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.
Future Research
The goal of this project is to determine the most efficient method of sedimentation. The two technologies that have been considered are floc blankets and lamella. There are some concerns about building a sedimentation tank that relies solely on a floc blanket. It takes time to build up a floc blanket that works well, especially if the incoming water has a low initial NTU. Also if an improper alum dose if given to the flocculator for any amount of time the floc blanket may disappear, removing the only method of sedimentation. Having some addition settling mechanism such as lamella would help with with settling during the times when the floc blanket is not performing well. It is the goal of this project for this summer and the upcoming semester to determine the reliability of floc blankets and determine how difficult would be to maintain a floc blanket even during times when turbidity is low, if alum is not being dosed correctly, or if only small flocs are being produced.
Other research projects include developing methods that would encourage faster formation of floc blankets. Other ideas included adding a mesh fabric filter at the top of the tank just below the launder to capture an tinp floc that escape above the floc blanket. This mesh would also allow for higher V up values to be used the nesh would act a secondary capture for the smaller floc not captured in the blanket. Also given that φ floc is the major factor affecting sludge blanket formation, techniques to affect φ floc could also created stronger and better floc blankets.
Possible modifications of the floc blanket tank include: lowering the continuous flow sludge removal rate to minimize water waste, optimizing floc blanket performance by modifying the blanket height and installing a clay solution feed to increase inflow turbidity to better mimic conditions in Honduras.
After construction of the lamella sedimentation tank, research will be done to see if anything can be done to improve setting in this design. We also want to analyze the possibility of forming a floc blanket underneath the lamella. The combination of these technologies seems like a viable option.
Inline Turbidity meters can hopefully be procured to test the outgoing turbidity from each sedimentation tank. This will allow testing how well sedimentation tanks perform over time and during various conditions, such as performance of the flocculator and raw water turbidity.
The operators at Ojojona experimented with a sludge judge to monitor the sludge level, but discovered that it broke flocs and resulted in a turbidity spike. They are no longer using the sludge judge. It will be very important that plant operators have an easy method to maintain and monitor the floc blankets while the flow rate through the plant varies.