Powerful steel teeth inside a waste disposal unit grind up food waste so that it can be mixed with cold water into a 'slurry' and washed down the drain. Despite these teeth, waste disposal units can only deal with soft waste such as vegetable peelings. They will not grind up hard items such as bones.

There are two types of unit: continuous feed units, which can have waste and water added while in operation, and batch feed units, which have to be loaded and covered before they can be switched on.

The unit. is powered by an electric motor and installed permanently just below the sink plughole. The sink outlet needs to be 90mm (3 1/2in): if your sink does not have this size outlet then you'll need a new sink or, if it's stainless steel or plastic, you can hire a special cutting machine to enlarge the outlet.

The unit must be permanently wired to the electricity supply via a fused connection unit fitted with a 13-amp fuse mounted above the kitchen work surfaces, where it cannot be accidentally switched on. If you are in doubt about undertaking electrical work, hire a qualified electrician.

Step by Step Instructions

1 Insert waste outlet and seal

Most units have a seal that fits over the tail of the plughole outlet. This usually comprises the outlet 'cover', a gasket and a back-up ring. Slot the outlet and seal onto the waste outlet.

2 Retaining collar

Under the sink, the waste outlet and seal are clamped, screwed or clipped by a collar to the waste disposal unit. A snap ring is attached that seals the unit and waste securely.

3 Attach the unit housing

Follow the manufacturer's instructions and bolt, clip or screw the unit housing the waste disposal to the collar under the sink.

4 Waste outlet

The waste outlet from the unit fits to a standard sink trap - but not a bottle trap. Make sure that if the wastepipe runs to a yard gully, it runs through the grille and just above the water in the trap. Wire the waste disposal unit to a fused connection unit.