High-rate ponding (HRP) processes have evolved from conventional oxidation ponds and could play an important role in the treatment of organic wastewaters in sunbelt communities requiring tertiary treatment. HRP systems are very efficient in removing biochemical oxygen demand, nitrogen, and phosphorus. Mineralized nitrogen is removed by algal uptake of ammonium ion (NH4+) and out-gassing of ammonia (NH3). High daytime pH generated in the pond due to algal uptake of bicarbonate shifts the equilibrium in favor of NH3. There are also two mechanisms for phosphorus removal in an HRP; algal uptake and chemical precipitation. Phosphorus uptake by algae is lower than nitrogen uptake because the nitrogen content of algae is approximately ten times more than the phosphorus content, which is approximately one percent of the 100 to 300 mg/l algal dry weights in an HRP. If sewage contains 10 mg/l of phosphorus, algal phosphorus uptake would be only 1 to 3 mg/l. Precipitation of phosphates with polyvalent cations such as calcium and magnesium also occurs in a HRP due to the high pH. This precipitation is sometimes called “autoflocculation”, which is often incomplete due to insufficient calcium and magnesium concentrations in the wastewater. In the case of Richmond, California, where the studies were conducted, the sewage has low magnesium and very low calcium concentrations. Enhancement of calcium and magnesium deficient autoflocculation was studied by adding 20 to 80 mg/l of freshly slaked lime to the pond during the continuous paddle wheel mixing. This simple procedure improved phosphorus, nitrogen and algae removal efficiencies to a level greater than 90%.

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.

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