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A comparison of four different established models along with parameter estimation was carried out in order to explain the aerobic biodegradation of acetate in an activated sludge system. These models were investigated using experimental OUR data from batch experiments of three different concentration studies. Model calibration reveals that ASM1 model is not suitable to explain the observed experimental OUR during the famine phase implying storage compounds could play an important role during that stage. Besides, the model corresponds to the accumulation concept and is not well fitted for all concentrations studies though it includes the storage phenomena. Both the ASM3 model and the model for simultaneous storage and growth on substrate can well describe the acetate biodegradation process, however the OUR data alone is not sufficient to justify the suitability of those models. Simulated profiles using the model outputs demonstrate that storage is overestimated while ammonia degradation is underestimated in ASM3 compared to simultaneous growth and storage model. The current study also gives reasonable outcomes related to parameter estimation as compared with previous study which is statistically interpreted in this paper.

In this paper, spatiotemporal analysis of groundwater level fluctuations of 32 piezometric wells using geostatistical analysis was done for Mymensingh district. A total nine years of weekly ground water level data were used for the analysis. Geostatistical analysis was performed using ordinary kriging and empirical Bayesian kriging (EBK) methods. The semivariogram models called spherical, exponential and Gaussian model were fitted with the experimental semivariogram in ordinary kriging while semivariogram fitting is automatic in EBK. Model performances were tested using root mean square standardised error (RMSSE), root mean square error (RMSE) and average standard error (ASE). The cross-validation results indicate that EBK performs better comparing to ordinary kriging in representing the spatial groundwater level fluctuation in the study area. The geostatistical analysis result shows that the Phulbaria, Trisal, Muktagachha, Bhaluka, Gafargaon and Mymensingh Sadar Upazila is comparatively more vulnerable than other parts of the district.

ab s t r ac tUrea biodegradation kinetics determination has been performed in the literature using a two-step nitrification model that was calibrated using on-line respirometric measurements. However, the model neglected the initial hydrolysis step that converts urea to carbon dioxide and ammonia nitrogen, and assumed constant carbon dioxide transfer rate (CTR), though it is inherently a nonlinear process which has an impact on the titrimetric modeling. Hence, in this paper, it is aimed to propose a complete two-step nitrification model for urea biodegradation paying attention to urea degradation pathway along with due consideration given for non-linear CTR process occurring in activated sludge system. Experiments were performed in a simple batch reactor equipped with respirometric and titrimetric set-up. Three different initial urea concentrations were added to the reactor for investigating the process kinetics. Proposed model was successfully calibrated with respirometric, titrimetric and combined respirometric-titrimetric measurements; and the estimated parameters were compared for model evaluation. Furthermore, the proposed model was validated with off-line ammonium, nitrite and nitrate measurements. The study revealed that urea was hydrolyzed at a faster rate in liquid phase. The maximum growth rates of the Nitrosomonas species and the Nitrobacter species were found to be 0.065-0.1 d -1 and 0.006-0.008 d -1 respectively.