How is Trade Effluent volume measured?

The volume of water used by a business is usually fairly simple to measure using a water meter but it’s not always as straightforward to measure Trade Effluent volume. You may have to consider a variety of factors which we’ll cover in this article.

Calculating water volume

Usually water volume is measured using a water meter. To work out the charge, this volume is multiplied by a unit rate. You can find this rate in our charges scheme.

Calculating sewage volume

Domestic sewage volume, is usually calculated based on the volume of water measured through the water meter, though some Wholesalers only charge a proportion of this as sewage and assume that some is not returned to sewer (i.e. used for drinking or lost in evaporation). 

Calculating Trade Effluent volume

Just like your business water, the simplest way to measure Trade Effluent volume is with a meter. But unlike water meters, which are usually provided by your water Wholesaler, it’s normally your decision whether to pay for and install one of these meters. It’s also worth being aware that it may be a requirement of your Trade Effluent Consent. 

If you do have a meter used to measure your Trade Effluent, it’s vital that you send us reads on a regular basis, so that you’re only charged for the volume discharged. If you don’t send regular reads through to us, we’ll use an estimate to calculate your bills, in the same way we do with water meters. There may also be an obligation in your Trade Effluent Consent to regularly have the meter calibrated to demonstrate its accuracy.

Calculating effluent volumes without a meter

Sometimes it’s not possible to install an effluent meter, but there are other ways of calculating effluent volumes. When your Consent is first granted (or following changes to the site or activity), your Wholesaler will determine the calculation method. There may also be regional differences in the preferred method or the approach taken.

Fixed volumes

Sometimes the activity that produces effluent is so consistent, it’s safe to assume the same volume will be discharged at a particular frequency throughout the year. A great example of this is swimming pools where filter backwashes generate the same volume of effluent to be discharged every week. In cases like this, the best approach may be to fix the volume for the year. This is a type of calculated discharge (see below).

Calculated discharges

Sometimes the effluent to be discharged doesn’t have a direct relationship with the water coming into the site. It can be hard to measure the volume of effluent discharged with meters and so a calculation is used. A good example of this is landfill sites, where the volume of discharged effluent is often calculated by multiplying monthly rainfall by the surface area of the site. Sending us your calculated discharge volumes on a regular basis (if relevant) is just as important in sending any meter reads to us, as it allows us to bill you based on the actual volume discharged at your site.

Water meter(s)

This is probably the most common method of calculating effluent volume if there’s no effluent meter installed. It’s done by calculating a proportion of the water coming into your business, as measured by one or more of your water meters. For example, if 75% of the incoming water goes into processes that produce effluent, this can be set as the volume of effluent produced, so the effluent volume is always 75% of the volume measured by the incoming water meter(s).

Water meter (plus allowances)

Similar to the above, but in this case you’d receive allowances for water that comes into your business but doesn’t return to the sewer. This is usually applied as either a fixed allowance (FA) or a percentage allowance (PA). Examples of non-return to sewer are:

  • Evaporative losses e.g. from cooling towers
  • Steam production
  • Loss in product e.g. food manufacturing
  • Ice-making
  • Off-site disposal e.g. where some effluent is tankered away for specialist treatment due to its toxicity

If you believe you have some water or effluent that’s not being returned to sewer and isn’t reflected in the way your Trade Effluent volume is currently being calculated, please get in touch with your account manager or contact us and we can help you to apply to the Wholesaler. 

How much water is my business using?

Knowing the amount of water/effluent in and out of a premises is often referred to as a mass balance or water balance. This illustration is a good way of visualising how water is used and lost in a business.

water usage graphic

It should be possible to account for every drop of water and in fact, this is essential to making sure the correct allowances are in place, the correct volumes are charged for, and it can also help you to understand whether there may be any leakage. 

Using sub-meters

Often sub-meters (water meters owned by the customer) can be used to measure the water used by different processes. If you use a sub-meter to calculate your Trade Effluent volume, please send us the reads on a regular basis, just as you would an effluent meter. Sub meters can also help identify which areas or processes of your site are using the most water, if this is something you'd like to know more about see our Water Efficiency Services.

Domestic Sewerage Allowance

In some situations, calculation of Trade Effluent volumes has an impact on the volume of domestic sewage that is charged for. As this is often charged at a different unit rate to Trade Effluent, it’s important to understand how this volume is being calculated and to make sure you’re being charged for the correct volumes of effluent and sewage. Sometimes, the volume of effluent discharged is subtracted from the water measured going into your site, and the remainder will be charged as domestic sewage. However, non-return to sewer allowances may mean not all of this is actually chargeable.

Domestic sewage is not always calculated based on volume of the incoming water meter, and instead a fixed annual volume is applied. Typically this is based on the number of people working full time. Whether or not there is a canteen and shower facilities for staff can also make a difference to this figure.  For some calculations of Trade Effluent volume, domestic, fixed and percentage allowances are all subtracted from the incoming water volume. If your domestic sewage is calculated based on head count it's worth reviewing this periodically. 


Depending on the nature and set up of your business, calculating Trade Effluent volume can be very simple or fairly complex. It’s certainly worth trying to understand the mass balance of your site to make sure all allowances are considered and your volumes accurately measured. If in doubt, please drop us a line at