The Floc Blanket Quest

AguaClara has a mission of creating high quality safe drinking water using sustainable, low cost water treatment plants. Since 2008, we have been researching and attempting to add floc blankets to our suite of safe drinking water technologies. A floc blanket is a fluidized bed of flocs that are maintained in the bottom of an upflow sedimentation tank. It looks like a snow globe with too much snow. The highly concentrated suspension acts like a filter (or flocculator) that intercepts incoming clay particles and thus reduces the turbidity of the settled water. The performance improvement created by a floc blanket can be quite dramatic. The settled water turbidity can be reduced by a factor of 10, for example, from 3 NTU to 0.3 NTU. Our goal was to add the floc blanket without significantly increasing the cost of an AguaClara plant. Last week Drew Hart confirmed that we have floc blankets at the new AguaClara plant in Atima! The water in the top of the sedimentation tanks is crystal clear!

In the spring of 2008 we first created floc blankets at laboratory scale. Because floc blankets have been used in drinking water treatment plants for many decades,  I thought that taking this idea to full scale would be easy. We tried at Cuatro Comunidades in 2010, Agalteca and Marcala's second plant  in 2011, and in Alauca in 2012. Each year we modified the design of the sedimentation tank in hopes of reproducing the success from our laboratory. As each plant would start up, it would perform well, but there was no dramatic improvement in water quality and no floc blanket. We learned something from each new version of the plant design.

At Cuatro Comunidades we learned that the velocities in the channels and pipes delivering water to the sedimentation tank had to be high enough to prevent sedimentation. At Agalteca we tried a simple inlet pipe with holes releasing the water into the sedimentation tank. This is a conventional design, so we were surprised to see that flocs came rising up one end of the sedimentation tank. A team tested this tank geometry at small scale in the lab by using dye in the water to reveal the circulation pattern. The tests
showed that the water leaving the bottom of the inlet manifold caused a large scale circulation in the sedimentation tank. You can see the dye moving to the left and then up to the top of the scale model of a sedimentation tank in the movie below.

To fix this circulation problem, we added small flow straightening tubes to the inlet manifolds at Agalteca, Marcala 2, and Alauca. This eliminated the problem of large scale circulation and improved settled water quality, but there was still no floc blanket.

Flow straightening tubes added to the inlet
manifold in the bottom of the sedimentation
tank at Alauca.

We realized that there was something else at full scale that we were missing and so we designed a laboratory reactor that is a 1.2 cm slice of a 50 cm wide sedimentation tank. We tested our full scale sedimentation tank bottom geometry and it became clear that the flat drain cover bottom at the center of the sedimentation was causing sludge to accumulate and the floc blanket to fail. You can see that failure mode in the video below. The flocculated water is entering downward from a tube at the center of the sedimentation tank. The floc blanket initially forms and then collapses due to the unstable sludge.

We learned that the bottom of the sedimentation tank needs to be designed to transport all of the settled flocs to a location where they can be resuspended by the incoming flocculated water.  We needed to eliminate the flat bottom in the sedimentation tank and add a jet reverser. We've experimented with many different geometries for the jet reverser and my favorite is the asymetric design shown in the video below. Flocs come sliding down the incline and then are immediately resuspended by the incoming jet of water.

We designed the first water treatment plant with a jet reverser and no flat spot in the middle of the sedimentation tank at Atima, Santa Barbara. Atima came on line a few weeks ago and last week Antonio Elvir and Drew Hart returned to provide more training and to investigate what was happening with the floc blanket. Below is a view of the sedimentation tanks showing the crystal clear water coming off the top of the plate settlers.

Sedimentation Tank with floc blanket at Atima, Santa Barbara, Honduras producing water that is less than 1 NTU.

Drew Hart reported that the operators hadn't drained any sludge from the bottom of the sedimentation tank in almost two weeks. Instead they were draining the sludge from the floc hopper every day or two. The floc hopper fills with flocs that it skims off the top of the floc blanket and thus proves that a floc blanket has formed.

We've finally achieved floc blankets in a full scale AguaClara plant! In the coming months we will learn what is required to operate an AguaClara plant with a floc blanket. We have much to learn about failure modes, provision of good feedback for the plant operator, and how to set the optimal coagulant dose. You can track performance of the Atima plant using the new open data project. 

The Floc Blanket Quest wouldn't have been possible without the Cornell students who conduct laboratory scale experiments, the design team that turns the research into AutoCAD drawings, Agua Para el Pueblo who builds the full scale water treatment plants, the communities who own and operate the plants, and the support of  friends of AguaClara who generously contributed to make this research and full scale implementation possible.  Contribute today to bring safe drinking water to more communities.