The ten-year loan is a first for Southern Water and proceeds from the deal will support the £63 million redevelopment of the Woolston wastewater treatment works in Southampton.
The redevelopment is to ensure treated wastewater meets new standards set by the Environment Agency.
The loan is the first support announced by the EIB to any UK water company for investment in the 2015-20 AMP6 regulatory cycle.
Southern Water chief executive Matthew Wright said: “Our business plan for 2015 to 2020 sets out how we will further improve water and wastewater services to our 4.6 million customers across Kent, Sussex, Hampshire and the Isle of Wight.
“During this period we will be investing £1.5 billion in looking after the environment, maintaining and improving our water supply and wastewater treatment works, and expanding and maintaining our network of water mains and sewers.
“The EIB is an important provider of the finance we need for the successful delivery of our plan.”
EIB vice president Jonathan Taylor added: “Long-term investment in water infrastructure is crucial to ensure a secure supply of safe drinking water and effective treatment of wastewater for years to come.
“The EIB is pleased to support significant new investment by Southern Water that will benefit millions of people across the region. We look forward to working with other UK water companies in the new regulatory period.”
The EIB provided a record £6 billion last year for long-term investment in key infrastructure across the UK, including support for new hospitals, better water and sewerage infrastructure, renewable energy and energy transmission and private sector investment.
A version of this story first appeared on wwtonline.