Ref # S2SMALL- 85630
A. Teodoro1, A. Machulek Júnior2, M. Á. Boncz3, P. L. Paulo4*
1 2 3 4Faculty of Engineering, Architecture and Urbanism, and Geography (FAENG), Federal University of Mato Grosso do Sul (UFMS), Cidade Universitaria, Campo Grande - MS, Brazil.
* Presenting Author: paula.paulo@ufms.br
Abstract: Greywater presents great potential for reuse, if treated correctly and efficiently, it can be used in a variety of residential uses. The objective of this work was to test advanced oxidation for greywater disinfection through UV/TiO2, UV/TiO2/H2O2, photo-fenton, UV/H2O2 and Photolysis (UV) processes, using Pseudomonas aeruginosa as an alternative indicator. In general, the processes with hydrogen peroxide (150 mg.L-1) mixed in the pretreated greywater and exposed to solar radiation or artificial radiation from UV lamps were the most efficient in the disinfection experiments, with total inactivation ofP.
aeruginosa. These processes (UV/H2O2 and photo-Fenton) were better fitted to the log-linear/caudal decay model with remaining microorganism for the hydrogen peroxide concentration of 25 mg.L-1. The use of P. aeruginosa as an alternative indicator for the greywater disinfection was very promising due to its high resistance and high natural concentration in the effluent used in the experiments. The treatment applied with the UV/H2O2 process with the hydrogen peroxide concentration at 150 mg.L-1 was the only one that showed acute toxicity, even though it removed a good part of the surfactant concentration from the pre-treated greywater.
Keywords: effluent treatment; pathogen indicator; source separation
INTRODUCTION
Greywater is defined as wastewater without effluent contributions from sanitary basins, which means only wastewater produced in bathtubs, showers, washbasins, washing machines and kitchen sinks. It is considered a highly recoverable water source, especially in countries suffering from water scarcity (Tsoumachidou et al., 2017). Light greywater refers to the wastewater produced by the previously mentioned sources, excluding greywater from the kitchen sink because it has a higher amount of undesirable compounds such as oils and fats (Allen et al., 2010). The relatively high concentration of the readily biodegradable fraction of organic matter may contribute in some cases to the presence of high levels of pathogens in greywater (Ridderstolpe, 2004).
For years, Escherichia coli, the main representative of the thermotolerant coliform subgroup, has been used as an indicator of fecal contamination. However its use for this purpose has been questioned. Some disadvantages are pointed out about the general use of Escherichia coli, such as its low tolerance to saltwater toxicity and accelerated bacterial decay, which may provide non-real data regarding the presence of other pathogenic microorganisms such as Giardia cysts, Cryptosporidium oocysts and human enteric viruses, which exhibit superior resistance to disinfectants. As with domestic sewage, due to the different characteristics of greywater, it is advisable to use more than one microorganism indicator. Mohammed et al.
(2012) found that non-enteric bacteria (Pseudomonas aeruginosa and Staphylococcus aureus) had a good ability to reproduce and persist, particularly in sterile dry sand, in addition to surviving a number of conditions. Thus, it is believed that these microorganisms can be used more reliably as indicators. P. aeruginosa are opportunistic bacteria, related to several cases of serious infections, mainly in hospital ones. They can be found widely in the environment, human skin and also in the feces of sick people (Toledo & Trabulsi, 2002). According to
Pitten et al. (2001), P. aeruginosa has a strong involvement in the most severe infections, such as those of the respiratory tract, urinary tract and the blood flow. Characterized by straight or curved bacilli, the genus Pseudomonas is restricted aerobic, gram-negative and has mobility through flagella. They have the capacity to grow in waters with low levels of dissolved solids and organic compounds, adapting easily in environments with low nutrient availability. Because of the natural resistance to several antibiotics, the presence of Pseudomonas aeruginosa is closely related to hospital infection cases and is capable of triggering several waterborne gastroenteritis outbreaks (Bier, 1994). The choice of disinfection indicators to be used for greywater reuse should indicate the true risks that this wastewater can pose. Pseudomonas aeruginosa is an example of microorganism that can be used with complementary pollution indicators in addition to the coliform group (Jones et al., 2013). Pseudomonas present in greywater can cause various diseases and compromise the organoleptic characteristics of water. Direct contact with water or effluents containing a sufficient number of these microorganisms can produce various infections on the skin, mucous membranes of the eyes, ears, nose and throat, thus requiring prior treatment to ensure safe reuse (WHO, 2006). In this sense, Advanced Oxidation Processes (AOP) can be used for the decontamination of water containing organic pollutants, classified as bio recalcitrant, and for the disinfection of emerging pathogens (Polo-López et al., 2012). These methods are based on the formation of highly reactive chemical species that degrade even the most recalcitrant molecules in biodegradable compounds (Malato et al., 2009), and have been widely studied in the last decades with several applications for effluent treatment (Dewil et al., 2017). Although there are different advanced oxidation processes, they are all driven by the same chemical characteristic: hydroxyl radical production (HO˙), which are capable of oxidizing and mineralizing almost any organic molecule, producing CO2 and inorganic ions under the proper conditions (Machulek Jr et al., 2013). Thus, the objective of this work was to test different greywater disinfection systems using advanced oxidation processes using an alternative disinfection indicator.
MATERIAL AND METHODS
The experiments were carried out on bench scale and pilot scale. For the bench tests, cylindrical photochemical reactors of borosilicate glass were used, with a useful volume of 1 L (first and second stages of tests). The UV radiation source was a 125 W high pressure mercury vapour lamp (without the protective bulb) with the maximum emission at 254 nm, positioned along the longitudinal axis of the reactor and protected by a quartz tube. The radiation intensity (4.30 mW.cm-2) was determined by actinometry with potassium oxalate, according to the methodology proposed by Harris et al., 1987. The greywater was recirculated in the reactor by pumping through an external auxiliary reservoir with a volume of 2 L. An isolation system consisting of a sealed wooden box was used to protect the operator from the radiation of the lamp. In the bench scale, PET bottles exposed to the sun were also used, functioning as photocatalytic reactors (third stage). For the pilot scale (fourth stage), baffled reactors with recirculation and solar radiation were used.
The intensity of the solar radiation was determined by means of a radiometer installed in the INMET meteorological station , near the locale of the experiment. These reactors were built with a 22º slope (approximate latitude of the city Campo Grande-MS), positioned on the roof of the Effluent Laboratory at the Federal University of Mato Grosso do Sul to receive solar radiation.
The use of Pseudomonas aeruginosa as an alternative indicator of disinfection in greywater was evaluated through the use of advanced oxidation processes with bench scale and pilot scale reactors, comparing the efficiency among the various processes: TiO2/UV,
TiO2/UV/H2O2, foto -Fenton, H2O2/UV and Photolysis (UV). The experiments were carried out in the following sequence:
- 1st Stage: Bench scale tests with a photochemical reactor and a UV lamp operating in batch;
heterogeneous photocatalysis with TiO2 supported in Pyrex glass microtubes (used as reactor filling) and UV, H2O2, UV/TiO2, UV/H2O2 and UV/TiO2/H2O2 processes. The hydrogen peroxide concentration for this stage was 150 mg.L-1.
- 2ndStage: Bench scale tests with a photochemical reactor and a UV lamp operating in batch;
homogeneous photocatalysis with the processes photo-Fenton, UV and UV/H2O2. Two hydrogen peroxide concentrations were used for this test stage: 25 and 150 mg.L-1.
- 3rd Stage: Bench scale tests with a solar photochemical reactor (photocatalysis in PET bottles exposed to the sun) operating in batch; homogeneous photocatalysis with the photo-Fenton, UV and UV/H2O2 processes. For this stage, two hydrogen peroxide concentrations were used: 25 and 150 mg.L-1.
- 4th Stage: Tests in a pilot reactor in a baffled reactor with acrylic tubes (continuous photochemical solar reactor with recirculation) operating in batch; homogeneous photocatalysis with the UV and UV/H2O2 processes. The hydrogen peroxide concentrations for this stage were 25 and 150 mg.L-1.
The greywater used in the experiments consisted of mixing the water from washbasins, washing machines, bathtubs and showers. This water did not contain the kitchen fraction, and was collected after passing through a pretreatment system composed of an evapotranspiration tank followed by a constructed wetland with horizontal flow. The plant species Heliconia psittacorum L.f. (popularly known as heliconia or andromeda) was chosen to be used in the constructed wetland. Table 1 shows the mean values of the physical-chemical parameters used for monitoring the greywater treatment system.
Table 1. Mean values of the parameters used for monitoring the greywater treatment system.
Parameters
For the analysis of Pseudomonas aeruginosa, the Pseudalert® method of IDEXX, USA, approved by the United States Environmental Protection Agency (USEPA) and included in the Standard Methods for the Examination of Water and Wastewater (APHA, 2012) was used.
All tests were performed in triplicate, with a duration of two hours, and samples were collected for analysis at 20, 40, 60, 90 and 120 minutes for the bench scale and at 30, 60, 90 and 120 minutes for the pilot scale. For the bacterial decay curves, the adjustments were verified through the correlation coefficient in two inactivation models: Chick-Watson and Log-Linear/Caudal. In order to predict the possibility of discharging greywater disinfected by advanced oxidation process (UV/H2O2) in streams and rivers, ecotoxicological tests were carried out since the reactor in the pilot scale (acrylic tubes) can be installed to disinfect effluents from small condominium treatment systems.
RESULTS AND DISCUSSION
The disinfection studies performed in this study showed that it is possible to disinfect light greywater for reuse purposes in a sanitary basin, with an alternative disinfection indicator (Pseudomonas aeruginosa) in a fast and economical way, by using UV radiation combined with some advanced oxidation processes. In the experiments, both the heterogeneous photocatalysis with titanium dioxide combined with hydrogen peroxide and the homogeneous photocatalysis with hydrogen peroxide and ferrous ion were able to inactivate significant amounts of Pseudomonas aeruginosa. The use of hydrogen peroxide without exposure to UV/solar radiation (blank) was ineffective for disinfection, with a reduction of less than one log unit for Pseudomonas aeruginosa. The results are presented below.
x 1st Stage
It was possible to obtain a homogeneous layer of TiO2 deposited in the small Pyrex glass tubes (used as reactor filling), with a mean thickness of 35.3 μm. However, only the use of TiO2 with UV radiation was not able to promote an increase in the inactivation of Pseudomonas aeruginosa. The photolysis performance (UV radiation only) was similar to the UV /TiO2 process (Fig. 3, first stage), with slightly higher inactivation for Pseudomonas aeruginosa (0.7 log unit). In studies by Robertson et al. (2005), the direct UV radiation was also more effective for the removal of Pseudomonas aeruginosa. On the other hand, the processes with hydrogen peroxide were much more efficient, with complete inactivation of the alternative indicator within one hour of the experiment. Pseudomonas aeruginosa disinfection processes without the addition of hydrogen peroxide had a better fit to the classic Chick-Watson model (1st order), with higher correlation coefficients (R2) (Table 2). The process with added hydrogen peroxide had the same adjustment for both models. This is because the process has reached the complete inactivation of Pseudomonas aeruginosa, with no remaining population of microorganisms. Without the remaining microorganisms’
component in the equation, the log-linear/caudal model becomes identical to the Chick-Watson model. For the processes with titanium dioxide, the unviability in the conditions tested in this study was due to the difficulty in obtaining a homogeneous and consistent layer of TiO2 deposited in the small Pyrex glass tubes. Even reaching optimum deposition of this layer, the UV/TiO2 process did not achieve complete inactivation of the indicator microorganisms. For this, it was necessary to add hydrogen peroxide combined with titanium dioxide (UV/TiO2/H2O2process).
x 2ndStage
The tests showed that the systems with the highest hydrogen peroxide concentration (150 mg.L-1) were the most efficient, reaching complete inactivation of Pseudomonas aeruginosa. It is believed that the generation of hydroxyl radicals by the Fenton reaction was responsible for the total inactivation, since even the H2O2/UV system without the addition of iron also reached this condition. This is due to iron present in the cellular metabolism of Pseudomonas reacting with the peroxide and producing HO. radicals (Ortega-Gómez et al., 2012; Agulló-Barceló et al., 2013). The UV experiments had efficiency very close to the systems with low peroxide concentrations, reaching a maximum of 3 log units of inactivation (Figure 1, second stage). The Fenton reaction with low peroxide concentrations was not so favored due to the peroxide consumption by organic matter contained in the effluent. In this case, we can observe that the disinfection reached was mainly through UV radiation. These results differ from those presented in some studies with gram-negative pathogenic bacteria, spores and bacteriophage viruses, in which the synergistic effect of low H2O2 and UV radiation concentrations are highlighted, reaching up to 6 log units of inactivation (Fisher et al., 2008; Polo-López et al., 2011). The best adjustments to the log-linear/caudal model, with
the remaining microorganisms at the end of the experiments, were found with the photo-Fenton and UV/H2O2 processes, both with a hydrogen peroxide concentration of 25 mg.L-1 (Table 2). For the tests with a concentration of 150 mg.L-1 in which total inactivation occurred, it was not possible to adjust the bacterial decay curve to the models compared in this study.
x 3rdStage
On a bench scale, with PET bottles functioning as photocatalytic reactors, the tests showed that for the highest hydrogen peroxide concentrations used in the experiments there was no difference in the levels of disinfection between the UV/H2O2 and photo-Fenton processes.
The UV process (photolysis) was not able to fully inactivate P. aeruginosa. Only a small inactivity occurred (less than one log unit). For the UV/H2O2 and photo-Fenton processes, both with a hydrogen peroxide concentration of 150mg.L-1, the inactivation of P. aeruginosa showed a low active initial phase followed by a marked development in the disinfection curve until complete inactivation occurred (Figure 1, third step). The low activity in the beginning of the test can be attributed to the resistance of bacteria to oxidative stress. It has also been observed in some cases that increasing the H2O2 concentration to 150 mg.L-1 broke the initial bacterial resistance and can achieve total disinfection in less time. The tests carried out showed that for the highest hydrogen peroxide concentrations used in the experiments there was no difference in the levels of disinfection between the UV/H2O2 and photo-Fenton processes. These processes with the highest hydrogen peroxide concentration (150 mg.L-1) were the most efficient in all tests, reaching total inactivation of the microorganisms in less time, in the same way as in the previous stages. As in the second stage of the tests, again the processes with hydrogen peroxide in the concentration of 150 mg.L-1 were the most efficient in the disinfection of the pretreated greywater. The processes with hydrogen peroxide concentrations of 25 mg.L-1 also reached total inactivation, however, in a longer time. In general, the processes did not have a good fit to the compared decay models, being the photo-Fenton process with a hydrogen peroxide concentration of 25 mg.L-1 that was better adjusted with the same correlation coefficients for total inactivation of P. aeruginosa (Table 2). It is believed that this is due to the smaller experimental control, mainly to the variations of solar radiation in this stage of the tests. It was also not possible to adjust the bacterial decay curve to the models compared in the hydrogen peroxide concentration of 150 mg.L-1.
x 4thStage
In this stage, the UV/H2O2 process with the hydrogen peroxide concentration of 150 mg.L-1 was also the most effective, reaching complete inactivation of P. aeruginosa in 30 minutes of solar exposure, with an applied dose of 1.08 MJ.m-2. Photolysis tests (UV process) were effective for the inactivation of P. aeruginosa (Fig. 3, fourth stage), in which complete inactivation occurred in only one hour of sun exposure. Studies by Ubomba-Jaswa et al., (2009) have shown that the use of the highest UV radiation over a short period of time is more advantageous than a lower intensity over long periods in terms of the bacteria's ability to respond and repair the damage caused by UV incidence. High UV radiation acts to negatively affect the efficiency of cell repair enzymes, attacking the defense mechanisms and preventing photoreception (Rincón & Pulgarin, 2006). For the tests with UV and H2O2
processes with a hydrogen peroxide concentration of 25 mg.L-1, an initial soft decay was observed, followed by an inactivation region with log-Linear until the total inactivation of the microorganisms, without any remaining population. As in the previous stages, it was not possible to adjust the bacterial decay curves to the inactivation models compared in the hydrogen peroxide concentrations at 150 mg.L-1. It is observed that for the conditions tested in this study, hydrogen peroxide in a synergistic effect with UV radiation was the key factor
for the increased disinfection rate. Although the processes did not present a good fit to the decay model for the 4thstage, it was possible to confirm the high disinfection efficiency of the UV/H2O2process, with simple use and low cost.
Figure 1. Inactivation of Pseudomonas aeruginosa for the four experimental stages. The hydrogen peroxide concentrations of 150 mg.L-1 and 25 mg.L-1 were used for stages 2, 3 and 4. For the first stage, only the concentration of 150 mg.L-1was used.
After the experiments, the samples kept at room temperature without the presence of light did not show bacterial regrowth within 24 hours of storage, thus showing the possibility of using treated greywater in domestic reuse.
Table 2.Pseudomonas aeruginosainactivation constants for the various disinfection stages.
Bacterial Decay Models C/C0 = e-k.t
(Chick-Watson) (C – Crem)/(C0 – Crem) = e-k.t
(Log-Linear/Caudal)
Process k
(min-1) R2 k
(min-1) R2
1st stage UV 0.04 0.98 0.05 0.88
UV/TiO2 0.03 0.99 0.05 0.86
UV/TiO2/ H2O2 0.08 0.77 0.08 0.77
2nd stage UV 0.04 0.94 0.05 0.96
UV/H2O2 0.06 0.96 0.07 0.97
photo-Fenton 0.06 0.85 0.09 0.99 3rd stage
UV 0.02 0.88 0.03 0.90
UV/H2O2 0.08 0.80 0.08 0.80
photo-Fenton 0.07 0.94 0.07 0.94
4th stage UV 0.06 0.76 0.06 0.76
UV/H2O2 0.06 0.78 0.06 0.78
x Ecotoxicity tests
For UV and UV/H2O2 processes with hydrogen peroxide concentrations at 25 mg.L-1 all test organisms survived. Only samples with a higher H2O2 concentration (150 mg.L-1) showed toxicity in the preliminary tests (100%). With this positive immobilization result, the dilutions were carried out at 50, 25, 12.5 and 6.25%. For the first three dilutions, no microorganisms
exposed to disinfected greywater survived. Only the 6.25% dilution decreased the immobility rate, with a value reduced to 90%. According to the results, the dilution of 6.25% of the greywater sample disinfected by the UV/H2O2 process with the hydrogen peroxide concentration of 150 mg.L-1 is no more toxic to the microorganism in question. Anionic surfactants are probably the main compound present in greywater capable of offering toxicity to Daphnia. However, the raw greywater used in this work presented a mean anionic surfactant concentration of 2.31 mg.L-1, below the lethal concentration range that according to the US Environmental Protection Agency (EPA, 2007) is 5.8 - 9.5 mg.L-1 for anionic surfactants regarding dodecylbenzene sulfonate. In general, greywater treatment reduces the acute toxic effect, as in studies by Hernández Leal et al., (2012). However, in the present study, the treatment applied with the UV/H2O2 process in the hydrogen peroxide concentration of 150 mg.L-1 was the only one that presented acute toxicity, even though it removed a good part of the concentration of surfactants from the raw greywater. This toxicity was probably due to the use of hydrogen peroxide, which is still present in greywater samples after sun exposure.
CONCLUSIONS
In general, the processes with hydrogen peroxide (150 mg.L-1) mixed in the pretreated greywater and exposed to solar radiation or artificial radiation from UV lamps were the most efficient in the disinfection experiments, with total inactivation of P. aeruginosa. These processes (UV/H2O2 and photo-Fenton) were better fitted to the log-linear/caudal decay
In general, the processes with hydrogen peroxide (150 mg.L-1) mixed in the pretreated greywater and exposed to solar radiation or artificial radiation from UV lamps were the most efficient in the disinfection experiments, with total inactivation of P. aeruginosa. These processes (UV/H2O2 and photo-Fenton) were better fitted to the log-linear/caudal decay