Tube settling of high-rate pond algae

Abstract

The effluent of a high-rate pond (HRP) may contain 200 to 400 mg/L of algal suspended solids (SS). Microalgae must be separated before the effluent is discharged. Gravity sedimentation is usually the first method considered in algal wastewater treatment systems. However, required overflow rates (OFRs) to remove algae in the conventional clarifiers are too low. Therefore, the shallow-depth sedimentation concept was studied for the separation of Micractinium from HRP effluent in order to increase the OFRs of gravity settling equipment. Using the orthogonal squares experimental plan, the design parameters, such as OFR, tube diameter, tube length, and inclination, were evaluated for algal SS removal. A circular upflow clarifier was run as a control. At an OFR of 8.15 L/m2 · min (0.2 gpm/ft2), SS removal efficiencies of tube settling and upflow clarifier were 80% and 18%, respectively. When the OFR was increased to 16.3 L/m2 · min (0.4 gpm/ft2), SS removal efficiencies decreased to 61% in tube settler and to 11% in upflow clarifier. SS removal efficiencies of tube settlers were 40% at 30.6 L/m2 · min (0.75 gpm/ft2) and 20% at 71.3 L/m2 · min (1.75 gpm/ft2). A linear relationship exists between removal efficiency and the diameter and length of tubes. The relationship is parabolic for the inclination of tubes and the OFR. The overall algae removal efficiency increased with the tube length but it was an inverse function of OFR, tube diameter, and inclination. At an OFR of 12.2 L/m2 · min (0.3 gpm/ft2), algae removal efficiencies were 7 to 8 times better than that of upflow clarifier. It was demonstrated that OFR in a gravity settler could be increased 4 to 5 times using tube settlers.