Sewage Treatment.

A septic tank separates solids and allows partially treated effluent to discharge to a drainage field for further treatment in the soil. A treatment plant uses an aerated process to treat the effluent to a much higher standard before discharge.

The practical consequence is that treatment plant effluent can generally discharge to a watercourse, while septic tank effluent cannot.

No. Under the General Binding Rules in England, septic tanks discharging directly to a surface water body had to be replaced or upgraded — typically by fitting a treatment plant or connecting to a drainage field.

Scotland, Wales and Northern Ireland have their own regimes with broadly similar intent. If you are buying a property with an existing septic tank, this is worth establishing early.

Sizing is by population equivalent, not bedrooms — although bedrooms are used as a proxy. A common approach counts the first two bedrooms as two people each and one person per additional bedroom, with a minimum of five.

Undersizing causes poor treatment and odour; significant oversizing can also treat poorly because the biological process needs consistent loading.

Small domestic discharges are often covered by General Binding Rules with no permit required, provided the discharge meets the conditions on volume, location and system type. Larger discharges, or those in sensitive areas such as groundwater protection zones, need a permit from the environmental regulator.

Check the position for your specific site before installing — retrospective permitting is considerably more difficult.

Annual servicing plus periodic desludging, typically every one to two years depending on loading and model. The blower or aerator is the main mechanical component and has a finite service life.

Neglecting desludging is the usual cause of poor performance and is often what fails a discharge sample if one is taken.

Not quite. Minimum distances from buildings, boundaries, watercourses and abstraction points apply, and the installation needs vehicular access for the desludging tanker.

Ground conditions matter too — high water table sites need anchoring to prevent the tank floating when empty, and shallow rock can make excavation prohibitive.