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 lenght of the tank. This was necessary to ensure an even upflow velocity.

Beginning with the Cuatro Comunidadies design water was even 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.

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. Designs are now based on the this closer spacing.

Drain

Entrance Tank Improvements

Floating Floc

Chemical Dosing and Rapid Mix

Fall 2008

Feedback from the interns in Honduras has shown that settling is occurring in the slopes at Marcala. This is thought to be occurring because the bottom angle is too shallow and because the exit velocity from the slopes is too low. Some literature review showed that to prevent settling and floc break up, the velocity should be around 5 cm/s. When possible, the lower angle for future designs was changed from 10 deg to 30-40 deg.
An algorithm was needed to automate the design of the slopes. After several versions, it was decided that exit ports would be needed at the slope exit. A description of this algorithm can be found here.