Bench Scale Model Design

In order to test the theory of the Stacked Rapid Sand Filter, a bench scale model was built. Each layer of the filter consisted of 20 centimeters sand. The model was built using a 4 inch PVC pipe. This diameter was chosen since the cross sectional area is small enough to feasibly deliver the required flow rate of 4800 L/min during back wash, yet the diameter is large enough to reasonably test the idea of using slotted pipes as the influent and effluent into the filter bed. The slots in the pipes are narrow enough the sand cannot enter them, and yet numerous enough that head loss through the slots is minimal. The filter consists of 3 influent pipes, and 2 effluent pipes, which thus creates 4 filter layers. The total depth of sand in the filter is 80 centimeters.

Figure 1: Diagram of the bench-scale stacked rapid sand filtration system. Arrows show expected flow directions during normal filtration.

In our experimental trials, we need to add clay and alum to the influent tap water line. Based on the flow rate, we have stock solutions of clay and alum that are pumped in to the tap water line to deliver a raw water supply of 5 NTU, with an alum dose of 1.5mg/L. The alum dose of 1.5 mg/L is calculated based on 30 mg/L dose per 100 NTU influent. Both the influent and effluent turbidities are recorded using Process Controller.

Figure 2: Process-flow diagram showing major system components for the proposed experimental trials