The Birmingham office of Hyder Consulting has secured a three- year framework agreement with Severn Trent Water.
It will help meet the future needs of more than eight million domestic and industrial customers across the Midlands.
The first focus for Hyder will be to study the West Bromwich area with a population in excess of 55,000.
Hyder will seek to assess the condition of the existing sewer systems, and predict rainfall patterns and sewer flows.
The firm's Birmingham water specialist division has expanded from 12 to 40 in the last three years. Nationally Hyder employs more than 270 water consultants.
Alastair Moseley, technical director of water environment for Hyder, said: "The aim is to discover precisely which sewers need replacing, which sewers require only new links to operate efficiently and which can be left untouched.
"It is a myth that every sewer installed during the 19th century and early 20th century needs to be dug up and replaced. Over the last ten to 15 years technology has moved on in leaps and bounds and we are now able to assess work needed on the networks much more accurately."
Hyder will build a computerised model and apply varying mathematical profiles of rainfall patterns to discover at what point various water flows will start to interact, when there will be a backlog and whether the sewers will overflow.
Severn Trent is the main water supplier in the West Midlands, the East Midlands and the Severn Estuary with an area which runs from Scunthorpe to Stroud and Leicester to Welshpool.
Hyder Consulting is a planning, engineering, environmental and management consultant, employing more than 2,500 people worldwide
It generated gross revenues of £122.3 million in the last financial year.