Tube Settlers

Tube settlers are used to test effluent turbidity at different locations. The tube settlers were designed to mimic the sedimentation tank that would traditionally follow the flocculator. Tube settlers were chosen because they provide an inexpensive way to sample and create a minimal disturbance within in the tank. Using this method, different locations of the tank can be sampled. Data gathered can be used to assess how each stage of the tank is affecting the final effluent turbidity. The three glass tube settlers used were of 60 cm length and 2.5 cm diameter. A peristaltic pump was used to transport the water to the three turbidimeters.

Once the equipment was gathered, the next step was to design the flow rate for the peristaltic pump. The assumptions used in the calculation for the flow rate were that the optimum angle for tube settler is 60° and the critical velocity is 10 m/day.

Sixty degrees is used because it is the angle at which the distance required for floc settling is minimized and still allows the solids that settled on the side of the tube to slide down. The critical velocity is taken from a range of accepted values and has been found to be the critical velocity in previous plants in Honduras. The flow rate was calculated using the following equations.

Schematic of the Data Collecting Tube Settlers

Dimensions and Variables chosen:

• w = diameter = 2.5 cm
• L = length of the tube settler = 60 cm
• alpha = optimum angle = 60°

The following equations were adopted from Shultz and Okun for determining critical velocity for up flow through a tube. The flow rate calculated for our initial configuration was 44 mL/min. There is a linear relationship between pump speed and the flow rate through the settling tube. The flow rate though the settling tube also has a linear relationship to the critical velocity of the sedimentation process in the tube. It is important to note that the critical velocity of the settling tubes is the same as the critical velocity that can be found in the sedimentation tanks at Ojojona.
The setup of the tube settlers in the tank was the next design step. Originally the tube settlers were to be hung from the edge of the tanks at designated locations. On further inspection however, when laid between the baffles on top of the connectors they are at the correct angle and so can easily be relocated and do not require any attachment to the tank.
The final design consists of the tube settlers nestled between the baffles, and then connected to the peristaltic pump. The peristaltic pump pulls water from the peristaltic pump at the correct velocity and the water is routed through a turbidimeter in order to measure the turbidity. Also installed is a turbidimeter that measures the influent turbidity of the water before it reaches the tank. The turbidimeter is gravity fed.

Moving the Tube Settlers

As is documented in Turbidity Profiles, when in an the experiment, the tube settlers 2 and 3 are moved to different places along the flocculator and turbidimeter 4 is always testing the turbidity of the water at the end of the flocculator. Tube settler 4 should be emptied along with moving tube settlers 2 and 3. 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.

Clogged Tube Settlers

Clogged tube settlers are the result of over alum dosing. Excessive alum doses result in the formation of large buoyant chemical flocs.