PPST Sludge Blanket Results

Sludge Blanket Results

At the end of June 2008 an inline turbidity meter was installed to compare the floc formation in the floc blanket tank to the results coming for the tube settlers at the end of the flocculator. The turbidity meter was set up to draw samples from the plant leveling tank set-up after the launder out of the sludge blanket tank. After this set up was completed the tank was run at up-flow velocities of 30, 50, 70, and 100 m/day. At all of these velocities the tube settlers out of the flocculator performed better than sludge blanket removal system. Graphs of the outlet turbidity versus time for the sludge blanket tank and the final tube settler in the flocculator can be seen below.

At 30 m/day there was very little suspension of the floc blanket, it was mostly settled sludge. All floc that weren't heavy enough to settle on their own rose through the tank and exited the tank through the launder. (To examine a larger picture of the graph, please click on the graph to the right)

At 50 m/day the tank performed decently. A good floc blanket formed in the tank and as long as the blanket was kept a few inches below launder. The suspension was thick and fairly stratified, thicker at the bottom and progressively thinner closer to the top. Still some fairly decently sized floc were escaping to the top. It was suspected that this flow rate was too low and that the shear in the blanket could possibly be breaking up flocs. This was a suspicion that was difficult to prove.

At 70 m/day the sludge blanket was thinner than at 50 m/day but still well developed and thick at the bottom. The floc rising through the blanket were about the same size as the in the 50 m/day tank and the results were in consistent. Sometimes better than the 50 m/day but also worse.

At 100 m/day the blanket was stratified and well formed but much thinner toward the top in comparison to the 50 m/day tank. The floc that were rising up through the tank were much smaller than at the other velocities suggesting that there was less shear in the blanket at this higher velocity and that flocs were not being broken up and rising through the blanket. The 100 m/day velocity needs to be re run to confirm the low shear theory given that changes were made to the flocculator during the testing of various velocities.

These results show that a sludge blanket alone is not as effective a method for settlement removal as the lamella simulated by the tube settlers. Thus now it is being explored if results can be improved by combining the technologies of sludge blankets and lamella.