The original water treatment facility was built and owned by the Ohio Water Service Company.  The City of Marysville purchased this facility in 1991 and it is now known as the Marysville Water Treatment Plant (WTP).

The Marysville Water Treatment Plant receives its water from two sources.  These sources are groundwater and surface water.

The groundwater source consists of four wells.  The oldest well was installed in 1927; this well is known as the Plant Well.  It has a capacity of 1.44 million gallons per day (mgd).  The second well was installed in 1966.  This well is known as the Dam Well and has a capacity of 1.23 mgd.  The third well was installed in 1975. This well has a capacity of 1.01 mgd.  The fourth well was installed in 1994 near the Plant Well.  This fourth well has a capacity of 0.86 mgd.

The surface water source is Mill Creek.  This is a relatively small creek serving a watershed of 178 square miles.  The flow of the stream varies from over 400 mgd to no flow at all.  The dam and intake systems were installed in 1928.  A wet well and a new 2.0-mgd pump were added in 1971.  Twenty-five percent of the water pumped tot he plant is groundwater; seventy-five percent is surface water.

The treatment plant is a lime-soda softening process and was originally built in 1928.  Plant capacity at that time was 0.67 mgd.  The plant was upgraded in 1956 with the addition of two new settling tanks, one new filter, and a 100,000-gallon clear well.  The plant was upgraded again in 1971 with the addition of two more settling tanks, a new chemical feed building, and two new filters, bringing the capacity to 2.31 mgd.

The plant upgraded again in 1995 with two additional settling basins and a new 420,000-gallon clear well, bringing the plant capacity to 3.17 mgd.  In 1998, the plant filters were upgraded from 2.0 gallons per minute per square foot (gpm/sf) to 4.0 gpm-sf, bringing the plant to its present daily capacity of 3.46 mgd.

In 2001, a pipe bypass was installed between the chemical feed building and the old plant.  This eliminated the water bottleneck between the buildings and increased the efficiency of the settling basins.  A standby generator was also installed for emergency power.