The Filtration Building is the northernmost of the three buildings that make up the plant, with the highest elevation from Lake Ontario. The treatment plant was named after Roland Caldwell Harris, who was the Commissioner of Works from 1912 to 1945.

The treatment plant is sometimes known as "The Palace of Purification" because of its Art Deco style. The building features buff brick, arching windows, and stylized frescoes. 

A wing was added to the filtration building in the 1950s, doubling its capacity. C. 2017, the plant was undergoing a major $22 million rehabilitation plan.

The building was designated a National Historic Civil Engineering Site by the Canadian Society for Civil Engineering in 1992. In 1998, it was designated as a Heritage Property by Scarborough City Council.

The plant has been a filming location for numerous television shows and movies. It also features in Michael Ondaatje's book "The Skin of a Lion."

• Architect - Thomas Canfield Pomphrey  <-- More buildings by this architect.
• Architect - Taylor Hazell Architects  <-- More buildings by this architect.
• Engineer - Gore, Nasmith, and Storrie