C4) Microbiological Drinking Water Quality in a Highrise Office Building of Hong Kong

Transcription

1 C4) Microbiological Drinking Water Quality in a Highrise Office Building of Hong Kong W.Y. Chan (1), L. T. Wong (2), K. W. Mui (3) (1) r@polyu.edu.hk (2) beltw@polyu.edu.hk (3) behorace@polyu.edu.hk (1-3) Department of Building Services Engineering, The Hong Kong Polytechnic University, Hong Kong, China Abstract Microbiological water quality of the drinking water is highly associated with enteric diseases. In this study, the water quality for the water supply system of a typical highrise office building in Hong Kong was examined. A comparison of the bacteria levels between the high and low water consumption periods was made. Tap water samples (0.1 L) were collected at the water supply system every 4 hours from 08:00 to 20:00 in a normal working day that had a working day prior and after and a normal working day that followed a long weekend. The results showed that the bacteria counts ranged from 80 to 1000 CFU L 1. It was found that the bacteria counts at a time after a low water consumption period were significantly higher. This was probably due to the multiplication and accumulation of bacteria in the water storage system in low occupancy hours. Among the bacterial genera identified, 59% were gram-positive bacteria genera composed of Bacillus (6%), Micrococcus (28%), Staphylococcus (6%) and others (19%), and 41% were gram-negative bacteria genera including Pseudomonas (13%), Escherichia coli (1%) and others (27%). A low quantity of the faecal indicating bacteria, E. coli, was detected with a relative abundance of %. Its presence was probably due to bioaerosols generated from nearby WC flushing. Future investigations regarding the probable transmission through a water supply system of high-rise office buildings in sub-tropical climate were recommended. Keywords Water supply, storage, water quality, high-rise office buildings, bacteria 149

2 1 Introduction Microbiological water quality is an indicator of the performance of a building s drinking water distribution system. Microorganisms including bacteria, fungi, viruses and protozoa, are natural inhabitants of water and could be associated with enteric diseases. Bacteria in drinking water distribution systems could be introduced from the water source, the water treatment plant, or the bacteria growth on the pipe walls (e.g. biofilms) which would be detached to the water by shear loss or erosion (Maul et al., 1990). The microbiological water quality can be assessed by measuring the amount of heterotrophic plate count (HPC) bacteria (Edberg and Allen, 2004). A per milliliter HPC limit of 500 colony-forming units (CFU) was adopted as a baseline of an acceptable drinking water quality at some potable water distribution systems (USEPA, 1989). Although most of the HPC bacteria recorded are not human pathogens, some genera are associated with opportunistic infections; such as Acinetobacteri, Aeromonas, Flavobacterium, Klebsiella, Legionella, Moraxella, Mycobacterium, Serratia, Pseudomonas and Xanthomonas (WHO, 2003; Edberg and Allen, 2004). Apart from causing disease, these microorganisms could degrade the water quality by depletion of dissolved oxygen, reduction of sulfate to hydrogen sulfide, corrosion of pipes, and occurrence of bad taste and color (van der Wende and Characklis, 1990). Two main sources of water in Hong Kong are rainfall from natural catchments and supply from The East River (Dongjiang) of China. It was reported that microbes would be removed by the treatment plants to an acceptable level and supplied to building water system through the city water distribution network (Ho et al., 2003). Water would be stored in buildings for probable interruption of water supply city main, accommodation of peak demands and to provide a pressure for gravity supplies in some cases (IOP, 2002). In Hong Kong, the minimum required volume of the drinking water storage for an office building is determined at a rate of 45L per each installed water appliance. Storage tanks would be constructed by reinforced concrete with access cover and periodic cleaning would be scheduled (once or twice per year). The stored water is open to air through pipes for pressure balance, overflow and some other engineering purposes. As Hong Kong has a sub-tropical climate, is densely populated and full of high-rise buildings, the stored water would easily become a favorable environment for proliferation of microorganisms and bacteria if the water systems in buildings are not designed and maintained properly. Water consumption at an office would be nonuniform over a day and closely related to the occupant loads (Wong and Mui, 2006; 2007). Low occupant loads were reported after business hours and in holidays and the water consumptions were expected low during these periods. The non-uniform water consumption in high-rise office buildings makes microbiological water quality of the stored drinking water a particular concern. 2 Methods and Materials Microbiological quality of drinking water from the water supply system of an in-use, high-rise, air-conditioned office building of Hong Kong was studied. The office building was 230 meters high and had 53 floors, consisting predominantly of open plan 150

3 offices. The water supply system was served by a roof storage tank and was physically separated from the city main by a break tank at ground level. Drinking water available at water taps in restrooms of each office floor was supplied from the roof tank by gravity through water pipes. In this office building, a total of 424 drinking water taps were installed (Laws of Hong Kong, 1997). A number of WCs of individual partition of about 1.8 m high were available in the same restroom at a distance from the taps. The estimated pipe length for the system was 7000 m (Wong, 2002). The total storage tank capacity was 19 m 3 and the estimated volume of whole water pipe system at the office tower was 5.5 m 3. Drinking water samples were collected from the water tap of a washbasin at level 11 at a time interval of 4 hours from 08:00 to 20:00 over a week. All samplings were taken on normal working days: some were taken on normal working days with a working day before and after, and some were on a working day following a long weekend (holidays of 3 consecutive days). The water consumptions in the holidays were expected to be lower as compared with the normal working days. The outdoor air temperature of the sampling period ranged from 15ºC to 23ºC and the daily average indoor air temperature was 17.5ºC to 19ºC. The daily average relative humidity in the period was from 50% to 80%. Daily sunshine was 8 hours to 10.5 hours and no rainfall was recorded. Upon each measurement, a water sample of 0.1 L was collected by using a 0.25 L sterilized bottle which containing sodium thiosulfate (1 ml 10% w/v solution). The water samples were filtered and then the filters were inoculated onto the R2A Agar (Difico). After being incubated at 35 C for 48 hours, the number of colonies (colony forming unit, CFU) was counted, and the bacteria genera were isolated and identified for the analysis of heterotrophic plate count (HPC) bacteria. Regarding the microbiological water quality of the drinking water supply system in high-rise office buildings, it is hypothesized that the bacteria levels in the sample water would be significantly higher after a low consumption period, e.g. overnight, long weekend or after a long holiday. The hypothesis was tested against the measured counts with the t-statistic at a level of significance p<0.05, where X (CFU L 1 ) is the expected counts, S (CFU L 1 ) is the sample standard deviation and n is the sample size respectively. X t = (1) S n 3 Results and Discussions 3.1 Bacterial count The HPC bacteria counts of the working days following a normal working day and the day following a long weekend are shown in Table 1. The HPC bacteria levels of the water samples varied from 80 CFU L 1 to 320 CFU L 1 for a day followed a normal 151

4 working day and from 200 CFU L 1 to 1000 CFU L 1 for a day following a long weekend. The HPC bacteria levels recorded in this study was much lower than the USEPA HPC limit (USEPA, 1989). The result was not surprising as the chlorination process for disinfection of drinking water in Hong Kong were reported satisfactory. The bacteria levels were low compared with some problematic cases of HPC ranged from <100 CFU L 1 to 20,000,000 CFU L 1 (Reasoner, 1990; Payment et al., 1994; Edberg et al., 1996; Cloete et al., 2003; Allen et al., 2004). Table 1 - The HPC bacteria levels of drinking water in a high-rise office building of Hong Kong Time of measurement Working day followed a normal working day (3 days) HPC bacteria count (CFU L 1 ) * Working day followed a long weekend (1 day) ± ± ±< ± Daily average 173±88 513±357 * values presented in arithmetic mean AM ± standard deviation SD The daily average HPC bacteria levels among the samples of the working days that followed a normal working day showed no significant difference (P>0.1) and the daily average was 173±93 CFU L 1. However, there were significant differences among different periods in a day. The results showed that the HPC bacteria levels in the morning session (227±88 CFU L 1 ) were higher than the afternoon session (120±38 CFU L 1 ) (p<0.05). Indeed, the bacteria levels of the samples taken at 08:00 were 65% and 100% higher than the daily average. The HPC bacteria levels of the day following a long weekend were 513±357 CFU L 1. This was significantly higher than the count for the days after normal working days (p<0.005). During the low water consumption periods, i.e. the night time, long weekend and in holidays, the water flow was low in the water distribution system with a long water retention time in the storage tank and pipes. This would provide a favorable condition for the multiplication and accumulation of bacteria to become biofilms attached on pipe walls. It was estimated that only 5% of overall biomass in the water distribution system was in the water phase and the remaining were attached to the pipe walls (biofilms) (Flemming et al., 2002). A high water flow velocity in the water pipe followed the low water consumption period would increase the detachment velocity of biofilms (Cloete et al., 2003). Besides, the bacteria concentrations in the water would be diluted from the water supply of the city mains and biofilms were therefore not accumulated. Lower HPC bacteria levels were therefore found at an afternoon session, the time after a higher water consumption period. 3.2 Bacterial composition The composition of HPC bacteria in the water samples is presented in Table 2. A total of 413 isolates were identified to 5 different genera. Similar pattern was observed in samples collected but with different occurrence frequency and relative abundance. 152

5 Overall, 59% of isolates were gram-positive and 41% of isolates were gram-negative. Micrococcus was the most dominant genera, presented in 94% of samples, with relative abundance of 28%. The other isolated gram-positive bacteria genera were Bacillus (6%), Staphylococcus (6%) and others (19%). These HPC genera were reported common bacteria in drinking water (Allen et al., 2004). The isolated gram-negative bacteria genera were Pseudomonas (13%), Escherichia coli (1%) and others (27%) respectively. Table 2 - The composition of HPC bacteria of drinking water in a high-rise office building of Hong Kong Count (CFU L 1 ) Composition (%) Overall (4 days) Day followed a normal working day (3 days) Day followed a long weekend (1 day) AM±SD Range AM±SD Range AM±SD Range 258 ± ± ± Occurrence frequency Relative abundance Occurrence frequency Relative abundance Occurrence frequency Relative abundance Micrococcus Bacillus Pseudomonas Staphylococcus Escherichia coli <1 Other (gram +ve) Other (gram -ve) Some species were considered to be opportunistic pathogen from oral ingestion for immunocompromised patients, such as Pseudomonas (Rusin et al., 1997). In this study, none of the water samples contained pathogenic bacteria except that E. coli was detected in three water samples with an average of 13±6 CFU L 1. This exceeds the WHO guideline for drinking water quality of zero E. coli per 100 ml water (WHO, 2004). The presence of E. coli indicated that the water was contaminated by human or animal wastes and would cause problems of probable infections. It was reported that only 1-2 colonies of E. coli were detected in the 0.1 L water samples collected at normal working days 08:00 and 12:00. It was probably due to bioaerosols generated from WC flushing. Investigations of the source of bacteria and the transmission path should be followed. In addition, Pseudomonas was commonly found genus and 38% of the samples was recorded. 4 Conclusions Microbiological water quality of drinking water available from the water supply system of an in-use, high-rise office building of Hong Kong was examined in the prospective of heterotrophic plate count (HPC). Samples were collected every 4 hours from 08:00 to 20:00 in working days that had a normal working day before and after or a working day that followed a long weekend. The results showed that the bacteria counts were correlated with the expected water consumption in a period. A high bacteria level would be obtained at a time following a low water consumption period due to low occupancy. 153

6 It was reported that the bacteria levels of the samples taken at 08:00 morning were 65% to 100% higher than the daily average. This was probably because the low water flow and a long water retention time in the storage tank and pipes provided a favorable condition for the multiplication and accumulation of bacteria to become biofilms attached on pipe walls. The bacterial genera in the samples were identified; 59% were gram-positive bacteria genera composed of Bacillus (6%), Micrococcus (28%), Staphylococcus (6%) and unidentified isolates (19%), and 41% were gram-negative bacteria genera including Pseudomonas (13%), Escherichia coli (1%) and unidentified isolates (27%). A low level of the faecal indicating bacteria, E. coli, was detected with a relative abundance of %. Investigations of the source of bacteria and the transmission path should be followed. Pseudomonas was recorded in 38% of the samples. Pseudomonas is, at some exposure levels, considered to be opportunistic pathogen from oral ingestion for immunocompromised patients. Investigation regarding the probable transmission through a water supply system in buildings was recommended. 5 Acknowledgement The work described in this paper was partially supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project no. PolyU5305/06E). The facilities described in this paper were partially supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project no. PolyU5248/06E). 6 References Allen, M.J., Edberg, S.C., Reasoner, D.J., Heterotrophic plate count bacteria what is their significance in drinking water? International Journal of Food Microbiology 92(3), Cloete, T.E., Westaard, D., van Vuuren, S.J., Dynamic response of biofilm to pipe surface and fluid velocity. Water Science and Technology 47(5), Edberg, S.C., Allen, M.J., Virulence and risk from drinking water of heterotrophic plate count bacteria in human population groups. International Journal of Food Microbiology 92(2), Edberg, S.C., Gallo, P., Kontnick, C., Analysis of the virulence characteristics of bacteria isolated from bottled, water cooler and tap water. Microbial Ecology in Health and Disease 9(2), Flemming, H.C., Percival, S.L., Walker, J.T., Contamination potential of biofilms in water distribution systems. Water Science and Technology: Water Supply 2(1), Ho, K.C., Chow, Y.L., Yau, J.T.S., Chemical and microbiological qualities of The East River (Dongjiang) water, with particular reference to drinking water supply in Hong Kong. Chemosphere 52(9), Institute of Plumbing (IOP), Plumbing engineering services design. Hornchurch, UK. 154

7 Laws of Hong Kong, Chapter 123I Building (standards of sanitary fitments, plumbing, drainage works and latrines) regulations. The Hong Kong Special Administrative Region, China. Maul, A., El-Sharrawi, A.H., Block, J.C., Bacterial distribution and sampling strategies for drinking water networks. In: McFeters, G.A. (Ed.). Drinking water microbiology, Progress and Recent Developments. Springer-Verlag, New York, Payment, P., Coffin, E., Paquette, G., Blood agar to detect virulence factors in tap water heterotrophic bacteria. Applied and Environmental Microbiology 60(4), Reasoner, D.R., Monitoring heterotrophic bacteria in potable water. In: McFeters, G.A. (Ed.), Drinking Water Microbiology Progress and Recent Developments. Springer-Verlag, New York, Rusin, P.A., Rose, J.B., Haas, C.N., Gerba, C.P., Risk assessment of opportunistic bacterial pathogens in drinking water. Reviews of Environmental Contamination and Toxicology 152, United States Environmental Protection Agency (USEPA), CFR Parts 141 and 142. Drinking Water: National Primary Drinking Water Regulations; Final Rule. Federal Register, v. 54. No. 124, , Thursday, June 29, World Health Organization (WHO), Heterotrophic plate counts and drinkingwater safety. In: Bartram, J., Cotruvo, J., Exner, M., Fricker, C., Glasmacher, A., (Eds.). IWA Publishing, London, UK. World Health Organization (WHO), Guidelines for drinking water quality. Vol. 1 (3 rd Ed.). WHO, Geneva. Wong, L.T., A cost model for plumbing and drainage systems. Facilities 20(11/12), Wong, L.T., Mui, K.W., Modelling transient occupant loads for offices. Architectural Science Review 49(1), Wong, L.T., Mui, K.W., Modeling water consumption and flow rates for flushing water system in high-rise residential buildings in Hong Kong. Building and Environment 42(5), van der Wende, E., Characklis, W.G., Biofilms in portable water distribution systems. In: McFeters, G.A. (Ed.). Dinking water microbiology, Progress and Recent Developments. Springer-Verlag, New York,

8 7 Presentation of Authors Miss W. Y. Chan is a PhD student at The Hong Kong Polytechnic University. Dr. L. T. Wong is an assistant professor in the Department of Building Services Engineering at The Hong Kong Polytechnic University. His research interests include environmental systems, safety systems and water systems in buildings. Dr. K. W. Mui is an assistant professor in the Department of Building Services Engineering at The Hong Kong Polytechnic University. His research interests include energy efficiency, building environmental performance, thermal comfort and indoor air quality. 156