How your wastewater is calculated (and why it might be wrong)

How wastewater charges are worked out

Your wastewater charge is normally linked to the amount of water recorded on your supply meter. Suppliers often assume that a large percentage of this water flows back into the sewer system as wastewater. For example, if your business uses 1,000 cubic metres of water in a year, the supplier might assume that 95% of it is returned to the sewer. They then multiply this assumed wastewater volume by a set price per cubic metre, which creates the wastewater element of your bill.

In some cases, this figure is adjusted based on information you’ve provided about your operations. For instance, if you have processes where water is lost through evaporation, incorporated into products, or used for irrigation, the volume of wastewater could be less than the supply meter reading would suggest. However, unless you actively give your supplier this data and get it agreed, the default assumption can remain in place. This means the calculation can be heavily based on generalised rules rather than your actual usage patterns, which is why it’s always important to check the basis of your charges.

4 factors that can cause miscalculations

Several different issues can lead to your wastewater charges being higher than they should be. In a nutshell, these include:

• Incorrect meter readings
• Overestimated return-to-sewer percentage
• Wrong drainage classification
• Human error in billing systems

So, let’s look at each of these in a little more detail. 

Incorrect meter readings often happen when your supplier inaccurately records your water usage. This can be due to estimated readings being used instead of actual figures from your meter, especially if the meter is in an awkward location or if access is restricted. Faulty meters can also give readings that are higher than the true consumption, and these inflated figures are then used to calculate wastewater volumes. Even small inaccuracies can add up to a significant overcharge over the course of a year.

Overestimated return-to-sewer percentage occurs when your supplier assumes a standard proportion of your incoming water is returned as wastewater without looking at your specific operations. For example, a manufacturing site may lose large amounts of water through evaporation in cooling systems or production processes. If this isn’t accounted for, the wastewater volume will be overstated. One good way to help avoid this is by giving your supplier with accurate operational data as far as you can.

Wrong drainage classification can lead to you being billed for services you don’t actually use. This usually happens if your property is recorded as sending surface water into the public sewer when it’s actually directed to a soakaway or watercourse. If the classification is wrong, you could be unnecessarily charged for surface water drainage.

Human error in billing systems can happen when incorrect tariff bands are applied, when old data is used for calculations, or when adjustments are entered incorrectly. A small mistake in data entry or account setup can create a long-term error that affects multiple billing periods until it’s noticed and corrected. It’s frustrating, and arguably shouldn’t happen, but it’s just one of those realities of life that it sometimes does. The important thing, of course, is how your supplier reacts. 

3 ways to check if your charges are accurate

You can take a few straightforward steps to make sure your wastewater charges match your actual usage.

• Compare wastewater volume against your water meter reading
• Review drainage charges on your bill
• Arrange an independent water audit

Again, we’ll take a look at these in more detail sequentially. 

Comparing wastewater volume against your water meter reading can highlight if the assumed return-to-sewer percentage is realistic. For example, if your bill suggests that 95% of your water is returned as wastewater but your operations clearly divert a significant proportion elsewhere, you may have grounds for an adjustment. If you can, try and keep accurate records of water use in processes, losses, or on-site reuse to strengthen your case.

Reviewing drainage charges on your bill is hugely helpful for helping you to see whether you’re unnecessarily paying for surface water drainage. If your site uses soakaways or channels rainwater directly to a watercourse, you may not need to be billed for this service. You can request a site assessment or provide drainage plans to help prove this to your supplier.

Arranging an independent water audit gives you an expert review of your charges. A water auditor can assess meter accuracy, confirm the correct drainage classification, and verify if the return-to-sewer percentage is set appropriately. They can also check historical billing data for patterns that suggest ongoing errors, giving you strong evidence to present to your supplier for corrections or refunds.

How you can correct errors

If you’ve identified possible inaccuracies, the most direct route is to contact your supplier with supporting evidence. Make sure to explain the issue clearly, provide data that shows the correct wastewater percentage or drainage setup, and request a formal review of your account. Having detailed records ready makes it easier for the supplier to adjust your charges and, if applicable, issue refunds for past overpayments.

You can also request a meter accuracy test if you suspect faulty readings. This may involve a site visit from the supplier or an independent contractor, depending on the terms of your account. In cases where the supplier’s records on drainage type are wrong, you might need to get written confirmation from your local water authority to support your claim. Taking action promptly means you can reduce the chance of further incorrect bills and get your charges aligned with the actual wastewater volumes from your site.

And if you ultimately decide that switching your business water supplier is the right solution, that’s exactly where we can help here at The Business Water Shop. We have a network of trusted suppliers at our disposal that spans across the UK, which makes us the perfect choice to get you the most competitive water quotes for your business. In fact, we’re able to save some customers up to 55% on their water bills, and you can count on us to use all our resources and industry expertise to get you the best deal.

It only takes a few minutes to get your online quote – so if you’re thinking about moving to switch business water suppliers, there’s never been a better time to do it!