United Utilities is entering the final stage of its £160m scheme to enhance Preston’s sewer system.
The water firm has built a 3.5km underground storage tunnel beneath the city - wide enough to drive a car through.
The work is part of a project to stop wastewater flowing into the River Ribble during heavy storms and improve the quality of coastal bathing waters.
It began in 2009 and United Utilities (UU) has paid almost £1m in total in loss of profit payments to local businesses.
Engineers have returned to two of the worst affected areas, around Watery Lane and Fishergate Hill, to make “final permanent connections”.
UU posted letters to customers explaining temporary changes to road layouts.
The work in Fishergate Hill will last until Sunday, March 2.
All traffic movements will be available at the junction, however Fishergate Hill slip road will be closed. Access to Broadgate from Fishergate Hill will be via the traffic lights.
The work in Watery Lane will last until Sunday, February 16. All vehicle movements will be available, however, lane restrictions will be in place at Watery Lane Grand Junction.
On the weekend of February 15 and 16, HGV vehicles will not be able to turn right from Strand Road into Water Lane. Diversions will be in place.
UU said alternative route signs would be in place, normal pedestrian access would continue and all shops would be open for business as usual.
A UU spokesman said: “The reliability testing has gone well and we are now ready to complete the final phase of work, to permanently connect the storm water tunnel into the existing live waste water network.
“There will be limited excavating in the road, however we need access to the existing manholes and the new tunnel.
“Carrying out this type of work requires digging and machinery, therefore it maybe noisy.
“Please note that this work is weather dependant and the connection into the network must be completed during “low flows in the sewer” so the dates may vary dependant of rainfall we get in Preston.
“However, we will still endeavour to remove the highway restrictions as soon as practically possible.
“We’re sorry if this work affects you and your daily routine and we thank you for your co-operation and patience.
“Our people on site will do all they can to help you should you have any problems.”