The proposed Swansea Bay Tidal Lagoon will be the world’s first energy-generating lagoon. The creation of the lagoon will modify the pattern of tidal flow within Swansea Bay, potentially altering the dispersion and distribution of pollutants discharged from Wastewater Treatment Works (WwTWs), Combined Sewer Overflows (CSOs) and local rivers. This situation is further complicated by enclosing the Swansea WwTW outfall. Intertek was commissioned by Tidal Lagoon Swansea Bay to assess the potential impacts of the lagoon on water quality in relation to the Water Framework Directive, specifically bathing water, shellfish water, and UK Technical Advisory Group (UKTAG) standards. The lagoon was predicted to enhance mixing in the adjacent areas of Swansea Bay leading to slight improvement in bathing water quality at the Swansea Bay and Aberafan Sands Bathing Waters. In most other areas, a no effect or slight improvement was predicted. This paper highlights the benefits that can be gained through detailed integrated modelling, namely: Understanding water quality issues; Optimising solutions; Predicting compliance with a range of water quality standards.