Sources of heavy metals in urban wastewater in Stockholm

The Science of the Total Environment 298 (2002) 131–145

Sources of heavy metals in urban wastewater in Stockholm a, ¨ L. Sorme *, R. Lagerkvistb a

¨ ¨ Department of Water and Environmental Studies, Linkoping University, S-581 83 Linkoping, Sweden b Stockholm Water Company, S-106 36 Stockholm, Sweden Received 2 April 2001; accepted 18 April 2002

Abstract The sources of heavy metals to a wastewater treatment plant was investigated. Sources can be actual goods, e.g. runoff from roofs, wear of tires, food, or activities, e.g. large enterprises, car washes. The sources were identified by knowing the metals content in various goods and the emissions from goods to sewage or stormwater. The sources of sewage water and stormwater were categorized to enable comparison with other research and measurements. The categories were households, drainage water, businesses, pipe sediment (all transported in sewage water), atmospheric deposition, traffic, building materials and pipe sediment (transported in stormwater). Results show that it was possible to track the sources of heavy metals for some metals such as Cu and Zn (110 and 100% found, respectively) as well as Ni and Hg (70% found). Other metals sources are still poorly understood or underestimated (Cd 60%, Pb 50%, Cr 20% known). The largest sources of Cu were tap water and roofs. For Zn the largest sources were galvanized material and car washes. In the case of Ni, the largest sources were chemicals used in the WTP and drinking water itself. And finally, for Hg the most dominant emission source was the amalgam in teeth. For Pb, Cr and Cd, where sources were more poorly understood, the largest contributors for all were car washes. Estimated results of sources from this study were compared with previously done measurements. The comparison shows that measured contribution from households is higher than that estimated (except Hg), leading to the conclusion that the sources of sewage water from households are still poorly understood or that known sources are underestimated. In the case of stormwater, the estimated contributions are rather well in agreement with measured contributions, although uncertainties are large for both estimations and measurements. Existing pipe sediments in the plumbing system, which release Hg and Pb, could be one explanation for the missing amount of these metals. Large enterprises were found to make a very small contribution, 4% or less for all metals studied. Smaller enterprises (with the exception of car washes) have been shown to make a small contribution in another city; the contribution in this case study is still unknown. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Heavy metals; Wastewater treatment plant; Sewage water; Stormwater; Sources; Households; Traffic; Goods

*Corresponding author. Tel.: q46-13-28-10-00; fax: q46-13-13-36-30. ¨ E-mail address: [email protected] (L. Sorme). 0048-9697/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 8 - 9 6 9 7 Ž 0 2 . 0 0 1 9 7 - 3

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1. Introduction A wastewater treatment plant (WTP) treats water from many sources. The two end products in the WTP are sludge and effluent. Sludge contains nutrients, mainly phosphorous and nitrogen. In Sweden the goal is that by 2010 at least 75% of the phosphorus recovered from waste and sewage sludge be recycled and used in agricultural or other productive lands without risk to health or the environment (Statens Offentliga Utredningar,

2000). If sludge (with the nutrient phosphorous) is to be used, its pollutant content—some heavy metals, for example—must be minimized. In Sweden the limits for allowable metal content in sludge have been lowered recently, a move that made it difficult for some WTPs to stay below the marginal value. There is also a limit on the maximum amount of metals to be spread per hectareyyear on agricultural land. This limit has been more difficult to achieve if the phosphorus is spread in amounts needed. Also, beginning in 2000

Fig. 1. The content of Cu and Pb (mgykg TS) in the sewage sludge of Henriksdal, Bromma and Loudden’s WTP, Stockholm, 1973– 1999.

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there has been a tax of 250 SEKyton (;US$25) on all waste. Sludge has been termed waste, and thus will be taxed. Historically, the sources of heavy metals to sludge have been industrial activities such as surface treatment with elements such as Cu, Zn, Ni and Cr. In recent years, industries have to a large degree moved out of the cities and the release of heavy metals and other compounds has decreased due to various pre-treatments of the effluent. This is reflected in the total amount of heavy metals in sludge from WTPs, see Fig. 1a,b. Generally, most of the metals in incoming water will end up in the sludge (except Ni); only a small amount is released via outgoing water. Cu content decreased in the late 1970s and the early 1980s, but since then the content has stabilized. For Pb, the same pattern can be seen, but after the early 1980s the rate of decrease has slowed. The other metals contained in sludge (Cd, Cr, Hg, Ni and Zn) follow the same pattern as Pb. A lower amount from industrial activities has been documented in ¨ Sweden. For example, Bergback (1989) showed that the ‘consumption emissions’ for Cd have been larger than ‘production emissions’ since circa 1980 ¨ et al. (2001) confirmed that in Sweden. Bergback the industrial sources were minimal in comparison to diffuse sources from different goods, in the city of Stockholm as of 1995. Diffuse sources are, for example, emissions from different goods in the traffic environment (e.g. brake linings, tires), from buildings (roofs), and from households (food). ¨ Sorme et al. (2001a) identified the major diffuse sources from goods and showed, for the city of Stockholm, that the goods in the traffic sector were a major contributor of the diffuse emission of heavy metals. The actual contribution from different goods to sewage and stormwater is relatively unknown although some important studies have been made. For example Malmqvist (1983) and Legret and Pagatto (1999) estimated sources to stormwater, and Boller (1997) and Boulay and Edwards (2000), as well as Koch and Rotard (2000) investigated some sources to stormwater and sewage water. The aim of this article is to analyse the sources of heavy metals (Cu, Zn, Pb, Cr, Ni, Cd and Hg)

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Fig. 2. Stockholm and the wastewater treatment plant Henriksdal. The reception area of Henriksdal is shown in a darker gray.

to a WTP. The Henriksdal WTP in Stockholm, Sweden, has been used as the object of the study. 2. Study area The Henriksdal WTP is one of four WTPs in Stockholm. It treats sewage water from the city of Stockholm, and parts of Haninge, Huddinge, Tyreso¨ and Nacka municipalities (see Fig. 2). It served 630 100 people in 1999. The flow of sewage water was 256 000 m3 y24 h in 1999. The average household consumption was approximately 203 lycapita for 24 h and the business consumption 85 lycapita for 24 h. The plumbing system is both combined and duplicated. For example, the amount of stormwater in the former system was 4.28 million m3 whereas in the latter it was 3.74 million m3 in 1999 (Stockholm Water, 1999, 2000). In Henriksdal’s reception area there are 200 activities (enterprises, usually) that need a permit according to the environmental code. There are also a large number of minor activities (dentists, art schools,

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Fig. 3. The different sources of heavy metals (see text) that are accounted for in this article, and the possible destination for each source.

600 car washes, for example). Although there are quite a few industrial activities, the character of the city is more administrative. The results of this study are from the year 1999 (otherwise indicated if not). The year of 1999 was a standard year in Stockholm in terms of precipitation, with a value of 549 mm. The average is 550 mm (Stockholm Water, 1999). This is of major importance, for example for analyzing the flow of stormwater and drainage water. 3. Method The aim of the study is to analyse the sources of heavy metals to the WTP Henriksdal. The sources have been classified into eight groups: households; drainage water; businesses; pipe sediments; chemicals; atmospheric deposition; traffic; and building materials. Emission from households, drainage water and businesses are transported in sewage water, emission from atmospheric deposition, traffic and building materials in stormwater (see Fig. 3). Emission from pipe sediments can be transported in both, see Fig. 3. Chemicals are added in the WTP.

No measurements were made for this study; all data was collected from different types of literature and personal communication. In households, we investigated the contribution from feces and urine, amalgam, detergents, pipes and taps, drinking water and artist paint, see Fig. 3. The numerical contribution was calculated using the general formula: metal content (mgyl)=water used (l) (in the case of pipes and taps and drinking water). In the other cases, data on emission in mgy24 h per person was derived or calculated directly from the literature. Drainage water is the water that leaks into the sewage system from surrounding soils, etc. The numerical contribution was calculated with the general formula: metal content in groundwater (mgyl)=drainage water (l). In the case of businesses, car washes, dentists, large enterprises (which submit information on how much is released to the sewage system) and drinking water are covered (see Fig. 3). Drinking water refers to the water consumption used for various purposes in businesses. Here the general formula: metal content (mgyl)=water used (l) was used in the case of pipes and taps, car washes (Cu) and drinking water. Pipe sediments means metals which have been deposited in pipes and make up a residue with other materials, both in sewage and stormwater pipes. Chemicals (which contain traces of some heavy metals) are added in the WTP to improve the treatment process. Data were derived from Stockholm Water. Atmospheric deposition is metals transported in to Stockholm from other areas. The contribution has been calculated with the general formula: deposition (gyha)=area (ha). Not all metals deposited in the reception area of Henriksdal will end up in the WTP, one part will enter the soil or water, since in Stockholm both combined and separated stormwater systems are used. Some particles could also be transported away again as aerosols. The different pathways are shown in Fig. 3. An assumption on the amount (%) to the WTP was made on the basis of other studies. From traffic the contribution from brake linings, tires, asphalt, gasoline and oil are covered (see

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Fig. 3). The emission is derived from the general formula: traffic work (km)=metal content in good (mgykg)=wear (mgyvehicles km). Not all the emission from traffic ends up in the WTP, see the text about atmospheric deposition, above. From building materials the contribution from roofs and fronts (Cu and Zn) and different galvanized materials (Zn) (such as crash barriers) has been investigated (see Fig. 3). The emission is derived from the general formula: runoff rate (mgy m2)=surface (m2). Not all the emission from building materials ends up in the WTP, see the text about atmospheric deposition, above. Overflow (untreated sewage water which finds its way into the watercourses) could be a source of off-transport of metals from the WTP, but the volumes are small, -1% of incoming water, hence this path is not further considered. It is assumed that the released metals from goods that enter the plumbing system also will be transported to the WTP within the year. This is a simplification because some particles form a sediment in the plumbing system, hence delaying the transport to the WTP. 4. Results 4.1. Sewage water—households Here, emissions from food, amalgam, detergents, pipes (taps included), drinking water and artist paint are included in the ‘household’ category. The drinking water itself contains some metals. The drinking water pipes are often of Cu and corrode and contribute with Cu to the WTP. It is assumed that the runoff rate from Cu pipes in other buildings is the same as in residences. Food contains some heavy metals, e.g. the essential Cu and Zn. Detergents contain small amounts of heavy metals. Artist paint in the colors of yellow–red can contain Cd. Amalgam, consisting mainly of Hg, Ag, Cu and Zn, is slowly released from fillings in teeth. But the emissions of Cu and Zn are negligible in comparison to other sources. Results are shown in Table 1. Results show, for example, that approximately 59% of the Cu at the WTP are explained by emission from household goods.

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4.2. Sewage water–drainage water Drainage water is here defined as the water that leaks into the sewage system from the surrounding soil, etc. The plumbing system has places where there are leaks, which drain the soil nearby. Total drainage water was estimated at 14.54– 15.01 million m3 in 1999 to Henriksdal (Stockholm Water, 2000). The average metal content in the drainage water has been assumed to be equal to the median metal content in groundwater. The metal content in groundwater varies considerably in Stockholm, and hence the contribution to the WTP is difficult to estimate. The metal content in groundwater has been measured to (median values in mgyl) Cu 8.6, Zn 31, Pb 0.6, Cr 0.8, Ni 7.1, ¨ ¨ Cd, Hg 0.0156 (Miljoforvaltningen, 1997). This gives a total contribution from drainage water to: 125 kg Cu, 450 kg Zn, 9 kg Pb, 12 kg Cr, 100 kg Ni, 0.7 kg Cd and 0.2 kg Hg (rounded numbers). This contribution is a few percent of the total for most of the metals, the contribution to the WTP is: Cu 2%, Zn 4%, Pb 1%, Cr 2%, Ni 10%, Cd 3%, Hg 1% wderived as contribution drainage water (kg)ytotal contribution (kg), see Table 1x. 4.3. Sewage water—businesses In Stockholm there are several types of businesses that emit heavy metals. Here the following are covered: car washes, dentists and large enterprises (which submit information on how much is released to the sewage system). Also the metals emitted due to the use of drinking water in business are included in the contribution from businesses. Here the consumption is averaged to 85 lycapita (Lagerkvist, personal communication). In Table 2 it is shown clearly that the major source of all heavy metals from businesses (except Hg, which comes mainly from dentists and Cu which comes from pipes and taps) is from car washes. There is no information from Stockholm about how much smaller enterprises release. All indications are that it is for most metals minor amounts, e.g. in Gothenburg the contribution from smaller enterprises was measured and the total contribution to the WTP was -3% for Zn, Hg, Ni, Cr, and Cd, 6% for Cu and 12% for Pb (GRYAAB, 1991).

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Table 1 The average emission from different goods in a household. A dash sign (–) indicates that the source is insignificant compared to other sources (in this study). Origin

mg y24 h and person Cu

Food Amalgam Powder laundry detergentsd Artist painte Pipes and taps Drinking waterh Total (rounded numbers) Total from households (rounded numbers) Total to the WTPi

Zn

1200

a

– 1.8 – 12 500f 610 14 300 kgyyear (1999)

Pb a

11 000 – 45 – 2100g 406 13 600

3300

3100

5560

10 300

% of total load to the WTP from households 59

30

Cr a

20 – 2.1 – – 6 28 6.4 680 0.9

Ni a

30 – 0.55 – – 20 51

Cd 80

a

– 0.55 – – 610 690

a

Hg

10 – 1.1 11 –

2b –6c 60b 1.4 – –

22

63–67

12

160

5.1

480

1000

25.6

31.6

20

44–47

2.4

16

14–15

a

Naturvardsverket (1995). ˚ Skare and Engqvist (1994). c Naturvardsverket (1993). ˚ d Jenkins (1998). e 2.6 kg total to Henriksdal (Lagerkvist, personal communication). f 203 lycapita. Cu: 68 mgyl (88%), 11 mgyl (10%), 42 mg (2%) Mohlander (1992). g 203 lycapita. Zn: 31 mgyl (10%), 8 mgyl (90%) Mohlander (1992). h 203 lycapita. i Stockholm Water (1999) 97% of Pb and 90% of Cr in sludge (average values) (Lagerkvist, personal communication). b

Larger enterprises contribute 4% or less of the total contribution to Henriksdal (see Table 2). Water from car washes (after oil separation) contains an average Cu value of 203 mgyl and 300 l per wash, which gives 61 mgycar (calculated from Stockholm Water, 1993). There are 220 000

cars, 1300 buses, and 22 000 lorries in Stockholm, and the average washing frequency is 20 times, ¨ and 300 times and 100 times per year (Bergback ¨ Sorme, 1998). This would give a total Cu emission of 426 kg. The total emissions for the other metals are 11 kg Cd, 60 kg Cr, 55 kg Ni, 340 kg Pb,

Table 2 The emission from businesses (kgyyear) and the businesses’ contribution (%) to total emissions to Henriksdal (1999) Activity

Known from large enterprises Car washes Dentists Pipes and taps Drinking water Total to WTP 1999

kgyyear (1999) Cu (kg)

Zn (kg)

Pb (kg)

Cr (kg)

Ni (kg)

Cd (kg)

87 300 – 1200 59 5560

200 2300 – 200 39 10300

21 240 – – 0.6 680

17 42

38

Hg (kg)

– – 2.0 480

32 39 – – 59 1000

0.47 7.7 – – – 25.6

6.5 – – 31.6

13

13

32

21

0.12

% of total load to WTP from businesses 30

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¨ and Sorme, ¨ 3300 kg Zn (Bergback 1998). Not all washes are done at an enterprise such as a gasoline station. In Sweden as a whole approximately 33% of all washes are estimated to be done at car washes (Naturvardsverket, 1996), but in this article ˚ it is assumed to be higher (70%), since in a city environment it is not so easy to wash a car manually. Dentists are allowed to emit 5 g Hgyyear and treatment unit. There are approximately 1300 treatment units in the Henriksdal reception area (Lagerkvist, personal communication). It is very difficult to estimate if the permitted emissions occur— some units probably release more, others less. 4.4. Sewage waterystormwater—pipe sediments Metals can be deposited in pipes and make up a ‘pipe sediment’ together with other materials, both in sewage and stormwater pipes. This sediment probably builds up over many years, so the emission to the WTP in 1 year could be the result of an historic emission. Stockholm Water regularly flushes the system and considerable amounts of Pb and Hg have, as a result, been released; for example, in one case it was 30 kg Pb and 2 kg Hg (Lagerkvist, personal communication). Past studies suggest the importance of these sediments as a source of particles and organic matter (Gromaire-Mertz et al., 1999). It has not been possible to estimate the amount that can be emitted from these sediments, see Section 7. 4.5. Stormwater—atmospheric deposition Heavy metals are transported from other areas as aerosols to the area of Stockholm. The deposition (wet and dry) in Stockholm (gyha per year) has been measured by Burman and Johansson (2000). It was measured to be Pb 30; 0.98 Cd; 6.7 Cr; 10 Ni; 256 Zn; 25 Cu gyha per year. The hard surface area where the system is combined and which delivers stormwater to Henriksdal is 950 ha (Stockholm Water, 2000). It means that approximately 28 kg Pb, 0.9 kg Cd, 6.4 kg Cr, 9.5 kg Ni, 243 kg Zn and 24 kg Cu will enter Henriksdal if all the deposited material is transported all the way to the WTP within the study

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year (which is the assumption made in this article). It contributes to the WTP approximately 3–4% Pb and Cd, approximately 2% Zn and 1% or less Cr, Ni and Cu. The remaining part of the deposition will be deposited on soil or water or on hard surfaces with a separate plumbing system. In the case it is deposited on soil it is possible that it will be a delayed transport to the WTP with soil particles as well. This has not been accounted for in this article but could be of importance. 4.6. Stormwater—traffic In the ‘traffic’ category, brake linings, tires, asphalt, gasoline, and oil are covered. The emission from traffic is in the form of particulates that can be transported to the WTP in stormwater in a combined system. The total traffic work in Stockholm city is approximately 3000=106 vehicle km (Burman, personal communication). Only traffic work in combined systems in the reception area of Henriksdal is of interest for this study. Burman (personal communication) calculated that traffic work to be 1100=106 vehicle km. When calculating total emission from each good, all traffic work is estimated to be made by private cars. Private cars do 96% of the traffic work in Stockholm (Burman, 1998). This gives a small underestimation of the results. 4.6.1. Brake linings An extensive study was performed in Stockholm to estimate the diffuse emissions from brake linings (Westerlund, 1998). OEM pads (original) and replacement pads (rear and front) from different brands were collected and analyzed (Westerlund, 1998). The metal content varied considerably, and the average values are shown in Table 3. The wear of brake linings was estimated to 10.5 mgyvehicle km (front) and 5.13 mgyvehicle km (rear) (Westerlund, 1998). Forty percent of the traffic work was estimated to be done with OEM pads and 60% with replacement pads. The Cd content in most of the brake linings was too low to detect. 4.6.2. Gasoline In Sweden since 1995, only unleaded gasoline has been allowed in vehicles. In gasoline, the

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Table 3 Metal content (mgykg) in traffic goods in Sweden. Good

Cu (mgykg)

Zn (mgykg)

Pb (mgykg)

Cr (mgykg)

Brake lining front, new Brake lining rear, new Brake lining front, old Brake lining back, old Tire rubber Asphalt wear Gasoline Oil

119 000 92 200 72 000 52 100 1.8 13 -0.01 -1

28 800 16 500 17 700 7200 10 000 52 -0.01 800–1400

9050 18 700 13 700 9110 6.3 24 -0.01 -1

137 73.4 92 151

metal content (Cd, Cr, Cu, Pb, Ni and Zn) has been measured to -0.01 ppm (Lotse, personal communication); the detection level was 0.01 ppm (mgykg). The density of gasoline is 745 kgym3. Gasoline consumption in Stockholm is approximately 300 000 m3. A metal content of 0.01 ppm would give a total emission of approximately 2 kg of each metal. In the case of Cd this amount would be of importance, but there is no indication that levels of Cd would be just under the detection limit. Gasoline is, therefore, assumed to be a negligible source of Cr, Cu, Pb, Ni, Zn and is most likely also negligible for Cd. Other research has found higher metal content in gasoline, Legret and Pagatto (2000) presented values of 1.3 mg Pbyl, 0.063 mg Cuyl, and 0.09 mg Znyl (in unleaded gasoline), all of which are considerably higher than what the Swedish references state that gasoline comprises.

? 4 -00.1 -1

Ni (mgykg)

Cd (mgykg)

Reference

141 69.6 182 122 ? 0.5 -0.01 -1

– – – –

Westerlund (1998) s s s Legret and Pagatto (1999) Alloway (1990) Lotse (personal communication) Kylberg (personal communication), ¨ Gothberg (personal communication)

2.6 0.09 -0.01 -1

4.6.3. Oils Zinc is added to motor and diesel oils (lubricating oils). The Zn content of the most popular brand for diesel fuel was measured at 800–1400 mgykg (Kylberg, personal communication) (see Table 3). The average consumption of motor oil in vehicles has been estimated at 1 l per 5000 km, and the traffic work as presented above. The density of the oil is 0.88 kgyl. The average wear can then be calculated, see Table 4. Of course the consumption of motor oil is dependent on the type of car, age of the car, etc., so the value is quite uncertain. For the other metals with -1 ppm, ¨ (detection limit 1 ppm) (Gothberg, personal communication), the emission would be less than 0.2 kg per metal. Metals other than Zn can be released through a secondary process, namely the corrosive action of oils in contact with air, which causes wear of alloys in vehicles, and which can contain

Table 4 Total emission (rounded numbers) from the traffic category in the reception area of Henriksdal where the wastewater system is combined (traffic work 1100 M Vehicle km) Good Brake linings Tires Asphalt Gasoline Oil

Cu (kg) a

1400 0.4b 28–42c -2d -0.2e

Pb (kg)

Zn (kg)

210 1 52–79 -2 -0.2

320 2300 110–170 -2 160–270

Cr (kg)

Ni (kg)

2

2

?

?

9–13 -2 -0.2

1–2 -2 -0.2

Cd (kg) ? 0.57 0.2–0.3 -2 -0.2

a Total emissionsemission front original pads wwear (mgykm)=metal content (mgykg)=part with original padsxqemission front replacement padsqemission rear original padsqemission rear replacement padss(10.5=119 000=0.4)q(10.5=72000=0.6)q (5.13=92200=0.4)q(5.13=52 100=0.6)s550q499q208q176s1433 kg. b Wear (mgykm)=metal content (mgykg)=traffic work (km)s20=1.8=1 100 000 000s0.396 kg. c Wear (gykm)=part with studs=metal content (gykg)=traffic work (km)s4–6=0.5=(00.013)=1 100 000 000s28.6–42 kg. d Metal content (mgykg)=density (kgym3)=consumption (m3)s-0.01=745=300 000s-2 kg. e Metal content (mgykg)=density (kgyl)=use (lykm)=traffic work (km)s1=0.88=0.002=11 000 000s0.19 kg.

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Zn, Cu and Cd. This has been proposed to be a source of metals in urban street dust by Miguel et al. (1997). 4.6.4. Asphalt The wear of asphalt in Sweden is mainly due to the use of studded tires, which increase the wear and break the surface. The wear has decreased in recent years due to the introduction of tires with lightweight studs and more wearresistant pavements (Lindgren, 1998). The wear when non-studded tires are used is insignificant compared to the wear when studded tires (November–April, approx. 1y2 of the total traffic work) are used. The average wear on roads in Stockholm has been estimated at 4–6 gykm and vehicle (estimated to be the average in Gothenburg, where the speed was 70 kmyh) (Jacobsson and Hornwall, 1999). In Stockholm the stone material used for pavement is mostly acidic kinds of rock, such as granite, quartzite and porphyry. The average metal content of the stone material varies with the type of rock, and presenting an average heavy metal content for the pavements of Stockholm includes great uncertainties. A first approximation is presented in Table 3, using values from an acid rock (Alloway, 1990). The total emission is derived as follows: traffic work when studded tires are used (vehicle km)=wear (gykm and vehicle)=metal content (mgykg). 4.6.5. Tires There are many reports on the estimation of the wear of tires (Cadle and Williams, 1980; Brinkmann, 1985; Malmqvist, 1983; Kim et al., 1990; Muschak, 1990). In this study, 0.2 gykm for personal cars has been used (KemI, 1994). 4.6.6. Total emission from traffic Table 4 presents the total emission from the category ‘traffic’ using the information above. It is clear that the emissions from brake linings and asphalt dominate. Also, in the case of Zn, motor oils and tires are important. The total emission ¨ from asphalt is considerably lower than Bergback ¨ and Sorme, (1998) made for Stockholm (study year 1995). However, the emission has steadily

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decreased due to lightweight studs and more resistant pavements. 4.6.7. Total emission from traffic to WTP There are three environmental pathways for pollutants emitted by vehicles: (a) deposition on the road surface with possible subsequent removal by runoff waters or by resuspension of particulate material; (b) deposition in the immediate vicinity of the road or on the central reservation (median) of a motorway; or (c) dispersal in the atmosphere away from the road (Hewitt and Rashed, 1990), which is also shown in Fig. 3. The roads which have stormwater connected to Henriksdal (combined system) are located mostly in the central parts of the city, where most of the surfaces surrounding the roads are hard surfaces. This means that particles from the traffic system are deposited on hard surfaces to a large degree and, in turn, means that precipitation will transport the particles to the stormwater system and eventually to the WTP. Physiochemical properties of particulates, sizes, wind speed and direction, and precipitation determine the fate of particulates released from automobile sources. Wind direction has been assumed to be of less importance in a city with mostly tall buildings. Total annual precipitation during 1999 was very close to the total annual average (see Section 1), therefore no corrections for this have to be made. This leaves particle size as the most determining factor. In general, the abrasive particulate matter of brake linings has a particle size of less than 10 mm (Brinkmann, 1985). This is also confirmed by Garg et al. (2000), which concludes that 86% of the particles are less than 10 mm in diameter. These particles are, therefore, rather small. Smaller particles are assumed to enter the plumbing system less than larger particles do. Garg et al. (2000) states that 35% of the brake pad mass loss was emitted as airborne particulate matter. This means that 65% could be deposited to stormwater if all was deposited on hard surfaces and transported to the WTP. Here, it is assumed that 20% of the total brake lining emission (on roads connected to a combined system) will end up in the WTP. Legret and Pagatto (1999) conclude that the main emission of Cu is from brake

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Table 5 The total amounts (rounded numbers) (kgyyear) of heavy metals from the traffic goods that enter Henriksdal in stormwater. Source Brake linings Tires Asphalt Oil Total emission from traffic Total emission to the WTP % of the total load to the WTP from traffic goods

Cu Pb kgyyear (1999) 280a 0.2b 11–17c

42 0.4 21–32

290–300 5560

63–75 680

5

9–11

Zn

Cr

64 920 44–68 32–54d 1060–1100 10 290 10–11

Ni

0.4 ?

Cd 0.4

?

?

4–5

0.4–0.8

0.23 0.08–0.12

4–5 478

0.8–1.2 1000

0.31–0.35 25.6

-1

-1

1

a

Total emission (kg)=part to WTPs1400=0.2s280 kg. Total emission (kg)=part to WTPs0.4=0.4s0.2 kg. c Total emission (kg)=part to WTPs28–42=0.4s11–17 kg. d Total emission (kg)=part to WTPs160–270=0.2s32–54 kg. b

linings, but only 2% are found in runoff waters (data from a major rural highway). It is likely that less will enter the WTP from a highway than from a city environment, which explains the difference. It has been postulated that smaller particulates (20%) that stem from oil burning enter into the WTP. Gasoline has been assumed to be a negligible source of Cr, Cu, Pb, Ni, Zn and most likely also negligible for Cd (see Section 4), therefore it is not included in Table 5. In the case of tire wear, the particles are likely to be larger. This seems to be confirmed by research by Legret and Pagatto (1999), which states that the main sources of Zn are wear and tear from tires, from brakes and the corrosion of galvanized safety barriers. The authors found 37% of estimated amounts in runoff-waters. In this article it is hence assumed that 40% of the total emissions from tires will reach stormwater (combined system) and the WTP. Particles from asphalt wear are assumed to be of a similar particle size to tires, and therefore 40% is assumed to reach the WTP as well. The emission to WTP from traffic goods is shown in Table 5. 5. Stormwater—building materials The building material that has been quantified is galvanized steel and roofs made of copper. Galvanized steel is used on, for example, crash barriers, lampposts and roofs.

The total Cu emission from roofs in Stockholm was estimated at 1200 kg (Ekstrand et al., 2001). It was based on a surface area of 0.623 million m2 (Ekstrand et al., 2001) and a runoff rate at an interval of 2.0 gym2 per year (He et al., 2001). Ekstrand (personal communication) estimated from maps that 90% of the total area of Cu roofs is in the reception area of Henriksdal and that 65– 85% of the roofs are located where the plumbing system is combined. This yields emissions to Henriksdal as indicated in Table 6. It has been claimed that Cu gets caught on the way from runoff to the sewage system in concrete, for example. In this study, we have not found any scientific evidence of this. Therefore, it is assumed in this article that no (or negligible) amounts will get caught on the way from runoff to the sewage system. The average use of unpainted galvanized steel has been estimated at 1 m2 yinhabitant on buildings and 2.33 m2 on other constructions (Tolstoy et al., 1989). The average runoff rate on a panel, inclination 458, facing south was estimated at 3 gym2 per year (He et al., 2001). This runoff rate has been used, although it is, of course, a simplification. It indicates the magnitude of the emission. The reception area of Henriksdal includes municipalities other than Stockholm (see Fig. 2). Those municipalities have almost exclusively duplicated systems, which means that no Zn from galvanized goods will reach the WTP. This means that

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Table 6 The emission from buildings (rounded numbers) (kgyyear) and the buildings’ contribution (%) to total emissions to Henriksdal (1999) Source

Cu emission to the WTP

Zn emission to the WTP

Cd emission to the WTP

–

–

kgyyear (1999) Copper roofs Galvanized steel (buildings) Galvanized steel (other constructions) Total emission from buildings Total emission to the WTP % of the total load to the WTP from buildings

700–920a – – 700–920 5560 13–17%

750b 1700c 2500 10 290 24%

0.074 0.17 0.25 25.6 -1%

a

Runoff in Stockholm (kg)=part that is in the reception area of Henriksdal=part where the plumbing system is combineds 1200=0.9=(0.65y0.85)s700–920 kg. b Area galvanized steelycapita (m2 )=connected people in the reception area where the plumbing system is combined=runoff rate (gym2 year)s1=w(630 100y170 000)=0.54x=3s750 kg. c Area galvanized steelycapita (m2 )=connected people in the reception area where the plumbing system is combined=runoff rate (gym2 year)s2.33=w(630 100y170 000)=0.54x=3s1700 kg.

630 100–170 000 (persons in other communities)s460 100 persons who are connected where the system could be combined. In the reception area to Henriksdal, which belongs to Stockholm municipality, 54% is combined (Lagerqvist, personal communication). This means that it is an area of 830 000 m2 galvanized steel where the system is combined. Using the surface and runoff rates, total emissions can be calculated (see Table 6). Zn does contain traces of Cd. The Cd in galvanized materials is difficult to estimate, since it varies over time. If we use a Cd content of 100 ppm in spite of this uncertainty, the release of Cd from galvanized goods can be estimated (see Table 6). 6. Chemicals used in the WTP In the WTP chemicals are added which contain traces of some metals, mainly Cr, Ni and Zn. These contribute to the total amount in sludge and effluent water. In the case of Henriksdal, the following amounts were used in 1999: Cd-0.02 kg, Cu 0.7 kg, Cr 22 kg, Hg-0.4 kg, Ni 310 kg, Pb-0.1 kg, Zn 250 kg (Stockholm Water, 1999). From this one can conclude that the only metals in chemicals that have a significant contribution to the total amount to the WTP are Ni, which has

31%, and further Cr and Zn, which have 5% and 2%, respectively. 7. Discussion Table 7 shows the total findings of the sources to Henriksdal from Section 4. In the case of Cu, the sources seem to be well understood, since 109–113% of the measured amount has been accounted for. Here, one or a few sources might be overestimated. The dominant sources are from Cu pipes and taps in residences and businesses, and Cu roofs. The large emission from traffic goods is not reflected in the WTP; it seems to be further transported as aerosols to soil or recipient water. Particles emitted on soils could have a delayed transport to the WTP, but this is not accounted for in this article (see Pb below). In the case of Zn the sources also seem to be well estimated (see Table 7). Large contributors are residences (food, pipes and taps), other buildings (galvanized goods), and businesses (car washes). The value from buildings is very uncertain since the spatial distribution of galvanized goods is uncertain. Pb is dominated by the emission from businesses (car washes). It is unknown which goods are the actual sources, but it is most likely particles from brake linings and tires (which are known to emit

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Table 7 The contribution in % from different sources and the sum of all sources estimated to Henriksdal, (1999) Source (%) Sewage water

Household Businesses Drainage water Pipe sediments Stormwater Buildings Traffic Atmos. deposition Pipe sediments Within the WTP Chemicals Total from all sources (incl. chemicals) Part from sewage water Part from stormwater

Cu (%)

Zn (%)

Pb (%)

Cr (%)

Ni (%)

Cd (%)

Hg

59 30 2 – 13–17 5 -1 – – 109–113 91 18–22

30 27 4 – 24 10–11 2 – 2 99–100 61 36–37

1 38 1 q – 9–11 4 q – 53–55 40 13–15

2 13 2 – – -1 1 – 5 23 17 1

16 13 10 – – -1 1 – 31 71 39 1

20 32 3 – 1 1 4 – – 60 55 5

44–47 21 1 q – – – (q) – 66–69 66–69 –

A dash sign (–) indicates that the source is insignificant compared to other sources (in this study). A plus sign (q) indicates that the source probably is significant but the amount has not been estimated.

considerable amounts of Pb). The missing sources of Pb could be pipe sediments and Pb that have accumulated in the urban street dust and which slowly emits to the stormwater system. The latter has been proposed to be a source by Miguel et al. (1997). This could also be the case with other metals, since large amounts of heavy metals are released to the traffic environment. Different kinds of water from washing (household and industrial activities) have had elevated Pb values, which could be an important source (Lagerkvist, personal communication). The sources of Cr seems to be poorly understood or underestimated, since only approximately 23% of the sources have been explained. However, even for Cr, businesses (car washes) are the largest contributors. Large amounts of stainless steel have been accumulated in Stockholm. In 1995 it was estimated to be approximately 4500 tons of Cr and ¨ 2000 tons of Ni (Sorme et al., 2001b). Hence, here is a large potential for Cr and Ni emission, but published emission rates from stainless steel in applicable environments are scarce. Therefore, it is very difficult to estimate if this potential source is of importance or not. Stockholm Water believes that a possible source of Cr is from smaller enterprises (Lagerkvist, personal communication). The case of Ni is rather unique. The chemicals added in the WTP account for approximately 30%.

In the case of Cd the sources are rather well understood. Businesses (car washes) are the dominant source, followed by households (food and artist paint). The results show that traces of Cd originating from galvanized goods or traffic do not seem to be an important source (only up to 1%) when the Cd content of 100 ppm is assumed. And lastly, in the case of Hg, households (amalgam) are the largest contributor. The emission from businesses (dentists) is very uncertain. One important missing source is probably pipe sediments. By comparing with other research, results can be verified. The metal content in sewage water has been measured extensively in a residential area ¨ (Skarpnack) by Stockholm Water. The metal content is (mgyl); Cu 78, Zn 150, Pb 3.6, Cr 4, Ni 6.2, Cd 0.23, Hg 0.1 (Lagerqvist, personal communication). In Table 8 the contribution to the WTP from measured values in stormwater and sewage water is compared with estimated values in this article. In the case of sewage water, measured values are always higher than estimated (except Cu). The higher measured values seem to indicate that the household sources are still not understood or are underestimated. This is confirmed when comparing with research done by Comber and Gunn (1996) in England. The residential contribution of the total contribution to the WTP (washing, dishwashing, bathing and toilet)

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Table 8 Comparisons between estimated values in this article and measured values Metal

Stormwater contr. Estimated (%)

Stormwater contr. Measured (%)

Sewage water contr. Estimated (%)

Sewage water contr. Measured (%)

Cu Zn Pb Cr Ni Cd Hg

18–22 37 13–15 2 1 5–10

5–23 6–17 9–27 2–11 1–5 5–17

59 30 1 2 16 20 44–47

65 67 24 39 29 41 15

¨ before Measurements made by Stockholm Water; stormwater (Ekwall, personal communication), sewage water from Skarpnack any pumping station (Lagerkvist, personal communication). Sewage water is here equivalent to household contribution, since measurements are made in a residential area.

was measured to (this article’s findings are in parentheses after each of Comber and Gunn’s findings): Zn, 46% (30%); Cu, 64% (59%); Pb, 17% (1%); Ni, 26% (16%); and Cr, 15% (2%). Plumbing systems partly made of Pb in England could be one explanation for the discrepancy in the Pb values. One missing source of Cr and Ni in households could be stainless steel, which emits Cr and Ni. Comber and Gunn (1996) state that ‘Data for hand dishwashing indicated no significant increase over tap-water levels for Cu but relatively high concentrations for Ni and Cr; release from metal cookware during cleaning may be a possible contributory factor’. Medicated shampoos contain Zn, which is one explanation for the missing Zn amounts, also shown by Comber and Gunn (1996). The plumbing system itself could be another important contributor (only Cu and Zn are accounted for in this article) which is shown in Koch and Rotard (2000). Further measurements in Stockholm are needed to ascertain if this is a possible source. In the case of Ni, drinking water is important as a source, but that is due to the metal content from the source of water supply ¨ (lake Malaren). Metal concentrations in stormwater are very variable. Therefore, it is very difficult and uncertain to give an average value, but the following values are a first approximation (based on extensive measurements): Pb, 15–45 mgyl; Cd, 0.3–1 mgyl; Cu, 70–300 mgyl; Zn, 150–400 mgyl; Cr,

2–12 mgyl; Ni, 2–2 mgyl (Ekwall, personal communication). In Stockholm, streets are ‘vacuumed’ in dry weather, which means that particles with heavy metals are transported away from the WTP. When this is the case, the estimations to what degree the particles enter the WTP might be too high (see Section 4.6.7). Since the estimations are higher (as for Zn, see Table 8) or of the same magnitude (Cu, Pb, Cd), the sources seem to be identified. The estimations of the amount that reaches the WTP might be too high, which could explain the discrepancy in Zn values. By these comparisons it appears that the sources to stormwater seem to be rather well understood and rather well estimated. 8. Conclusion Results show that it was possible to track the sources of heavy metals for some metals such as Cu and Zn (110% and 100% found, respectively), Ni and Hg (70% found). For the other metals, sources are still poorly understood or underestimated (Cd 60%, Pb 50%, Cr 20% found). Mostly for these last three metals, further research is needed. Contributors to sewage water are households, drainage water, businesses and pipe sediments. In sewage water, households are an important source of Cu (tap water systems and food), Hg (amalgam) and Zn (food and tap water systems). Households are, according to measurements made, also

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an important contributor of Pb, Cr, Ni and Cd, but here not all the sources have been identified or the sources known are poorly estimated. Drainage water had only a minor contribution to sewage water for all metals. Businesses include large enterprises, and the known emission from those was -4% for all of the metals studied. Car washes were an important contributor of Pb, Cd, Cr and Zn. Pipe sediments are probably important for Pb and Hg, but actual estimations have not been possible to make. Contributors to stormwater are pipe sediments, atmospheric deposition, traffic and building materials. There seems to be an agreement between measurements (although very uncertain) and estimations made. One interpretation is that the sources are rather well identified and well estimated. Atmospheric deposition and traffic have a relatively minor contribution. Building materials are an important source of Zn (galvanized goods) and Cu (roofs). If the WTPs (here exemplified by Stockholm Water) want to decrease the metal load on a shortterm basis, non-built-in or ‘loose’ metals in their control must be tackled. This is the case only with chemicals (which are responsible for approx. 30% of the Ni load). Another short-term change (but where the WTPs might not have control) is to decrease the load from, for example, artist paint, enterprises (if due to the process). Goods that are built in the society and that often have a long lifespan cause more difficulty. Here two different strategies can be taken; firstly, different types of treatment and secondly, decreasing the metal content in goods. Treatment seems to be applicable to stormwater or car washes, for example. Examples of goods in the second strategy, which would influence the metal content to the WTP, are among others amalgam, pipe materials, roof materials, tires and brake linings. Treatment and decreasing of metals content in goods must be seen as longterm strategies, and the WTP itself might not have the power to enforce these changes. There seem to be rather few goods and activities that can be changed to decrease the metal load to WTP on a short-term basis (with the exception of Ni). In order for the WTPs to decrease the metal load, the solution seems to be to establish a long-term

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