Pressure Filters provide two stage water treatment consisting of a
flocculator and filter vessel. Direct filters are only recommended for
raw waters with turbidity less than 10 NTU and colour less than 20 TCU.
First, chemicals are rapidly mixed into the raw water and then gently agitated
in a detention tank called the flocculation chamber. The main group of
chemicals added are referred to as coagulants. Coagulants when mixed
with raw water form precipitates in the water that trap contaminant
particles (dirt, bacteria, viruses and other microorganisms).
The detention time in the flocculator allows these precipitates to form and
capture the contaminates. The colder the water the longer the detention
time must be. Agitation of the water is required to ensure collisions
between the forming precipitates and the remaining contaminant particles.
In pressure flocculators the agitation is provided by hydraulic means as
the water flows around baffles, constrictions, or special contact media.