Sludge Blanket Concerns
One concern when implementating a floc blanket into the AguaClara treatment plants is the length of time it takes to develop a fully functional floc blanket. The water that goes through this sedimentation tank probably has a high turbidity during the formation period. It needs to be determined how long this could take, and explore the option of using a combined floc blanket and lamella design to continue sedimentation when the floc blanket is not fully functional. Another issue is the reliability of the floc blanket. If there is a period of time when either the influent water is clean, or for some reason alum is not being added, it is important to know if the floc blanket will be able to sustain itself, or what might happen. If the floc blanket does not sustain itself, and a new floc blanket has to be created, then this could make a substantial period of time when the sedimentation tank is not producing clean water. This is also a reason that a combined floc blanket and lamella tank may be superior.
When and how excess sludge in the floc blanket is drained is an additional issue that will need consideration. It seems unlikely that a continuous drain, like the one we are currently using, will be a good idea, because there is likely to be too much water wasted. An adequately sized hopper will be necessary and hopefully located in an area that will minimize the disruption of uniform upward flow. It will also be important to determine what may happen if the hopper is not emptied in the designated time frame. There should be an adequate safety factor before the floc blanket rises to the point that it affects the output of clean water.
Ojojona is currently using a sludge judge to monitor the sludge level, but even with the use of the "sludge judge" it may still be difficult to see all that is happening with the floc blanket, and because floc blankets seem to be fragile, it will be very important that plant operators adequately understand how to maintain floc blankets.