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Article

Assessment Impacts of Ozone on Salmonella Typhimurium and Escherichia coli O157:H7 in Liquid Dairy Waste

by
Ruixue Chang
1,2,
Pramod Pandey
1,*,
Peja James
1,3,
Prachi Pandey
1,4,
Yanming Li
2,
Ruihong Zhang
4 and
Bart C. Weimer
1
1
Department of Population Health and Reproduction, School of Veterinary Medicine, University of California, Davis, CA 95616, USA
2
Department of Environmental Sciences and Technology, College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, China
3
Department of Biology, Howard University, Washington, DC 20060, USA
4
Department of Biological System Engineering, University of California, Davis, CA 95616, USA
*
Author to whom correspondence should be addressed.
Appl. Sci. 2022, 12(13), 6527; https://doi.org/10.3390/app12136527
Submission received: 16 May 2022 / Revised: 18 June 2022 / Accepted: 19 June 2022 / Published: 28 June 2022
(This article belongs to the Special Issue New Trends in Waste Management Models)
Browse Figures
Versions Notes

Abstract

:
Liquid dairy manure, which is produced in enormous quantities in flush dairy manure management systems, is commonly used as an alternative to chemical fertilizers. It provides nutrient benefits to crops and soils. While dairy waste is a well-accepted and widely used fertilizer, the presence of indicator organisms and human pathogens in manure may lead to pathogen contamination in crops and soils. This study is focused on the examination of ozone gas-based sterilization. In the past, ozone (O3) has been used for sanitizing various foods and solid surfaces, but the potential of O3 for eliminating human pathogens in liquid dairy waste is not studied yet. Pathogens such as Salmonella Typhimurium and Escherichia coli O157:H7 are reported to be present in liquid dairy manure, and this research evaluated the effects of various levels of ozone on the survival of these two pathogens. We designed a continuous type O3 treatment system that has four major components: (1) ozone generator using oxygen; (2) ozone concentration control by mixing with pure air; (3) continuous monitoring of ozone concentrations; and (4) ozone experiment chambers. Various levels of ozone (43.26, 87.40, and 132.46 mg·L−1) were produced in the ozone system, and subsequently, ozone was diffused through liquid manure. Liquid manure was exposed to ozone for multiple durations (30, 60, and 120 min). To determine the effectiveness of O3 in eliminating pathogens, time-series samples were collected and analyzed for determining the levels of S. typhimurium and E. coli O157:H7. Preliminary results showed that ozone concentrations of 132.46 mg/L, and exposure time of 120 min resulted in the reduced levels of E. coli and Salmonella. Low levels of ozone and limited exposure time were found to be less effective in pathogen removal potentially due to high solid contents. Additional studies carrying out experiments to evaluate the impacts of solids in combination with ozone concentrations will provide further insights into developing full-scale ozone-based treatment systems.

1. Introduction

In general, livestock manure provides valuable nutrients such as Nitrogen, Carbon, Phosphorous, Potassium, and micronutrients required for plant growth [1,2]. Previous studies showed that manure contains high levels of bioavailable carbon and nitrogen, and the application of manure in cropland can help in replenishing the soil nutrients [3,4,5,6]. While animal manure provides nutrients, animals such as dairy cattle are also known to shed pathogens, which is a concern and poses risks to animal and public health. Livestock waste-borne pathogens can be transmitted to the soil, and subsequently, these pathogens may be transported to surface and ground water through runoff and leachate causing contamination [7]. The results of these previous studies showed that the significant numbers of zoonotic pathogens (E. coli, Salmonella, Listeria monocytogenes, Campylobacter, Cryptosporidium parvum, Giardia intestinalis) are produced in livestock systems, which has the potential to contaminate water and soil [8,9]. Both fresh and stored animal waste (cattle, pig, poultry, and sheep) were found to contain various types of pathogens, which poses risks to food safety and fresh produce [7,10,11].
Over the past decades, the farm sizes have increased [12,13] and with the increasing size of farms, there is a large amount of animal waste in a confined area, which requires treatments and disposal. Because of public health concerns, and increasing emphasis on improving water quality, environment, and food safety, recently particular attention has been given to controlling pathogens from animal waste (i.e., dairy manure, swine manure, and poultry litter) [14,15]. To control public and animal health risks posed by animal waste, there is a requirement for effective disinfection tools and treatment methods, which can reduce the pathogen loads [16,17] in the environment. This will reduce the risk of zoonotic disease transmission [18,19]. Based on current guidelines, at least a 5-log10 pathogen reduction in animal waste is suggested to improve food, farm biosecurity, and control the spread of infectious diseases caused by zoonotic pathogens [20,21,22].
There are various treatment methods such as composting and anaerobic digestion, which are used for treating animal waste [23,24,25]. Temperature-based treatment methods, such as composting, which produces high temperature (>55 °C) and thermophilic anaerobic digestion, were found to be suitable for reducing pathogens in animal waste [26,27,28]. In many instances, however, pathogens such as E. coli O157:H7, Salmonella spp., and L. monocytogenes were present in composted organic waste [29,30,31,32,33] and animal-waste amended soil [34]. One possible reason for this could be that the composting process did not attain a high enough temperature for a sufficient duration [35,36]. In addition to this, cross contaminations are also known to cause pathogen contamination in cross products [37,38].
Because of pathogen survival issues in existing waste treatment methods, there is a growing demand to identify new technologies or improved methods capable of removing pathogens from waste such as dairy manure. Ozone treatment could be an option [39,40]; however, further studies are needed prior to its widespread application in treating dairy waste. Ozone has been used to sanitize a number of products including food products and ozone treatment has been studied in various food and waste treatment industries. Ozone is used for treating sewage and killing harmful bacteria [41,42,43]. Application of ozone treatment for wastewater treatment at a pilot scale showed significant removal of contaminants such as pharmaceuticals, nitrate, and micropollutants [44,45,46]. Ozone treatment is considered a cost-effective, and eco-friendly method for the removal of various contaminations including mycotoxins in grain products [47], pharmaceutical removal from water [48], and contaminants removal in food products [49].
The triatomic form of oxygen (O3) [ozone] is detrimental to pathogens and bacteria in food products because of the strong oxidation property of ozone. Ozone is considered to be highly effective against bacteria in many products, even with a low concentration of ozone (0.01 ppm). Further, ozone treatment is an approved food additive by the U.S. Food and Drug Administration (US FDA), which facilitates its use for treating food products for the removal of bacteria, viruses, and protozoa [7,39,50,51].
Ozone reduces microbial populations by 2–4 orders of magnitude and ozone treatment increases the shelf life of fruits and vegetables [22]. It has been found that E. coli would be more readily killed by ozone in juices used for human consumption. Pathogens reductions in wastewater have been widely reported when wastewater was exposed to ozone [42]. Further, the effects of ozone on reducing the populations of Salmonella, E. coli O157:H7, and L. monocytogenes on the surface of the lettuce, spinach, and green onion were studied and results showed that pathogens populations were reduced substantially [52].
During ozone treatment, the survival of pathogen depends on the O3 concentration and duration of exposure [53,54]. In addition to time and concentrations, the effect of O3 in pathogen reduction is influenced by the physicochemical characteristics of the material (i.e., water content, pH, microbial load, and organic matter content) [22,53,55,56,57]. Pathogens such as E. coli and Salmonella typhimurium in food products were reduced substantially after exposure to O3 [51,55]. The goal of this study was to understand the impacts of ozone on pathogen removal in dairy wastewater. Specific objectives of this study were: (1) to investigate the impacts of ozone concentrations and exposure time on E. coli O157:H7 reductions; (2) to determine the impacts of ozone S. Typhimurium reductions; and (3) to understand the impacts of ozone on the degradation of organic content of manure.

2. Materials and Methods

2.1. Bacterial Strains and Inoculum

In this study, we did use two pathogen strains: (1) S. typhimurium LT2 ATCC 700720, and (2) E. coli O157:H7 ATCC 35150. These pathogens were grown in Luria Bertani (LB) broth (Becton, Dickinson and Company, Sparks, MD, USA) with shaking (250 rpm) at 37 °C for 24 h prior to starting each experiment. Inoculums of these pathogens were prepared using the fresh pathogen cultures of E. coli O157:H7 (ATCC #35150), and S. typhimurium LT2 (ATCC #700720). The fresh cultures were prepared in the lab prior to starting the experiment. Difco LB Broth Miller (Luriae Bertani) growth media was used for growing E. coli O157:H7 and S. typhimurium LT2. The overnight growth of these pathogens in respective media was used to inoculate feedstock. The pathogen inoculated mixed feedstock was used for testing the pathogen levels in feedstock. Once the experiment started, ozonated samples were collected at regular intervals, and analyzed within 24 h using a petri dish with selective agar media. Samples for E. coli O157:H7, and S. Typhimurium were streaked (i.e., plated) on MacConkey II Agar (Becton, Dickinson and Company, Sparks, MD, USA), and Difco XLD Agar (Becton, Dickinson and Company, Sparks, MD, USA), respectively. All ozonation experiments were conducted at room temperature.

2.2. Dairy Manure Characteristics

Liquid dairy wastewater used in this study was collected from three dairy farms. These dairy farms were located in the Central Valley of California. The average number of cows on these farms is around 2000. Prior to starting the experiment, manure from three dairy farms was mixed to form homogenized manure streams representative of manure produced in the flush manure system in California. All these three dairy farms follow similar animal waste management practices (i.e., flush manure management system and mechanical liquid-solid separation). Each of these three farms has around 2000 dairy herd sizes and uses a flush system. Further, each of the three dairy farms have two lagoons and uses a liquid-solid separation system for separating the liquid and solid. The nitrate analysis of manure showed nitrate concentrations between 330 and 400 ppm, and the pH was 6.5–7.3. In manure, total solid (TS) was 1–2%, and volatile solid (VS) was 20–25% of TS. During the experiment, we formed two types of dairy wastewater for preparing feedstock for treatment: (1) raw liquid manure (no pretreatment); and (2) sterilized raw manure (pretreatment) followed by pathogen inoculation. Sterilization of manure feedstock was done by autoclaving raw manure for 15 min. Subsequently, pathogens were inoculated prior to the ozone treatments. Experiments with post-sterilization-inoculated pathogens (i.e., pretreated manure) were done to compare the impacts of ozone treatment on manure with pretreatment and without pretreatment.

2.3. Ozone Experiment Setup and Samples Analysis

A bench-scale experiment shown in Figure 1 was used to test pathogen inactivation under different treatment conditions. The experiment setup has various components including an ozone generator, ozone monitor, ozone distributor, ozone concentration controller, and treatment chamber. The O3 was generated using an O3 generator (Model NANO, AbsoluteOzone®, Edmonton, AB, Canada). The concentration of O3 was controlled by controlling the inflow of ultra-pure air. Mixing of ozone with air produced different levels of ozone, and concentrations of ozone were monitored continuously using an O3 monitor (Model 106-H Ozone Monitor™, 2B Technologies, Boulder, CO, USA). Three round bottom flasks (each flask volume of 250 mL named as a treatment chamber) with 20 mL of feedstock (i.e., liquid dairy manure) in each chamber were connected with the ozone tubes, which provided continuous and constant ozone flow to the chamber. The O3 concentration was set as 3%, 6%, and 9%, which were equal to 43.26, 87.40, and 132.46 mg·L−1, respectively. We selected this range because the ability of the ozone generator was to produce ozone concentrations within this range. The time of ozone exposure to manure was set to 0, 30, 60, and 120 min at each concentration of ozone. In the inoculated sterile manure, 1 mL per 10 mL of feedstock (1/10 dilution) of the pathogen solution (either E. coli or Salmonella) was added 12 h prior to O3 treatment and incubated for 12 h statically at 37 °C to obtain ~ 106–8 CFU/mL of each pathogen prior to treatment.
After the O3 treatment, 1 mL sample from the treatment chamber was collected, transferred to the centrifuge tube, and diluted 1/10 with 9 mL PBS. Subsequently, pathogens in the samples were examined by plating serially diluted samples onto specific agar plates. To enumerate S. typhimurium LT2, samples were plated on Difco XLD Agar (Becton, Dickinson, and Company) plates. To determine E. coli levels, samples were plated on MacConkey II Agar (Becton, Dickinson, and Company). The concentrations of the bacteria were reported as colony-forming units (CFU)/mL of dairy manure. The moisture of the samples was analyzed by weight loss on 102–105 °C heating. Total organic matter was determined by weight loss on ignition of dried ground samples at 450 °C.

2.4. Statistical Analysis

All experiments were done using three biologicals, in which each sample was duplicate-plated after extraction and dilution. Observations were analyzed with one-way ANOVA using the SPSS statistics 23 (IBM Institute, Armonk, NY, USA) and Duncan’s multiple range test. This analysis provided significant differences (p < 0.05) in mean values of pathogen reductions in different ozone exposure times. Bacterial counts were converted to log10 before statistical analysis.

3. Results and Discussion

3.1. Effects of O3 Concentration and Duration on E. coli O157:H7 Inactivation

Previous studies have shown that the ozone treatment is effective in pathogen reduction in wastewater or sludge [58,59,60]. The current study sought to determine the effects of O3 on the survival populations of E. coli O157:H7 in liquid dairy manure by using different O3 concentrations and exposure times (Figure 2).
The ozone treatment reduced the amount of E. coli O157: H7, and the reductions increased with the exposure time, particularly at 87.40 or 132.46 mg·L−1 (Figure 2). As shown in Figure 2B,C, the pathogen reduction percentages were similar regardless of the initial concentrations of pathogens. To remove the potential interference between the native microbial population and inoculated pathogens, first dairy manure was autoclaved, and then inoculated with E. coli O157: H7 prior to treatment by O3 (Figure 3). In this condition, the reduction percentages at O3 concentrations of 87.40 and 132.46 mg·L−1 were significantly higher than that of 43.26 mg·L−1. O3 is reported to eliminate bacteria by attacking the glycoproteins and glycolipids in the cell membrane, and the sulfhydryl groups of certain enzymes, therefore, the effects of ozone in complex feedstock such as manure where multiple microorganisms are present depends on the pretreatment of manure [22].
The results of this study suggest that when potential interference between the native microbial population of manure and inoculated pathogens was reduced by autoclaving the manure, the effects of O3 were more apparent. Further, pretreatment such as autoclave may have an influence on the structure of the organic contents of manure, which may have allowed increased pathogen reduction [59,60,61]. While pretreatment of manure (i.e., inoculation followed by sterilization) was more effective in pathogen removal, the application of pretreatment requires additional cost and facilities. Therefore, for manure management, the application of sterilization could be cost-prohibitive. In this study, we implemented a sterilization process to understand the impacts of ozone on mixed species versus known species, and complex waste material such as manure. To understand the full potential of pretreatment, and its cost-effectiveness in manure treatment, additional studies carrying out pilot-scale implementation are needed.
Under standard temperature and pressure conditions, the O3 concentration of 132.46 mg·L−1 could be converted to ≈61,855 ppm, which was significantly higher than the concentrations of O3 used in previous experiments to remove the pathogens in foods [22,62,63,64,65,66], and in liquid organic wastes [21]. Pathogen reductions in foods were considerably higher than those in our study. This indicates that the impacts of ozone may vary depending on the feedstocks, and a higher level of ozone concentrations may be needed for waste material. The previous study, while investigating O3 treatments for pathogen reduction, also suggest that the needed ozone concentrations depend on material types [60].

3.2. Effects of O3 on S. typhimurium Inactivation

Effects of ozone on S. typhimurium reduction were estimated (Figure 4), and results showed that more than 1–2 log reduction was achieved depending on the ozone concentrations and exposure time. The S. typhimurium reduction was compared in the dairy manure with and without autoclave (i.e., pretreatment) to understand the impacts of microbial interference in treatment. As shown in Figure 4, the bacteria counts were significantly decreased along with the exposure time. The reduction percentages in treatment with ozone concentrations of 132.46 mg·L−1 were significantly higher than that at 43.26 or 87.40 mg·L−1 (p = 0.05). Figure 5 showed Salmonella reductions in the dairy manure, which was autoclaved before O3 treatment. The reduction percentages in treatments of 87.40 and 132.46 mg·L−1 were significantly higher than that in 43.26 mg·L−1. The reduction percentages in autoclaved dairy manure were significantly higher than the reductions in the material without an autoclave, and similar findings are presented elsewhere [67]. These results showed that the effects of ozone on the removal of the studied two pathogens were similar for both species (E. coli and Salmonella), and ozone may eliminate a range of pathogenic species [68].
In previous studies, pathogen removal through ozone treatments has been demonstrated on various surface types and bacteria. As an example, ozone treatments were used on personal protective equipment (PPE) to study coronavirus 2 (SARS-CoV-2) and ozone impacts on it. To generate O3 concentrations a medical ozone generator that produced higher ozone concentrations at 500 and 40,000 ppm (1–80 g/m3) was used and a special ozone chamber was designed that produced low concentrations of ozone 8–12 ppm (0.016–0.024 g/m3). While at high concentrations, there was a virucidal effect after a few minutes of exposure, the effects of ozone were low at low concentrations. Further, the virucidal effects of ozone treatment on SARS-CoV-2 were also dependent on humidity levels [69]. Other studies displayed reductions of Escherichia coli O157:H7 and Salmonella in apple cider and orange juice at thermophilic conditions. E. coli and Salmonella populations were reduced below the detection limit dependent on temperature conditions. In apple cider, impacts of ozone were greater in E. coli and Salmonella reductions at 50 °C than at 4 °C (p < 0.05). In orange juice, however, the effectiveness of E. coli and Salmonella reductions between 4 °C and ambient temperature did not differ substantially. These results were statistically insignificant (p > 0.05). Also, the type of juice (apple cider or orange juice) did not alter the results [70].
In surfaces such as wheat, ozone levels (0–20 µg m = 0–120 ppb v/v 7 h per day)−3 degraded the leaf pigments prematurely [71]. Specifically, in medium, hard wheat, Triticum aestivum L., ozone gas levels at 5 mg/L at 3.3 L/min in time intervals of 0, 0.5, 1.0, 1.5, and 2.0 h were influential in the degradation of the wheat endosperm starch granule but not against the protein or fat contents in the wheat (p < 0.05) [72].
In foods, ozone treatments of 0.35 ppm for 3 days at 2 °C were partially effective in preventing fungal growth in strawberries [73] and reduced E. coli and Listeria spp. in spinach when it came in contact with ozone exposure of 1 ppm for 10 min and 10 ppm for 20 min [64].
Other studies have shown that ozone treatments were efficacious against multiple pathogenic bacterial strains such as Escherichia coli, Staphylococcus aureus, Enterococcus faecalis, Klebsiella pneumoniae, Acinetobacter baumannii, and Pseudomonas aeruginosa. The concentrations of the O3 treatments were split into separate groups: an ozone-oxygen mixture group containing 20 μg of O3/mL for 5 min, 100% oxygen group for 5 min, and a baseline group where no gas was used. As a result, the ozone-oxygen mixture stopped all the bacterial strains present [74]. Ozone treatments have also been proved to reduce antibiotic resistance genes and pathogenic bacteria in wastewater [73,74,75,76,77].
Using 1 g of ozone/g, the ozonation reduced the micro-pollutants like antibiotic drugs or biocides and effectively removed approximately 2 log units of the micro-pollutants [40]. Previous studies have tested the effectiveness of a domestic ozone generator in reducing bacterial and fungal species of E coli, Proteus sp., Pseudomonas aeruginosa, Serratia sp., three strains of Staphylococcus aureus, Candida albicans, and Aspergillus fumigatus using ozone concentrations of 0.3 to 0.9 ppm for 4 h. In these conditions, there was a bactericidal effect in all of the species except Candida albicans which showed more resistance. In other experiments that dealt with 0.9 ppm ozone exposure for 0–4 h, bacteria was most rapidly reduced in S. aureus within the first few hours until the bactericidal effect rate was slowed significantly. At 1 ppm, ozone effectively reduced the bacterial numbers of most of the tested species by 95% [78]. Pathogen reduction by ozone treatment is likely to be affected by the type of material to be treated, its physical properties, and the mode of application. In a previous study [53], we designed an ozone chamber to expose solid poultry litter to ozone gas. Because liquid dairy manure has a higher amount of moisture, the current research needed substantial modification in previous experiments for diffusing ozone uniformly inside the liquid manure. In contrast to the liquid dairy manure treatment experiment presented here, diffusion of ozone inside of poultry litter was not needed because poultry litter moisture content was considerably low, and poultry litter was in the form of solid pellets. Published results showed that pathogen reduction was reduced when moisture level was increased [53], which indicates that the ozone impacts on pathogen removal may vary from one material to another depending on moisture level and type of material (liquid vs. solid surface). In addition to bacteria removal, ozone treatments are also proven suitable for virus removal. For example, several human pathogenic viruses in sewage were reduced to undetectable levels, when sewage was exposed to ozone treatment suggesting that ozone can be a promising technique for reducing the transmission of human viruses [21].

3.3. Effects of O3 Treatments on Organic Content Degradation

The effects of O3 in pathogen removal are influenced by the material characteristics, like pH, water temperature, free chlorine, moisture, and the organic load of the materials [56,57]. To assess the effects of O3 treatments on the organic matter of dairy manure, we evaluated moisture content and organic matter during O3 treatment. Results showed that the increase in O3 concentrations resulted in decreased organic matter contents in dairy manure (Figure 6). The results suggest that increased ozone exposure reduces the organic matter in manure. During treatment, O3 may firstly be consumed by the dissolved organic carbon of organic material, which results in high chemical oxygen demand (COD). This increased COD demand can lower pathogen reduction during ozone treatment [52]. Another study reported that aqueous O3 can directly react with the dissolved organic compounds and generate radical species such as a hydroxyl radical (•OH) that have higher oxidative potential (2.83 volts) than O3, and ozone treatment can influence available organic carbon in the feedstock [79]. The increased oxidative potential may be due to the fact that conversion of O3 to aqueous O3 resulted in a low concentration of O3 in the materials [61].

4. Conclusions

To determine the impacts of ozone treatment on pathogen reduction in dairy manure, we executed a series of experiments at a range of exposure times and ozone concentrations. The effects of O3 concentrations and exposure times on E. coli O157:H7 and S. Typhimurium reduction in liquid dairy manure were determined. Results showed that the O3 treatment could be used as a potential option for liquid dairy manure treatment. The effects of O3 are likely to change with moisture and concentrations of ozone, therefore, the treatment time and ozone concentrations will vary with the characteristics of feedstocks. Under the tested conditions, the increase in O3 concentration and prolonged exposure time resulted in more reduction of E. coli O157:H7 and S. typhimurium.

Author Contributions

Conceptualization, P.P. (Pramod Pandey), Y.L., R.Z. and B.C.W.; Funding acquisition, P.P. (Pramod Pandey); Investigation, R.C., P.J. and P.P. (Prachi Pandey); Methodology, P.P. (Pramod Pandey); Project administration, P.P. (Pramod Pandey); Resources, B.C.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the USDA National Institute of Food and Agriculture, Hatch Project. Accession Number 1019323.

Institutional Review Board Statement

This study did not require ethical approval.

Informed Consent Statement

This study did not require informed consent.

Data Availability Statement

This study did not require data availability statement.

Acknowledgments

The authors thank the Center for Food Animal Health (CFAH), University of California, Davis, and the Division of Agriculture and Natural Resources (ANR), University of California, Davis, for supporting this study.

Conflicts of Interest

The authors declare no conflict of interest.

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Applsci 12 06527 g001 550
Figure 1. Schematics of bench-scale ozone treatment system.
Figure 1. Schematics of bench-scale ozone treatment system.
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Figure 2. Change in E. coli levels: (A) E. coli O157:H7 counts; (B,C) reduction percentages in the dairy manure with E. coli inoculation (B) or without E. coli inoculation (C) under different ozone concentrations and durations (note: the letters above columns showed the differences among different lasting times, which was the same as the next figures; means denoted by a different letter indicate significant differences between treatments (p < 0.05)).
Figure 2. Change in E. coli levels: (A) E. coli O157:H7 counts; (B,C) reduction percentages in the dairy manure with E. coli inoculation (B) or without E. coli inoculation (C) under different ozone concentrations and durations (note: the letters above columns showed the differences among different lasting times, which was the same as the next figures; means denoted by a different letter indicate significant differences between treatments (p < 0.05)).
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Figure 3. Change in E. coli levels in autoclaved manure. (A) E. coli O157:H7 counts; (B) reduction percentages in the autoclaved dairy manure inoculated with E. coli O157:H7 under different ozone concentrations and durations (means denoted by a different letter indicate significant differences between treatments (p < 0.05)).
Figure 3. Change in E. coli levels in autoclaved manure. (A) E. coli O157:H7 counts; (B) reduction percentages in the autoclaved dairy manure inoculated with E. coli O157:H7 under different ozone concentrations and durations (means denoted by a different letter indicate significant differences between treatments (p < 0.05)).
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Figure 4. S. typhimurium (A) counts and (B) reduction percentages in the dairy manure inoculated with S. typhimurium under different ozone concentrations and durations (means denoted by a different letter indicate significant differences between treatments (p < 0.05)).
Figure 4. S. typhimurium (A) counts and (B) reduction percentages in the dairy manure inoculated with S. typhimurium under different ozone concentrations and durations (means denoted by a different letter indicate significant differences between treatments (p < 0.05)).
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Figure 5. S. typhimurium (A) counts and (B) reduction percentages in the autoclaved dairy manure inoculated with S. typhimurium under different ozone concentrations and durations (means denoted by a different letter indicate significant differences between treatments (p < 0.05)).
Figure 5. S. typhimurium (A) counts and (B) reduction percentages in the autoclaved dairy manure inoculated with S. typhimurium under different ozone concentrations and durations (means denoted by a different letter indicate significant differences between treatments (p < 0.05)).
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Figure 6. The change in moisture content and organic matter of dairy manure during O3 treatment (left side figures showed moisture content and organic matter in raw manure; right side figures showed moisture and organic matter in autoclaved manure, and black dots indicate outliers).
Figure 6. The change in moisture content and organic matter of dairy manure during O3 treatment (left side figures showed moisture content and organic matter in raw manure; right side figures showed moisture and organic matter in autoclaved manure, and black dots indicate outliers).
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Chang, R.; Pandey, P.; James, P.; Pandey, P.; Li, Y.; Zhang, R.; Weimer, B.C. Assessment Impacts of Ozone on Salmonella Typhimurium and Escherichia coli O157:H7 in Liquid Dairy Waste. Appl. Sci. 2022, 12, 6527. https://doi.org/10.3390/app12136527

AMA Style

Chang R, Pandey P, James P, Pandey P, Li Y, Zhang R, Weimer BC. Assessment Impacts of Ozone on Salmonella Typhimurium and Escherichia coli O157:H7 in Liquid Dairy Waste. Applied Sciences. 2022; 12(13):6527. https://doi.org/10.3390/app12136527

Chicago/Turabian Style

Chang, Ruixue, Pramod Pandey, Peja James, Prachi Pandey, Yanming Li, Ruihong Zhang, and Bart C. Weimer. 2022. "Assessment Impacts of Ozone on Salmonella Typhimurium and Escherichia coli O157:H7 in Liquid Dairy Waste" Applied Sciences 12, no. 13: 6527. https://doi.org/10.3390/app12136527

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