Enhanced nutrient removal in high-rate ponds

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%.