Media enquiries arrive in two shapes. One says "we need 200 m3 of tube settler" and asks for a price. The other says "our clarifier is 12 by 6 metres, flow is 2,400 m3/day, what do we need?" Only the second one can be answered without guessing, because a quantity is an output of a calculation, never an input.
This article sets out how the quantity is actually derived for the five families we supply most, with the arithmetic shown. Every published figure used below comes from the product pages on this site, so the numbers can be checked rather than taken on trust.
1. Tube settler and lamella modules
Settling media is counted in cubic metres, but the calculation starts from an area. The chain has three steps.
- Area. Settling area = design flow divided by hydraulic loading. Our published loading for this product family is 3-5 t/m2·h, so the number you pick inside that band drives the area directly.
- Volume. Volume = settling area multiplied by the layer height. Published stacking height is 4-5 m.
- Allowance. Add cutting waste for troughs, walls and scraper openings, then round up to whole modules of 1000 × 500 × 1000 mm.
Worked example. Design flow 2,400 m3/day is 100 m3/h. At a hydraulic loading of 4 t/m2·h the settling area is 100 / 4 = 25 m2. At a 4.5 m stack the net volume is 25 × 4.5 = 112.5 m3, which is 225 modules before any cutting allowance.
The tube settler calculator runs these three steps and shows the intermediate values.
2. MBBR carriers
Carriers are counted as a share of reactor working volume, and the share is derived from the load they have to remove, not from the reactor size.
- Load. Mass of pollutant removed per day = flow × (inlet - outlet) concentration.
- Carrier volume. Volume = mass load divided by the removal rate published for the carrier series, in g/m3·day.
- Check the fill fraction. Carrier volume divided by reactor working volume must sit inside the published 15-67 % band. If it does not, the reactor volume or the series is wrong.
Worked example. Flow 1,000 m3/day, BOD 300 mg/L falling to 30 mg/L. That is 270 kg/day, or 270,000 g/day. At 4,000 g/m3·day the carrier volume is 67.5 m3. In a 300 m3 reactor that is a 22.5 % fill - inside the band, so the design closes.
Run the same calculation for ammonia as well, and take the larger of the two answers. A reactor sized on BOD alone is the reason plants pass carbon tests and fail ammonia tests.
3. Random and structured tower packing
Tower packing is bought as bed volume, and bed volume is a geometry decision rather than a loading calculation: cross-sectional area from the gas or liquid rate, multiplied by bed height. What the buyer is really choosing is surface area per cubic metre, because that is what sets the transfer or contact capacity for a given bed.
Two consequences follow. First, when two suppliers quote the same bed volume at different surface areas, they are not quoting the same product - compare m2/m3 before comparing price. Second, liquid distribution above the bed matters as much as the packing: a bed starved on one side has spare capacity that never gets used. The tower packing guide works through the alternatives.
4. Filter nozzles
Nozzles are a count, not a volume, and the count comes from the plate area divided by the area each nozzle serves.
- Choose a pitch. The grid spacing is a design decision driven by backwash rate and allowable head loss through the false floor.
- Area per nozzle. For a triangular layout that is about 0.866 × pitch squared. A 150 mm pitch gives roughly 51 nozzles per m2.
- Total. Plate area × nozzles per m2, then round up to whole rows so the grid stays regular at the edges.
Worked example. A 20 m2 plate on a 150 mm triangular pitch needs about 1,030 nozzles. Edge trimming usually adds a few, so order by rows rather than by the bare number. The nozzle count calculator does this per pitch.
5. Fibrous and ball-type filter media
These are bought by bed volume, but the honest conversion to mass needs the bulk density from the supplier's own datasheet - bulk density varies with how the material is packed, and a figure borrowed from a different product will be wrong. The method is simple: bed volume = filter area × bed depth, then mass = bed volume × published bulk density. Ask for the packing density in writing, and state whether it refers to loose fill or to a settled bed after backwashing.
What to send, per family
| Family | Inputs that decide the quantity | Unit you order in |
|---|---|---|
| Settling modules | Design flow, hydraulic loading, basin plan, stacking height, layer arrangement | m3 and module count |
| MBBR carriers | Flow, inlet and outlet concentrations for every limited parameter, reactor working volume | m3 of carrier and % fill |
| Tower packing | Gas and liquid rates, tower diameter, bed height, required surface area per m3 | m3 of bed |
| Filter nozzles | Plate area, pitch, backwash rate, allowable head loss | pieces, by rows |
| Fibrous media | Filter area, bed depth, published bulk density and packing state | m3, or kg when density is fixed |
One habit removes most of the risk: write the unit next to every number in the enquiry, and write the assumption the number depends on. "200 m3" is a quantity somebody guessed. "25 m2 of settling area at a 4.5 m stack, cut for the layout attached" is a specification that can be priced, checked and built.
Questions buyers ask
Why is settling media counted in cubic metres and not square metres?
Because you buy the stack, not the floor. The settling area that the design needs sets the footprint, and the layer height sets how much material fills that footprint. Two plants with the same footprint can need 30 % different material volumes simply because one has deeper basins. Quoting a clarifier retrofit on floor area alone is the single most common arithmetic error we see.
Should I add a cutting allowance on tube settler modules?
Yes. Modules are cut to fit around troughs, walls, scraper bridges and outlet launders, and offcuts are not recoverable. Practical projects carry a small allowance on top of the net calculated volume, and how much depends on how irregular the basin is. Tell us the layout and we will state the allowance we are applying rather than leaving it hidden in the price.
Is the MBBR fill fraction a percentage of basin volume or of water volume?
Of the working volume of the reactor - the water volume at operating level. Fill fraction is the bulk volume of carriers as a share of that. It is normally kept inside the range published for the carrier type, because below it the biomass inventory is too small for the load and above it the mixing energy available is no longer enough to keep every carrier moving.
How do I count filter nozzles for a false floor?
Divide the plate area by the area served by one nozzle, which follows from the pitch of the grid. For a triangular layout the area per nozzle is about 0.866 times the pitch squared, so a 150 mm pitch gives roughly 51 nozzles per square metre. The pitch itself is a design decision driven by backwash rate and allowable head loss, not a catalogue number.
Can you size all of this for me?
Yes, and it costs nothing to ask. Send flow, basin dimensions, water analysis and the discharge limits you must meet. We return the calculated quantities, the assumptions we used and the material options, so you can check the arithmetic instead of trusting it.