Sanitary sewage disinfection of a municipal treatment plant: use of individual and combined ozonation with UVC-LED radiation

Desinfecção de esgoto sanitário de uma estação de tratamento municipal: uso da ozonização individual e combinada com radiação UVC-LED

Abstract

The systematic articulation of water resources involves the use of technical and management instruments capable of minimizing environmental impacts with optimization of available capital and constitution of sewage treatment plants (STPs) to seek greater efficiency in the removal of contaminants, including microbiological ones, and enhance the quality of the water to be released into the receiving bodies. In systems where tertiary treatment is present, disinfection methods such as chlorination and UV radiation are commonly used as a final polishing of the effluent, and the monitoring of their effectiveness in disinfection is extremely important to avoid unnecessary expenses. Thus, this research sought to analyze the efficiency of disinfection of effluent samples from an STP of the sanitation system in the municipality of Indaial-SC using ozonation methods and the process combined with UVC-light-emitting diode (LED) radiation, on a bench scale, in batches. The determination of treatment efficiency was carried out by quantifying colony-forming units (CFU), and variables such as pH (5, 7, and 9) and application time (10, 20, and 30 min) formed the configuration of the analysis. The optimal disinfection point was reached within 20 min of the process, regardless of the disinfection method. In 10 min time, whenever the combined application was used, it showed improvement in efficiency at all pHs, most notably in alkaline medium. The application of UV radiation through LED is a recent technology and, therefore, has aroused interest in research worldwide.

Keywords:
UVC-LED; ozonation; energy efficiency; disinfection; plasmid denaturation

Resumo

A articulação sistemática de recursos hídricos envolve a utilização de instrumentos técnicos e de gestão capazes de minimizar impactos ambientais com otimização do capital disponível. As estações de tratamento de esgoto (ETE) são constituídas buscando a maior eficiência na remoção de contaminantes, incluindo microbiológicos, potencializando a qualidade da água a ser lançada nos corpos receptores. Em sistemas em que o tratamento terciário está presente, métodos de desinfecção como cloração e radiação ultravioleta (UV) são comumente utilizados como polimento final do efluente, e o acompanhamento da sua efetividade na desinfecção é extremamente importante para evitar gastos desnecessários. Desse modo, esta pesquisa buscou analisar a eficiência em desinfecção de amostras de efluente de uma ETE do sistema de saneamento do município de Indaial (SC), utilizando os métodos de ozonização e o processo combinado com a radiação UVC-LED, em escala de bancada, em batelada. A determinação da eficiência dos tratamentos foi realizada pela quantificação de unidades formadoras de colônia (UFC), e as variáveis pH (5, 7 e 9) e tempo de aplicação (10, 20 e 30 min) formaram a configuração da análise. O ponto ótimo de desinfecção foi atingido em 20 minutos de processo, independentemente do método de desinfecção. Em tempo 10 min, sempre que a aplicação combinada foi utilizada, apresentou melhora na eficiência em todos os pHs, mais notoriamente em meio alcalino. A aplicação da radiação UV por meio de diodos emissores de luz (LED) é uma tecnologia recente e, por isso, vem despertando interesse em pesquisas mundiais.

Palavras-chave:
UVC-LED; ozonização; eficiência energética; desinfecção; desnaturação de plasmídeos

INTRODUCTION

In the recent approach to environmental governance in Brazil, one of the main concepts is water security, which can be defined as ensuring access to quality water to preserve human health and ecosystems. In this way, different actors are involved in the process of governing natural resources, be it the business sector, the government, or civil society (Jacobi; Fracalanza; Silva-Sánchez, 2015).

In Brazil, it is a fact that several hospital admissions are linked to water-related diseases. This situation is generally due to the lack of basic sanitation and the dumping of sanitary effluents without going through a disinfection process (Assirati, 2005).

Water disinfection aims to inactivate pathogenic organisms contained in this liquid to ensure that water bodies are not contaminated. Furthermore, the attributes of disinfection include the inactivation of all pathogens at levels that minimize the risk of disease transmission; the prevention of by-product formation; the prevention of recovery from damage to the body; and being minimally affected by variations in the physical and chemical properties and characteristics of water (Cairns, 1995 apud Souza; Daniel, 2008).

Chlorine, which is widely used for this purpose, performs this water disinfection function well and has been widely used for several decades; however, in recent years, new pathogens have been discovered, such as drug-resistant microorganisms and others that have acquired new viral factors (Beattie; Skwor; Hristova, 2020).

Among several researched products with disinfectant action, ozone has been shown, in several studies, to be a powerful alternative disinfection product to chlorine, especially in the treatment of effluents due to its high reaction activity (Mahmoud; Freire, 2007; Wang et al., 2017; Miklos et al., 2018).

On the contrary, several combinations of advanced oxidation processes (AOP) can be used in order to improve disinfection based on ozone (O₃), among which the combined use of O₃ with the hydroxyl radical (OH·) stands out for UV radiation, H₂O₂, and ultrasound (Mahmoud; Freire, 2007).

In the search for effective disinfection processes, historically, DNA or RNA have been the main target in several organisms, and when UV light is absorbed by them, it ends up causing cell death (Li et al., 2019). On the contrary, photon emission can initiate hydroxyl radicals in the presence of catalysts or oxidants; thus, the OH· yield can be significantly improved during UV irradiation (Deng; Zhao, 2015).

In recent years, studies on the use of ultraviolet light-emitting diodes (UV-LED) for water disinfection have shown advantages and gained power in replacing lamps made with mercury (Li et al., 2017). A UV-LED is a solid-state semiconductor device with the advantages of no need for stability time, no mercury, flexible configuration, variety of wavelengths, longer lifespan, low power consumption, and the ability to be switched on and off at high and adjustable frequencies (Wang et al., 2022). The UVC wavelength is the one with the maximum absorption of UV light by DNA; however, in most cases, the absorption peak varies and is dependent on the target organism (Li et al., 2019).

Based on this information, the present study sought to develop an innovative approach through the creation of a bench-scale disinfection system, via the combination of ozone and UV-LED to obtain water free of microorganisms.

Sanitary sewage treatment plant

The water sample in this study comes from a sanitary sewage treatment plant (STP) that currently uses the disinfectant chlorine to deliver the final product; however, the use of this product in disinfection has shown little efficiency, for example, in bacterial removal/reduction, despite complying with current legislation.

The STP (WWTP) in question is of the continuous type, and its plant currently features a pumping station for raw sewage, preliminary treatment units (grating, grit chamber, and grease trap), biological, anaerobic, and aerobic treatment units in sequence (UASB reactor and submerged aerated biofilter), and subsequent final disinfection.

Based on bacterial quantification and the perception of degradation of the bacterial plasmid in the effluent sample, this study evaluated the proposed disinfection methods in solitary and combined application, seeking to optimize the techniques in an advanced oxidative process, combining the power of generating the hydroxyl radical (OH·) with the disinfection power of O₃ and UVC-LED irradiation that produces direct damage to the DNA of microorganisms through its inactivation mechanism.

MATERIALS AND METHODS

Microbiological activity: CFU

The quantification of the microbiological activity of the water sample was evaluated by the number of CFUs. The colony count was determined by the number of plaques per 100 μL of diluted sample in established culture form.

An aliquot of each concentration was taken for the quantification of CFU in the vortex. In total, 100 μL of sample from each concentration was taken and diluted in 900 μL of 0.85% saline solution (w/v) in 1.5-mL microtubes. At the end of the dilutions, 100 μL of the diluted sample was collected and transferred to the center of Petri plaques, previously containing the solid LB culture form (Luria–Bertani) prepared according to Döbereiner, Andrade, and Baldani (1999). The sample was then spread on the plate and incubated. After 24 h of incubation at 37°C, the colonies formed on the plates were counted. For each concentration, CFU was quantified in triplicate (COSTA et al., 2016).

Ozone generation

Ozone was generated from the corona effect in equipment with a working power of 43.8 W (0.0438 kW), and always operating in such a way as to produce residual ozone greater than 2 mg.min^-1, ensuring O₃ saturation in the reactor (Paula; Urruchi; Freire, 2021).

To quantify and establish the ozone dosage of the equipment used for this study, the iodometric method (indirect quantification method) was used according to Clescerl, Greenberg, and Eaton (1999) with minor modifications.

The working power of the ozonizer was determined by a specific electrical quantity measuring device (Fluke 87 V True-RMS Digital Multimeter).

UVC-LED

In systems using UV radiation as a microbial disinfection mechanism, the wavelength is very important and directly influences the disinfection efficiency (Li et al., 2019). Three LT5050UVC-XPC UVC-LEDs with wavelengths of 275 nm (Para Light Corp., Ltd.) were used in this study, their characteristics are shown in Figure 1.

Figure 1
(A) Wavelength of the UVC-LED with a peak radiant flux of 275 nm. (B) Viewing angle of the UVC-LED LT5050UVC-XPC; the red dotted line indicates where 50% of the radiation peak is reached, setting the beam angle (60 + 60 degrees).

The drive circuit was made on a phenolite electronic board composed of a 7.4 V power supply (Vcc), TIP41C transistor (Q), and 2 kΩ resistors (R). The board with LEDs, with a total power of 8.88 W, was placed next to the reactor for UVC irradiation during the tests.

Experimental apparatus

The samples were stored in opaque bottles and refrigerated until the analysis began. All collections and storage that followed were carried out in the same way, based on the ABNT/NBR 9898 (1987) standard.

Figure 2 shows the illustrative scheme of the process configuration used to disinfect the samples.

Figure 2
Illustration of the disinfection process.

The disinfection methods were applied separately (O₃ and UVC), combined, at different pHs (5, 7, and 9), and at different application times (10, 20, and 30 min).

Preparation, plasmid DNA extraction, and electrophoresis

The preparation of plasmids was carried out on a small scale by alkaline hydrolysis, based on the method described by Birnboim and Doly (1979) and Sambrook, Fritsch, and Maniatis (1989).

The separation of DNA fragments or intact plasmids was carried out by electrophoresis on a 0.8% (w/v) agarose gel in TBE buffer (Tris base 90 mmol.L^-1; boric acid 90 mmol.L^-1; and EDTA 20 mmol.L^-1, pH 8.0) containing ethidium bromide (0.5 μg.mL^-1). The electrophoretic run was performed in the same buffer and at the same concentration as described above. For gel application, DNA samples were mixed with 0.2 volume of FSUDS dye solution (bromophenol blue 0.08% (w/v); SDS 1% (w/v), EDTA 1.8 mmol.L^-1 pH 8.0; Tris-HCl 65 mmol.L^-1 pH 8.0; Ficoll 400 10% (w/v); and xylene cyanol 0.4% (w/v)), and the run was carried out at 5 V.cm^-1 of gel for 1–2 h (Sambrook; Fritsch; Maniatis, 1989). The parts were visualized under UV light (312 nm) on a transilluminator.

Electrical energy per order (E_EO)

Electrical energy per order (E_EO) is a parameter that was conceptualized by Bolton et al. (2001), published by the International Union of Pure and Applied Chemistry (IUPAC), and, according to Daneshvar, Aleboyeh, and Khataee (2005), is defined by the amount of electrical energy in kilowatt-hours (kWh) required to reduce the logarithmic concentration of a pollutant in order of magnitude in one (1) cubic meter (m³) of contaminated water. The E_EO formula (kWh.m^-3.order^-1) is given according to Bolton et al. (2001) (Equation 1):

(1) $$E_{EO}=\frac{P\cdot T\cdot 1000}{V\cdot \lg \mathbf{\left(}\frac{C_i}{C_f}\mathbf{\right)}}$$

Where:

P: the nominal power (kW);

V: the volume (L) of water in the reactor;

T: the time (h) necessary to reduce the pollutant concentration;

ci and cf: the initial and final values of the pollutant concentrations;

lg: the symbol for the decadic logarithm.

RESULTS AND DISCUSSION

The volume used in the analysis was 1000 mL, aiming to fill the entire reactor in search of process optimization. Notably, 10, 20, and 30 min were standardized for applying the disinfection methods in this study. Figure 3 illustrates the different results obtained.

Figure 3
Representative summary of results.

UVC-LED application

According to the current state of the art, UVC-LEDs represent a good compromise between the luminous power achieved and the ozone absorption coefficient (Schäfer et al., 2022).

The radiant power of the LEDs (40 mW) was decisive in their choice, and, aiming for better cost–benefit, we opted for LEDs with a higher power compared to other works.

Despite several attempts in different combinations, the direct application of LEDs individually to disinfect the raw water sample from the STP had no effect. After analysis, this application method using only UVC-LED radiation can be ignored as it does not present any change in the CFU values that did not show a statistically significant difference. Based on this finding, the UVC-LED system was only applied in combination with ozone.

The inefficiency of direct application in our setting is consistent with previous studies such as Galezzo and Susa (2021) and Wang et al. (2022). Most likely, it is related to the quality of the water sample and the desired volume for treatment combined with the small quantity of LEDs available, which can directly affect UV radiation, causing it to be weakened by key factors, such as water turbidity or dissolved organic matter.

Furthermore, Li et al. (2019) corroborated the idea that due to current technical difficulties, several aspects remain underdeveloped, and most UV-LED applications are in the research and development stage. There are no standard measurement and description schemes that determine how UV-LEDs can reach the disinfection mechanism.

3.2 Ozone application

According to works such as Preethi et al. (2009) and Deng and Zhao (2015), O₃ has been widely used in process industries at various levels to remove pollutants in effluents and is a strong oxidant with high oxidation potential. Table 1 presents the results of the individual application of ozone to the samples in this study.

Table 1
Result of applying O₃ to a water sample from an STP at different pHs. Average of triplicate repetitions.

Parcels	Unit	Raw	10 min	20 min	30 min	pH
CFU	x 104	2.51a	0.26b	0.001c	0.001c	5
CFU	x 104	3.07a	0.68b	0.001c	0.001c	7
CFU	x 104	3.07a	1.13b	0.001c	0.001c	9
Removal efficiency CFU	%	-	89.74A	≥ 99.9	≥ 99.9	5
Removal efficiency CFU	%	-	77.99B	≥ 99.9	≥ 99.9	7
Removal efficiency CFU	%	-	63.22C	≥ 99.9	≥ 99.9	9

• *Different lowercase letters in the row and different capital letters in the column differ by 5% by method and Dwass–Steel–Critchlow–Fligner—DSCF (p < 0.05).
Source: Author (2023).

It is noted that the efficiency of the treatment reaches its optimum condition after 20 min of application, regardless of the pH of the sample; however, analyzing the treatment in 10 min shows that the efficiency decreases with the increase in pH, having registered its greatest efficiency in sample disinfection (89.74%) at pH 5. This fact is explained by Khadre, Yousef, and Kim (2001), Jung et al. (2017), and Ferreira Filho (2021), who discussed the subject of favoring the presence of ozone in the liquid phase in samples with more acidic pH values and close to neutrality, a factor that influences a longer contact time with the aqueous medium causing greater inactivation of microorganisms.

Determining E_EO is an important tool for analyzing the amount of electrical energy (kWh) consumed to reduce the desired concentration of pollutants in contaminated water. By comparing E_EO with the different efficiencies obtained in the shortest application time (10 min), it is possible to analyze the effect of pH on the energy consumption of the disinfection method. Thus, at pH 5 and 7 the lowest value of E_EOpH5 = 7.41 kWh.m^-3.order^-1, E_EOpH7 = 11.26 kWh.m^-3.order^-1 was observed compared to the highest value found at pH 9 E_EOpH9 = 16.82 kWh.m^-3.order^-1. recording the lowest CFU reduction (pH 9). In this case, changing pH would become very interesting in situations with alkaline effluents to improve disinfection efficiency and electricity consumption.

O₃ + UVC-LED combination

According to Schoenell et al. (2022), the combination of ozonation with a photocatalytic oxidation system, compared to the direct application of ozone, can increase the degradation rate, depending on the pollutant and the radiation source.

Table 2 presents the results of this application, and what can initially be seen is that similar to the application of O₃, the combined treatment reaches its optimal condition after 20 min of application, regardless of the pH of the sample. However, unlike the direct application of ozone for 10 min, in which the efficiency observed in reducing the number of CFU gradually decreased with the increase in pH, in the combined application the efficiency at pH 7 and pH 9 is statistically the same (83.29% and 84.47%, respectively), including a considerable improvement in reducing the number of CFU (approximately 21% more efficient) at pH 9 when compared to the individual application of O₃ (63.22%).

Table 2
Result of applying O₃ + UVC-LED to a water sample from an STP at different pH. Average of triplicate repetitions.

Parcels	Unit	Raw	10 min	20 min	30 min	pH
CFU	x 104	3.68a	0.24b	0.001c	0.001c	5
CFU	x 104	3.35a	0.56b	0.001c	0.001c	7
CFU	x 104	3.72a	0.58b	0.001c	0.001c	9
Removal efficiency CFU	%	-	93.54A	≥ 99.9	≥ 99.9	5
Removal efficiency CFU	%	-	83.29B	≥ 99.9	≥ 99.9	7
Removal efficiency CFU	%	-	84.47B	≥ 99.9	≥ 99.9	9

• *Different lowercase letters in the row and different capital letters in the column differ by 5% by method and Dwass–Steel–Critchlow–Fligner—DSCF (p < 0.05).
Source: Author (2023).

As, in a time of 10 min, pH 5 was once again the most efficient in disinfecting the samples, the electrical energy consumption per request followed the efficiency response, where, in descending order, it remained that pH 5 > pH 7 = pH 9. Bringing the information in numbers, the value found at pH 5 was E_{EO pH5} = 7.40 kWh.m^-3.order^-1, followed by E_{EO pH9} = 10.87 kWh.m^-3.order^-1 and E_{EO pH7} = 11.19 kWh.m^-3.order^-1, and, comparing the disinfection methods studied in this work, electrical energy consumption is more advantageous in the combined process, achieving better disinfection rates with lower energy costs.

Electrophoresis: response of treatments to the bacterial plasmid

Gene transfer between bacteria occurs through the passage of genetic material to individuals of the same species, different species, and even different genera (Bennett, 2008). In this mechanism, plasmids play an important role, through the conjugation process. The environment polluted by sanitary effluents can be favorable to the manifestation of these plasmids, allowing the exchange of genetic material between bacteria due to the large presence of organic matter (Dodd, 2012).

The effect of the treatments on the integrity of the genetic material with possible resistance genes existing in the raw effluent sample was then analyzed after 30 min of application of the disinfection methods, as shown in Figure 4.

Figure 4
0.8% agarose gel electrophoresis of plasmid samples extracted from sanitary effluent samples subjected to different disinfection methods.

In all conditions tested, there is a clear decrease in viable cells after the treatment time (30 min) compared to the raw effluent. Therefore, it can be stated that all disinfection methods and conditions analyzed showed positive synergy in the degradation of plasmid DNA, possibly due to the increase in the concentration of radicals (Somensi et al., 2015).

Just like the application of UVC-LED for sanitary effluent disinfection, the analysis of bacterial plasmids in a process combined with UV-LED is an innovative work that demonstrates a promising field for the development of new studies.

It is worth highlighting here that, even though the disinfection process reached the optimum point of efficiency (> 99.9%) in 30 min in all the configurations analyzed, it is clear that, even with a decrease, plasmid DNA is still present in the electrophoresis analysis presented in Figure 4. This shows that disinfection is not directly linked to the degradation of genetic material; however, the processes presented here demonstrated promising results in this degradation, unlike chlorine. According to Dodd (2012), chlorination is not a good process when seeking to degrade genetic material, and, based on the available data on the degradation of extracellular and intracellular DNA by chlorine, the author reports that it will probably only be possible to achieve measurable levels of bacterial plasmid degradation through chlorination with concentrations 10 times (10×) higher than those commonly applied in treatments. This leads to other concerns related to the quality of the receiving body, such as the amount of residual chlorine released and the excessive formation of byproducts.

The application of the combined method showed an evident and significant improvement at pH 9, where the reduction in the number of CFU colonies was 21% more efficient than when applying O₃ directly alone. We can also say that, at all pH levels, there was an improvement in disinfection when the combined process was applied.

The application time is decisive for the energy efficiency of the disinfection process, and for the purpose of comparison and reproducibility of the studies, the electrical energy consumption of all configurations was analyzed within 10 min.

Although the effluent analyzed has a naturally neutral pH, in treatment situations where the pH of the water is alkaline or acidic, the application of the combined disinfection method also presents itself as a possible solution with lower energy consumption, compared to the application of ozone alone. However, taking into consideration all the information presented, it is suggested that further studies and analyses, to complement this work, be developed to determine the quantification of UVC-LED lamps and O₃ consumption, aiming at the development of a disinfection system in shorter time, ozone, and electrical energy consumption. And from this, we observe that constant analyses are needed to verify the possibility of changing the pH of the effluent to achieve greater efficiency in a shorter process time, evaluating the best cost–benefit.

Furthermore, it is important to emphasize that, as shown in Figure 4, all treatments showed the power to degrade bacterial plasmids, requiring further studies on this issue, even with the quantification of this degradation for each configuration used. Therefore, it is clear that for each application, the objectives to be achieved with the desired disinfection method must be evaluated, since, according to Somensi et al. (2015), disinfection is not synonymous with the degradation of genetic material.

CONCLUSIONS

The best treatment in 10 min was the combined process at pH 5. In order of disinfection efficiency: (O₃ + UVC-LED)_pH5 > O_3pH5 > (O₃ + UVC-LED)_pH9 > (O₃ + UVC-LED)_pH7 > O_3pH7 > O_3pH9. This has shown a greater difference between the application of ozone alone to the combined process at pH 9. However, in the analysis of electrical energy consumption per E_EO request in 10 min of process, the order of efficiency resulted in: (O₃ + UVC-LED)_pH5 = O_3pH5 > (O₃ + UVC-LED)_pH9 > (O₃ + UVC-LED)_pH7 = O_3pH7 > O_3pH9.

The combined process (O₃ + UVC-LED) proves to be suitable for disinfection processes of sanitary effluents, even as it presents (albeit discreetly) a greater efficiency in inactivating bacteria with the same consumption of electrical energy per request as disinfection by ozone alone.

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• Funding:

  Instituto Federal Catarinense; SETEC/MEC-Brasil (Public Call Notice 02/2020 - IFES).

Publication Dates

• Publication in this collection
  21 Oct 2024
• Date of issue
  2024

History

• Received
  03 Mar 2024
• Accepted
  19 June 2024