Energy consumption and performance of a rotating biological contactor treating high-strength wastewater

Abstract

Clean water availability, energy costs and the environmental impact of energy usage are major concerns all over the world. At the same time, the Rotating Biological Contactor (RBC) has emerged as a low energy-consuming technology used in wastewater treatment which compares favorably with other treatment methods. RBC is a fixed-film bioreactor employing rotating discs to provide support medium for the microbial growth and to supply dissolved oxygen. RBCs, when applied in the treatment of high strength wastewater, demand some modifications such as the addition of aeration systems or change the flow configuration. Aeration systems certainly reduce the footprint but at the cost of energy consumption. Therefore, the optimization of energy consumption in a modified RBC is a very relevant research objective. This thesis is an investigation on energy optimization in a commercial scale RBC modified with an aeration system and treating high strength synthetic wastewater. The coarse bubble diffuser was replaced by fine bubble air diffusers. To study energy consumption a mono-block main drive system and the central compressed air supply were replaced by a three phase motor with variable frequency drive and an aeration blower respectively. Removal performance and unit energy consumption were studied at various combinations of rotating speed (2.5-5 RPM) and rate of aeration (0-15 SCFM). Constant hydraulic (0.017 m³/m²-day), organic (86.1 gCOD/m²-day) and ammonia (3.444 gNH₃-N/m²-day) loadings were maintained throughout the study. The modified RBC was able to remove 34 to 96% COD and 21 to 68% ammonia depending on the aeration rate and angular velocity. The suspended growth section of the modified RBC contributed 47 to 85% and 38 to 87% of the total removal of COD and ammonia respectively. Conversion of ammonia-nitrogen to nitrate-nitrogen was observed very negligible at 0.26 to 1.59%. The angular velocity, 3.66 RPM and the rate of aeration 8.13 SCFM, were found to be the optimum parameters to achieve minimum unit energy consumption of 1.31 KWH/kg CODr. A mathematical model correlating energy consumption per unit oxygen demand with the rate of aeration and the angular velocity was developed.