case study swiming pool
TRANSCRIPT
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Swimming pool water disinfection
Case study
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Introduction
When going to the swimming pool are you thinking about water purification method there and
how water quality can affect your health?
Swimming pool situated in London is the object of current case study. Swimmers have
complained about skin and eye irritation and chlorine smell after bathing. Pipe corrosion is an
increasing problem. Pool workers have more sick leaves due to breathing problems. Average
number of bathers is 700 per day. Water is circulated continuously between 6am and 7pm. Three
pools (main, outdoor and pool for teaching) were constructed according one scheme with the
only difference in pool size, scheme is presented in figure 1. Pool specification is presented in
table 1.
Figure 1. Scheme of main, outdoor and teaching pools [1]
Table 1. Swimming pool specification
Main pool (indoor), size: 25 x 13 m
Capacity 490 m3
Duty 198 m3/h
Outdoor pool, size: 2 x 10 m + free form
Capacity 312 m3
Duty 243 m3/h
Teaching pool (indoor), size: 13 x 7 m
Capacity 68 + 6 m3
Duty 105 m3/h
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Table 1. Swimming pool specification
Water treatment
Primary filtration Sand filters
Primary chemical treatment
Disinfection Chlorine gas
pH Control Soda ash
Flocculation Polyaluminium chloride
As it can be seen from the table, water disinfection method is chlorination for main pool, outdoor
pool and teaching pool. Chlorination is the most common method for swimming pool water
treatment due to low costs and quite high efficiency [2]. However, this method has few important
drawbacks such as carcinogenic by-products formation (trihalomethanes, haloacetic acids, etc.).
Formation of trihalomethanes (THMs) in water depends on amount of organic matter in water,
pH, temperature, contact time between water and chlorine, presence of bromide in source water.
Chloroform (CHCl3), bromdichloromethane (CHCl2Br), dibromochloromethane (CHClBr2) and
bromoform (CHBr3) constitute THMs. According to USEPA among these only CHClBr2 is
probable human carcinogen (type C), others are human carcinogens (type B2) [2].
Chloroform which was used in the past for medical purposes (anesthesia) basically caused death
followed by respiratory and cardiac arrhythmias [3]. Those patients who could survive after
chloroform induced anesthesia had other symptoms such as nausea, vomiting, prostration,
jaundice, coma, etc. It was reported that mean lethal oral dose of chloroform for an adult is about
45g however serious health problems can be caused by digestion of 7.5g [3].
Bromoform was used in the past as a sedative for children who suffered from whooping cough.
Bromoform overdosing has lead to several fatal cases. Dwell described the death of 2 years and 9
month girl suffered from whooping cough due to overdosing of bromoform (bromoform
concentration was 445 mg/kg/day and exposure time 1day). [4]
Dibromochloromethane and bromdichloromethane were not sufficiently studied. However there
are some information concerning acute toxicity of dibromochloromethane, it was defined that
LD50 (oral) for rats is 370 mg/kg [5].
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Swimming pool water contaminated by THMs is especially dangerous for swimmers. It was
reported that level at which people would be exposed to THMs during 1h in the pool is 141 times
than having shower for 10 min under tap water [2]. Currently maximum permissible
concentration for THMs in swimming pools does not exist. However, there are some defined
Maximum Contaminant Levels (MCLs) for chlorination by-products including total THMs in tap
water; for instance, WHO published that concentration of these compounds should not be higher
than 100g/Land the Maximum Contamination Levels Goal (MCLG) for THMs in water is less
than 40g/L [2].
Some attempts were made to evaluate level of THMs concentration in swimming pools in
different countries. Thus 114 residential swimming pools in the USA were examined by Sandel
and it was found that mean chloroform concentration was 67.1g/L with maximum level313g/L.According to the WHO investigations level of chloroform in the USA pools was 4-
420g/L [2]. Italian swimming pools THMs pollution level was reported to be 17.8 - 70.8g/L.
According to Chu and Nieuwenhuijsen concentration of total THMs in Londons pools is
125.2g/L and chloroform concentration is 113.3g/L. Linear correlation was defined between
number of people in the pool and THMs concentration in water. More data concerning THMs
concentration in indoor swimming pool water in various countries is represented in table 2. [6]
Table 2. THMs concentration measured in swimming pool water [6]
CountryTrihalomethanes concentration, g/L
Chloroform Bromoform Bromodichloromethane Dibromochloromethane
Poland 35.9-99.7 0.2-203.2 2.3-14.7 0.2-0.8
Italy
25-43 0.1 1.8-2.8 0.5-10
9-179
19-94
USA
4-402
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water intake is higher than that for adult women and constitute 22 ml while for women it is only
12 ml. For boys water intake is also higher (45 ml) than that for girls (30 ml). [6]
However contaminated water is not the only problem associated with THMs. Another important
point is that THMs are volatile compounds and they can be found in the air above the indoor
pools. Basically concentration of THMs in the air of the swimming pool depends on the
concentration in water, temperature, amount of splashing, ventilation system, etc. Data
represented in table 3 gives an idea about THMs concentration level in the air of indoor
swimming pools.
Table 3. THMs concentration in the air above the swimming pool water surface [6]
Country Trihalomethanes concentration, g/m
3
Chloroform Bromoform Bromdichloromethane DibromochloromethaneCanada 597-1630
Italy
66-650 5-1000 0.1-14
49-280 2-58 4-30
39-195 16-24 9-14
USA
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THMs are considered as one of the most dangerous contaminants in bathing water however it is
necessary to consider other pollutants as well. Possible sources of swimming pool water
contamination and some examples of pollutant are presented in figure 2.
Figure 2. Possible sources of swimming pool water contamination
Most obvious problem for swimmers and pool workers is caused by chloramines as they are the
source of chlorine odor. They induce skin, eye and lung irritation and are a big risk factor for
occupational asthma. They are formed when too small amount of chlorine is added to water.
Chloramines are products of free chlorine and nitrogen derived from urine and sweat. [7, 8]
THM measurements
As a response to swimmers complaints, some studies were carried out recently in all pools and
concentration of THMs was measured in the water and in the ambient air 25 cm above the water
surface, data presented in table 5 and 6.
Table 5. Mean THMs concentration in the water of swimming pools, g/L
Pool name Chloroform Bromoform Bromdichloromethane Dibromochloromethane
Main pool 150.2 1.5 15.6 4.8Outdoor 215.6 8.6 5.4 125.8
Teaching 138.8 14.3 20.2 15.5
Table 6. Mean THMs concentration in the ambient air of the swimming pools, g/L
Pool name Chloroform Bromoform Bromdichloromethane Dibromochloromethane
Main pool 215.4 0.5 46.2 19.5
Outdoor 5.1 0.9 0.1 0.6
Teaching 187.5 35.7 36.8 42.7
Results of studies show that THMs concentration in pools water and ambient air is quite high
which could be risky for bathers. Thus acute problem of water treatment technique should be
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solved. Main engineer of the swimming pool should decide if it is reasonable to change water
purification system. There are several commonly used methods for water disinfection to choose
from.
Chlorine disinfection
It is well known that chlorination is widely used for wastewater disinfection since 1914, when it
was applied first time for this purpose in the USA [9]. There are few main components used for
chlorination: chlorine, sodium hypochlorite, calcium hypochlorite and chlorine dioxide. Recently
quite many water treatment plants started to use sodium hypochlorite instead of chlorine due to
safety concern (handling and storage of hypochlorite is much more easy).
Solubility of chlorine in water is moderate. Chlorine is considered to be highly toxic componentand it could be dangerous for treatment plant workers and for public in case if it accidentally
released. By-products of chlorination are known as carcinogens and mutagens, residual chlorine
in water after disinfection is toxic for aquatic life. [9]
Sodium hypochlorite used instead of chlorine allows to avoid many problems related to
transportation, storage and feeding. It is usually presented as a liquid with 12.5 to 17 % available
chlorine at the moment of manufacture. One of the main disadvantages is that solution
decomposition is affected by high temperature and light. For instance solution (16.7%) stored at
26.7C lose 10% in 10 days. Another drawback is relatively high cost. During handling it can
cause some problems because of chlorine fumes presence and corrosiveness. [9]
Some remarks:
Very effective as both primary and secondary disinfectant; Cheap chemical (equipment (piping etc.) maintenance does increase disinfection cost); Easily measurable by residual chlorine; Residual disinfection effect; Harmful byproducts, which cause skin, eye and lung irritation; Chlorine gas is toxic (high risk for workers).
Ozone disinfectionDue to short life of ozone molecule it is not delivered to target unit (in this case swimming pool),
but is produced on site, usually with a high voltage corona discharge or vacuum ultraviolet ozone
generator. Thus safety problem associated with shipping and handling are diminished. Like
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chlorine, ozone is a strong oxidizing agent. It is effective against viruses and bacteria. Ozone has
very short half-life, so there are not residuals to be removed after ozonation.
There is a certain risk of pool operators to be exposed to ozone due to e.g. leakage. Even very
low concentrations of ozone can be harmful to the upper respiratory tract and the lungs. The
severity of injury depends on both by the concentration of ozone and the duration of exposure.
Some remarks:
No harmful residuals; Short contact time; No regrowth of microorganisms; Complex disinfection system; Very corrosive; Toxic; Relatively high cost; No measurable residual to indicate the efficacy of disinfection. [10, 11]
UV disinfection
On contrary to chlorination and ozonation UV disinfection is a physical disinfection method. As
ozone treatment, ultraviolet will not leave any residuals in water. UV radiation will inactivate
any organism in water, there are no resistant species. Inactivation rate is proportional to UV
dose.
UV radiation causes DNA base thymine dimerization, which leads to inactivation of organism.
Most effective wavelength is somewhere between 260-270 nm [12]. Also production of ozone
and hydroxyl radicals occurs, when wavelength is lower than 200 nm. These compounds are
germicidal, but may cause health issues if people are exposed. A typical low pressure Hg-lampproduces wavelengths around 254 nm - possibly bit off the target. Medium pressure Hg-lamp
produces multiple wavelengths, which might be waste of energy. LEDs can produce any narrow
band of wavelength. UV-lamps contain toxic mercury. This harm could be avoided with UV-
LED systems as they arrive to the markets. At present mostly visible LEDs are used and
probably in near future they will totally replace traditional incandescent and fluorescent lamps.
There are no commercial applications using UV LEDs yet, however extensive research is taking
place at the moment and probably it will be a major green water disinfection method.
Some remarks:
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No byproduct formation (no smell or health effects); No risk of overdosing; No resistance; Easy to operate; Turbidity of water can drastically reduce the disinfection effectivity: prefiltering needed
(as applied with chlorination and ozonation methods);
Some microorganisms may be able to repair damage caused by UV-radiation, when theyare exposed to visible light shortly after the ultraviolet treatment: photoreactivation;
There is no significant residual disinfection, so additional disinfection (e.g. small amountof chlorine) possibly needed;
No means for measuring efficiency in various water conditions; No standardized mechanism to measure, calibrate and certify equipment for efficiency
before and after installation;
Lamps contain toxic mercury and are not that durable [13]In tables 7, 8 and 9 disinfection efficiency of UV radiation is presented.
Table 7. UV-doses (253.7 nm) required for inactivation of different microorganisms [14]
Microorganism Log reduction mWs/cm2
Bacillus subtilis3 20-40
4 60-90MS-2 4 87
Hepatitis A 3 30
Rotavirus 4 40
Giardia lambliacysts1 40
2 180
Table 8. Average log reduction of microorganisms using combined carbon block filter with
ultraviolet disinfection unit (>128 mWs/cm2 / 253.7 nm) [15]
Microorganism Log reduction
Poliovirus 4.28
Rotavirus 4.29
HAV 3.92
MS-2 6
Giardia lambila cysts 3.99
Cryptosporidium parvum 4.3
Vibrio cholerae 5.96
Shigella dysenteriae 6.7
Escherichia coli 6.3
Salmonella typhi 6.52
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Table 9. UV-doses required for inactivation of different microorganisms [15]
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Comparison of disinfection methods (effectiveness and cost)
Every disinfection technique has its specific advantages and disadvantages. In the table 10 some
of these are shown. By-products of discussed methods are presented in table 11.
Table 10. Advantages and disadvantages of different methods [16]
Technology
Friendliness
to the
environment
By-
products
formation
Efficiency InvestmentOperational
costsFluids Surfaces
Ozonation + + ++ - + ++ ++
UV* ++ ++ + +/- ++ + ++
Chlorination:
Chlorine gas -- -- - + ++ +/- --
Chlorine
dioxide+/- +/- ++ ++ + ++ --
Hypochlorite -- -- - + ++ +/- --
* Hg-lamps evaluated. Operational costs with LED-lamps should decrease as the energy demand of LEDs is minimal compared
to Hg-lamps. Investment costs on the contrary would be higher due to new technique and more difficult manufacture.
Table 11. Predominant chemical disinfectants used in pool water treatment and their
associated disinfection by-products [6]
Disinfectant Disinfection by-products
Chlorine/hypochlorite trihalomethaneshaloacetic acids
haloacetonitriles
haloketones
chloral hydrate (trichloroacetaldehyde)
chloropicrin (trichloronitromethane)
cyanogens chloride
chlorate
chloramines
Ozone bromate
aldehydes
ketones
ketoacids
carboxylic acids
bromoform
brominated acetic acids
Chlorine dioxide chlorite
chlorate
Bromine/hypochlorite
BCDMH
trihalomethanes, mainly bromoform
bromal hydrate
bromamines
*UV is generally not considered to produce by/products
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Comparative efficiency of considered water disinfection methods are presented in figure 3.
Figure 3. Comparison of water disinfection methods[17]
Estimated total production costs of different disinfection methods according to US
Environmental Protection Agency (USEPA) are presented in table 12. The disinfection facilities
are categorized to 5 classes based on the designed water flow through the facility.
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Table 12. Cost comparison of different disinfection methods [14]
Total production costs ($cents/m3)
USEPA Flow
Category
Design Flow
m3/d
UV dose
40 mJ/cm2
Chlorination dose
5 mg/l
Ozonation dose
1 mg/l
UV dose
140
mJ/cm2
1 91 5 75 93 72 329 2 18 26 5
3 1022 1 6 9 4
4 2461 1 5 6 3
5 6814 1 2 2 3
Task
Results of studies show that THMs concentration in pools water and ambient air is quite high
which could be risky for bathers. Thus acute problem of water treatment technique should be
solved. Main engineer of the swimming pool should decide if it is reasonable to change water
purification system. Final decision should based on economical calculations (investment and
daily expenses), advantages and drawbacks of chosen method, also environmental issues (waste
utilization, etc.) should be considered for all possible pools water treatment methods.
Take the role of main engineer, discuss about following points:
Which disinfection method would be economically most profitable? Which disinfection method would be most environmentally friendly? Which method would have least health issues (swimmer / poolside worker / machine
operator)?
Decide if it is reasonable to change the disinfection system! Which of the alternative methods would be best for the situation? Reason your decision!
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References:
[1] How swimming pools work. Document available at:
http://home.howstuffworks.com/swimming-pool.htmDate of access: 4.01.2012
[2] Panyakapo Mallika, Soontornchai Sarisak, Paopuree Pongsri. Cancer risk assessmentfromexposure to trihalomethanes in tap water and swimming pool water. Journal of
Environmental Sciences 20(2008), p.372-378
[3] Concise International Chemical Assessment Document 58. Chloroform. WHO. 2004.
Document available at: http://www.who.int/ipcs/publications/cicad/en/cicad58.pdf. Date of
access: 22.12.2011
[4] BROMOFORM AND DIBROMOCHLOROMETHANE. Document available at:
http://www.atsdr.cdc.gov/toxprofiles/tp130-c3.pdf Date of access: 12.12.2011
[5] Material Safety Data Sheet. Dibromochloromethane. Document available at:http://www.sigmaaldrich.com/catalog/DisplayMSDSContent.do SIGMA-ALDRICH. Date of
access: 19.12.2011
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[8] Jacobs J.H., Spaan, S., van Rooy, G.B.G.J., Meliefste, C., Zaat, V.A.C., Rooyackers, J.M.and Heederik, D., Exposure to trichloramine and respiratory symptoms in indoor swimming poolworkers, Eur Respir J 2007 29(4)690-698
[9] George Tchobanoglous, Franklin Louis Burton, H. David Stensel, Metcalf & Eddy.
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[10] Health Effects of Ozone. Article available at:
(http://www.ccohs.ca/oshanswers/chemicals/chem_profiles/ozone/health_ozo.html)Date of
access: 04.01.2012
[11] National Environmental Services Center, Tech Brief Fact Sheet: Ozone Disinfection.
Available at:http://www.nesc.wvu.edu/techbrief.cfm Date of access: 8.01.2012
[12] Chen,R., Craik, S. and Bolton, J., Comparison of the action spectra and relative DNAabsorbance spectra of microorganisms: Information important for the determination of
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[14]USEPA 1997, Small System Compliance Technology List for the Surface Water TreatmentRule EPA 815-R-97-002
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[15]USEPA 1996, Ultraviolet light disinfection technology in drinking water application Anoverview EPA 811-R-96-002
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