Water In is Not Equal to Water Out (WIWO)

The amount of water supplied by Uisce Éireann to a customer's premises can vary from the amount of wastewater discharged to the public sewer.

Why are there variations in the volume of water supplied and wastewater returned to the sewer?

Below are some common examples where the amount of water supplied might vary from the amount of wastewater discharged to the public sewer:

1. Grounds irrigation;
2. Water used in products;
3. Water loss by evaporation;
4. Where wastewater may be tankered offsite and not returned to sewer;
5. Alternative treatment of wastewater onsite such as a septic tank.

How do I get a Water In Is Not Equal to Water Out (WIWO) variation?

Under legislation, the amount of wastewater discharged from a premises is deemed to be equal to the amount of water supplied to the premises, unless Uisce Éireann and the customer agree otherwise. You can complete a WIWO application form and, in doing so, request confirmation from Uisce Éireann that the amount of water supplied to that premises is a different volume to the amount of wastewater discharged from the premises. This form must be completed with adequate information and appropriate evidence to enable Uisce Éireann to assess the application and consider whether entry into an agreement might be appropriate.

In order to make a valid application, you must have no outstanding arrears on your account before submitting an application. Once this has been verified, the application will be processed and a detailed assessment will be carried out. No WIWO agreement will exist between you and Uisce Éireann until such time as a written agreement is entered into.

From 1st October 2026 Onwards

Following implementation of the trade effluent charging framework, Uisce Éireann may enter into a WIWO agreement from 1st October 2026 with a customer who has, or whom Uisce Éireann considers ought to have, a trade effluent discharge authorisation. Click here for more details on how to apply for a trade effluent licence.

[1] "Trade Effluent Discharge Authorisation" means a licence or consent issued by Uisce Éireann under Section 16 of the Local Government (Water Pollution) Act, 1977, as amended, or a consent issued by Uisce Éireann under Section 99(e) of the Environmental Protection Agency Act 1992, or Section 52 of the Waste Management Act 1996, as amended.

Exclusions

All applications will vary and each one will be assessed on its own merits. In processing an application, Uisce Éireann will apply assessment criteria, including a requirement that the difference between water supplied and wastewater discharged is greater than 15%. This threshold will enable Uisce Éireann to efficiently process priority applications in the initial phase of operation.  Uisce Éireann will keep this threshold under review as further information is gathered over the coming years.

Note: Uisce Éireann will not enter into a WIWO agreement with a customer in circumstances where:

1. Water is consumed by customers, visitors, spectators, staff or members of the public on-site and is assumed to enter the wastewater system, even if it may ultimately be discharged at a different location, or
2. Licensed Trade Effluent is billed using a Trade Effluent meter as the measured volume is deemed to represent wastewater discharged to the public sewer after any non-return-to-sewer losses have occurred. The measured volume discharged is therefore used as the basis for charging. 

Requirements

Uisce Éireann may request additional supporting information as part of the water in is not equal to water out application assessment process. This may include evidence of water use, wastewater discharge, site processes, water balance information, or the installation and operation of metering or sub-metering by the customer where required. Any such information or assessment must be provided by the customer to Uisce Éireann’s satisfaction before a determination can be reached on an application.

Complex Sites Requiring a Water Mass Balance Assessment

Where a premises has multiple water sources, multiple wastewater discharge points, complex treatment processes, recycling systems, seasonal operations, or other operational complexities, a standard WIWO assessment may not provide sufficient evidence to accurately determine the volume of water that does not return to the public sewer.

In these circumstances, a detailed water mass balance study is required to demonstrate the relationship between water supplied, water consumed, water lost through process activities, and wastewater discharged. The study provides an evidence-based methodology for determining non-return-to-sewer volumes and ensures that any wastewater billing adjustment is supported by verifiable operational data.

Requiring the assessment to be undertaken over a representative period, including seasonal variations, helps ensure that any allowance accurately reflects the site's typical operating conditions rather than a short-term snapshot. Independent sign-off by a suitably qualified engineer provides assurance regarding the accuracy and integrity of the methodology and findings.

Where information submitted as part of the water mass balance materially differs from information previously provided to Uisce Éireann for Trade Effluent licensing purposes, Uisce Éireann may review the Trade Effluent Licence to ensure the licence remains accurate and appropriate for the site's operations.

Key information on the WIWO application: 

Uisce Éireann is under no obligation to enter into an agreement with a customer. If Uisce Éireann enters into a WIWO agreement with you, the following should be noted:

1. You must enter into a written WIWO agreement with Uisce Éireann which records the terms of the agreement that have been reached. Until such time, there is no agreement.
2. The applicable WIWO variation to your bill will only come into effect from the date the agreement was signed (and will not be applied retrospectively).
3. Where an agreement has been made with an applicant who is on a non-domestic water, wastewater or trade effluent transition tariff, the amount of the WIWO variation that applies will be reduced by the same % as the % difference between the transition tariff and the enduring tariff that would otherwise apply if there was no transition tariff.  

Next Steps:

To make an application see the steps below:

1. Please download the WIWO application form from the below link.
2. Before completing the form, please review all information on the Uisce Éireann website and application form.
3. Once complete, return to wiwo@water.ie for assessment. In certain circumstances we may request further information, and/or a site visit may be required.
4. If applicable to your business, a trade effluent licence may be required. Uisce Éireann will advise you of this at the earliest opportunity after receiving your application. For further information on trade effluent click here.
5. Once the assessment is completed Uisce Éireann will inform you of the outcome of the assessment.

Send any further WIWO enquiries to wiwo@water.ie