Methods
In order to determine the effect of the inner-diameter of the tube settlers on settling efficiency, a three-part experiment will be completed to vary flow rate, inner tube diameter, and tube inlet location. The experiment will consist of running a range of flow rates through tubes, with inlets both in and above the floc blanket, with a range of diameters. Table 1 below illustrates the parameters for the proposed experiment.
The diameters of the tubes were chosen to explore a range of spacings based on the current plate spacings used in plants of two inches. The flow rates were chosen to explore a range of capture velocities from 5-20 m/day.
Table 1: Tube flow rates vary due to the critical velocity and the diameter.
As the tube diameter increases the necessary flow rate to achieve the same Vc also increases. Note the high flow rates for the 9.5 mm diameter tubes and the 12.7 mm diameter tubes. These flow rates actually represent the flow rate through the manifold holding six tubes, there for the per tube flow rate can be determined by dividing the flow rate by six. The final three tube sizes 19.5, 25, and 36.1 mm, will only be tested using one tube at the appropriate flow rate.
The main process controller states for the experiment are floc blanket formation, two flow states for high and low floc blanket heights, a floc blanket equilibrium state, two states devoted to incrementing the flow rates. Several other states, such as drain, are used for system maintenance.
We expect to see failure in the lower diameter tubes with the high Vc values because the flocs have a smaller area over which they can settle. The smaller diameters are expected to with flocs more quickly, and are less likely to have flocs waste out of the tube as the flow state is running.
Results
Priminary experiments have been conducted for the purpose of determining the appropriate time interval for the flow and increment states, as well as determining which Vc and floc blanket combination cause failure. Based on these preliminary findings we are modifying the experiment to produce quality results in an efficient manner.
The plot below displays the effluent turbidity vs. critical velocity graphed on a semi-log plot. Each flow state was run for 6 hours, however, the floc blanket was at the high setting of 60 cm, as opposed to the desired low setting, due to air blocking the flow through the solenoid valve. There were also error in both the initial calculations of the flow rate and the tubing size. Instead of testing at a Vc range of 5 to 20 m/day a much higher Vc range of around 39 to 131 m/day. The results are display in the graph below.
The graph clearly indicates that at the first Vc, the 9.5 mm tubes performed very well. The next three Vc settings result in very high effluent turbidities, over 100 NTU over the influent. This high values are most likely cause by a combination of extremely high Vc and the tubes position in the floc blanket at the upper height of 60 cm.