Please note that the data from the 15.1 mm tubes at the high floc blanket was a convincing outlier..it's data are not included in the results section below and we should consider doing that trial again.
Methods
In order to determine the effect of the inner-diameter of the tube settlers on settling efficiency, a three-part experiment was completed to vary flow rate, inner tube diameter, and tube inlet location. A range of flow rates were run through the tube settlers, 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. Critical velocity table.
As described on our apparatus design page, this experiment used the process controller program to automate the system. The main process controller states for the experiment were floc blanket formation, two flow states for high and low floc blanket heights, a floc blanket equilibrium state, and two states devoted to incrementing the flow rates. Several other states, such as drain, were used for system maintenance.
Results
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.
Preliminary 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.
Graph 1 The average effluent turbidity of the 9.5 mm tubes at a high floc blanket, with very high Vc values.
The graph clearly indicates that at the first Vc, the 9.5 mm tubes performed very well, with effluent turbidities around 0.5 NTU. 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.
Conclusions and Future work
From the preliminary results we were able to troubleshoot many of the errors in the system. Future work includes testing all of the parameters in the above chart at the two floc blanket heights. The results were not conclusive enough to determine the appropriate time interval for the flow states, so we will stick with the 6 hour time period.