disposed of at €50 per wet tonne. A complete pack change costs €2,000 (labour and lost production). Two cloths are compared:
- Cloth A: €100 each, lasts 6 months, cake at 60 % dry solids.
- Cloth B: €150 each, lasts 9 months, cake at 65 % dry solids.
| Item, per year | Cloth A | Cloth B |
|---|---|---|
| Cloths | 100 × €100 × 2 = €20,000 | 100 × €150 × 12/9 ≈ €20,000 |
| Changeovers | 2 × €2,000 = €4,000 | 12/9 × €2,000 ≈ €2,667 |
| Cake disposal | 1,667 t × €50 ≈ €83,333 | 1,538 t × €50 ≈ €76,923 |
| Total | ≈ €107,333 | ≈ €99,590 |
| Cost per dry tonne | ≈ €107/t | ≈ €100/t |
Cloth B costs 50 % more per unit, but works out about €7,700 cheaper per year. In this example, most of the difference does not come from the cloths: it comes from the filter cake.
The calculation can also work the other way: if cloth B did not improve dry solids content, the advantage would shrink to the changeovers avoided. Hence the importance of measuring dry solids content and service life on site, through a comparative trial, before drawing conclusions.
The pitfalls
- Comparing on claimed service lives rather than measured ones: only the site's own history counts (see chapter 4.5).
- Forgetting the washes: a cloth that blinds quickly costs in stoppages and chemicals (see chapter 4.4).
- Changing several things at once during a trial: you no longer know what produced the result.
- Looking only at the cloth budget: it often belongs to a different department from the one that pays for cake disposal or bears the stoppages.
The worksheet to fill in
Dry tonnes per year · number of cloths · unit price · measured service life · cost of a changeover · measured dry solids content · disposal cost per wet tonne · cost of washes · energy. With these nine values, the cost per tonne can be calculated in a few minutes.
Sources
This chapter presents a calculation method; the worked example is fictitious and is not based on any site or supplier data.
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