Feasibility of Glow Discharge Nonthermal Plasma as an Alternative Pretreatment for Low-Carbon Wastewater in a Biological Nutrient Removal Plant

Abstract

Biological treatment is insufficient for infield treatment of domestic wastewater that contains low amounts of carbon. This study proposes a pretreatment process for enhancing the nutrient removal efficiency by improving the biodegradability of organic matter using glow discharge nonthermal plasma (GDNTP), which generates several strongly oxidizing active species. A series of batch experiments was conducted at pH 5.0 and 9.0 to investigate removal of nitrogen based on either physiochemical effects or improvement of the biodegradability of organic matters by the GDNTP pretreatment. At pH 5.0, total nitrogen (TN) was not removed by chemical reactions with active species from the GDNTP generator, whereas more TN was removed by chemical reactions or air stripping with the gas discharged from GDNTP at pH 9.0. Part of nitrogen removal was not achieved by ammonia air stripping, but by the improvement of biodegradation due to the GDNTP pretreatment for each operational sequencing batch reactor cycle. Findings indicate that GDNTP pretreatment improves the denitrifying of nitrate rather than producing a better carbon/nitrogen (C/N) ratio.

Introduction

Biological nutrient removal processes for treating a certain type of wastewater such as a low-carbon wastewater are more complicated and difficult to operate than chemical treatment processes. It is known that for real wastewater with limited carbon sources (low C/N), the rapid enrichment of phosphorus-accumulating organisms is hard to be achieved (Wang et al., 2015). For instance, a simultaneous nitrification–denitrification and phosphorous removal sequencing batch reactor (SBR) was developed to achieve simultaneous nutrient and carbon removal from domestic wastewater with a low carbon/nitrogen ratio (C/N ratio ≤3.5; Wang et al., 2015). In the case of domestic wastewater with high ammonia but low carbon content (C/N ratio: 4.03), Zhao et al. (2016) achieved a high denitrifying and phosphorus removal efficiency of 96.86% using a predenitrification anaerobic/anoxic/aerobic nitrification SBR. Considering the operational difficulty and system complexity of treatment processes, conventional biological tertiary treatment is insufficient to remove nitrogen and phosphorus from wastewater with a low C/N ratio in the field (Tunçal et al., 2009; Mahvi et al., 2011). Therefore, a simple chemical pretreatment process is required to improve the C/N ratio of wastewater with high nitrogen or low carbon content.

Advanced oxidation processes (AOPs) have been studied extensively owing to their strong oxidizing power and high processing efficiency (Belkheiri et al., 2011; Epold et al., 2012; Liu et al., 2015). Among them, nonthermal plasma (NTP) based on glow discharge can effectively oxidize organics that are difficult to biodegrade by generating different radicals (Muhammad et al., 2001; Schiavon et al., 2017). NTP has been introduced as a promising technology to remove nitrogen oxide from the atmosphere over the past several years (Talebizadeh et al., 2014). Ercan et al. (2016) reported that plasma treatment of an N-acetylcysteine solution creates a predominantly reactive nitrogen species rather than reactive oxygen species, and the generated peroxynitrite causes significant bacterial inactivation. For wastewater treatment, previous studies have applied this to the treatment of biologically persistent degradable matters, whereas a few studies focused on nitrogen removal or nitrification of water and wastewater. Many studies have reported that the degradation of persistent organic pollutants in wastewater was enhanced by the active species generated from the NTP process (Cheng et al., 2007). However, little else is known about nitrogen removal from wastewater, with the exceptions of air stripping (Abdullahi et al., 2016) and nitrification (Summerfelt, 2003).

Active species released by plasma discharge include hydroxyl radicals (^•OH), atomic oxygen, ozone, and hydrogen peroxide, which are the most important for the treatment and removal of unwanted organic compounds from water (Yang, 2011). The oxidation potentials of various active species produced by plasma in water range from 1.78 to 2.8 V (hydrogen peroxide: 1.78 V, ozone: 2.07 V, atomic oxygen: 2.42 V, and hydroxyl: 2.8 V; Sun et al., 1997). During AOP treatment of wastewater, a sufficient quantity of ^•OH is generated to remove refractory organic matters or to increase wastewater biodegradability as a pretreatment to an ensuing biological treatment (Watts et al., 1993; Deng and Zhao, 2015). The NTP process also includes the generation of ozone as well as high-energy electrons and active species that can maintain high oxidizing power based on the presence of various radicals. Ozone has a high oxidation potential (E^θ = 2.08 V), but that of ^•OH is stronger (E^θ = 3.06 V; Jiang et al., 2015). The rate of ^•OH production using ozone is linearly proportional to the ozone concentration, and theoretically, 0.24 moles of ^•OH are produced per mole of ozone (Andeozzi et al. 1999).

Gas-phase discharges generated over a water surface produce strongly oxidative species, such as ^•OH, ^•O, and their reaction products (O₃, H₂O₂) in the gas or at the gas–liquid interface, which can dissolve into the water to initiate oxidation (Cheng et al., 2007; Benetoli et al., 2012; Gu et al., 2016). Jiang et al. (2014) reported that various electrical discharge types of plasma applied for wastewater remediation have certain common chemical reaction mechanisms and physical phenomena, such as formation of molecular and radical species and ultraviolet light generation. The main reaction for air discharge in the plasma generator is summarized as follows (Lieberman and Lichtenbeg, 2005). \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm{E}} \; + \;{{ \rm{N}}_2} \; \to \;{ \rm{e}} \; + { \;^ \bullet }{ \rm{N}} \; + { \;^ \bullet }{ \rm{N}} \tag{1} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm{e}} \; + \;{{ \rm{N}}_2} \; \to { \;^ \bullet }{{ \rm{N}}_2} \tag{2} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm{e}} \; + \;{{ \rm{O}}_2} \; \to \;{ \rm{e}} \; + { \;^ \bullet }{ \rm{O}} \; + { \;^ \bullet }{ \rm{O}} \tag{3} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm{e}} \; + \;{{ \rm{O}}_2} \; \to { \;^ \bullet }{{ \rm{O}}_2} \tag{4} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} ^\bullet { \rm{O}} \; + \;{{ \rm{O}}_2} \; \to \;{{ \rm{O}}_3} \tag{5} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} {\rm{e}} \; + \;{{ \rm{H}}_2}{ \rm{O}} \; \to \;{ \rm{e}} \; + { \;^ \bullet }{ \rm{H}} \; + { \;^ \bullet }{ \rm{OH}} \tag{6} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} {{ \rm{H}}^ \bullet } + \;{{ \rm{O}}_2} \; \to { \;^ \bullet }{ \rm{H}}{{ \rm{O}}_2}. \tag{7} \end{align*} \end{document}

Here, we conducted a series of experiments on a sample obtained from a municipal wastewater treatment plant to investigate the feasibility of glow discharge nonthermal plasma (GDNTP) as an alternative pretreatment process for the low-carbon wastewater in a biological nutrient removal plant. Artificial wastewater was also treated to elucidate the optimal GDNTP operation conditions through aeration under different conditions.

Experimental Protocols

Experimental apparatus

The type of glow discharge used here is an electrical plasma discharge (Kong et al. 1998; Wang et al., 2012), which can generate various reactive species (e.g., hydroxyl radical, atomic oxygen, ozone, and hydrogen peroxide) that enable active chemical reactions. When electrical plasma discharges are supplied through aeration, oxidation is more effective and homogeneous as various radicals directly and quickly come into contact with pollutants in wastewater. In the experimental apparatus, pollutants, such as nitrogen and organic matter in wastewater, were in direct contact with bubbles containing various radicals that were generated from the GDNTP of this study. Aeration was conducted by an air pump (Model No. 2546C-10; WELCH) and air diffusers for the contact reaction with a strong oxidizing-power plasma, as shown in Fig. 1.

FIG. 1.

Schematic representation of experimental apparatus and applied GDNTP generator. 1, Air filter; 2, flow rate gauge; 3, air pump; 4, GDNTP generator; 5, electric power supplier; 6, air valve; 7, ceramic diffuser; 8, water bath; 9, GDNTP contact tank; 10, SBR tank; 11, water pump. GDNTP, glow discharge nonthermal plasma; SBR, sequencing batch reactor.

The plasma generator was manufactured to carry out this study using laboratory-scale 10 mA and 2.2 W glow discharge. The whole treatment processes of experiment apparatus consisted of (I) aeration in GDNTP contact tank, (II) oxidation of active species in GDNTP contact tank, (III) aeration in SBR tank, and (IV) biochemical treatment in SBR tank. The pretreatment of wastewater refers aeration (I) and oxidation (II) in GDNTP contact tank.

The closed space where plasma was generated was supplied with sufficient air through the pump. The discharging number was 1–9 pins, and the capacity of the reactor was 2 L. The air flow rate was maintained by the air flow meter (Model No. RMA-22-SSV; Dwyer) in each reactor. Experiments were conducted by placing the ceramic diffuser on the bottom of the reactor.

After the GDNTP pretreatment process, the pretreated wastewater from the GDNTP contact reactor was moved to the SBR for the biological nutrient removal processes. The operational cycle is shown in Table 1.

Table 1.

Operational Mode of Each Sequencing Batch Reactor Cycle

	Time sequence (h)
Variables	0.5	1.0	1.5	2.0	2.5	3.0	3.5	4.0	4.5	5.0	5.5	6.0	6.5	7.0	7.5	8.0∼
Aeration	None			Aeration					None		Aeration		None
Processes	Fill				Reaction								Settle			Draw

Experiment and measurement methods

Raw water used in this study was retrieved from the flow control tank of the Y domestic wastewater treatment plant (600 m³/day; Nonsan, Republic of Korea). The quality of raw water from July to October 2016 is summarized in Table 2. The total dichromate chemical oxygen demand (TCOD_Cr), soluble dichromate chemical oxygen demand (SCOD_Cr), total suspended solid (SS), and total nitrogen (TN) removal efficiencies were measured under various reaction times and pH conditions.

Table 2.

Water Quality of Raw Water Used

Analysis item	Maximum	Minimum	Average
TCOD_Cr (mg/L)	578.8	164.5	283.0
SCOD_Cr (mg/L)	221.1	52.1	120.6
TN (mg/L)	97.5	32.8	51.5
TP (mg/L)	37.8	15.4	26.2
SS (mg/L)	240.3	160.3	181.2

SCOD_Cr, soluble dichromate chemical oxygen demand; SS, suspended solid; TCOD_Cr, total dichromate chemical oxygen demand; TN, total nitrogen; TP, total phosphorus.

An indirect confirmation method can be adapted to measure ^•OH formation through a decomposable substance that is known to selectively react with ^•OH, as the actual amount of ^•OH is hard to quantify (Li et al., 2009). Of the various indicators, N-dimethyl-4-nitrosoaniline (RNO) is often used because it is visible through its bleaching effect in water and easy to analyze (Meyers and Montgomery, 1995). RNO (97%; Sigma-Aldrich) was added as an indicator to measure the concentration of ^•OH in the reactive gas discharged from the GDNTP. In the indirect ^•OH measurement using 9 mg/L RNO, the plasma reactor was operated with 1, 3, 5, 7, and 9 pins, with a reaction capacity of 2 L, air pump flow rates of 3.5, 5.0, and 10.5 L/min, and reaction times 5, 20, 40, 60, and 75 min. The variable code values for the three central composite designs are listed in Table 3 and Fig. 2.

FIG. 2.

Schematic representation of central composite design.

Table 3.

Actual Values of Variables Used for the Coded Values

	Actual values for the coded values
Variables	−1.682	−1.000	0	+1.000	+1.682
Flow, L/min (X₁)	3.5	5	7	9	10.5
Pin, number (X₂)	1	3	5	7	9
Time, min (X₃)	5	20	40	60	75

An additional experiment was conducted to elucidate the fraction of nitrogen removal by air stripping through simple aeration supplied only with air. The air flow was adjusted to 9 L/min, pH 5.0 and 9.0 using 1 mole H₂SO₄/L and 1 mole NaOH/L, respectively, the reaction times 0.5, 2, 4, 6, 8, 20, 30, and 44 h, and the sample volume was adjusted to 2 L. Based on the Standard Methods for the Examination of Water and Wastewater (APHA, 2005), we determined the chemical oxygen demand (COD) concentration following the method 5220 B, TN concentration following 4500-N, and SS concentration following 2540 D. The pH and conductivity were analyzed using a pH and conductivity meter (LAQUA F-74, Horiba, Japan).

Results and Discussion

Comparison between aeration and oxidation as pretreatment of GDNTP

Kuba et al. (1996) reported that a C/N ratio exceeding 3.4 was required to achieve a nitrogen removal efficiency higher than 80%, and a carbon supply was required for domestic wastewater with a low C/N ratio. The high nitrogen content was due to ammonia in wastewater (Drzewicki and Kulikowska, 2011). Zhang et al. (2012) reported that ammonia removal by ammonia stripping was strongly dependent on the pH and aeration rate.

Concentrations of TCOD_Cr, SCOD_Cr, and SS decreased at both pH 5.0 and 9.0 due to organic matter degradation by contact with the gas that contained active species from the GDNTP generator, as shown in a previous study (Wang et al., 2014). The results of GDNTP pretreatment showed that TCOD_Cr, SCOD_Cr, and SS removal was similar in terms of the contact time, as shown in Fig. 3a, b. However, the TN concentration drastically decreased at pH 9.0, whereas it slightly increased at pH 5.0 in the GDNTP contact reactor. The TN removal efficiency was −12.4% and 34.3% at pH 5.0 and 9.0, respectively, when contact time was equal to 70 min.

FIG. 3.

Variation of water quality by GDNTP pretreatment with different contact times. (a) pH 5.0. (b) pH 9.0.

At the water interface of nitrogen or air in NTP, different nitrogen species are formed in water at pH <7 (Lobachev and Rudakov, 2006; Cadorin et al., 2015). The reactions of NTP as shown in Equations (8)–(16) could also influence on the nitrogen species so that the TN removal efficiency of pH 5.0 was lower than that of pH 9.0. \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm{N}} \; + \;{ \rm{OH}} \; \to \;{ \rm{NO}} \; + \;{ \rm{H}} \tag{8} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm{NO}} \; + \;{ \rm{OH}} \; \to \;{ \rm{HN}}{{ \rm{O}}_2} \tag{9} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm{N}}{{ \rm{O}}_2}^ - \; + \;{{ \rm{H}}_2}{{ \rm{O}}_2} \; + \;{{ \rm{H}}_3}{{ \rm{O}}^ + } \; \to { \rm{ \;ONOOH}} \; + \;2{{ \rm{H}}_2}{ \rm{O}} \tag{10} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm{ONOOH}} \; + \;{{ \rm{H}}_2}{ \rm{O}} \; \leftrightarrow \;{ \rm{ONO}}{{ \rm{O}}^ - } \; + \;{{ \rm{H}}_3}{{ \rm{O}}^ + } \tag{11} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm{ONOOH}} \; \to \;{ \rm{HN}}{{ \rm{O}}_3} \tag{12} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm{ONOOH}} \; \leftrightarrow \;{ \rm{N}}{{ \rm{O}}_2} \; + { \rm{ \;OH}} \tag{13} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm{HN}}{{ \rm{O}}_2} \; + \;{{ \rm{H}}_2}{ \rm{O}} \; \leftrightarrow \;{ \rm{N}}{{ \rm{O}}_2}^ - \; + { \rm{ \;}}{{ \rm{H}}_3}{{ \rm{O}}^ + } \tag{14} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} 2{ \rm{HN}}{{ \rm{O}}_2} \; \to \;{ \rm{NO}} \; + \;{ \rm{N}}{{ \rm{O}}_2} \; + { \rm{ \;}}{{ \rm{H}}_2}{ \rm{O}} \tag{15} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm{HN}}{{ \rm{O}}_2} \; + \;{{ \rm{H}}^ + } \; \to \;{{ \rm{H}}_2}{ \rm{N}}{{ \rm{O}}_2}^ + \; \to { \rm{ \;N}}{{ \rm{O}}^ + } \; + \;{{ \rm{H}}_2}{ \rm{O}}. \tag{16} \end{align*} \end{document}

A high TN removal efficiency at pH 9.0 could be achieved by the air stripping as well as the oxidative deamination reaction (Abdullahi et al., 2016). Therefore, we conducted additional experiments to determine the fraction of TN removal efficiency by simple aeration in the contact reactor without GDNTP pretreatment, which led to ammonia air stripping and the biological degradation of organic matter. Although the pattern of TN variation was similar between the GDNTP pretreatment and the simple aeration at pH 5.0 (Fig. 4a), the TN removal efficiency of simple aeration at pH 9.0 was lower than that with GDNTP pretreatment, as shown in Fig. 4b. The TN removal efficiency of simple aeration and GDNTP pretreatment was 9.4% and 34.3%, respectively, at pH 9.0 and contact time 70 min.

FIG. 4.

Variation of water quality by aeration with different contact times. (a) pH 5.0. (b) pH 9.0.

Removal fractionation and C/N ratio change

Figure 5 shows the bias distributions of the experimental results as a Youden plot, which is intended for interlaboratory comparisons. The scatter points of SCOD removal fraction were highly biased toward the GDNTP pretreatment compared with aeration. The “GDNTP pretreatment” of wastewater refers oxidation (II) by the operation of plasma device in GDNTP contact tank, whereas “aeration” refers aeration (I) in GDNTP contact tank when the plasma device is turned off. In addition, the removed fractions of TCOD and SCOD were biased toward the “GDNTP pretreatment,” whereas the bias of TN differed depending on the pH conditions. The scatter points of TN were concentrated around zero and indicated that TN was not removed by chemical reactions with the active species from the GDNTP generator. Meanwhile, at pH 9.0 with GDNTP pretreatment, more TN was removed by the chemical reaction or air stripping with the gas discharged from GDNTP, rather than biological degradation. The sum of the removed fraction of TN removal by GDNTP pretreatment (II) was 0.34 and that by aeration (I) was 0.09; thus, a TN removal fraction of 0.25 was achieved by GDNTP pretreatment.

FIG. 5.

Comparison of removal fractions between GDNTP and aeration.

Total organic carbon (TOC) concentration was also measured to elucidate changes in the C/N ratio through treatment with GDNTP gas in terms of contact time. The C/N ratio ranged from 1.31 to 2.32 at pH 5.0 and from 1.82 to 2.34 at pH 9.0. The required C/N ratio for nitrogen and phosphorus removal in a biological wastewater treatment plant is 3.5. However, the C/N ratio decreased depending on contact time with GDNTP gas at pH 5.0 because the removal efficiency of TOC was much larger than that of TN. However, after a contact time of 40 min, there was a gradual increase in the C/N ratio in proportion to the significant increase of the TN removal efficiency at pH 9.0, as shown in Fig. 6.

FIG. 6.

Variation of C/N ratio by GDNTP with different contact times. (a) With GDNTP, no pH adjustment. (b) Without GDNTP, no pH adjustment.

Nutrient removal with GDNTP pretreatment

Preceding results (Hashim et al., 2016) reported that a higher 5 day-biochemical oxygen demand (BOD₅)/COD ratio was achieved using plasma treatment, which indicates better wastewater biodegradability. To make a comparison between with and without GDNTP pretreatment under the same conditions described above, two series of SBR experiments were conducted to investigate the feasibility of GDNTP as a pretreatment process, as shown in Table 4. The influent of SBR reactor was induced from GDNTP reactor after contacting for 80 min, and the effluent was measured after treating one cycle (8.0 h). First of all, TN (16.6%) was much higher than that of other water quality criteria for the improvement of the removal efficiency. For the SBR operation cycle, TCOD, NH₃-N, and total phosphorus (TP) were increased during the first 2 h owing to organic matter degradation and then decreased gradually, as shown in Fig. 7. The concentration of NO₃-N in effluent that underwent GDNTP pretreatment was lower than that without, indicating that nitrogen removal was accomplished by the improvement of biodegradation by GDNTP pretreatment for each 8-h operational cycle and not by ammonia air stripping. At the same time, the concentration of TCOD treated with GDNTP was decreased greater than that without, which was considered that the organics pretreated with GDNTP were consumed as a carbon source in the denitrification phase of SBR. In conclusion, the findings indicate that GDNTP pretreatment improves the denitrifying degradation of nitrate. However, from an engineering perspective, further studies are necessary to apply GDNTP process as a pretreatment in biological wastewater treatment field, where it is required to enhance bioavailability and denitrification efficiency.

FIG. 7.

Variation of nutrient concentrations with and without GDNTP pretreatment based on operational mode of SBR cycle. (a) With GDNTP pretreatment. (b) Without GDNTP pretreatment.

Table 4.

Removal Efficiency of Sequencing Batch Reactor With and Without Glow Discharge Nonthermal Plasma Pretreatment

	TCOD	TN	TP
Without GDNTP pretreatment
Influent (mg/L)	402	34.9	17.8
Effluent (mg/L)	55	16.5	2.15
Removal efficiency (%)	86.3	52.7	87.9
With GDNTP pretreatment
Influent (mg/L)	387	34.2	18.5
Effluent (mg/L)	17	10.5	1.67
Removal efficiency (%)	95.6	69.3	91.0

Operation condition: GDNTP pretreatment (contact time: 80 min) and SBR one cycle (contact time: 8.0 h).

GDNTP, glow discharge nonthermal plasma; SBR, sequencing batch reactor.

Conclusions

To investigate the feasibility of GDNTP as an alternative pretreatment process for low-carbon wastewater, a series of experiments was conducted for the nutrient removal in the biological wastewater treatment. These included chemical treatment by GDNTP and biological treatment removing nutrients through SBR using laboratory-scale experimental apparatus. From the results of our experiments, we conducted C/N ratio tracking and removal fractionation of the denitrifying reaction in terms of contact time and drew the following conclusions.

TN concentration was sensitive to pH as well as contact time in the reaction tank that supplied active gas from the GDNTP generator. After a contact time of 30 min, TN in the GDNTP contact reactor was significantly reduced at pH 9.0, whereas it increased slightly at pH 5.0. The pattern of TN curve changed after a contact time of 70 min, at which the TN removal efficiencies were −12.4% at pH 5.0 and 34.3% at pH 9.0.

At pH 5.0, TN was not sufficiently decreased by chemical reactions of active species, whereas at pH 9.0, more TN was decreased by the chemical reaction or air stripping with the gas discharged from GDNTP.

Due to the TOC removal by the active oxidation species, the C/N ratio decreased in terms of the contact time with GDNTP gas at pH 5.0. However, after a contact time of 40 min at pH 9.0, the C/N ratio gradually increased as the TN removal by the air stripping.

Concentration of NO₃-N in effluent from the SBR reactor that underwent GDNTP pretreatment was lower than that without GDNTP, indicating that the nitrogen removal was accomplished through the improvement of biodegradation by GDNTP pretreatment for each operational SBR cycle, rather than ammonia air stripping.

With GDNTP pretreatment, the removal efficiency of TN was achieved much higher than TCOD and TP in the SBR. It was considered that the physicochemical ammonia stripping and the biological denitrification improvement by the GDNTP pretreatment gave rise to the removal efficiency increase of TN in biological nutrients removal process.

In conclusion, the findings from the experiment results in this study revealed that various reactive species (e.g., hydroxyl radicals) are not directly involved in the denitrifying degradation of nitrate but help the bioavailability of the organics pretreated with GDNTP as a carbon source, improving the denitrification efficiency.

Footnotes

Acknowledgment

This research was supported by Research Base Construction Fund Support Program funded by the Chonbuk National University in 2016 and partly supported by the National Research Foundation of Korea (NRF) with grants (NRF-2015R1D1A3A03020597) from the Ministry of Education in Korea, 2015.

Author Disclosure Statement

No competing financial interests exist.

References

Abdullahi

M.E.

, Hassan

M.A.A.

, Noor

Z.Z.

, and Ibrahim

R.K.R.

(2016). Integrated air stripping and non-thermal plasma system for the treatment of volatile organic compounds from wastewater: Statistical optimization. Desalin. Water Treat., 57, 16066.

Andeozzi

, Caprio

, Amedo

, and Marotta

(1999). Advanced oxidation process (AOP) for water purification and recovery. Catal. Today, 53, 55.

APHA (2005). Standard Methods for the Examination of Water and Wastewater, 21st ed. Washington, DC; New York: American Public Health Association.

Belkheiri

, Fourcade

, Geneste

, Amrane

(2011). Feasibility of an electrochemical pre-treatment prior to a biological treatment for tetracycline removal. Sep. Purif.Technol., 83, 151.

Benetoli

L.O.B.

, Cadorin

B.M.

, Baldissarelli

V.Z.

, Geremias

, and Debacher

N.A.

(2012). Pyrite-enhanced methylene blue degradation in non-thermal plasma water treatment reactor. J. Hazard. Mater. 237–238, 55.

Cadorin

B.M.

, Tralli

V.D.

, Ceriani

, Benetoli

L.O.B.

, Marotta

, Ceretta

C. D

, ebacher

N.A.

, and Paradisi

(2015). Treatment of methyl orange by nitrogen non-thermal plasma in a corona reactor: The role of reactive nitrogen species. J. Hazard. Mater., 300, 754.

Cheng

H.H.

, Chen

S.S.

, Wu

Y.C.

, and Ho

D.L.

(2007). Non-thermal plasma technology for degradation of organic compounds in wastewater control: A critical review. J. Environ. Eng. Manage., 17, 427.

Deng

, and Zhao

(2015). Advanced oxidation processes (AOPs) in wastewater treatment. Curr. Pollut. Rep., 1, 167.

Drzewicki

, and Kulikowska

(2011). Limitation of sludge biotic index application for control of a wastewater treatment plant working with shock organic and ammonium loadings. Eur. J. Protistol., 47, 287.

10.

Epold

, Dulova

, Veressinina

, and Trapido

(2012). Application of ozonation, UV photolysis, fenton treatment and other related processes for degradation of ibuprofen and sulfamethoxazole in different aqueous matrices. J. Adv. Oxid. Technol., 15, 354.

11.

Ercan

U.K.

, Smith

, Ji

H.F.

, Brooks

A.D.

, and Joshi

S.G.

(2016). Chemical changes in non-thermal plasma-treated N-acetylcysteine (NAC) solution and their contribution to bacterial inactivation. Sci. Rep., 6, 20365.

12.

, Huang

, Xu

, Ma

, and Pan

(2016). Dissolved organic matter as a terminal electron acceptor in the microbial oxidation of steroid estrogen. Environ. Pollut., 218, 26.

13.

Hashim

S.A.

, Samsudin

F.N.

, Wong

C.S.

, Abu Bakar

, Yap

S.L.

, Mohd Zin

M.F.

(2016). Non-thermal plasma for air and water remediation. Arch. Biochem. Biophys. 605, 34–40.

14.

Jiang

, Hu

, Zheng

X. Z

, heng

, Tan

, Wu

, and Xue

(2015). Rapid oxidation and immobilization of arsenic by contact glow discharge plasma in acidic solution. Chemosphere, 125, 220.

15.

Jiang

, Zheng

, Qiu

, Wu

, Zhang

, Yan

, and Xue

(2014). Review on electrical discharge plasma technology for wastewater remediation. Chem. Eng. J., 236, 348.

16.

Kong

S.H.

, Watts

R.J.

, and Choi

J.H.

(1998). Treatment of petroleum contaminated soils using iron mineral catalyzed hydrogen peroxide. Chemosphere, 37, 1473.

17.

Kuba

, Van Loosdrecht

M.C.M.

, and Heijnen

J.J.

(1996). Phosphorus and nitrogen removal with minimal COD requirement by integration of denitrifying dephosphatation and nitrification in a two- sludge system. Water Res. 30, 1702.

18.

, Feng

, Hu

, Zhang

, and Sugiura

(2009). Electrochemical degradation of phenol using electrodes of Ti/RuO2-Pt and Ti/IrO2-Pt. J. Hazard. Mater., 162, 455.

19.

Lieberman

M.A.

, and Lichtenbeg

A.J.

(2005). Principles of Plasma Discharges and Materials Processing, 2nd ed. New York: John Wiley & Sons, Inc. Publication.

20.

Liu

, Wang

, Zheng

, Lei

, Tang

, Hu

, Xu

, and Wu

(2015). Radiation induced degradation of antiepileptic drug primidone in aqueous solution. Chem. Eng. J., 270, 66.

21.

Lobachev

V.L.

, and Rudakov

E.S.

(2006). The chemistry of peroxynitrite. Reaction mechanisms and kinetics. Russ. Chem. Rev., 75, 375.

22.

Mahvi

A.H.

, Malakootian

, Fatehizadeh

, and Ehrampoush

M.H.

(2011). Nitrate removal from aqueous solutions by nanofiltration. Desalin. Water Treat., 29, 326.

23.

Meyers

R.H.

, and Montgomery

D.C.

(1995). Response surface methodology: process and product optimization using designed experiments. John Wiley & Sons Inc., New York, NY, USA.

24.

Meyers

R.H.

, and Montgomery

D.C.

(1995). Response Surface Methodology: Process and Product Optimization using Designed Experiments. John Wiley & Sons, Inc.

25.

Muhammad

A.M.

, Abdul

, and Salman

A.M.

(2001). Water purification by electrical discharges. Plasma Sour. Sci. Technol., 10, 82.

26.

Schiavon

, Torretta

, Casazza

, and Ragazzi

(2017). Non-thermal plasma as an innovative option for the abatement of volatile organic compounds: A review. Water Air Soil Pollut. 228, 388.

27.

Summerfelt

S.T.

(2003). Ozonation and UV irradiation—An introduction and examples of current applications. Aquacult. Eng., 28, 21.

28.

Sun

, Sato

, and Clement

J.S.

(1997). Optical study of active species produced by a pulsed streamer corona discharge in water. J. Electrostat., 39, 189.

29.

Talebizadeh

P. B

, abaie

, Brown

, Rahimzadeh

, Ristovski

, and Arai

(2014). The role of non-thermal plasma technique in NOx treatment: A review. Renew. Sustain. Energy Rev., 40, 886.

30.

Tunçal

, Pala

, and Uslu

(2009). Determination of microbial responses to seasonal variations of wastewater composition in the Izmir wastewater treatment plant. Fresenius Environ. Bull., 18, 2114.

31.

Wang

, Zheng

, Jiang

, Lung

, Miao

, and Wang

(2014). Pilot-scale treatment of p-Nitrophenol wastewater by microwave-enhanced Fenton oxidation process: Effects of system parameters and kinetics study. Chem. Eng. J., 239, 351.

32.

Wang

, Wang

, Xue

, Li

, Dai

, and Peng

(2015). Treating low carbon/nitrogen (C/N) wastewater in simultaneous nitrification-endogenous denitrification and phosphorous removal (SNDPR) systems by strengthening anaerobic intracellular carbon storage. Water Res. 77, 191.

33.

Wang

, Zhou

, and Jin

(2012). Application of glow discharge plasma for wastewater treatment. Electrochim. Acta, 83, 501.

34.

Watts

R.J.

, Udell

M.D.

, and Monsen

R.M.

(1993). Use of iron minerals in optimizing the peroxide treatment of contaminated soils. Water Environ. Res., 65, 839.

35.

Yang

(2011). Plasma Discharge in Water and Its Application for Industrial Cooling Water Treatment. PhD thesis of Drexel University, Philadelphia.

36.

Zhang

, Lee

Y.W.

, Jahng

(2012). Ammonia stripping for enhanced biomethanization of piggery wastewater. J. Hazard. Mat. 199–200:36.

37.

Zhao

, Zhang

, Lv

, Wang

, Peng

, and Li

(2016). Advanced nitrogen and phosphorus removal in the pre-denitrification anaerobic/anoxic/aerobic nitrification sequence batch reactor (pre-A₂NSBR) treating low carbon/nitrogen (C/N) wastewater. Chem. Eng. J., 302, 296.