17) Suggested TZ web
page - Load estimations for mercury in wastewater from point source
contributors in Christchurch
[The information on
this page has been prepared in the form of a suggested
web page for Target Zero’s section of the Christchurch City Council website at http://www.ccc.govt.nz/TargetZero].
Mercury loads in
Christchurch wastewater have been estimated as follows:
1) Dental surgeries: 19.9 kilograms of mercury per annum
2) Human wastes from dental amalgam: 2.0 kilograms of mercury per annum
3) Hospitals: 1.1 kilograms of mercury per annum
4) Industrial sources: 1.1 kilograms of mercury per annum
5) Laundry greywater: 1.0 kilograms of mercury per annum
In view of the large
estimated mercury load from dental surgeries, Target Zero has:
• Consulted with dentists on dental surgery wastes [hyperlinked to a suggested Target Zero web page, which is web page 18 on this website]
• Investigated the environmental standards and costs of locally available amalgam retention units [hyperlinked to a suggested Target Zero web page, which is web page 21 on this website] designed to capture amalgam wastes in the dental surgery
• Investigated the environmental standards and costs of locally available services for the disposal of dental amalgam wastes [hyperlinked to a suggested Target Zero web page, which is web page 22 on this website]
Calculations used to
estimate the loads from point source contributors in Christchurch
For the sake of
estimating the mercury load from dental surgeries, dental amalgam discharges to
wastewater are assumed to be approximately equivalent to the amount of fine
amalgam wastes that could be expected to be captured from exiting wastewater if
amalgam retention units were to be fitted in all dental surgeries. An
approximate average figure of between 700 and 800 grams of amalgam per dental
surgery per year has been provided by Advanced Recycling Australasia Pty Ltd, a
dedicated mercury wastes recycling service that accepts and processes dental
amalgam wastes from amalgam retention units installed in dental surgeries in
Australia. For the sake of this estimation, it is assumed that, on average,
dental surgeries in Australia have approximately the same level of dentist
staffing, and therefore generate approximately the same weight in fine amalgam
wastes, as those in Christchurch. In Australia, surgeries range from single-dentist
surgeries in the outback with only an occasional part-time presence of the
dentist right through to dental surgeries with 5+ full-time dentists. Probably
Australia has more surgeries with a lower staffing level, based in the outback,
but in light of the higher population density in Australian cities these may
well be counterbalanced by a larger proportion of multi-dentist surgeries in
the cities.
In the last batch of 80
kg of fine amalgam wastes from amalgam retention units that Advanced Recycling
Australasia processed, a total of 36 kg in mercury was recovered. The portion
of mercury in these amalgam wastes was therefore 45% by weight. On the basis of
an estimated 59 dental surgeries in Christchurch generating 750 mg of fine
amalgam wastes per dental surgery per annum, with the wastes containing a
mercury content of 45% by weight, a total of approximately 19.9 kg of mercury
per annum could be expected to be generated and discharged to wastewater by
Christchurch dental surgeries:
59 surgeries x 750 g wastes per annum per surgery x 45% Hg content = 19.9 Hg kg per annum
Further confirmation of
these figures is provided by information on a web site of the Metasys Group, a
manufacturer of amalgam retention units and a recycler of dental amalgam wastes:
“Amalgam consists of approximately 50% mercury. 800 g of amalgam residue is produced per year per treatment unit” [1].
In addition, the result
of this calculation base appears to be consistent with results based on estimates
of mercury discharges per dentist per day (in surgeries without amalgam
retention units) given in 10 published research studies, which range from 35 Hg
mg/dentist/day to 773 Hg mg/dentist/day [2].
The following calculations are based on an estimate of 130 F.T.E. (full-time
equivalent) dentists practising in Christchurch, and an assumption that
dentists work an average of approximately 240 days per annum:
0.035 Hg g/dentist/day x 240 days x 130 dentists = 1.1 Hg kg per annum
0.773 Hg g/dentist/day x 240 days x 130 dentists = 24.1 Hg kg per annum
2)
Human wastes from dental amalgam
The mercury load from
human wastes (urine and faeces) from dental amalgam is estimated from a figure
of 17.2 Hg ug/day/person [17.2 micrograms of mercury per person per day. A
microgram is a thousandth of a milligram], derived from an analysis of various
research studies on human wastes and outlined in a report prepared for the U.S.
Association of Metropolitan Sewerage Agencies [2]:
“The value calculated by Barron, 17.2 ug/day/person, was used because it appears to be the most representative of the U.S. population. This number accounts for all people; the fact that some people have amalgam fillings and some don’t, has been factored into the 17.2 ug/day/person value.”
On the basis of 17.2 Hg
ug/day/person, a total of approximately 2.0 kg of mercury per annum could be
expected to be contributed to Christchurch biosolids from the population of
Christchurch:
17.2 ug/person/day x 365 days x 322,000 Christchurch population = 2.0 Hg kg per annum
Note that the
calculations above solely refer to the mercury in human wastes from the dental
amalgam in that proportion of the population who have amalgam fillings. On the
basis of an estimated mercury loading for human wastes from dietary sources in
the same U.S. report, the total for Christchurch can be expected to be:
1.4 ug/person/day x 365 days x 322,000 Christchurch population = 0.2 Hg kg per annum
Calculations for the
annual mercury load contributed to Christchurch biosolids from hospitals are
based on:
1) A mercury concentration in discharged wastewater of 4.39 Hg ug/L, taken from a report prepared for the U.S. Association of Metropolitan Sewerage Agencies that includes wastewater load estimation figures [2].
2) An estimated discharge rate for all hospitals in Christchurch of 254,587,000 L/yr. For the sake of estimating the total litreage of wastewater discharged per year for all hospitals in Christchurch, the litreage discharged is assumed to be approximately equivalent to water consumption. This has been calculated on the basis of C.C.C. water consumption figures, plus the volume of water drawn from Christchurch Hospital’s own wells. A narrow definition of ‘hospital’ (only medical and/or surgical) was applied in line with the U.S. report:
4.39 Hg ug/L x 254,587,000 L/yr = 1.1 Hg kg per annum
Calculations for the
annual mercury load contributed to Christchurch biosolids from industrial
sources are based on:
1) A mercury concentration in discharged wastewater of 0.21 Hg ug/L, taken from a report prepared for the U.S. Association of Metropolitan Sewerage Agencies that includes wastewater load estimation figures [2].
2) An estimated discharge rate for industrial sources in Christchurch of 5,000,000,000 L/yr. The litreage discharged from industrial sources is estimated by the C.C.C. wastewater treatment plant as 10% of the total of all wastewater discharged:
0.21 Hg ug/L x 5,000,000,000 L/yr = 1.1 Hg kg per annum
Calculations for the
annual mercury load contributed to Christchurch biosolids from laundry
greywater are based on a mercury concentration in discharged wastewater of 8.4
Hg ug/person/day, taken from a report prepared for the U.S. Association of
Metropolitan Sewerage Agencies that includes wastewater load estimation figures
[2]:
8.4 Hg ug/person/day x 365 days x 322,000 Christchurch population = 1.0 Hg kg per annum
In the preparation of
these load estimations, ten other point sources were investigated and/or
estimated as possible contributors of
mercury to Christchurch biosolids [hyperlinked to a suggested Target
Zero web page, which is web page 20 on this website].
References
[1] Metasys web site [hyperlinked to http://www.ecotwo.com], which provides information on fine amalgam wastes per dental surgery per annum. Click on “Mercury” in the flash demo, or access the frameless web page directly [hyperlinked to http://www.metasys.co.at/purewater/2mercury/mercury.htm]
[2] Mercury Source Control & Pollution Prevention Program Evaluation [hyperlinked to http://www.amsa-cleanwater.org/advocacy/mercgrant/], U.S. Association of Metropolitan Sewerage Agencies, 2002. The mercury concentrations used above are taken from “Appendix B – Mercury Data and Calculations: Description of Calculation”. The main body of the report cites the results of 8 published research studies on mercury discharges per dentist per day.
Two recent, and particularly rigorous, studies were not included in this evaluation:
A Study of the Fate of Mercury from the Placement and Removal of Dental Amalgam Restorations [hyperlinked to http://www.rcdso.org/pdf/amalgam_watson.pdf], Royal College of Dental Surgeons of Ontario, 2002. The figures in this study would translate into 14.2 Hg kg per annum from Christchurch dentists:
755 Hg Kg per annum from 6,914 dentists = 0.109 Kg per dentist per annum
0.109 Kg per dentist per annum from 130 F.T.E. dentists = 14.2 Hg kg per annum
Characteristics and Treatment of the Dental Waste Water Stream [hyperlinked to http://www.dentalmercury.com/dentalmercury/Publications/WMRC_DUWW_Report.pdf], University of Illinois Chicago, 2002. The figures in this study would amount to 15.5 Hg kg per annum from Christchurch dentists:
0.498 Hg g/dentist/day x 240 days x 130 dentists = 15.5 Hg kg per annum