Anaerobic digestion effluent from sewage treatment plants (STP) is challenging because of high organic matter and inorganic nitrogen concentrations, which require an effective process for biological treatment. The study aimed to introduce a UV-VIS spectrum representative index by discrete Fourier transform (DFT) and apply UV/Vis spectrophotometric techniques for the real-time organic compounds monitoring in the AOP process using anaerobic digestion tank effluent wastewater. However, the effect of advanced oxidation on the organic compounds of effluent using ozonation was examined. Therefore, in this research, after treating secondary treated water with the UV-AOP process and anaerobic digestion effluent with ozone microbubble systems, changes in organic substances were expressed by the UV-VIS spectrum and compared with conventional water quality parameters. The anaerobic digestion tank effluent was subjected to ozonation, had a high curve, and fell gently from 230 nm to 667 nm. Discrete Fourier Transform (DFT) was applied to obtain representative values from the obtained spectrum. Among the coefficients obtained by analyzing the UV-Vis spectrum by DFT, an expected value was selected, and the correlation between CODMn and a3 was the highest (correlation function = 0.694, RSQ = 0.482). Therefore, a linear regression analysis was performed to determine which water quality factor it was related to.
Research Article
Real-time organic compounds monitoring in advanced oxidation process (AOP) of sewage digestion tank wastewater: introduction of UV-VIS spectrum representative index by discrete Fourier transform
https://doi.org/10.21203/rs.3.rs-2631657/v1
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Advanced oxidation
Digestion tank effluent
Organic compounds
UV-VIS spectrometry
DFT
- Changes in organic substances were reached through the efficient UV-AOP process and ozonation with a microbubble nozzle.
- DAS decreased in Ozone treatment and increased in the UV-AOP process.
- Spectral parameters range 436-254 nm can evaluate the changes in DOM.
- Discrete Fourier Transform (DFT) was used to analyze the UV-Vis spectrum.
- DFT coefficients were extracted from each UV-Vis spectrum.
Nowadays, effluents from municipal wastewater treatment plants' discharge regulations have been strict.
It was maintained (Mecha et al., 2016). Municipal wastewater, generally generated from different domestic sources such as toilets, showers, and the kitchen, is widely treated through the activated sludge process (Zhou et al., 2017; Demir & Filibeli, 2012). In the domestic wastewater treatment process, monitoring before and after treatment is performed by directly sampling raw water and chemical analysis; otherwise, it is operated through automated devices and sensors. Although automated equipment that analyzes TOC or COD has been developed, it is an improved version of the existing analysis equipment, which is substantial, expensive, and time-consuming.
The sewage in the sewage treatment process contains many organic substances, and successful decomposition is possible through advanced oxidation treatment (Rekhate et al., 2020). A representative technique is an ozonation using irradiation of a UV lamp and dissolution of ozone gas. Because UV-AOP significantly affects turbidity, it should be applied to raw water with UVT (UV Transmittance) secured. There have been many reports of cases in which the treatment technology is used mainly for the secondary treatment of sewage and subsequent processes (Audenaert et al., 2013). On the other hand, ozone treatment has no side effect on the water quality of the target water, and there is no restriction on the treatment of high concentrations of organic substances. In particular, a process that simultaneously treats suspended solids (hereinafter referred to as SS) and organic matter based on the ozone-flotation principle linked to microbubbles is advantageous in wastewater treatment. Anaerobic digestion effluent has a highly concentrated organic compound and contains a lot of SS and nitrogen components (Lee et al., 2011; Akter et al., 2022).
Some cases have been reported, including the present study on anaerobic digestion effluent treatment using AOP (Akter et al., 2022; Lee et al., 2011). The water quality of anaerobic digestion effluent is challenging to quantify, and even if oxidation treatment is continued, the expected results are not obtained. In a previous paper, it was found that TOC and T-N increased when ozone injection was continued. CODMn and CODCr also continuously increased organic matter indicators. Ammonia fluctuated or showed a tendency to increase, and nitrate and nitrite continued to increase, but they were minimal in composition. Highly concentrated organic wastewater may be diluted to observe consistent treatment trends (Lee et al., 2011). As a result, securing a particular water quality and treatment rate was possible, but it was not easy to calculate the treatment effect on undiluted wastewater (Lee et al., 2011).
Monitoring of organic substances in sewage treatment water using the UV-Vis spectrum is described by (Audenaert et al., 2013) is well introduced. Recent advances in optical and electronic technology allow the mixing or combining of all UV-Vis spectroscopy methods. Also, small, rugged sensors monitor water quality online and in situ. Online UV-Vis spectrometry can be used to monitor and control water quality systems (Van den Broeke, 2007) and for the monitoring of wastewater treatment plants (WWTPs), especially for the different treatment stages, in order to assess pollutants’ loads and the efficiency of organic materials treatments (such as chemical oxygen demand (COD) and biochemical oxygen demand (BOD5)), nitrates and nitrites, as well as total suspended solids (TSS) (Rieger et al., 2004).UV/Vis technology is rapid and can be used for the monitoring of the essential water quality parameters: total suspended solids, dissolved organic carbon, the chemical oxygen demand, the biochemical oxygen demand, and measure of detergents and nitrates since these materials absorb electromagnetic radiation at different wavelengths (λ). (Radzevicius et al., 2020). The UV-VIS spectrum was used to compare the reduction of organic substances by ozone and UV-AOP in the secondary treated water of the sewage treatment process. A case in which an empirical equation was derived with a multiple linear regression (MLR) model by extracting values from the entire range or multiple wavelengths of UV-VIS rather than a specific wavelength of the UV spectrum is reported (Kim et al., 2016). UV-VIS spectrum does not take time and can be analyzed in real-time. With the development of electronics industry technology, it can be connected to a personal computer or IoT (Internet of Things) device so that it can be used for web monitoring and process automation as well as securing DB.
In this study, the literature review shows some of the valuable measurements used to describe organic matter's properties during (advanced) oxidation. Examples are UV-visible (UV-VIS) absorbance measurements (Gong et al., 2008; Wert et al., 2009; Nanaboina & Korshin, 2010; Ratpukdi et al., 2010; Tripathi et al., 2011), polarity measurements (Gong et al., 2008; Rosario-Ortiz et al., 2008; Ratpukdi et al., 2010) and high-performance size exclusion chromatography (HPSEC) (Ratpukdi et al., 2010; Gonzales et al., 2012; Sharif et al., 2012). Nanaboina and Korshin (2010) state the differential absorbance spectra (DAS) from UV-VIS spectra of secondary effluent and explain that some wavelength regions (e.g., 260-280 nm) were more affected by oxidation than others. In this study, DAS will be used for the first time to compare organic matter transformations induced by ozone treatment using conventional water quality parameters. To the best of our knowledge, the advanced oxidation process to anaerobic digestion tank water will be used for the first time to study real-time monitoring of organic matter. This could be interesting because, besides a decreasing overall absorbance, there are also dynamic shifts between different during (advanced) oxidation. In this study, after treating secondary treated water with UV-AOP and anaerobic digestion effluent with ozone microbubble, organic substances were expressed by UV-VIS spectrum and compared with conventional water quality. Discrete Fourier Transform (DFT) was applied to obtain representative values from the obtained spectrum. A representative value was selected among the coefficients obtained by analyzing the UV-VIS spectrum by DFT, and a linear regression analysis was performed to determine which water quality factor it was related to.
The objective of this study is to real-time organic compounds monitoring by ozone spectral response, concentration, and biodegradability using a set of different surrogates, including DAS, compare with conventional water quality parameters and establish correlations between the different measurements performed, which provide insight into the process and can be used as a starting point for mathematical modeling.
2.1 Sample collection and Ozone treatment
Anaerobic digestion effluent samples were collected from the anaerobic digestion tank of a domestic WWTP in South Korea. The target raw water was the effluent from the anaerobic digestion tank obtained by dewatering the sludge discharged from A sewage treatment plant (STP) and B STP for partial comparison. The characteristics of the digestion tank effluent samples are presented in Table 1.
Ozone treatment was a 20 L reactor-sized circulation method, and ozone gas was supplied through a microbubble nozzle. The influent gas flow was the maximum output value of the ozone generator. The concentration through the meter was 91.5~144.0 g/m3, initially 91.5~129.1 g/m3, and then gradually stabilized to 120.2~144.0 g/m3 within a few minutes.Table 1. Water quality parameters of two STP
|
Parameter |
Concentration |
|
|
|
‘A’ STP |
‘B’ STP |
|
pH |
8.05 |
8.16 |
|
CODCr (mg/L) |
230.00 |
142.00 |
|
CODMn (mg/L) |
136.50 |
71.00 |
|
TOC (mg/L) |
40.71 |
73.48 |
|
T-N (mg/L) |
365.00 |
|
|
NH4 +-N (mg/L) |
345.60 |
364.68 |
|
NO3 −-N (mg/L) |
2.70 |
0.60 |
|
NO2 −-N (mg/L) |
0.100 |
0.001 |
|
BOD (mg/L) |
2.90 |
52.37 |
2.2 Experimental set-up and analytical process
consists of an ozone generator, reaction column with an effective volume of 20 L, sample collection port, residual ozone meter (electrode method, measuring range 0.5 ppm), flowmeter (20 LPM), ejector, ozone gas monitor, gas flow meter, and ozone destructor. Ozone was generated from high-purity oxygen using conversion in the ozone generator and simultaneously injected into the ozonation reaction column.
In this experiment, the ozone process was performed in a batch reactor and was fixed with a microbubble
generator. Samples were taken at regular intervals after starting the ozonation using 1 L plastic bottles from the bottom sample collection ports of the reactor. Temperature and pH were measured using a pH meter (S-610H) on-site immediately after the collection of the samples. The water quality parameters as soluble CODCr, CODMn, T-N, NO2−-N, and NO3−-N were measured by the kit (CMAC) standard method using the DR-5000 spectrophotometer. TOC analysis using the total organic high-temperature carbon combustion oxidation method (ES 04311.1b) was applied to the water pollution process test standards.
2.3 UV-VIS spectrum and DAS
For the UV-Vis spectrum, a mini-module type spectrometer (STS UV, ocean insight) was used. The analysis range was 186.232~667.856 nm for UV and visible light region, the resolution was 1024 pixels, and the pixel interval was 0.442~0.494 nm.
The differential adsorption spectrum (DAS) is obtained by subtracting the value of the initial spectrum from each wavelength after treatment and can indirectly know the degradation characteristics after AOP treatment.
2.4 Discrete Fourier Transform
Discrete Fourier Transform (DFT) was used to analyze the UV-Vis spectrum. Odd function and half range expansion spectrum were applied for waveform analysis. Fourier transform coefficients that can be obtained during decomposition are as follows. A is relative absorbance, resolution is 1024, λi is the symbol of the wavelength corresponding to 186.232~667.856 nm, and serial number i is 1024 resolution. λmax is 667.856 nm and λmin is 186.232 nm.
The UV-Vis Spectrum re-build-up by the above formula is as follows. Since Ai is the amplitude at each wavelength, the original waveform can be drawn by combining the wavelengths corresponding to the range with Ai.
3.1 Changes in organic matter
Ozone reacts with organic and inorganic matter because of its high reproduction potential and reactivity (Hoigne & Bader 1983a; Hoigne & Bader 1983b, Hoigne & Bader 1985). As a result of the experiment under ozone injection conditions, the changes in the organic matter were as follows (Fig. 1). CODMn dropped significantly from 167.6 mg/L to 86.7 mg/L at 40 min, then decreased gently to 46.9 mg/L at 180 min. The final removal rate was 72.0%. CODCr decreased by 69.1% from 230.0 mg/L to 71.0 mg/L at 180 minutes. TOC showed a similar trend to CODMn, which fell sharply from 148.5 mg/L to 76.8 mg/L in 5 minutes, then decreased gently, and finally decreased by 68.7% to 46.5 mg/L. Although there was a difference for each indicator, the organic matter decreased significantly between 5 and 20 minutes and decreased at a relatively small rate after that. Ozone utilization is more effective at lower ozone concentrations, but longer retention times are required for optimal efficiency (Shin & Lim, 1996). Only a slight nitrogen reduction was observed at the highest oxidant doses over time, indicating that only minor mineralization occurred.
On the other hand, organic nitrogen decreased following a linear trend, and reductions varied between 20 and 30% after 120 min of treatment. Due to the constraint characteristics of ammonia nitrogen, during 20 min of reaction, the NH3-N concentration was increasing steadily, and after 20 min, it decreased by around 28%. However, the ozone had a nominal effect on T-N, and the results show a variation in the removal of Total Nitrogen. The summary of these results showed that the ozone treatment mainly had a crucial changing effect on organic nitrogen.
3.2 UV-vis spectrum change
The curve change of the UV-vis spectrum after ozone treatment of the anaerobic digestion wastewater is shown in Fig. 2. The anaerobic digestion wastewater subjected to ozonation had a high curve and fell gently from 230 nm to 667 nm. After that, the curves were sequentially stabilized until 20 minutes, and after that, almost similar results were shown. As a result of tracking the treatment results after 20 minutes, there was no significant difference, but rather a shoulder between 270 nm and 340 nm appeared. (Noethe et al. 2009) stated that highly concentrated organic compounds absorb at higher wavelengths (e.g., 436 nm), and possible to change compounds that still have absorption at lower wavelengths (e.g., 254 nm) after ozonation. Most important is that during UV/H2O2 treatment, some of the change compounds formed and absorbed in the higher UV region, that is, as opposed to ozonation. Presumably, these changes in organic compounds were also formed during ozone treatment, but the rate of decomposition exceeded the rate of formation due to their high reactivity to ozone. The distinction of ozone is also exemplified by the fact that the shape of the normalized DAS changes appreciably as a function of time. In comparison with the results of (Nanaboina & Korshin, 2010), at higher wavelengths, the normalized DAS values were initially higher and gradually decreased with ozone dose. This appears to be a result of the strong reactivity of organic intermediates produced as by-products after the ozone reaction. (Audenaert et al., 2013)
The graph of DAS showing the differential value of UV-vis spectrum in Fig. 3a. DAS increased by 20 minutes and there was no significant difference thereafter. Rather, Fig. 3b, showed it decreased little by little. According to (Audenaert et al., 2013), comparably DAS decreased in ozone treatment and increased in UV-AOP process.
Changes in the above spectrum are visible, but it isn't easy to compare them by representing them with a single numerical value. Therefore, the representative values of the spectral changes were taken as the sum at each wavelength and the absorbance at 254 nm and are shown in the following graph. As a result, there was no significant difference after the sum of the wavelengths decreased sharply up to 20 min, and after 20 min, it increased slightly at the 254 nm wavelength. However, the curves during ozone treatment almost coincide, and this phenomenon means that during the ozonation process, DAS curves changed uniformly. Hence, the absorbance removal efficiency at a particular wavelength was almost independent of oxidant exposure. Therefore, it was hypothesized that the change of organic compounds that were initially absorbed in 30 min contributed to the formation of products absorbing until 180 min. After 30 minutes of ozone treatment, the normalized DAS overlapped further as the areas preferentially charged by ozone were almost completely oxidized. The decreasing trend of the sum of these wavelengths is shown in Fig. 4. It appeared similar to CODMn and TOC curve of Fig. 1.
3.3 Comparison of UV-Vis representative extraction and water quality index
Spectral analysis is used to find the periodicity contained in a time series. For this, DFT was selected to go from time domain to frequency domain. This approach converts a finite number of equally spaced samples into multiple coefficients of a finite combination of frequency-ordered complex sinusoids (components) with values of the same number of samples (Proakis &Manolakis, 2007). DFT coefficients were extracted from each UV-Vis spectrum. For the spectrum shown in Fig. 5, the coefficient values from a1 to a10 are shown in the graph. The most representative curves were a1 and a3, which decreased sequentially with time.
To compare the correlation between the actual organic matter water quality index and the DFT coefficients a1 to a10, Pearson's product-moment correlation (Correlation) and Root Mean Square Slope (RSQ) index were used in Fig. 6 and Fig. 7. Among organic material indicators, the correlation was highest in the order to express CODMn, TOC, and CODCr values. In particular, the correlation between CODMn and a3 was the highest (correlation function = 0.694, RSQ = 0.482).
CODMn and coefficients correlation shows that there are a3, a6, a8, a10 specifically higher values. Summary of UV 254-Abs absorbance values (UV254-Abs) of UV-VIS indicators and the smallest values from 186.232 to 667.856 nm. Moreover, the sum of UV254-Abs and UV-Abs for CODCr and TOC, 0.694 more than the sum of UV254-Abs and UV-Abs, and a8 values are showing 0.688, 0.652, and 0.572 accordingly.
The effects of different practices on the UV–Vis absorption spectrum of CODMn in different regression Curve are presented in Fig. 8. Linear regression analysis was performed for the data with the absolute value (UV254-Abs, Fig. 8a) and, sum of all data (Sum of UV-Abs, Fig. 8b) was used to normalize the data before performing linear regression analysis. According to the results of UV-DFT correlation analysis (Fig. 8), it was found that the correlations between UV254-Abs, CODMn and UV-FT, CODMn spectral parameters were used to determine the organic contents in wastewater. Due to the large variation in organic content in anaerobic digestion tank effluent different field experimental treatments, both the individual correlations of each treatment and the overall correlations were investigated. Due to the large variation in organic content in anaerobic digestion tank effluent different field experimental treatments, both the individual correlations of different 4 samples and the overall correlations were investigated. From the overall regressions, the determination coefficient (R2) was determined and the value of 3rd coefficients was most fitted to the CODMn among the other cases.
This study investigates the effect of advanced oxidation on the properties of organic matter using the ozone oxidation process. The study results can be presented as follows:
-
DAS were applied for the first time to compare with conventional water quality parameters conversions induced by ozone treatment. These spectral calculations proved to be helpful in unraveling mechanistic differences between Ozone and HO-induced changes in organic contents and significantly contributed to process understanding.
-
Ozone preferentially reacted with organic compounds absorbing at higher wavelengths, while during the UV-VIS, spectral changes were more uniform. Additionally, the absorption of transformation products impacted the spectral results for both oxidation techniques differently.
-
The preliminary results of the present work showed that the UV-VIS spectrum of wastewater samples contains information that can be extracted and used for qualitative WWTP monitoring. This information can support the selection of monitoring investments in quality assessment and provide data for process and WWTP optimization. From these preliminary studies, it can thus be concluded that UV spectroscopy associated with simple DFT is a valuable tool for wastewater quality monitoring.
Acknowledgements
The authors are very grateful for the funds [Project, NRF-2021K1A4A8A01079319] provided by the “National R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT”.
Author contributions
Jesmin Akter analyzed and collected the data, and wrote the first version of the manuscript. Wonjae Kim conceived the study, and commented on the results, and contributed to fund acquisition. Ilho Kim also conceived the study, contributed to designing the experimental plan, and commented on the results. Jaiyeop Lee contributed to revising the manuscript, managed the data; the calculations and interpreted the results, and improved the writing. All authors reviewed the results and approved the final version of the manuscript.
Funding
This research is financially supported by a grant from National Research Foundation of Korea (NRF) (NRF-2021K1A4A8A01079319).
Data availability
The authors confirm that the data supporting the findings of this study are available within the article [and/or] its supplementary materials.
Conflict of interest
The authors declare no competing interests.
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No competing interests reported.
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