After water mains are in service for a long time, there will be deterioration in structural, water quality, and hydraulic performance. Structural deterioration may be caused by many factors. Metal-based pipes develop internal and external corrosion, causing the pipe walls to thin or degrade. They can eventually leak or burst. Cement-based pipes are subject to cement matrix and reinforced steel deterioration. All pipes are subject to joint failures. Water quality deterioration includes scaling, sedimentation, and biofilm formation. Scaling is the formation of hard deposits on the interior wall of pipes. This can be a by-product of pipe corrosion combined with calcium in the water, which is called tuberculation. Sedimentation is when solids settle within the pipes, usually at recesses between scaling build-ups. When there is a change in the velocity of water flow (such as sudden use of a fire hydrant), the settled solids will be stirred up, causing water to be discolored. Biofilms can develop in highly scaled and thus rough-surfaced pipes where bacteria are allowed to grow, as the higher the roughness of the interior wall, the harder it is for disinfectant to kill the bacteria on the surface of the pipe wall. Hydraulic deterioration that affects pressures and flows can be a result of other deterioration that obstructs the water flow.
When it is time for water main renewal, there are many considerations in choosing the method of renewal. This can be open-trench replacement or one of the pipeline rehabilitation methods. A few pipeline rehabilitation methods are pipe bursting, sliplining, and pipe lining.
When an in-situ rehabilitation method is used, one benefit is the lower cost, as there is no need to excavate along the entire water main pipeline. Only small pits are excavated to access the existing water main. The unavailability of the water main during the rehabilitation, however, requires building a temporary water bypass system to serve as the water main in the affected area. A temporary water bypass system (known as temporary bypass piping) should be carefully designed to ensure an adequate water supply to the customers in the project area. Water is taken from a feed hydrant into a temporary pipe. When the pipe crosses a driveway or a road, a cover or a cold patch should be put in place to allow cars to cross the temporary pipe. Temporary service connections to homes can be made to the temporary pipe. Among many ways to make a temporary connection, a common one is to connect the temporary service connection to a garden hose. The temporary pipe should also add temporary fire hydrants for fire protection.
As water main work can disturb lead service lines, which can result in elevated lead levels in drinking water, it is recommended that when a water utility plans a water main renewal project, it should work with property owners to replace lead service lines as part of the project.