Entrapped mixed microbial cell process for combined secondary and tertiary wastewater treatment

An entrapped mixed microbial cell (EMMC) process was investigated for simultaneous removal of carbon and nitrogen from a synthetic wastewater in a single bioreactor. The influent had a soluble chemical oxygen demand (SCOD)/nitrogen ratio varying from 4 to 15 and an alkalinity of 140 and 230 mg/L as calcium carbonate. An alternating schedule of intermittent aeration was used for two sizes of carriers: 10 x 10 x 10 mm3 and 20 x 20 x 20 mm3. The medium carrier (10 x 10 x 10 mm3) was found to achieve higher removals of nitrogen, chemical oxygen demand, and 5-day biochemical oxygen demand of 92, 95, and 97%, respectively. These higher removal rates occurred at a 12-hour hydraulic retention time (HRT), an aeration/ nonaeration ratio of 0.5:2 hours, and an SCOD/nitrogen ratio of 15 in the influent. Influent alkalinity concentrations of 140 and 230 mg/L as calcium carbonate were found to have minimum effect on the removal of carbon and nitrogen. However, the oxidation-reduction potential, ranging from -100 to 400 mV (during air-off period), provides better nitrogen removal efficiency to maintain the total nitrogen (nitrate, nitrite, and ammonium) less than 10 mg/L when an SCOD/nitrogen ratio of 10 or 15 is maintained in the influent. Nitrogen removal efficiency increased with increasing ratios of SCOD/nitrogen in the influent (i.e., SCOD/ nitrogen ratio of 15 > 10 > 7 > 4.0). The results of actual wastewater operation based on a study of synthetic wastewater show that organics removal efficiencies (94.6 and 94.2%) and nitrogen removal efficiencies (61.0 and 60.9%) are similar for reactors packed with medium and large carriers, respectively, when they are operated with a 9-hour HRT, an air on/air off ratio of 1 hour:2 hours and an SCOD/nitrogen ratio of 4.6.