Chemical Doser Design

h1. Design Modifications and Improvements

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h3. Balancing the T connection at the pressure break
The flow control module (FCM) outflow tubing connects to the T in the selector, which created an unbalance in the T, causing it to tip towards the FCM.  This was fixed by balancing the T connectors on the FCM side with two weighted nuts on the other side.

h3. Reducing erroneous variation in lever arm-induced driving head
The chemical dosing tube and its connection to the lever arm were too heavy for the fishing line used.  The combined weight was causing the lever arm to sag down and cause many more centimeters of driving head at the farther distances down the arm than closer to the pivot.  This makes logical sense based on the principle of moments around a pivot.  The whole connection was taken off of the dosing tube and replaced with a simple string that tied around the lever arm instead of clamping to it.  The string was taped in place to prevent slipping, and it appeared to greatly reduce the problem of changing driving head from apparatus weight

The chemical doser (CD) has three principle components: a float, a lever arm, and a flow controller (FC).

The float currently used in the lab for data collection is a 45.5 cm long 4 in PVC tube capped at both ends and partially filled with sand so that it weighs 8 lbs. It rests 33.5 cm below and 12 cm above the water line in a virtual grit chamber, which is a deep bucket with a pressure sensor at its base. This pressure sensor provides readings of the water level in the bucket.

The float is connected via low-elasticity fishing line to the lever arm. The lever arm is a 68 cm length of 1 1/4 in schedule 40 PVC pipe with a pivot point at 34 cm (the center). Opposite of the pivot there are notches cut into the pipe at 5 cm length intervals. The chemical doser hangs from these notches.
A 1 1/2 in schedule 40 PVC pipe attached to a flange base holds up the lever arm.

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The FC should be attached to the base pipe with two clamps that are interconnected so that they firmly anchor the FC at the appropriate height for zeroing the CD. Only one clamp was used during experimental testing because the FC was being continually checked and re-zeroed, and it was being used in a very controlled environment. Additional clamps would be necessary for robust plant operation.

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The chemical dosing tube is a tube with an open T connection at the top, which allows the FC outflow tubing to drain through a pressure break T. The tubing exiting the pressure break T has a larger diameter than the tubing entering it. The pressure break and larger outflow tubing allow a free surface at the same height as the discharge to the rapid mix (minus head loss) to form inside the larger tube while air can also flow through the tube. Currently ¼ inch outer diameter tubing is used to connect the flow controller to the pressure break T, but it is not known what size tubing is necessary to allow air to flow after the T. Experience has indicated that ½ inch outer diameter tubing is large enough, but it is possible that tubing with an outer diameter as small as 3/8 inch could work as well. The larger tube is then connected to 1/4 in outer diameter tubing, which connects the CD to the rapid mix. It is unclear if this final connection is needed, though, and it may be removed so that the larger tubing connects the T to the rapid mix directly.

For laboratory testing, water flows from the bottom of the chemical dosing tube to a plastic column with a pressure sensor at the bottom. This pressure sensor gives readings of water height in the column, which can be converted into flow rates from the FC.