Vertical Flow Hydraulic Flocculator
Overview
Alum and raw water flow through a short series of pipe elbows acting as a rapid mix. This disperses the alum throughout the raw water stream, ideally coating a significant amount of the suspended particles. This mixed solution then enters the vertical flocculator. The flocculator is comprised of a series of channels with alternating up and down baffles. The baffles force water to change direction through an amount of space that is calculated to provide target mixing.
This mixing is vital to the water treatment process because it is the engine that drives floc formation. When alum is added to water it decreases the net charge on suspended dirt particles in the flow. When particles collide in an un-coagulated stream (one not treated with alum), they tend to repel one another. Conversely, particles in a coagulated stream are more likely to actually stick together and form larger conglomerates. These larger clumps of particles are referred to as flocs.
At the beginning of flocculation the particles are small. The vertical flocculator has many more tight turns, and therefore creates much more mixing in the early sections of the tank as compared with the later sections. As the stream mixes more and larger flocs form, the number of baffles per unit length of the tank decreases to reduce the strain on the larger flocs. The exact values for mixing parameters at various stages of floc formation is currently a central focus for the AguaClara laboratory research team.
Research
Developing Turbity Profiles along the Flocculator
Construction History
Since 2004, Cornell University's AguaClara team has worked in conjunction with Engineers for a Sustainable World (ESW), and Agua Para El Pueblo (APP) to design and build four water treatment plants in Honduras. In addition to providing clean water to La 34 and Ojojona, a plant in Marcala is under construction and a design for a plant in Tamara is also being completed. The plant at Ojojona also functions as a pilot operation, demonstrating successes and potential problems for future plants.
In spring 2007, the Vertical Flow Pilot Plant sub-team worked with various Cornell University staff to design and build a larger-scale vertical flow flocculator at the Cornell University Water Treatment Plant (CUWTP), to facilitate testing under turbulent flow conditions. The new flocculator more closely models Ojojona's existing configuration, hopefully allowing for more practical testing. This experiment will also allow verification and possible reduction of the large Gθ range (20,000 to 150,000) recommended for community-based flocculation. Flocculation effectiveness is influenced by a number of factors, including coagulant dosage, mixing value, influent turbidity and velocity gradient.
Design
Flocculator Maintenance
Constant communication with the Cornell Water Treatment Plant was needed to attempt to integrate our pilot plant into their facility. Some modifications would be necessary after transportation and set up in the tank at the plant. We fabricated a frame to hold the rapid mix in place on the side of the tank. The inlet pipe had to be redirected to fit into the rapid mix pipe. Directing the inlet pipe directly straight down into the top of the rapid mix pipe would give us the maximum flow from that pipe. The outlet pipe was reconstructed to prevent leakage. Water from the treatment plant is directed to us before treatment. After the water is treated in our system the effluent exits the tank and reenters the treatment plant at the same point through a nearby drain. Assembly and installation of the turbidimeters, sedimentation tubes, and alum feeder were completed in the same day.