This article summarises the approach to be adopted for Waste Water Treatment. In the UK disposal of sewerage where no mains drainage is available is covered under the Pollution Prevention Guidelines (PPG) 4 and it is important that this is followed both for the effective implementation of a system and for the most hygienic installation. The following procedure summarises current working methods and best practice which should be adopted.

A septic tank is a two or three chamber system, which retains sewerage from a property for sufficient time to allow solids to form into sludge at the base of the tank. The remaining liquid drains from the tank by means of an outlet pipe. This effluent is normally disposed of by soakage into the ground via a soakaway, provided that the disposal does not generate a pollution risk to surface waters or groundwater resources.

For domestic properties the capacity of the septic tank should be calculated using the following formula:

Capacity of tank in litres = ( 180 * number of people in the house ) + 2000

For non-domestic properties the following per capita sewerage volumes (litres per day) should be used:

Boarding School 180
Campsites 75
Caravans 120
Day School 50
Domestic 180
Factories 65
Hospitals 450
Hotels 200
Offices 55
Public Houses 15
Rest Homes 300
Restaurants 25

The satisfactory biological performance of the sewerage system relies on routine desludging and maintenance. A regular desludging and servicing programme should be established to ensure that the system continues to work efficiently and comply with the conditions of the discharge consents.

Desludging should be carried out every 12 months by an operator registered for the carriage of sewage waste by the Environmental Regulator. A check on the nature of the effluent draining from the tank can indicate whether it needs emptying. The effluent will normally be light grey in colour but, as the sludge content of the tank increases, the effluent will increasingly contain dark solids.

Cesspools, septic tanks and package treatment plants should ideally be desludged when they are half full of sludge to maximise efficiency. Approximately 1/6 of the sludge should be left in the tank to maintain the community of bacteria that digest the sewage.

In an ideal world every time a tank or treatment plant is “serviced” effluent samples should be taken and then analysed to determine the efficiency of the system.

As previously mentioned septic tanks should only be used in conjunction with a form of secondary treatment such as a drainage field, drainage mound or constructed wetland.

In accordance with the guidance note PPG4 the septic tank and soakaway area should be sited:

• Not less than 10 metres from any ditch, drain or water course
• Not closer than 15 metres to any dwelling
• At least 50 metres from the point of abstraction of any ground water supply (any well or borehole)
• Not in any Zone 1 ground water protection zone
• Sufficient vehicular access to allow tank emptying

Under the provisions of the Water Resources Act 1991 in England and Wales, the Control of Pollution Act 1974 (as amended) in Scotland and the Water Act 1972 in Ireland, a consent from the relevant Agency is required for any discharge of sewerage effluent into “controlled waters”. A consent may also be required for any / all discharges into a soakaway.