Last updated: (February 2005)

Introduction

Companies that discharge liquid effluents or wastewater to a sewer are likely to require a trade effluent consent to do so from the local water company or other sewerage undertaker. This Reference Note describes some of the legislative and technical issues involved.

Background

To discharge liquid effluent or wastewater to a sewer, your company will need to obtain a trade effluent discharge consent from the appropriate authority, usually the water company or sewerage undertaker. These bodies are required by law to accept your effluent as long as it conforms to certain standards, and they are allowed to levy a charge appropriate to the service provided. The makeup and level of these charges are regulated. Where the volume of effluent is small, a minimum trade effluent charge may apply. Where no process liquors are discharged, a separate fixed charge will be made for flows from toilets and restaurants etc. Appropriate environmental management can help to reduce trade effluent charges.

Which legislation and regulations apply to discharges to sewer?

In England and Wales the water companies provide these services according to the Water Industry Act 1991 (as amended). In Scotland the sewerage authorities act under the Sewerage (Scotland) Act 1968, and in Northern Ireland the Water Service acts under the Water and Sewerage Services (Northern Ireland) Order 1973.

If your process is authorised under Integrated Pollution Control legislation in Britain or under Integrated Central Control in Northern Ireland, the discharges that you may make to sewer will also be considered as part of that authorisation. Your interface with the water company remains.

The sewage works will ultimately discharge your treated or partially treated effluent to a watercourse. At that stage, the discharge from the sewage works will be regulated by the Environment Agency, SEPA or the Environment and Heritage Service.

What is a trade effluent consent?

The water company will apply standards or restrictions to the flow, strength and character of your discharge. These restrictions form part of your trade effluent consent. They are primarily intended to ensure the safety of the sewerage workers, to preserve the sewers and other infrastructure, to allow the sewage works to process the effluent successfully, and to achieve compliance in its final discharge to a watercourse.

Which parameters are included in typical consent conditions?

The trade effluent discharge consent that covers the flow from your premises will usually state some of the following conditions:

  • the maximum permitted daily flow (m3/day)
  • the maximum hourly flow (m3/hour)
  • the permitted pH range, typically pH 5-9
  • the maximum biochemical oxygen demand (BOD) or chemical oxygen demand (COD) value, expressed in milligrams of oxygen requirement per litre (mg/l)
  • the maximum concentration of suspended solids (SS) in mg/l.

The BOD or COD limit will depend upon the scale of your discharge relative to the size of your local sewage works, and the amount of spare treatment capacity that is available there.

Which other parameters may be included in your consent conditions?

In addition, and according to the nature of your process, or your discharge history, there may be stated limits on the amounts of:

  • oil (mg/l)
  • dissolved metals such as copper, zinc, etc (?g/l).
  • organic chemicals such as phenols (mg/l).

Such substances could affect the operation of the sewage works. The absence of stated limits does not imply any right to discharge such pollutants.

In addition the consent conditions may reflect the concerns of the central regulatory agency regarding the composition of the final discharge from the sewage works itself, and its effect on the environment. Your consent may expressly contain limits on:

  • prescribed substances such as cadmium, mercury, or permethrin (?g/l).

How is the trade effluent charge calculated?

The water company will provide you with an analysis of your discharge, perhaps on a quarterly basis, and this may serve as the basis for deriving the trade effluent charge. The charge for treatment of your effluent will be derived from a modified MOGDEN FORMULA. The parameters that affect the final charge are the total volume flow in that quarter, the average COD or BOD concentration, and the average suspended solids concentration. The other parameters in your consent do not affect the charges. The full formula used will be available from your water company, but the following formula is used where the sewage works offers biological treatment of the effluent.

C = R + VB + (Ot/Os ? B) + (St/Ss ? S), where

C is the total charge per m3 of your effluent (?/m3)

R is the charge per m3 of your effluent for the infrastructure related to conveyance and reception, that is the sewers and inlet works (?/m3)

VB is the charge per m3 of your effluent for the sewage works infrastructure and operation where there is biological treatment (?/m3)

Ot is the COD of your settled effluent (mg/l)

Os is the COD of the average settled sewage reaching your water company's works (mg/l)

B is the charge for the biological oxidation of a cubic metre of average settled sewage, being mainly related to electricity, labour, biological sludge disposal (?/m3)

St is the total suspended solids in your effluent (mg/l)

Ss is the total suspended solids in the average sewage reaching your water company's works (mg/l)

S is the charge for the treatment and disposal of the primary sludge from a cubic metre of average sewage (?/m3).

The calculated charge for a cubic metre of effluent is then multiplied by the total flow during the quarter.

Can you reduce your trade effluent charges?

Volume

The volume flow of your effluent is the most important factor in determining the bill. However, it is very common for effluent flows not to be measured. In such circumstances the volume of effluent is assumed to be the same as the volume of water purchased, as recorded on the site's incoming water meter. This figure may be used as it stands, or after some modification for losses as steam or in product.

It follows that any water that leaks away on the premises, from a badly maintained water distribution system, or is used carelessly in processes, bears not only a purchase charge, but also a trade effluent charge, although it does not actually flow to the sewer.

It is therefore very important to take all steps to reduce your water consumption, and where practical you should measure or make a reliable estimate of the flow of effluent that enters the sewer. You should map and analyse the flow of water and effluent through your premises. You should note all the opportunities for reducing water use. You should determine whether any cooling water or rainwater that flows to sewer would be sufficiently clean for direct discharge to a ditch.

BOD, COD and SS

Should the BOD or COD concentration or the suspended solids concentration be a significant component of the bill you should take steps to ensure that no product or other material enters the effluent stream unnecessarily.

Where the suspended solids or COD concentrations are high, you should consider the provision of an on-site separation stage or biological treatment process to reduce their concentrations in the final effluent flow. The cost-effectiveness of such a development can be determined by comparing its capital and operating costs with the annual saving in trade effluent charges.