Effect of C/N ratio and support material on heterotrophic denitrification of potable water in bio-filters using sugar as carbon source
Introduction
Human activities associated with food production, including agricultural waste and industrial effluents, currently generate high-strength nitrogenous wastewaters. Uncontrolled disposal of these effluents introduces nitrogen compounds into surface and groundwater environments, causing pollution (eutrophication) and threatening drinking water quality (Carrera et al., 2004). Water polluted by nitrate-nitrogen has become a major problem for water management authorities due to the human health risks induced from ingestion of contaminated drinking water (Nolan et al., 1998). Therefore, the successful removal of nitrates from drinking water is of paramount importance. The potable water standard for nitrate and nitrite recommended by the World Health Organization (WHO, 1991) is 11.3 mg NO3−-N/L and 0.91 mg NO2−-N/L, respectively.
Among the competing technologies used for NO3−-N-rich water treatment, biological heterotrophic denitrification has received great attention as it is an efficient, environmentally-friendly and cost-effective process (Liu and Koenig, 2002). For these reasons, the heterotrophic denitrification process has been widely studied in order to optimize the factors that could enhance the biological removal of nitrates from water. The type of treatment configuration (rotating vs. fixed-bed bioreactors) (Mohseni-Bandpi et al., 2013), the hydraulic retention time (HRT) (Kesseru et al., 2002, Suhr et al., 2013, Mirbagheri et al., 2014), the C/N ratio (Her and Huang, 1995, Bi et al., 2015, Ucar et al., 2016) and, most importantly, the type of carbon source (Magram, 2010, Hu et al., 2013, Luo et al., 2014), have all been extensively studied. Commonly used carbon sources are acetone, acetic acid, glucose, methanol and ethanol (Mohseni-Bandpi et al., 2013). It is been observed that the type of carbon source used for the heterotrophic denitrification process may reduce the operating cost of the treatment system (Suhr et al., 2013). In an effort to reduce the cost of the process, a number of researchers have used organic substrates extracted from natural materials (wheat straw, plant prunings, etc.), however the pretreatment required was complicated and lengthy (Aslan and Turkman, 2005, Su and Puls, 2007). Consequently, the selection of a low cost and readily available carbon substrate together with an optimal C/N ratio is the subject of ongoing research. One promising carbon source is sucrose in the form of commercial white sugar, which is widely available at low cost. However, to date, limited studies have been conducted using white sugar as electron donor and carbon source for the heterotrophic denitrification of drinking water (Mercado et al., 1988, Nurizzo and Mezzanotte, 1992).
Mathematical modeling has been widely used to simulate and predict the processes involved in heterotrophic denitrification and the kinetics of the biological removal of nitrates and nitrites have been investigated thoroughly. Many approaches have proposed Monod kinetics to describe nitrogen compounds removal, including the effect of carbon source and oxygen concentration on the model (Kornaros and Lyberatos, 1998, Marazioti et al., 2003, Beline et al., 2007, Samie et al., 2011, Zheng et al., 2014, Bi et al., 2015, Leyva-Díaz et al., 2015, Peng et al., 2016). Moreover, biofilm growth as well as diffusion of nutrients and nitrogen compounds in the biofilm have been considered in several mathematical models regarding attached growth reactors (Ergas and Rheinheimer, 2004, Matsumoto et al., 2007, Wichern et al., 2008, Vazquez-Padin et al., 2010, Seifi and Fazaelipoor, 2012). However, there is still a lack of information on the kinetics that could describe the effect of nitrites on the elimination of nitrates and vice-versa. This effect has been previously described during hydrogenotrophic denitrification (Vasiliadou et al., 2006, Vasiliadou et al., 2009) with growth kinetics on substitutable substrates (nitrate and nitrite). Results suggested that the modeling of biofilm growth considering nitrates and nitrites as substitutable substrates, could describe the denitrification process better than Monod kinetics. Considering that mathematical models can be effective tools for the optimum design and control of the heterotrophic denitrification process, failure to account for this two-way influence may lead to erroneous predictions.
Based on the above, the aim of the present work was to evaluate: a) carbon source (commercial white sugar) efficiency, b) the type of support material (silicic gravel and hollow plastic tubes), c) the number of layers of support material (monolayer and multilayer), and d) the effect of C/N ratio, using suspended and attached growth pilot-scale packed-bed reactors (bio-filters) for heterotrophic denitrification under anoxic conditions. A mathematical model was then developed to describe heterotrophic denitrification under optimal operating conditions and various NO3−-N concentrations. The model includes diffusion of dissolved components in the biofilm and growth kinetics of substitutable substrates (nitrate and nitrite). In addition, sugar by-products released from the lysis of bacterial cells were also included in the model as a second carbon source.
Section snippets
Cultivation of microorganisms
All test runs were inoculated with a mixed culture of acclimated heterotrophic bacteria, enriched from sludge taken from the wastewater treatment plant of Agrinio city, Greece. The enrichment process was performed by inoculating a suspended growth reactor (SGR: 3 L flask with working volume 2 L) with sludge liquor, and subsequent incubation under anoxic conditions using synthetically produced contaminated water (SCW) as growth medium. The SCW was prepared using tap water, KNO3 (0.722 g/L), KH2PO
Suspended growth reactors
The mathematical model developed describes biomass processes (growth and death), sugar degradation, transformation of nitrates to nitrites, and nitrites to nitrogen or nitrogen oxide gases. In addition, the interaction between nitrites and nitrates, being treated as substitutable substrates, was also included (Vasiliadou et al., 2006). Moreover, the growth kinetics of denitrifying bacteria was assumed to be subjected to nutrient limitation by nitrates, nitrites and sugar, with inhibition by
Effect of support medium
Two pilot-scale bio-filters, packed with different types of material, were operated under SBR mode with recirculation. Reactors R1 and R2, packed with silicic gravel and hollow plastic tubes, respectively, were used to determine the best support medium regarding maximum efficiency without operating problems. As shown in Table 1, both bio-filters were operated under a wide range of nitrate-nitrogen concentrations (20–400 mgNO3--N/L), while the ratio of C/N was maintained constant at 13.5. The
Conclusions
The aim of the present work was to study heterotrophic denitrification of drinking water using an effective and low cost bioreactor scheme. Commercial white sugar was used as a cheap and readily available carbon source. The effect of the support medium (silicic gravel and plastic tubes) and the C/N ratio on the performance of two bio-filters was evaluated. Results indicated that for a C/N ratio of 13.5, support material of silicic gravel and feed concentrations up to 60 mgNO3--N/L, the system
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