Biological removal of hexavalent chromium: evaluation of the metabolic activity of native and Cr(VI)-acclimated activated sludge using a respirometric method

Hexavalent chromium becomes in one of the tops internationally concern environmental issues due to its wide usage in several industrial activities. There are two stable oxidation states of chromium in the environment which differ significantly on its toxicity; Cr(III) has lower solubility, mobility and lesser biological toxicity in comparison with Cr(VI). While Cr(VI) is a well-known carcinogen, Cr(III) is an essential dietary element. For this reason, most technologies focus attention on the reduction of Cr(VI) to Cr(III). On this context, the ability of microorganisms to reduce Cr(VI) to Cr(III) has gained attention. The objectives of the present work were to analyze the effect of Cr(VI) on the activated sludge community in a continuous reactor, and to evaluate the differences on the metabolic activity of native (NAS) and Cr(VI)-acclimated activated sludge (CrAAS) using a respirometric method. Results showed that the activated sludge community had the capability to acclimate to the presence of Cr(VI). On the other hand, the increase of the initial Cr(VI) concentration from 0 to 100 mgCr/L leads to a decrease in the specific exogenous respiration rate (q_Ex ) values, but this reduction was more noticeably in the case of NAS in comparison with CrAAS. The respirometric curves were well described by the proposed mathematical model. It was concluded that the CrAAS tolerated a Cr(VI) concentration about one order of magnitude higher than NAS, which was positively reflected in the respiration rate first-order decay constant (k_d ), the specific maximum exogenous respiration rate (q_Exm ), and the observed oxidation coefficient (Y_O/S ) values.

Steady-State Methodology for Activated Sludge Model 1 (ASM1) State Variable Calculation in MBR

The complexity of Activated Sludge Model No. 1 (ASM1) is one of the main obstacles slowing its widespread use, particularly among wastewater treatment plant (WWTP) professionals. In this paper, a simplification procedure based on steady-state mass balances is proposed for the conventional activated sludge process (ASP) configuration, consisting of an aerated bioreactor and a perfect settler (without particular compounds in the outlet). The results do, in fact, show perfect suitability to a membrane bioreactor process (MBR). Both organic carbon and nitrogen removal were investigated. The proposed approach was applied to ASM1, and simple analytical expressions of the state variables were obtained. These analytical expressions were then validated by comparison to simulations given by the original ASM1 (implemented in GPS-X software). A strong match (less than 4% of error overall) was obtained between both results in the steady-state; consequently, these analytical expressions may be useful as tools for quickly estimating the main state variables, feeding the filtration models, or identifying the interaction between operating parameters. Moreover, this enables a sensitivity analysis, covering relevant factors such as kinetics or operating parameters. For instance, the sludge retention time (SRT) effect is lower on XBH and XS at high SRT (˃20 days), while it is more pronounced on XP and XI as their variations with SRT are linear.

A Methodology for Global Sensitivity Analysis of Activated Sludge Models: Case Study with Activated Sludge Model No. 3 (ASM3)

The main objective of this study was to demonstrate a computational approach of global sensitivity analysis (GSA) integrated with functional principal component analysis (fPCA) for activated sludge models through aggregation of time-dependent model response patterns into time-independent coefficients of functional principal components (PCs). This proposed approach addresses the main issue of time-varying character of GSA indices when calculated solely on the time-dependent model outputs. The GSA-fPCA methodology was implemented using the rigorous model Activated Sludge Model No. 3 (ASM3) as case study. The approach transforms the time-dependent model outputs into functional PCs prior to calculation of GSA indices to remove the time-varying character of the calculated GSA indices. This work focused on the evaluation of the following key computational factors that may significantly influence the performance of the GSA-fPCA methodology: (a) model parameter sampling range, (b) model simulation period, (c) basis functions system, and (d) state of the system being modeled-batch or continuous activated sludge process. Results show that first few functional PCs capture up to 100% of the curve patterns in the time-dependent model outputs. The sensitivity indices calculated from the PC scores via Morris' GSA technique elucidated parameter sensitivity patterns inherent to the complex mathematical structure of ASM3. PRACTITIONER POINTS: Functional principal components-mediated GSA technique to remove time-varying character of sensitivity indices derived from time-dependent dynamical models. Technique amenable to improving efficiency of capturing response patterns into few functional principal components through various basis functions. Identifying priority parameters for ASM3 model calibration requires specification of target model outputs to which parameter sensitivities are calculated. GSA-fPCA offers a comprehensive numerical approach to manipulating models depending on the intended applications: simple fast-responding models to complex models.