Monitoring of microbial storage products and the effciency of an activated sludge plant performing anoxic phosphorus removal under different operational conditions

Abstract

The present work focuses on the operation of a continuous flow laboratory scale plant for enhanced biological phosphorus removal (EBPR) from synthetic wastewater. After the acclimatization period, basic operational conditions, namely the influent COD concentration and the anoxic to anaerobic tank volumes ratio, varied, in order to evaluate their impact to the system performance. The progressive increase of the influent COD concentration from 200 mg COD/l to 300 and 400 with constant influent phosphate concentration of 15 mg PO -3 4 -P/l, led to proportional increase of the intracellular PHAs used during the anoxic phosphorus uptake process and to the respective amount of nitrate denitrified. However, the highest phosphorus uptake rate and the maximum net phosphorus removal were observed for the influent COD/P concentrations ratio of 20 (300 mg COD/l). Interestingly, this was also the ratio where the maximum carbohydrates synthesis and biomass P content during the anoxic phase were observed, indicating this, as the optimum ratio for the system efficiency. In a second operational period and for this particular COD/P ratio, three different anaerobic hydraulic residence times (HRT) were imposed, by increasing the volume of the anaerobic tank, in order to achieve anoxic to anaerobic HRT ratios of 2.33, 1.75 and 1.4. The experimental results indicated that decreased ratio values led to increased PHAs utilization for phosphorus removal. Furthermore, despite the considerable increase of the specific P uptake rate, the specific anaerobic P release rate remained almost constant, irrespective to the anoxic to anaerobic HRT ratio. The maximum net phosphorus removal was observed for the minimum ratio, indicating that increased anaerobic contact time enhances the activity of polyphosphate accumulating organisms (PAOs) in the EBPR plant. The biomass growth yield was not influenced by the anoxic to anaerobic HRT ratio and remained constant at 0.27 gVSS/gCOD. Phosphorus removal under alternative anaerobic/anoxic conditions proved its practical feasibility and satisfactory efficiency in a continuous flow activated sludge system.