A COMPARATIVE EVALUATION of BOAEROSOLS BETWEEN VACUUM and GRAVITY TOILETS. by Elizabeth Nyarkoa Osei a Research Dissertation
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A COMPARATIVE EVALUATION OF BOAEROSOLS BETWEEN VACUUM AND GRAVITY TOILETS. by Elizabeth Nyarkoa Osei A research dissertation submitted in partial fulfilment of the requirements of the award of the degree of Master of Science at Loughborough University August 2019 Supervisors: Professor M. Sohail Khan Dr. Sola Afolabi ACKNOWLEDGEMENTS I am grateful to God for granting me good health to complete this dissertation. I would like to express my sincerest gratitude to the following people, for their immense support and encouragement which have helped make this project a success: • My family • My Supervisors, Professor Sohail M. Khan and Dr. Sola Afolabi • Lorraine Withers, Andrew Hay, Kris Wojcik and the entire team at Otter Vacuum Systems • Mrs Jayshree Bhuptani • Mr. Bernard Agyeman • Mr. Kobina Paintsil • The entire WEDC class of 2019. i ABSTRACT Toilet flushing is known to generate and disperse bioaerosols which can contaminate surfaces in the washroom and increase the risk of infections through contact with the contaminated surfaces or inhalation of these bioaerosols. Whilst this phenomenon is well known for gravity toilets, vacuum toilets have been hypothesized not to generate bioaerosols and have been tagged as more sanitary compared to the gravity toilet. This study investigates whether a vacuum toilet produces bioaerosols and how the bioaerosols and particle concentrations of a vacuum and a gravity toilet differ from each other. The study further looks into the reasons and behaviour of the measured concentrations based on the main difference between the toilets - their flushing mechanism. Experiments were conducted in a warehouse-office building using two identical toilet cubicles with dimensions of 1.5 meters by 1.17 meters which were constructed for the purposes of this study. 12 experimental conditions this were framed around two main toilet-use scenarios; using the toilet during an incidence of diarrhea and using the toilet when there was no diarrhea (faecal matter of solid consistency). Faecal matter was not used in this study; rather, Escherichia coli bacteria, an indicator organism for faecal contamination on surfaces and in toilet water was used in seeding the toilet bowls to represent the experimental conditions in the study. The bioaerosol concentrations in this study were measured according to the ACGIH and ASTM standards. Particle concentrations were measured using an optical particle counter. Based on the results of this study, it can be concluded that, gravity toilets produce bioaerosols via visible splashes or overspray when faecal matter of loose or solid consistency is flushed with the lid open, but a vacuum toilet would not produce bioaerosols or visible splashes under the same conditions. Keywords: Bioaerosols, Toilet Plume, Gravity-Flush Toilets, Vacuum Toilets, Droplet Nuclei, Bioaerosol Sampling. iv EXECUTIVE SUMMARY The aim of this research is to generate quantitative and qualitative evidence to test the hypothesis that, gravity toilets produce toilet plume which contains bioaerosols when flushed but vacuum toilets do not produce toilet plume which contains bioaerosols when flushed. Toilet flushing is the most common source of bioaerosols in indoor settings (Darlow and Bale, 1959). Even though the microbiological profiles of bioaerosols differ from setting to setting, bioaerosols are usually made up of high concentrations of bacteria and fungi (Zemouri et al., 2017). Literature review alone cannot provide enough qualitative and quantitative data to compare bioaerosol and particle concentrations between gravity and vacuum toilets. It was therefore necessary to supplement data collection with experiments to generate enough quantitative data for the testing of the hypothesis of this study, and qualitative data to support the results of the hypothesis test. The experiments for this study were conducted in a warehouse-office building. For the purpose of this study only, two identical toilet cubicles with dimensions of 1.5 meters by 1.17 meters were constructed in a large storage room on the site where the experiments were to be conducted. The experimental conditions for this study, labelled A-L, were chosen to represent the several scenarios which may exist in an actual toilet cubicle which is left idle or in use. The conditions were then framed around two main toilet-use scenarios; using the toilet during an incidence of diarrhea and using the toilet when there was no diarrhea (faecal matter of solid consistency). Faecal matter was not used for the experiments in this study; instead, Escherichia coli bacteria, an indicator organism for faecal contamination on surfaces and in water was used. To determine the concentration Escherichia coli that would be seeded into the toilets, a membrane filtration test was performed for successive dilutions of the E. coli broth used in this study. The procedure used for measuring bioaerosol concentrations in this study are according to the ACGIH (1999) and ASTM (2019) standards and guidelines for collecting culturable aerosolized v bacteria unto agar plates using inertial impaction for indoor air quality investigations. A total of 56 trials were conducted across all the experimental conditions for both the gravity and vacuum toilet (28 trials for each toilet). Line, violin, distribution and strip plots were used to observe the distribution of bioaerosol concentrations across the experimental conditions and particle concentrations across the experimental conditions and bin sizes for the two toilet types. A t-test was performed to determine the statistical differences between bioaerosol and particle concentrations at an experimental condition for the test gravity toilet and at the same condition for the test vacuum toilet. Paired t-tests (significance level 5%) were conducted for each toilet type, within pre and post seeding, pre and post flush, lid open and lid closed and first and subsequent flush experimental conditions. For the testing of the hypothesis, separate t-tests were performed using the average bioaerosol and particle concentrations for the flush conditions. The generated p-values were then compared with the desired significance level of the t-test (p=0.05). For the gravity toilet, the agar plates retrieved from the impactor showed growth after the incubation period, for only experimental conditions which included flushing the toilet with the lid open with no disinfection prior to flushing. For the vacuum toilet, all agar plates retrieved from the impactor for all the experimental conditions showed no growth after the 24-hour incubation period. he zero bioaerosol concentrations for the vacuum toilet implies a variance of zero making it deterministic. For the gravity toilet, 75-85% of all the particles generated in each experimental condition were of bin size 0.3 µm or less in all the three minutes of sampling. For the vacuum toilet, 70-81% of all the particles generated in each experimental condition were of bin size 0.3 µm or less in all the three minutes of sampling. For the test gravity toilet, all surfaces tested in experimental conditions A and L (no seeding with E- coli and disinfection prior to flushing) tested negative for E-coli or total coliforms. For the vi test vacuum toilet, all surfaces tested in experimental conditions A and L tested negative for E-coli or total coliforms. Bioaerosol concentration data for the gravity and vacuum toilet suggest that, the vacuum toilet did not produce any significant concentrations of aerosolized E. coli to be captured by the impactor. The only time the vacuum toilet seat was contaminated, were in experimental conditions where the lid was closed, and the toilet was flushed. Flushing with the lid closed contaminates the lid for both vacuum and gravity toilets and this supports the reasoning that the toilet lid may be a source of bacteria and not desirable to touch. For the gravity toilet, subsequent flushes may continue to produce bioaerosols because, bacteria residues may remain in the toilet bowl water and the insides of the bowl after the first flush even though a reduction in the concentration of bowl water contamination by subsequent flushes and hence a proportionate decrease in bioaerosol concentrations was observed. There is no complete vacuum; every vacuum is hypothesized to be made up of appearing and disappearing ghost particles which may interfere with real particles and contradict the contribution of real particles to any space (Sakharov, 1991). Because bacteria had been flushed through the sewer pipes, it cannot be totally overruled that the vacuum sewer pipes were contaminated with bacteria. This could also mean that, particle in the vacuum sewer pipes may have been made up of bacteria in the sewer pipes and the ghost particles mentioned by Sakharov (1991). With this theory and the relatively low vacuum percentage used for the vacuum toilet pump, this study suggests that, the vacuum that existed in the sewer pipes contained bacteria particles which floated (against gravity) into the sampling area anytime the flush button pressed, opening the discharge valve connecting the sewer pipe to the toilet bowl. This could explain why the particle concentrations in the vacuum toilet increased immediately after flush. vii LIST OF TABLES Table 1.1 Outline of research objectives, questions and data collection methods…………………....5 Table 3.1. Experimental conditions and their relevance to the objectives of the study………………15 Table 4.1. Results for hypothesis tests conducted using the particle concentrations for