Methods
To determine the effects of inner-diameter, flow rate, and floc blanket height on settling efficiency, we completed an experiment to vary these three parameters. Based on the two-inch lamella spacing currently used in AguaClara plants, we chose a range of diameters less than two-inches to push the lower-limit of lamella spacing. Similarly, the range of flow rates that we tested was based on the current capture velocity used in the plants. The final variable in this experiment, the floc blanket height, was set to both fully submerge the inlet of the tube settlers and to leave the inlets resting above the top of the floc blanket. Table 1 below illustrates the parameters for the experiment.
Table 1. Parameter 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.
Preliminary experiments were conducted to determine the appropriate time interval for the flow and increment states. These experiments consisted of running the system for an exorbitant length of time to identify the timeframe necessary to reach constant effluent turbidity. Ultimately, we decided to form the floc blanket for three hours; run each flow rate for six hours; stop effluent flow for ten seconds between flow rates; and allow about twenty minutes for the floc blanket to grow from its low to high depths. The floc blanket was reformed each time a new diameter was tested.
Results
Let us begin by discussing the failure. As stated in the methods section above, this experiment primarily tested tube settlers of three diameters. That is, three diameters that did not exhibit utter failure under nearly every condition. In fact, tubes of 9.5 mm and 12 mm inner diameter were tested as well. These narrow tubes exhibited failure readily. The tubes would clog with flocs and then, eventually, the water flow would push the clog through to the effluent. This process of clogging did, however, lead to a few interesting observations.
To begin, we observed a "rolling" movement of the floc as it travels up the tube. This movement can begin to be described by a quantitative drag analysis which is in the process of being completed. It is hypotheiszed that the flocs in the smaller diameter tubes experience enough drag force tooverpower the settling force due to gravity.
The wider tube settlers, as expected, provided slightly more sensitive data. That is, the effluent turbidity was measurably impacted by changes in flow rate, floc blanket height, and tube diameter. The table and graph below illustrate the average effluent turbidity for the three diameters described in the methods section above.
_Figure 1. Results
Table 1. Parameter table.
These results are displayed graphically below.
_Graph 1. Effluent Turbidity vs. Critical velocity
!PSSAvgEffTurb vs Vc.jpg |width=700px height=1200px!
As the results show, the second lowest capture velocity for each diameter tube resulted in the optimum settling efficiency at the low floc blanket heights. The results also show a general trend of increasing effluent turbidity once the capture velocity exceeds 11.0 m/day. This indicates that the capture velocity designed to in the current plants is appropriate.
However, as the graph clearly illustrates, there are no well-defined trends between diameters and floc blanket height at the capture velocities where effluent turbidities are under ~.5 NTU. Perhaps this is because the effluent turbidity is exceptionally sensitive to disturbances when it is at such a low level.
what are the newest small tube data? this data could probably be reworked to fit better.
_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, 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._