Design Modifications

Flocculator Improvements

The original flocculator design that was constructed in La34 was a hydraulic baffle flocculator that utilized horizontal flow and had uniformly space baffles. Flocs were thought to have a higher probability of settling out in the horizontal flow design. Therefore, when Ojojona was built a horizontal and vertical flocculator were both installed to test the floc fall out theory. Although the horizontal flocculator at Ojojona was never fully tested, the vertical flow flocculator was found to perform well and used for the general plant algorithm.
All of these designs were based on a floc formation theory of G and Gtheta which are measures of shear and fluid mixing respectively.
Flocculators built after Ojojona were built with a tapered baffle spacing configuration. This was change over the previous design where all baffles were uniformly spaced throughout the flocculator. The tapered ideas was founded on the fact that as flocs grow in size they can withstand less shear stress, so the thought was to have the shear gradient decrease over the length of the flocculator.
With the construction of the Cuatro Comunidades, the theory behind flocculator design was changed. Now flocculators are designed based on the energy dissipation rates. Different floc sizes are believed to be able to with stand different amounts of energy dissiaption. Therefore the spacings in the tapered sections are based on energy dissipation rates.

Sedimentation Improvements

Tank Inlet

The early designs have water entering the sedimentation tank through a series of pipes connecting from the bottom of the inlet channel down to the bottom of the sedimentation tank. The pipes were designed to be of different lengths to allow for water to be even distributed evenly down the length of the tank. This was necessary to ensure an even upflow velocity. This design proved to be very expensive and took up a lot of room in the sed tanks.

Beginning with the Cuatro Comunidadies design, water was evenly distributed to the bottom of the tank through two triangular manifolds extending the length of the tank. These manifolds are connected to the inlet channel by concrete chimneys with ports exiting into the manifolds. The water flows out of the manifolds through ports cut into the slopes. An even upflow velocity is maintained by the geometry of the ports.

Lamella

Lamella are set to be angled at 60 degrees. This angle was found to optimal for floc fall out. In older designs lamella were set to be about 5cm apart, but lab experimentation has shown that a minimum spacing of 2cm is still acceptable. Beginning with the Gracias plant, designs are now based on the this closer spacing.
In LA34 and Ojojona, lamella were kept in place via nails that were embedded in the concrete. All designs after Ojojona had lamella that were held in place with a pvc frame sitting on a concrete ledge built into the sides of the tank walls. This would enable operators to easy take the lamella out for cleaning.

Sludge Drain

A launder situated at the bottom of the sedimentation tank is used to drain the tanks. All designs prior to the Cuatro Comunidades have a pvc pipe with holes drilled along its length running the length of the tank that acts as the drain launder. The Cuatro Comunidades has a small concrete channel running the length of the channel to act as the drain. This channel is covered by a flat concrete plate with holes formed down its length to create a launder from the channel. The use of concrete instead of pvc is much cheaper.

Entrance Tank Improvements

The entrance tank is where raw water enters the plant. At some point in the entrance tank alum has to be dosed and mixed into the water before entering the floc tank.

Floating Floc

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 and 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.