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Code of Practice

Flows and Loads – Sizing Criteria, Treatment Capacity for Small Wastewater Treatment Systems (Package Plant)

This code of practice was produced by the British Water Package Sewage Treatment Plant Focus Group – for membership see the company logos on page 6.

Each manufacturer is aware of the capabilities of their own systems with respect to the individual wastes, but guidance is given so that the user is aware of performance inhibitors.

The Environment Agency, the Environment and Heritage Service (Northern Ireland) 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.

The code provides a new table of loadings which allows the total daily load fed into a sewage treatment system to be calculated. These criteria will be widely disseminated and it is recommended that designers of small wastewater treatment systems should use this table of loadings.

The flow and load figures given represent current best practice within the UK but may change with time. British Water welcomes data to improve the content of this document.

Guidance notes are provided to assist the user to consider the nature of the sewage being received for treatment and to identify the various sources of the sewage.

The table of loadings may be used for all sizes of package sewage treatment plant up to a maximum of 1000 population equivalents.

Use of this table by all UK manufacturers will

  • Encourage collection of accurate and complete loading information

  • Provide a consistent data collection approach

  • Define site activity and sewage sources

  • Provide consistent information to system purchasers regarding treatability and difficult effluents

  • Reduce the problem of undersized systems

The loadings in this code of practice are revised and more comprehensive than previously published guidance (CIRIA PR72 2001, Environment Agencies PPG4): ammonia figures are included and loading figures are generally higher.

1 Scope The purpose of this code of practice is to provide an improved table of loadings to allow the total daily load fed into a system to be calculated. The volume factors given are very similar to those previously published. BOD loading factors have been increased. Ammonia figures are included within the table.

2 Regulations Early contact / license application with the Regulator to discuss the proposed discharge of sewage effluent is advisable.

  • Planning requirements, eg DETR Circular 3/99, site survey, etc.

  • Building regulations, eg part H DTLR England & Wales, part M Scotland, Water Order (Northern Ireland) 1999 and Northern Ireland building regulations.

  • Discharge consent, Environmental Regulators (Agencies).

  • Planning permission (Local Authority Planning Guidance).

3 D e f i n i t i o n s o f t e r m s Population (P) – Number of inhabitants, e.g. of a settlement area.

4 S e l e c t i o n C o n s i d e r a t i o n s A l l a p p l i c a t i o n s Values and conditions required by any regulatory permit or consent.

  • The loading chart includes individual figures for each

specified load.

Biochemical oxygen demand (BOD ) – Mass concentration of dissolved oxygen consumed under specified conditions (5 days at 20° C with nitrification inhibition) by the biological oxidation of organic and/or inorganic matter in water.


Ammonia expressed as mg/l N – Ammonia is NH , Ammonium is NH OH. In wastewater we frequently refer to and use the word/symbol, ammonia/NH . The term ammonia usually includes ammonium as well.




  • Some water treatment equipment effluents eg softeners, chlorinated backwashes, may not be acceptable; system designers should specifically accept or exclude their use. Many treatment plant designs will accept regenerants into their units, however this must be checked and agreed.

(continued on page 4)

L o a d i n g f i g u r e s a r e g i v e n f o r F l o w , B O D a n d N H 3 .

The user/purchaser of the system must declare ALL ACTIVITIES to enable all loads entering the treatment to be identified and evaluated. The user/purchaser should be made aware that there is a risk of poor performance from the equipment if loads are understated. The accuracy of the declared loads is of paramount importance.

Guidance points given under each category suggest questions to enable the specifier to recognise variable or unusual loads, particular to that site, to improve correct system selection.

  • Total daily loadings are calculated based on the anticipated final maximum capacity of the site. New sites initially may have a reduced business level but the system suggested should reflect the full business potential, e.g. a system suggested for a hotel or caravan site or any other application, with an average 80% occupancy rate should be able to handle 100% occupancy. The equipment selected by the specifier should reflect the maximum potential of the site. Where a specifier is instructed to use lower occupancy rates, this should be recorded. Flow balancing should be considered where appropriate

  • Excess disinfectants, chemicals, etc can affect the biological processes as can specific toxic substances from site activities

    • e.

      g. photographic chemicals, weed killers, motor oils. It is assumed that these substances are excluded from the wastes to be treated.


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