British Water Sizing Criteria

Sewage Treatment Systems Code of Practice
Flows and loads (sizing criteria)

This code of practice was prepared by the British Water Package Sewage Treatment Plant Focus Group comprising manufacturers, suppliers and service companies of all types of small wastewater treatment systems.

The Environment Agency, the Northern Ireland Environment Agency and the Scottish Environment Protection Agency support the use of this code of practice, but the Agencies do not specifically endorse any particular manufacturer’s product.

This code of practice provides a table of loadings which allows the total daily sewage load from properties to be calculated and it is recommended that all designers should use this table when sizing and designing non-mains sewage treatment systems.

The flows and loads values given represent current best knowledge within the UK but may change with time in line with per capita water use.

Where proposed alternative usage rates or methods of sizing might be more appropriate for a particular application this should be supported by the collection of data or additional site specific evidence to validatethe proposal. Professional judgment is required and may be used to compare alternatives especially when assessing sewage strengths and treatability.

Guidance is provided to assist the user to identify the various sources of sewage, to consider the nature of the sewage to be treated and to make users aware of issues which may affect treatability and system performance. Each manufacturer is aware of the capabilities of their own systems with respect to different situations.

The table of loadings may be used to design all sizes of sewage treatment systems serving up to 1000 population.

Use of this code of practice by all UK manufacturers and system designers will:

  • Help to clearly define site activity and sewage sources
  • Promote a consistent approach to collecting accurate and complete loading information
  • Provide consistent information about problem effluents and treatability
  • Promote the design and installation of appropriate treatment systems and so reduce the problem of undersized systems causing environmental contamination

The loadings in this code of practice are more comprehensive than in previously published guidance, they are generally higher and include values for ammonia.

The owner of the treatment system holds the permit, consentor authorisation to discharge and should be aware that he is responsible for the effluent quality discharged. Thus all sources of discharge into the system must be declared. It is an offence if the effluent fails to comply with the regulators requirements.

Full details of the guidelines are enclosed on the attached PDF, but please do not hesitate to call us on 01773 767611 should you require any further assistance.