Determining TP

Author: AguaClara Pilot Plant Spring '08 Sub-Team

Sections on Testing of the Tapered Flocculator were written by:

• Rebecca Thompson: rnt3@cornell.edu
• Narayana Pappu: nvp4@cornell.edu

Developing Turbidity Profiles along the Flocculator

Profile Test Procedure

The purpose of this experiment was to develop a profile of flocculation at different places along the flocculator. In order to do this, we moved the tube settlers to different points along the length of the flocculator and tested the turbidities of the water after it passed through the tube settler and reached the turbidimeter. In Experimental Set-up of the Tube Settler Placement above, you can see a schematic of the experimental set-up. A photograph of the same set-up is also shown. The experiment has three parts, A, B, and C, each lasting for 45 minutes. During the experiment, the location of tube settlers 2 and 3 were moved to different places along the flocculator as shown above. Turbidimeter 1 was always testing the incoming water, and turbidimeter 4 was always testing the turbidity of the water at the end of the flocculator (location 4 above). Along with moving tube settlers 2 and 3, we emptied tube settler 4 of water between parts A, B, and C of the experiment. This is because when the tube settler is filling with water, plug-flow conditions exist in which velocity gradients cannot develop, and flow up the tube settler is more even. So, by emptying tube settler 4 of water, we ensured that the potential effects of this condition in the tube settler were even across all the tube settlers.
Likewise, when performing the data analysis after the experiment, we found an increase in the turbidity to unreasonably high levels (on the order of 100 NTU) for about 10 minutes after moving the tube settlers. This was because the air in the tube settlers which was being pumped through the turbidimeters. Therefore, only the data at the end of each part of the experiment was used (approximately after 10 minutes of running). The removal of the air can be easily observed in the data when the system appears to have reached a steady-state.

Results from Profile Testing of Tapered Flocculator Set-up Spring '08

In figure the Combined Turbidity Profiles from Three Experiments graph, it can be seen that a general trend exists in which the turbidities taken from the end of the tube settlers first spike at the beginning of the flocculator to about 2 times what they were in the incoming water. Then, they take a rapid dive to a fraction of the original turbidity. The most likely reason for this is the formation of flocs which are not large enough to settle in the tube settlers at the beginning of the flocculator but are large enough to greatly increase the deflecton of light in the turbidity meters. Later in the flocculator, larger flocs form, and these have a settling velocity of greater than 10 m/day. Therefore, they settle in the tube settlers and do not add to the turbidity in the turbidimeters.
It appears that a trend also exists towards the peak turbidity happening earlier in the flocculator when incoming turbidity was high, and later in the flocculator when turbidity was lower. This result suggests that the flocculator is more effective when turbidities are higher. This is consistent with the expectation that the collision rate is proportional to the floc volume fraction, φ _floc. However, the settled water turbidity from the end of the flocculator was lower when incoming water was lower. The outgoing turbidity was 0.9 NTU, 1.3 NTU, and 1.8 NTU for the trials where the incoming turbidity was 2.0, 2.5, and 7.5 NTU, respectively. It is important to examine the absolute turbidity at the outgoing points because this is the parameter that determines the effectiveness of chlorine and overall safety of the water produced by our system. We need to achieve water that is consistently safe to drink because the reliability of our water treatment plants affects the health of our beneficiaries on a daily basis.

The graph above shows the flocculation profile versus Gθ as calculated with the model developed by Leslie Campbell of the Design Team. This model is as follows:

Where totbaffle is the number of baffles before the testing plant, Qplant is the flow rate of 100 L/min, and FlocTankwidth was the width of the flocculator sections, or 12 inches. The other parameters were set as follows: Π _cell = 2, kb =3, and the viscosity of water was 1 * 10 ^-6 m ^2/s.

By examining this graph, we can see that the total Gθ of this set-up was about 5000, which similar to the Gθ of the flocculator at Ojojona, which is about 4000.