PROCESS DESIGN AND OPERATIONAL MODIFICATIONS OF OXIDATION DITCHES FOR BIOLOGICAL NUTRIENT REMOVAL

An oxidation ditch system was modified for excess biological phosphorus and nitrogen removal by connecting two oxidation ditches in series with a small anaerobic cell (10 percent of total reactor volume). The process was operated with 40 to 50 percent of the influent flow entering the anaerobic cell. The rest of the influent was bypassed to the first oxidation ditch. The detention time in the anaerobic cell was adjusted as a process control parameter to develop low oxidation reduction potentials in cold weather or in situations where the influent flow entrained substantial quantities of dissolved oxygen. Average performance levels of 0.6 mg/L for total phosphorus and 4 mg/L for total nitrogen were achieved under temperate conditions. Secondary effluent alkalinity, turbidity and ortho-phosphorus measurements were used as alternative control parameters in addition to dissolved oxygen measurements in the oxidation ditch. Performance testing was carried out at a second oxidation ditch system to estimate the temperature sensitivity of nitrification. It was found that the temperature sensitivity coefficient was only 1.066 1/Celsius in the temperature range between 15 and 25 Celsius.