Mixed-Cell Raceway: Engineering Design Criteria, Construction, and Hydraulic Characterization

A prototype raceway was constructed in a research greenhouse at the Conservation Fund's Freshwater Institute to study engineering design criteria and the hydraulics of a large mixed-cell raceway. The raceway measured 16.3 × 5.44 × 1.22 m and was constructed of structural lumber with a high-density polyethylene liner. The basic rationale of a mixed-cell raceway is to be able to operate it as a series of square or octagonal tanks, each having a center drain for continuous removal of solids and sludge. A series of vertical manifolds along the sidewalls directed the water through orifice discharges parallel and perpendicular to the walls to establish the desired rotary circulation. This was then combined with the concept of the “Cornell-type” dual-drain system, where 10–20% of the total flow into a tank is removed from a center bottom drain and 80–90% of the flow is removed from the side upper drain. Settlable wastes and sludge are then removed from the center drains and collected in a settling sump. Velocities were measured in three dimensions in representative cells at three depths (5–10 cm off the bottom, middepth, and 10 cm below the water surface) on a 0.5-m grid pattern over the raceway floor. The raceway mean rotational velocity was 5–6% of the inlet jet velocities. The results from this study can be used to further refine the engineering design criteria and demonstrate the potential for employing mixed cells as a new design concept both for production systems and for retrofits of existing raceway systems.