Passive and automatic units solve the same problem in different ways, and the choice is about labour and volume rather than about performance. A passive trap holds and separates, and somebody empties it. An automatic unit separates and then skims the grease into a container continuously, so the kitchen handles less and pays for the mechanism.
The passive unit
No moving parts, no power, and a low purchase price. It relies entirely on being emptied often enough, which makes it a good fit for smaller kitchens with a reliable service schedule and a poor fit for high volume sites.
The automatic unit
Heats or skims the separated grease into a collection container as it accumulates, so the trap itself does not fill in the same way. It costs more to buy and needs power and servicing, and it suits high volume kitchens where a passive unit would need emptying constantly.
What the choice really turns on
Volume of grease, the labour available, access for emptying and the cost of a tanker visit at your site. Where a passive unit would need weekly attendance, an automatic unit is usually cheaper across a few years.
Either way, the waste still leaves
Both produce waste grease that has to be collected by a registered carrier under a transfer note. Automatic does not mean self disposing, and a container of skimmed grease is still a duty of care obligation.
Questions people ask about passive grease trap
Is an automatic grease trap better?
It is better suited to high volume kitchens. For a small site with a good service schedule, a passive unit does the same job for less money.
Do automatic units need power?
Most do, and that has to be part of the installation plan. Check the running cost as well as the purchase price.
Which do water companies prefer?
They are concerned with what reaches the sewer rather than with the mechanism. Either is acceptable where it is correctly sized and actually maintained.