A novel modeling approach for evaluating microbial mechanism and design of contact stabilization process

Abstract

BACKGROUND The paper offers a novel interpretation of microbial mechanisms and evaluates the assets of contact stabilization (CS) using multi‐component modelling. A model structure re‐defining microbial processes is adopted, where the function of the contact tank was limited to the utilization of soluble substrate fractions and that of the stabilization tank essentially involved endogenous decay and removal of the adsorbed particulate substrate. RESULTS The model was used to simulate the microbial behaviour and performance of CS in comparison with a conventional activated sludge system (CAS), at an SRT range between 6 and 15 d. The results were confirmed by a stoichiometric description of the process using relevant mass balance relationships, which identified the role of major parameters in system behaviour and performance. A rational process design procedure was proposed based on modelling and process stoichiometry. CONCLUSION The CS process achieved effective carbon removal with a much smaller footprint and/or aeration volume with respect to CAS, due to its ability to work at significantly higher volumetric organic loadings; its flexibility to be operated at much higher SRT compared with CAS with the same HRT, allowed minimizing sludge production and sustaining a microbial composition also supporting nitrification. However, process stoichiometry reflected that the CS process, despite its significant advantages, should not be considered a suitable candidate for maximizing sludge harvest and energy recovery. © 2017 Society of Chemical Industry

Which activated sludge configurations qualify for maximizing energy conservation – Why?

“…Only a small fraction of the sludge age can be associated with the contact tank. The contact sludge age clearly becomes insufficient to secure a sufficient level of substrate removal, especially in high‐rate systems operated at overall sludge ages of lower than 2.0 d. The kinetic basis of these arguments was explained in great detail in the recent literature . Furthermore, the vivid proof of the shortcomings in soluble COD removal is presented in Fig.…”

“…This way, the organic particulate components in sludge include X H , X P , X I and sometimes X SR , in case a fraction of X S remains unutilized in high‐rate AS systems. Table summarizes a typical COD fractionation of sewage, which may be used in the numerical evaluation of different expressions derived below, reflecting the process stoichiometry in this study …”

Assessment of anoxic volume ratio based on hydrolysis kinetics for effective nitrogen removal: model evaluation

“…The selected model defines the hydrolysis of X S by the generally adopted surface type of reaction mechanism involving two rate coefficients: the maximum hydrolysis rate parameter k hX and half saturation coefficient K XX ; k hX was defined as 1.60 day −1 and K XX as 0.08 g COD g −1 cell COD in the selected process kinetics profile as given in Table . While these values properly define the hydrolysis process for a typical domestic sewage, a wide range of values are reported for these two coeffcients . This part of the model evaluation tested the impact of k hX and K XX on the hydrolysis of X S in a pre‐denitrification system operated at a sludge age of 5.5 days with a V D / V T value of 0.3.…”

“…Only a model evaluation can predict the exact level of N DP and the corresponding V D / V T ratio for system design that will provide the desired nitrogen removal efficiency. It should be noted that the structure of the selected model should account for major COD fractions with different biodegradation characteristics as model components …”

“…It should be noted that the structure of the selected model should account for major COD fractions with different biodegradation characteristics as model components. 17,18 In this context, the main objective of this study was to set forth a model simulation approach for an accurate calculation of N DP and selection of the appropriate anoxic volume V D that would achieve complete removal of available oxidized nitrogen. In the study, specific emphasis was placed upon the fate of the slowly biodegradable COD component X S both under anoxic and aerobic conditions, because it is the major COD component in sewage and plays a decisive role in nitrate utilization in the anoxic volume.…”