Stockholm

In order to get a general overview of the metal fluxes to and from the STPs in Stockholm, as well as their relative significance compared to, for example, the total metal flow from Lake Mälaren through the Stockholm Stream to the Baltic Sea (according to Lindström et al., 2001), or the total emissions of the same metal from the metal stock, a compilation of data from Bergbäck et al. (2001) has been made i Table 3.10.

Metal fluxes to the water recipient (A) are greater than the difference between the influx to the STPs and the amount removed to the sludge, because of the portion of stormwater that is discharged directly to the water recpient and the minor metal fluxes with groundwater (see Aastrup and Thunholm, 2001). According to Bergbäck et al. (2001), the percentage of metals removed to sludge was 80-90% for chromium and copper, but only 67% for zinc. The removal efficiency for nickel cannot be calculated from the data, because some nickel input with chemicals used in the STP disturbs the picture. It is questionable to regard the metal fractions ending up in sewage sludge, which is either recycled in the farming system (by all standards a man-made, artificial system) or disposed of in landfills, as being emitted to the biosphere (the natural ecosystem). This should relate only to the metal fractions that are emitted to the water recipient.

Table 3.10. Flows of metals in Stockholm: the influx of metals to the STPs, the effluxes via sludge and treated sewage, including stormwater and groundwater (emitted to surface waters, A), fluxes through the Stockholm Stream (B) and calculated fractions of metals emitted compared to total transports in the recipient and compared to total emissions from the metal stock in Stockholm, respectively. (Data from Bergbäck et al., 2001).

Metal flux, t/y To STP In sludge To recipient From L. Mälar A/B, % A/emiss.,%

Chromium

1.1

0.9

0.40

1.0

- 1.4

29

40

47

Copper

10

9

2.63

11 -

18

15 -

24

24

Nickel

2.2

0.8

1.53

10 -

11

14 -

15

235

Zinc

18

12

9.12

17 -

22

41

54

38

In absolute numbers, the metal (among those presented in Table 3.10) showing the greatest flux from the anthroposphere of Stockholm to the water recipients around the city is zinc, followed by copper and nickel, while the flux of chromium is much less. However, when these fluxes are compared with the large-scale flux of metals from Lake Malaren to the Baltic Sea, the contribution from the city of Stockholm is 15-25% for copper and nickel, 3040% for chromium and 40-55% for zinc. If, on the other hand, the metal discharges to the water recipient are compared with the total emissions from the metal stock in Stockholm, the input to the aquatic ecosystem represents 24% for copper, 38% for zinc, while the figure is higher for chromium. For nickel, those comparisons are not relevant, because the nickel emissions from the metal stock are very low compared to the influx via atmospheric deposition and food and, moreover, the direct input of nickel to the STPs tends to invalidate the comparison.

Table 3.11. Contribution of metals from different sources (%) to the STP of Henriksdal in 1999. After Sorme and Lagerkvist, 2002.

Major route Source Cu Cr Ni Zn

Sewage

Households

59

2

16

30

Businesses

30

13

13

27

Drainage

2

2

10

4

Stormwater

Buildings

13-17

-

-

24

Traffic

5

<1

<1

10-11

Atm. deposit.

<1

1

1

2

Added in STP

Chemicals

-

5

31

2

Total, all sources,

%

109-113

23

71

99-100

Total load, kg/y

5560

478

1000

10290

In a later paper, Sorme and Lagerkvist (2002) made a detailed inventory of all the various sources of trace metals that were transported to the biggest STP in Stockholm, Henriksdal. Compared to the previously reported data (e.g. Table 3.10), the percentage of metals entering the Henriksdal STP in 1999, out of the total metal influx to all STPs in Stockholm was, for copper, 56%, for chromium, 43%, for nickel, 45% and for zinc 57%. By making independent estimates of the contribution by every significant source to the mass flow of each of the metals, it was possible to get a general picture of how well relevant sources of metal fluxes were tracked down. The overall results of the estimates for the 4 actual metals are shown, first, in percent of the total amount of metals entering the Henriksdal

STP (Table 3.11), and secondly, in kg/year for the various sources (Table 3.12).

If, based on the figures in Tables 3.11 and 3.12, we compare the rather well-known amounts of incoming metals to the Henriksdal STP, for the year 1999, with the sum of the individual fluxes from all the identified sources, we can find that there is an excellent correspondence in the case of zinc. This was interpreted as practically all the relevant sources of zinc emissions in the reception area of Henriksdal were identified and quantified (Sorme and Lagerkvist, 2002). In the case of chromium and nickel, the correspondence was poor, with only 23% and 71%, respectively, of the incoming metal (to the STP) being accounted for. In the case of copper, it is interesting to notice that the sum of the individual fluxes is greater than the measured total influx to the STP, i.e. some 109-113% of the incoming copper was explained.

Table 3.12. Average emissions in the year 1999 (kg/year) of the four metals, copper, chromium, nickel and zinc, from various sources in Stockholm, within the catchment of the STP of Henriksdal. Estimated according to methods described by Sorme and Lagerkvist, 2002.

Major source

Origin

Cu

Cr

Ni

Zn

Households

Food

275

7

19

2507

Drinking-water

140

5

141

93

Water pipes, etc.

2885

-

-

500

Businesses

Large enterprises

87

17

32

200

Car washes

300

42

39

2300

Drinking-water

59

2

59

39

Water pipes, etc.

1200

-

-

200

Traffic

Brake linings

280

0.4

0.4

64

Tyres, asphalt, oil

15

4

0.6

1020

Infrastructure

Buildings, roofs, etc.

700-920

-

-

2450

Other sources

Drainage, atmos.,chem.

160

38

420

820

Sum of individual contributions

6100-6320

115

710

10190

This phenomenon with a surplus of copper emissions would merit a brief analysis. First, it can be pertinent to compare some of the emission figures in Table 3.12 with emissions estimated in earlier published reports. Looking at the greatest contribution, releases from tap water pipes, heat exchangers and other plumbing materials, originating from both the housing and business sectors, together 4,085 kg/year, it can be noted that this figure is clearly higher than a similar estimate of copper corrosion in the Henriksdal catchment, made for the year 1995, which came to 3,770 kg/year (Landner et al., 2000). The new figure for 1999 is also in poor correspondence with the figure given by Bergbäck et al. (2001) for the whole of Stockholm, 4,300 kg/year. Because Henriksdal deserves about 67% of the total population in Stockholm, and assuming that copper plumbing materials are approximately evenly distributed over the whole urban area, the fraction of the total copper emission from water pipes going to Henriksdal would be about 2,890 kg/year, and not 4,085 kg/y as estimated by Sörme and Lagerkvist (2002).

The traffic sector contributes with a surprisingly low amount of copper in the account made by Sörme and Lagerkvist (2002), i.e. with less than 300 kg/year (Table 3.12). This may seem to be a unreasonably low figure, considering that Bergbäck et al. (2001) reported the total copper emission from traffic in Stockholm to be about 4,100 kg/year. However, Sörme and Lagerkvist start from an estimate of about 1,440 kg/year for the emission from the traffic in the reception area of Henriksdal, where the stormwater collection system is combined with the household sewage collectors. This figure corresponds to the estimate made by Landner et al. (2000) for 1995. Thereafter the authors set out for a reasoning that is somewhat difficult to follow: They start by noting that "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." Meaning that deposited particles from the traffic system to a large degree are transported to the stormwater system and eventually to the STP. In spite of this consideration, they later assume that only about 65% of the brake pad mass loss could be deposited to stormwater (the remaining part obviously remaining as airborne particulate matter). To be sure, Sörme and Lagerkvist, do not even use the latter fraction in their final calculation, but according to a second assumption, they state that only 20% of the total brake lining emission will end up in the STP. Therefore, the traffic contribution to the copper transport to Henriksdal would be only 5% (295 kg/year) of the total.

When Sörme and Lagerkvist (2002) discuss the contribution from copper roofs, however, they use quite a contrasting assumption, namely that there is no retention whatsoever or no losses to other compartments. All the copper released from the roofs situated within the Henriksdal reception area, where there are combined sewer systems (65-85% of the total sewer system area), i.e. 700-920 kg/year, would end up in the STP, according to the authors. They even point out that they have found no scientific evidence of any retention of copper in e.g. sewers made of concrete. Obviously, the authors are unaware of the experimental work carried out by Bertling and others at the Royal Institute of Technology in Stockholm, who have shown that the retention of copper occurring in fresh runoff from copper-clad roofs in soil or concrete varied from 18 to 99.8% (typically in the range 80-90% of Cu retention) (Bertling et al., 2002b). This indicates that runoff water passing a street gutter and/or a collector made of concrete will be significantly impoverished in both free cupric ions and in total copper. It is therefore very abstruse why Sorme and Lagerkvist (2002) have chosen to postulate that 100% of the copper emitted with runoff water from roofs will reach the STP, while only 20% of the copper emitted by traffic in urban streets will have the same fate.

As a matter of fact, the correspondence between total influx to the STP and the sum of the various emissions, that seemed to be very good for copper and zinc at first sight, might not in reality be that good at all, because the conformity with several of the figures presented by other researchers participating in the project "Metals in the Urban and Forest Environment" is poor. However, if a more realistic (higher) figure for the contribution from traffic to the copper transport to Henriksdal is given, and necessary revision of the contributions from water pipes and roofs are made, it is quite possible that the individual fluxes will sum up to close to 100% for copper, as was the case for zinc.

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