Kinetic evaluation and process analysis of COD and nitrogen removal in UAASB bioreactor
Graphical abstract
Introduction
Nitrogen, phosphorus and their derivatives are essential for normal function of aquatic ecosystems and they are considered as a critical food source for aquatic plants in the food network. Disrupting the nutrient bioavailability and discharging in excess to water bodies have brought out a wide range of ecological effects such as eutrophication [1]. It can be noted that as a result of human activities, eutrophication has been rapidly expanding. The immense growth of phytoplankton and macroalgae, oxygen depletion, loss of benthic areas and reduction of water quality are the consequences of this process [2], [3]. In order to avoid eutrophication, it is necessary to provide suitable conditions in wastewater treatment plants to eliminate nutrients from wastewater. Most researchers have focused on biological nutrient removal (BNR) as a sustainable method and cost-effective to build or upgrade wastewater treatment plants [4]. In addition, low sludge waste and solids as well as low energy consumption can be achieved by this process [5]. Activated sludge process has attracted much attention among BNR methods. It is a unique biotechnological phenomenon and combination of diverse microbial species that are responsible to remove organic matters and nutrients from wastewater. This technology has high efficiency and low operational costs, which has been used for municipal, industrial and domestic wastewaters treatment [6], [7]. The growing trend of this process is not only because of its ability to provide precise criteria but also due to its flexibility [4].
The kinetic models have currently been developed to aid environmental engineers for designing and optimizing the wastewater treatment process. The majority of kinetic models are basically nonlinear and the nonlinear regression method is used for approximation of kinetic constant values. It should be pointed out that the nonlinear model transformed into linear one for determination of empirical constants at a limited sample size [8]. Hence, kinetic models regulate and organize the experimental data in a certain framework to better control and monitor the system performance [9], [10]. Various kinetic models have been applied to study the removal of nitrogen and COD in wastewater treatment bioreactors. First-order, Grau second-order, Monod and Stover–Kincannon are popular and fundamental models to determine substrate removal rate [11], [12], [13]. The Stover–Kincannon model is beneficial for modeling of continuous and semi-continuous mesophilic bioreactors [14]. The significant point is that this model determines the output concentrations and bioreactor operation using organic loading rate. Tomar and Gupta [15] indicated the suitability of Grau second-order and Stover–Kincannon models for prediction of nitrogen removal in anammox hybrid reactor. On the other hand, the Monod model has been introduced as the most widely used model in aerobic bioreactors and the basis of activated sludge models because of its simplicity and high accuracy [16]. The Monod model has facilitated the interpretation of substrate concentration influences on substrate consumption rate and biomass growth [17].
Although the kinetic models of activated sludge were studied in details, but UAASB has not been used for treatment of synthetic wastewater with the aim of kinetic model evaluation. There is no complete information about kinetic parameters and interaction of multiple variables in this kind of reactor feeding with synthetic wastewater. In the present study, the COD and ammonia-nitrogen removal rates were estimated using kinetic models. In addition, the effects of HRT and F/M ratio on carbon and ammonium elimination were experimented.
Section snippets
Bioreactor configuration
The UAASB bioreactor was made of Plexiglas with a total volume of 5 L and operated under anoxic-aerobic conditions. According to the schematic diagram (Fig. 1), the column had a cylindrical structure with 4 sampling ports on the reactor body at different heights. It consisted of two sections: the main part which wastewater and activated sludge were in contact and the upper section that embedded for gas emission. Aeration was done by a compressor and air diffuser from the bottom of the column.
Batch performance
Batch experiments were performed in four phases with different CNP loadings as seen in Table 2. Fig. 2 represents the system performance in terms of COD removal. As seen in the Fig. 2, at CODin of 1500 mg/L, very small amounts of COD remained after 3 days operation which showed rapid acclimation of biomass. In the next cycle with CODin of 2000 mg/L, more than 90% COD removal was obtained after 7 days. By further increase in COD in up to 3000 mg/L, a lag phase in the first 2 days was observed
Conclusion
Simultaneous removal of TKN and COD was investigated in the UAASB reactor under anoxic/aerobic phases. The maximum COD and ammonium removal were 92.4% and 86.7%, respectively. In addition, the optimum HRT and F/M ratio were 0.64 days and 0.8 mg COD/mg MLSS day, respectively. Four kinetic models were applied to characterize the system operation. The high coefficients of Grau second-order model and Stover–Kincannon model distinguished them from others to determine the maximum substrate utilization
Acknowledgments
This research was carried out with the financial support of Iran National Science Foundation (INSF grant No. 95850048) and Tarbiat Modares University (TMU), Iran. The authors wish to thank Mrs Haghdoust for her assistant (Technical Assistant of Environmental Laboratory) and Tarbiat Modares University, Ministry of Science and National Science Foundation for their financial support.
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