Abstract
A biological nutrient removal was investigated as a continuous flow (orbal) system to study the efficiency of system for removing phosphor and nitrogen compounds, and to predict the effect of internal and external recirculation rates on its performance. The pilot plant consisted of three concentric circular channels: outer, intermediate and inner in capacities of 24, 16.8 and 8.7 m3 respectively. The last sedimentation basin has sludge return system to the outer channel. Pilot plant also has an internal cycle to return the mixed liquor from the inner channel to the intermediate one. Continues Flow was used to feed the plant with raw municipal wastewater from Al-Khadraa residential quarter at an average flow rate of 66.7 m3/day. The plant was equipped with a modified surface aeration system operating in total power equal to 0.48 kW to achieve the required dissolved oxygen concentrations in the outer, middle, and inner channels: 0.38, 1.3, and 2.4 mg /L respectively. The three-channel was operated on two levels of returned activated sludge (RAS): 60% and 80%. It has three levels of internal cycle (IR): 100%, 150% and 200%. The results showed: the total removal efficiencies of phosphorous, and ammonia were in the range of: (93.3% - 88.5%) and (93.5% - 92.4%), respectively. The results meet the Iraqi standards for wastewater disposal into rivers. This study revealed there is no significant correlation between removal efficiency of ammonia with the internal and external recirculation ratios, while the effluent phosphor concentrations significantly affected by RAS and IR ratios. This study adopted the hypothesis: the process of simultaneous nitrification and denitrification (SND) occurs in the outer channel, while the phosphorus removal by the pilot plant was performed by the mechanism of enhanced biological phosphorous removal (EBPR).
