Nitrate removal from pharmaceutical wastewater using microbial electrochemical system supplied through low frequency-low voltage alternating electric current☆
Introduction
Due to anthropogenic activities, some industrial and agricultural sources discharge their wastewater containing high nitrate and low organic biodegradable into the environment [1]. Although various techniques have been proposed for the simultaneous removal of nitrate and organic compounds, they are not free from drawbacks [2]. Some industrial processes generate wastewater with high levels of nitrate and organic compounds that must be removed simultaneously [3]. For example, wastewater of pharmaceutical factories has high levels of toxic, complex, and poorly biodegradable compounds accompanied by nitrogenous compounds [4]. The toxicity of the organic compounds on microorganisms is a major problem during biological treatment [5]. The type of organic material used as a carbon source for heterotrophic denitrification is the main parameter affecting nitrate removal [6]. To increase denitrification efficiency, various methods have been proposed; e.g., addition and tuning of nutrients and electron donor/acceptor ratio, electrochemical hydrogen production for autotrophic denitrifying bacteria, and enhancement of bacteria activity by electrostimulation [1], [7], [8]. Available data show that the induced electrical current can stimulate bacteria metabolism and enhance biochemical performance [9], [10], [11]. For example, Hao et al. reported a single chamber microbial fuel cell and a bioelectrical reactor for vanadium removal. The results showed that the highest removal efficiency of vanadium was 93.6% at 12 h operation. Recently, microbial electrochemical systems (MESs) have been proposed for bioremediation of various environmental pollutants. MESs refer to a set of biological systems in which bacteria catalyze the oxidation or reduction reactions [12]. Electric currents can be divided into two branches: alternating current and direct current. Applying direct current to electrochemical systems can produce highly active chemical byproducts such as OH, O3, H2O2, and Cl2 [13]. So, the current in bioelectrochemical systems can lead to toxic byproducts for microorganisms [14]. In the alternating current, on the other hand, the magnitude of frequency changes periodically while net charge is zero. Utilizing a low voltage-low-frequency with an alternating current may solve some problems such as anode corrosion and cathode inactivation [15]. Previous studies have shown that applying a low voltage-low frequency alternating electric current in a bioreactor can create several changes including dense and fast-settling sludge granules, increase in enzyme activity, changes in morphological and biochemistry characteristics [16]. It has been reported that biomass activity could be enhanced as a result of alternating current utilization. In the present study, a low voltage very low frequency alternating current at different waveforms was applied on bacteria community as electrostimulation. The effect of inlet nitrate concentration, retention time (h), the magnitude of a waveform [AMPL / Vp − p], an adjustable DC voltage added to the signal output [OFST/V], alternating current frequency, and waveform was studied on the biological denitrification process. Several studies have been conducted on electrostimulation of bacteria using direct current [13]. However, to the best of our knowledge, rare studies have rarely reported on low voltage very low-frequency alternating currents to enhance biological activities. It is expected that experimental results could be used as a reference for applying MES through a low frequency-low voltage alternating electrical current for biological denitrification in industrial wastewater treatment.
Section snippets
Materials and bioreactor configuration
The experimental bioreactor consisted of a glass vessel with 5 L effective capacity. Cylindrical carbon cloth and stainless steel mesh with 2 cm inter-electrode distance were mounted in the wall of the bioreactor. The bioreactor was stirred manually to ensure complete mixing. The MES configuration is shown in Schematic 1. Nutrients including potassium nitrate, ibuprofen, KH2PO4, and K2HPO4 were inoculated in the reactor at a C:N:P ratio of 3:1:0.2 for microbial growth [17], [18]. The chemical
Start-up of the bioreactor
Fig. 2a shows nitrate reduction during the start-up period. Nitrate was added to the bioreactor at various concentrations (50–600 mg L− 1). Nitrate reduction efficiency was obtained as 90.1 ± 0.1% at a steady state with a maximum inlet nitrate concentration of 600 mg/L after 45 days of operation. The bioreactor had some instabilities in nitrate reduction as result of loading nitrate from 50 to 400 mg L− 1 at the start-up periods (Fig. 2a). However, after reaching the steady state, high nitrate removal
Conclusion
Based on the results of the present study, we have reached the following conclusions:
Biological denitrification in the proposed MES using ibuprofen as a carbon source was enhanced with low frequency-low voltage alternating electric current. Nitrate removal efficiency of 95.6% was obtained over 6 h. AMPL and OFST have significant effects on nitrate reduction and dehydrogenase activity. A frequency of 10 Hz could lead to highest nitrate removal efficiency. The results obtained showed 15.24 μgTF mg
Acknowledgments
This paper resulted from PhD thesis of Mr. Edris Hoseinzadeh, which was done by the financial and technical support provided of Tarbiat Modares University, by grant No. 1261868 Tehran, Iran. This study was partially financially supported by grant No: 95.06.02 of the Biotechnology Development Council of the Islamic Republic of Iran.
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This is a part of VSI: Bioelectrosynthesis.