Apparatus
Overview
The complete experimental assembly consists of synthetic raw water (SRW) and coagulant metering system, rapid mix and tube flocculator, and flocculation residual turbidity analyzer (FReTA). (Figure 1) The turbidimeter with a feedback loop is used to measure the influent turbidity and the turbidimeter of FReTA measures the effluent turbidity.
Figure 1 Schematic of the complete experimental assembly. (Updated from Ian Tse's MS thesis, Aug. 2009)
SRW and coagulant metering system
The SRW and coagulant metering system includes a reservoir of clay suspension (10g/L), a reservoir of tap water and three pumps (Figure 2). The clay suspension is added to the tap water periodically by the regulation of a solenoid pinch valve. Both alum stock and raw water (with added clay suspension) are drawn by the pumps to mix together. A portion of raw water is pumped into a turbidimeter for the measurement of influent turbidity and then flows back into the raw water feedstock to maintain a constant volume. A flow accumulator connected to the pump dampens the periodic pulses caused by the pump rollers. 
Figure 2 Synthetic Raw Water (SRW) and coagulant metering system. (Updated from Ian Tse's MS thesis, Aug. 2009)
Rapid Mix and Tube Flocculator
After the flow of the raw water and alum is merged, they enter into a spiral “rapid mix coil ” to accelerate their blending. The mixture then flows through the laminar tubular flocculator. The tube flocculator is comprised of three spiral tubing units (3/8” ) wrapped in a figure eight around six parallel support cylinders (two cylinders per unit, see figure 3). The tubes are arranged in a helical coil because the flocs would stay at the bottom in a straight tubing system, restricting the growth of the flocs. A calibrated in-line pH probe is inserted at the beginning of the flocculator tube and provides a readout of the mixed suspension's hydrogen ion activity. A pressure sensor is also connected to the tubing to protect the system. Velocity gradients in the tubular flocculator cause particles to collide and form flocs. For coiled tube flocculators, velocity gradient is a function of the flow rate and the inner diameter of the tube. 
Figure 3 Tube flocculator. (Updated from Ian Tse's MS thesis, Aug. 2009)
FReTA
FReTA is an apparatus for measuring the sedimentation velocity and the effluent residual turbidity from the flocculator ( Figure 4). There are basically three components of FReTA: an inline turbidimeter, a settling column, and an computer-actuated ball valve. The plastic housing of the HF Scientific MicroTOL 2 IR inline nephelometric turbidimeter was changed in our apparatus to allow a glass column to fit through the entire housing and through the measurement area. Before use, FReTA is carefully calibrated using a HF Scientific, Inc. Primetime Calibration Standards kit. 
Figure 4 FReTA. (From Karen Swetland's dissertation, Aug. 2012)
The glass settling column serves as a chamber for floc settling without affecting the structure of flocs. Before measurement of floc settling velocity, it is important to minimize the movement of fluid inside the column. This is accomplished by ramping the flow to a stop and closing the valve at the top of the tube.
The function of the electrically actuated ball valve (Gemini Valve model 630) was to prevent flocs in the tube flocculator from falling into the settling column once the measurements begin and to hydraulically isolate the water in the tube. The distance between the bottom of the ball valve and the center of the LED zone of the turbidimeter is used to calculate floc settling velocities. The equation for sedimentation velocity of flocs is given below.
Vs= Z / t where Vs = sedimentation velocity
Z = distance between the bottom of the ball valve and the center of the LED zone of the turbidimeter
t = settling time