Experimental Set-Up
In an attempt to recreate surface foam in a lab setting, similar to that found in AguaClara Plants in Honduras, we first had to recreate a similar design in the lab. We did so by first having a tank which mixed tap water and clay in order to have a constant turbidity of approximately 100 NTU. This tank represented the incoming plant water. Water from this tank was then mixed with Natural Organic Matter (NOM) and sent into Tank 2 which simulated the rapid mix tank and first baffle of the flocculation tank. Once in this tank, the turbidity of the water was once again measured, and alum was dripped onto the surface of the water. The water in the rapid mix side of tank 2 was mixed with a stirrer before traveling into the first baffle portion of the tank. It was here that the water was able to settle a bit and a webcam took pictures of the surface water every two minutes to determine if surface foam was created. When an aerator was added, it was placed in the first baffle portion of tank 2.
In order to calculate the energy dissipation rate at the rapid mix due to the tubing, first we calculated the head loss (h) of the tubing using the equation,
Unknown macro: {latex}
\large
$$
h = \left( {{Q \over
Unknown macro: {ka}
}} \right)^2 {1 \over {2g}}
$$
Then we calculated the residence time (θ) using,
Unknown macro: {latex}
\large
$$
\theta = {{d_
Unknown macro: {tube}
} \over v}
$$
Where,
Unknown macro: {latex}
\large
$$
v =
Unknown macro: {Q over a}
$$
Finally energy dissipation (ε) was calculated using the following equation,
Unknown macro: {latex}
\large
$$
\varepsilon = {{gh} \over \theta }
$$
Where:
h= the head loss due the tube,
Q = is the flow rate in mL/min
a = the area of the tube
k = constant magnitude of 0.6
g = gravity
θ = the residence time
dtube = the length of energy dissipation of the jet, which was estimated on the length scale of the diameter of the tubing.
V = speed the flow
ε = energy dissipation
We obtained the same order of magnitude for energy dissipation as the AguaClara plants.
Unknown macro: {float}
!Experimental_Design_Setup.jpg|width=300px,align=centre!
Illustration of Experimental Setup
Our experiment included the following parameters: Coagulant(Alum) dosage, concentration of Natural Organic Matter (NOM), location of alum addition, aeration of water and addition of a surfactant. In the first week of our experiment, we tested for alum dosage in order to determine whether it caused foam formation. Alum dosages of 35 mg/L, 45 mg/L, 55 mg/L and 65 mg/L were added to the rapid mix chamber.
The following week the group used Humic Acid in order to determine the effects of NOM on surface foam formation. Varying the dosage at 1mg/L, 2mg/L, 5 mg/L and 10 mg/L, Humic acid was mixed in the rapid mix chamber with 45 mg/L of alum.
After deciding to add an aerator, we kept the parameters the same as in the NOM experiments, however we added an aerator to the first baffle portion of tank 2.
Finally to study the effect of hydraulic jump on bubble formations, we varying the height of the water fall at 12.4cm, 14cm, and 15.4 cm. (Did you try much greater heights and much smaller heights to observe a difference?)
After determining both the cause of the foam and theoretical ways to prevent the water from aerating, our next task was to produce designs for retrofitting the current AguaClara plants in operation.