Biological nutrient removal using an IACOD process: determining the combined effects of low temperature and long solids retention time.

This study was undertaken using an intermittently aerated cylindrical oxidation ditch (IACOD) process for biological nutrient removal. The kinetic aspects of nitrification, denitrification, and phosphorus release/uptake were investigated by conducting a pilot-scale operation under various contact times for the aerobic/anoxic reactor, SRTs, and HRTs. The effects of temperature were also evaluated. The results of the study revealed that a cycle time of 180 min. (i.e., the aerobic contact time of 60 mim. followed by the anoxic contact time of 120 min.) enhanced the biological nutrient removal at an aerobic SRT of 8.3 days and an HRT of 24 hours in the aerobic/anoxic reactor. Even during the winter months with mixed liquor temperatures between 9.6 degrees C and 12 degrees C at an aerobic SRT of 10 days, the IACOD process was capable of almost completely nitrifying the influent NH4- -N. The IACOD process was also capable of denitrifying the NO3- -N in the aerobic/anoxic reactor, yielding effluent NO3- -N concentrations of less than 2.0 mg l(-1) N. Furthermore, the release and abundant uptake of phosphorus successfully occurred at this low temperature range. The enhanced biological phosphorus removal rates increased steadily as the temperature increased from 9.6 degrees C to 22 degrees C. However, an inhibition of phosphorus release was observed at a temperature range of 18 degrees C to 20 degrees C. This inhibition might have been caused by the sudden increase in the NO3- -N concentration of the return sludge, which was induced by the rapid nitrification of the influent NH4+ -N at a relatively elevated temperature. The inhibition was not prolonged due to the subsequent increase in the denitrification rate as the temperature increased further to 20 degrees C and above.