Sampling Method
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 tube settlers were designed using the following equipment.
Equipment:
- Glass tube settlers (3)
- Length: 60 cm
- Diameter: 2.5 cm
- Peristaltic pump
- Turbidimeters (3)
Once the equipment was gathered, the next step was to design the flow rate for the peristaltic pump. The following assumptions were used in the calculation for the flow rate:
Assumptions:
- The optimum angle for tube settler is 60°
- 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 Settling Tubes.
Dimensions and Variables chosen:
- b = diameter = 2.5 cm
- L = length of the tube settler = 60 cm
- alpha = optimum angle = 60°
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LaTeX Markup:
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$$
V_\alpha = V_c \left( {{L \over b}\cos \alpha + \sin \alpha } \right)
$$
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. This turbidimeter is gravity fed.
Data Collection
Process Controller was used as our main data collection tool. MathCAD programs were then used to analyze the data that was collected. Process Controller is a software package that is used to control the raw water pump, the alum pump and data collection. For the raw water pump Process Controller only controls the on/off status of the pump. When the flocculator is running the raw water pump is turned on and the flow rate is controlled by a valve that can only be changed in increments. The flow rate was calculated by partially draining the flocculator to below the outlet pipe height. The valve was then opened to a noted location and the time it took for the water to rise 5 cm was recorded. To calculate the flow rate increase in the volume of water in the tank was divided by the time it took for that volume to fill in the tank. The volume was calcuated by multiplying the height the water rose in the tank by the cross sectional area of the tank. When this technique was used the head loss over the flocculator was small and did not affect the measurement. If the head loss increases then a new way to measure the head loss will need to be created.
This choice of location for the tube settlers was chosen originally to establish general information about how flocs were forming in each individual section of the flocculator.
The states utilized by process controller allow the flocculator to run continuously and data to be collected about how alum dose and changes in Gθ affect flocculation and settled water turbidity With the configuration of the baffles spaced evenly throughout the flocculator G remains constant through the 3 sections. However, by sampling at different locations the volume of the flocculator that the water travels through changes which changes θ and thus Gθ. The tube settlers were placed one at the end of each section. Original Tube settler set-upThis figure shows the configuration that was originally tested and used to collect data. This configuration was chosen to get a general understanding of how each section contributed to floc formation and final settled water turbidity.