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You are here: Home > Services > Trade effluent


Consent conditions

The aim of trade effluent control is to ensure that discharges either alone or in combination with other effluents or contents of the sewer cannot harm:

  • the sewerage network
  • the sewage treatment works
  • employees and the general public
  • the environment.

To achieve these objectives conditions are written into consents to control trade effluent discharges on an individual basis.

Protection of sewers

 Conditions are set to prevent:

  • corrosion of the sewer fabric
  • overloading of sewers and possible flooding of properties
  • blockages of sewers
  • the formation of explosive, flammable and poisonous gases in sewers
  • hazardous situations developing in sewers for maintenance workers such as high temperatures
  • unacceptable discharges from storm water overflows on the sewer network.

Protection of sewage treatment works

Conditions are set to ensure that:

  • the sewage arriving at the works can be treated effectively and economically
  • damage does not occur to the structure of the works or to mechanical or electrical plant
  • employees operating the works are not harmed
  • biological treatment processes are not affected by toxic effluents
  • the treated effluent produced by the works are suitable for discharge to river
  • the biosolids produced by the works are suitable for disposal to agricultural land.

 Indicative standards to which trade effluent discharges should normally comply with prior to discharge to the public foul sewer in the Wessex Water region are given below. These limits may be varied depending on local circumstances taking into account the volume and rate of discharge, the size and flow of the receiving sewer network and treatment works. Additional requirements may be necessary to control the discharge of toxic and other hazardous substances.

Limits are normally set as maximum permitted concentration. In certain circumstances limits are applied on the load of a particular substance which can be discharged in a given period (usually 24 hours).


Ammonia can cause unsafe sewer atmospheres and toxicity problems in watercourses.

Flammable substances

Flammable substances or substances which can produce flammable or explosive atmospheres will be prohibited or controlled to safe levels.

Fats, oils & greases (FOG)

FOG can build up within the sewer which can lead to smell problems, blockages and subsequent foul flooding. FOG can also build up on equipment such as pumps and cause operational difficulties at pumping stations and treatment works.

200 milligrams per litre (mg/l) limit is usually applied.

Free cyanide

The limit is not greater than 3 mg/l. Cyanide is toxic and can inhibit treatment processes.

Organic load

Chemical oxygen demand (COD) and biological oxygen demand (BOD) may be limited to prevent overloading of the biological wastewater treatment processes.

Other substances

Other substances that may be present in the discharge will be assessed and controlled on an individual basis.


The normal range of pH allowed is 6 to 10. Extremes of pH can lead to an unsafe working environment, affect biological treatment systems and damage equipment. Low and high pH can result in damage to the materials of construction of the sewer network.

Prescribed substances

These substances are the most dangerous substances to the water environment based on their toxicity and bio-accumulation. The list includes the metals cadmium and mercury, chlorinated solvents and a range of pesticides.

Any trade effluent application containing these substances above background levels must be referred to the Environment Agency for determination.


The limit for substances that can produce hydrogen sulphide upon acidification is normally 2 mg/l. Hydrogen sulphide is a toxic gas that can build up in the sewer atmosphere, leading to hazardous working conditions. Hydrogen sulphide also has a very pungent odour which can cause smell nuisance.


Sulphate is normally limited to 1000 mg/l because it can cause damage to concrete structures.

Suspended solids

Suspended solids can cause siltation and blockages in the sewerage system. The normal limit is 500 mg/l.


This is limited to a maximum of 43.3ºC (110ºF).

Toxic metals

This includes chromium, nickel, copper, zinc, lead, silver, tin and molybdenum. Toxic metals can inhibit biological treatment processes and may accumulate in the environment. The Environment Agency regulates the metals content of sewage works discharges and sludge disposal to land.


This parameter is normally limited to a number of cubic metres (m3) per 24 hour period and a rate of discharge in litres per second.

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