Abstract
Oxic-settling-anaerobic (OSA) process has been introduced into the treatment line of wastewater in order to upgrade activated sludge processes and to reduce the production of excess sludge. The aim of the present study was to simulate the performance of an OSA pilot plant by implementing the Activated Sludge Model No.2d (ASM2d) into a mathematical modelling software (BioWin). The stepwise calibration, performed both by off-line experiments and software dynamic calibration, was carried out in a heuristic way, adjusting the parameters values that showed a major influence to the effluent and internal concentrations. All the reduction factors introduced into ASM2d to simulate the processes occurring in anoxic and anaerobic conditions were lowered in order to reproduce the concentrations of interest. In addition, the values of parameters of the PAOs (polyphosphate accumulating organisms)-related process (namely qPHA and YPO4) were found lower than those usually adopted. In general, theoretical results were in good agreement with the experimental data obtained from plant’s operation, showing an accurate predictive capacity of the model. Good performance was achieved considering the phosphorus removal related process, while some failures were detected in COD and ammonia simulations.
Highlights
The substrate removal process was simulated with the Activated Sludge Model No.1 [14], and the results showed that the heterotrophic maximum growth rate and the maximum hydrolysis rate were lower for the OSA system compared to the control system (CAS), while no differences were found for the endogenous decay rate
During the dynamic calibration the ηPAO value increased from 0.07 to 0.16. This was done under the hypothesis that the denitrification process accomplished by polyphosphate accumulating organisms (PAOs) bacteria improved over time due to an increased amount of denitrifying PAO (dPAO) fraction or an enhancement of anoxic process rates
The final found value of 0.16 is lower than 0.60, as proposed by Activated Sludge Model No.2d (ASM2d) developers [18], but comparable to those found by García-Usach et al [19], ranging from 0.23 to 0.25
Summary
A portion of the recalculated sludge is treated in the SHT, and it is introduced into the aerobic reactor. This cyclic stress condition alters bacterial metabolism, leading to biomass reduction [3]. The implementation of an additional tank in the return sludge line increases the investment cost, OSA process is an environmental-friendly technology with several advantages: (i) the proven excess sludge reduction [1,4,9,10], (ii) the process stability, and (iii) the easy implementation in current conventional activated sludge system (CAS) associated to low operational costs [3,11]
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