A water treatment plant for water purification or the removal of organic matter, suspended solids and other pollutants from water or wastewater is formed from multiple sections of polymer that are assembled to form a pre-treatment chamber, a treatment chamber and a settling chamber. The plant is preferably formed of a polymer, such as HMWPE. The treatment chamber is preferably a biofilm-aeration chamber. An outer tank is comprised of first and second upper sections and first and second lower sections secured by mating flanges. A divider baffle forms an inner wall separating the pre-treatment and treatment chambers. A hanging clarifier baffle is formed to be suspended from the outer tank between the treatment chamber and the settling chamber and includes a lower wall with specific angled portions near an angled wall of a lower section of the tank.