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Starting with the Tamara plant, the entrance tank is also the place where dissolved air is removed from the water. Raw water was becoming supersaturated prior to entering the plant. The dissolved oxygen was coming out of the water in the flocculator and causing flocs to break up and float up with the air bubbles. These "Floating Floc" where causing plant performance to deteriorate. Sand was added to these later designs to encourage the bubbles to rise out before alum is dosed and flocs are formed.
Chemical Dosing
Alum is dosed at some point prior to the flocculator. Chlorine is dosed as water is leaving the exit channel on the way to the distribution tank. These chemicals are added to the water at a specific dose via a specific height differential and a float valve. More details on the this technology can be found on the Flow Control Module Page. Plants with high flow rates needed to run float valves in parallel to handle the necessary chemical dose so a large float value utilizing a toilet valve was designed.
Rapid Mix
Alum has to be well mixed for flocs to form well. LA34 and Ojojona have rapid mix tubes that the water flocs through. These tubes had small pvc crosses connected in series to act as agitators and encourage mixing.
The Gracias plant has a long shallow channel built into the top of the entrance tank. Water in this channel has to flow over a waterfall and then through a narrow orifice before entering the floc tank. These two processes have enough energy dissipation to completely mix the alum dosed higher up in the channel.