Frequency follows the grease storage volume against the rate the kitchen produces grease, not a fixed interval. A unit sized tightly for its flow will fill its grease layer faster than a generously sized one on the same duty.

The usual operating rule is that the separator is emptied before the accumulated grease layer reaches a set proportion of the liquid depth. Past that point the retention volume is no longer available and grease begins to carry over.

In practice that lands somewhere between weekly and monthly for most commercial kitchens. Skipping cycles does not simply delay the work: hardened grease has to be broken out rather than pumped, which costs more than the missed service.

It is a rating derived from the peak wastewater flow the separator must handle, and it is the number that units are selected and compared by rather than a physical dimension.

Flow is calculated from the appliances discharging into it, their usage pattern and the way the kitchen operates, then adjusted for the density of the grease and the temperature of the water, because hot water keeps grease liquid and mobile.

Two units with the same tank volume can carry different nominal sizes if their internal baffling separates at different rates, which is why the rating rather than the capacity is the specification.

Because separation depends on the grease being buoyant and distinct from the water. Hot water keeps fats dissolved and emulsified, so they pass through the unit still in suspension and cool in the pipe beyond it.

That is where they solidify, which is why a kitchen with a correctly sized separator can still block its drain if dishwasher discharge at high temperature dominates the flow.

The remedies are sizing for the actual discharge temperature, allowing cooling before the separator where the layout permits, and avoiding detergents that hold grease in emulsion, since a chemically stabilised emulsion will not separate at any residence time.

Space, access and the invert level of the drain. An under-sink or free-standing unit inside the kitchen is cheap to install and easy to inspect, but it occupies floor area in the most valuable room in the building and is emptied in the working space.

A buried unit outside takes the smell and the emptying operation away from the kitchen and can be far larger, but it needs excavation, a cover rated for the traffic above it and a tanker route.

Drain invert usually settles the argument. If the kitchen waste already falls to a level below the slab, a buried unit is the natural fit; if it does not, pumping to reach one is a cost that recurs.

No. Dosing introduces bacteria or enzymes that digest grease over time, and it works on the residue clinging to pipe walls rather than on the bulk grease arriving in the flow.

Used alongside a correctly sized separator it can reduce odour and slow the build-up in the run downstream. Used instead of one, it is asked to act on grease that has already left the building faster than it can digest it.

Some discharge consents explicitly require a physical separator, in which case dosing is a supplement and cannot be substituted regardless of how well it performs.

Skimming and, on larger units, emptying. A powered skimmer draws the floating layer off into a separate collection vessel on a timer, so the grease is removed continuously rather than accumulating until a service visit.

That keeps the retention volume available and largely removes the risk of a unit quietly running past its capacity between services, which is the common failure of a manual trap in a busy kitchen.

The cost is a machine in a hostile environment: pumps, heaters and controls that need power, maintenance and a route for the collected grease. The collection vessel still has to be emptied by somebody.

Frequently, though the constraint is rarely the unit. Falls have to be maintained through it, which needs headroom under the appliances or depth outside, and retrofitting into a slab that has none is where projects stall.

Under-sink units are the usual retrofit answer because they work within existing waste runs, at the cost of capacity and of being emptied in the kitchen.

Where several appliances discharge separately, combining them upstream of one unit is normally better than fitting several small ones, since one correctly sized separator on the combined peak flow outperforms a set of individually undersized traps.