Bad odours are the result of a poorly maintained or badly designed grease trap. A good quality bacterial treatment will reduce bad odours but there is no substitute for a well-designed grease trap.
The lid of the unit should be mechanically secured to create a 100% airtight and watertight seal.
It should only be opened a few times per year for professional emptying.
It should be properly vented to allow the controlled escape of gases and bad odours.
A maintenance plan or SLA (Service Level Agreement) is recommended