The reservoir at Kington St Michael can now store an extra five million litres of water.
Over the past 12 months work has been taking place to increase the storage capacity at Allington Reservoir.
Project manager Joe Edmunds said the project would ensure customers had a reliable supply of water for the future.
He said: “This important scheme forms part of a larger water supply grid programme of works that will help us meet future demand for water without the need to develop new resources. The new storage facility was completed on time and on budget.”
The work forms part of Wessex Water’s Water Supply Grid improvement programme, which comprises more than 20 schemes across Wiltshire, Somerset and Dorset and will take eight years to complete.
For more information visit www.wessexwater.co.uk/grid