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Sink waste disposal units

waste disposal unitWaste disposal units are normally fitted directly to the bottom of the kitchen sink and grinds the soft waste (such as vegetable peelings, tea bags, fruit stones, fruit skins, nutshells, corncobs, crust etc.) to a pulp before it is flushed down the waste pipe by flowing water to the drain. The pulped waste can then flow down the drain without blocking it.

Fitting to sink

Most models of waste disposal units are designed to fit to a 89mm (3½ inch) sink outlet which is larger than a standard 38mm (1½ inch) outlet. Some sinks come with the larger outlet; double sinks often have a larger outlet in one bowl and a standard outlet in the other - this suits most installations as one bowl will be a 'normal' sink with the other one having a waste disposal unit.

Some waste disposal units are designed to fit a smaller 38mm (1½ inch) outlet, there are not so many on the market as for the larger outlet and tend not to work so well. It may be possible to enlarge a smaller outlet hole in an aluminium sinks to 89mm - a local hire shop may be able to provide a suitable cutter.

Most disposal units do not cater for connecting the overflow outlet from the sink, if required the hose from the overflow on the sink should be routed into the waste trap after the disposal unit.

Waste disposal units vary in height from model to model; sometimes the gap between the underside of the sink and the shelf inside the kitchen unit will be too small for the waste unit to fit without either removing the shelf completely or cutting the shelf directly under the waste unit to allow room.

Waste connection

Most waste disposal units are designed to connect to38mm (1½") waste plumbing (i.e. waste trap and pipe work). Usually Bottle or Grease traps must NOT be used as these can easily become blocked by the pulp.

Depending on the model of waste disposal unit purchased, and the style of waste trap already fitted, it may be possible to connect a new unit to the existing waste pipe without major changes.

The waste pipes in the run from the waste unit to the drains should have a fall of at least 15 degrees (26cm per metre) to aid the flushing of the waste pulp along the pipe work

If the waste pipe work discharges into a gully, the outlet of the pipe must be below any grating covering the gully -where necessary, the grating should be cut away and the bottom of the waste pipe extended to below the grating.

Electrical connection

The electrical supply to the waste disposal unit must be made in accordance with Part P of the building regulations.

Waste disposal units normally require a220/240 volt AC supply (UK), the current rating will depend upon the model and the supply wiring must be protected by a fuse (or other device) in accordance with the manufacturer's installation instructions.

Operation of waste disposal units will depend upon the model but are usually controlled by a manual switch, a wireless remote control or automatically (i.e. every time water begins to flow through the unit). Wireless remote control switches are available separately and can be fitted with most types of non-automatic waste disposal units.

Fitting a waste disposal unit.

When fitting a new waste disposal unit to an existing sink:

  • Empty the cupboard under the sink to give space to work.
  • Disconnect and remove the existing waste trap- take care not to spill the water in the trap, dispose of it into something like a bowl.
  • Remove the existing plug outlet from the sink - the method will vary depending on the arrangement fitted, it could be by undoing a large nut under the sink or a screw through the middle of the outlet from above - see examples of the different sink outlets used.
  • Clean the inside and under the sink to remove all old signs of sealing compound.

Hold the waste disposal unit under the sink and check:

  • If there is enough vertical space under the bottom of the sink for the waste unit to fit; if the space is not enough, either remove the shelf completely or mark the shelf and cut away a section to suit.
  • Under the sink, that the gap around the outlet is sufficient for the inlet of the waste unit to fit without obstruction - the most common problem is that the sound deadening material under the sink is too near the outlet. If necessary, mark and cut away enough for the inlet flange to fit flat to the underside.

Different models of waste disposal units use different methods for attaching to the sink but generally they involve:

  • A special fitting to the sink outlet.
    and
  • The waste disposal unit being attached to the sink outlet fitting.

So, follow the manufacturer's instruction to:

  • Attach the sink outlet fitting using any appropriate gaskets supplied.
  • Attach the waste disposal unit to the sink outlet fitting, often the unit can be rotated to line up its outlet with the waste trap/pipe.

Once the waste unit has been fitted and its outlet aligned, the waste pipe can be modified or made up to a waste trap (a 'P' or an 'S' trap - NOT a bottle trap) and then the pipe run to the drains- ensure that there is a good fall along the waste pipe to help the flow of the pulped waste.

Connect a suitably fused electrical supply to the waste disposal unit together with a suitable switch above and away from the sink if required.