Spring 2008 Sed Tank Research

Spring 2008 Experiment

(Sara Schwetschenau , Kevin Borden, T. David Mcevoy)

The first sedimentation tank containing the floc blanket has been constructed. Flocs enter the tank and do not appear to be breaking up, so it has been determined that our entrance pipe was large enough. The hydraulics are working correctly and the water levels are as calculated. We also measured the flow rate of the sedimentation tank and the sludge hopper, and both were very close to the design rates.

Over the past week we have been observing how the floc blanket has been forming. Sludge has been piling up slowly and a suspension of flocs has begun to form. By the end of the first week it was very clear that a thin floc blanket had formed in the tank. Although some floc were noticed to still be making it up to the effluent manifold a large number of flocs were being captured by the floc blanket. The flocs forming the blanket were relatively large, up to 5 mm in diameter. The top of the blanket was forming around the level of the sludge hopper. This tells us that the hopper is successfully controlling the level of flocs in the tank. Hopefully with more time for development the floc blanket performance will improve as the floc blanket thickens, and the smaller flocs noticed toward the top of the tank will be captured.

It was observed that upward flow through the tank seemed slightly faster in the front corner of the tank by the effluent manifold. Upon tank draining, it was observed that the sludge at the bottom of the tank formed a conical shape. The evenness of the conical shape demonstrates that flow seems to be flowing uniformly out of the inlet and up through the tank, and thus the slight increase in flow noticed at the top of the tank is not significant, and it is not leading to a large amount of scour at the bottom of the tank. Flow irregularity is a concern because it is one of the main causes of blanket instability and poor blanket formation.

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.