310
G. Söderlund
Water sampling
An automatic sampler came into use when the rainfall intensity exceeded 1 mm/hour.
Usually one sample was taken every fifteen minutes. The water was shared between four
bottles, one each for general analysis, oil analysis, BOD and bacteriological examination.
The sampler stopped automatically one hour after the rain had ceased. It was loaded with
24 batches each of 4 bottles. Since the study was begun in January, 1969, samples from
about 50 rainfalls have been collected and analysed. The rainfall intensity ranged from 1
to 25 mm/hour. Samples were also taken during the spring thaw.
Analytical methods
Chemical and bacteriological analysis were conducted according to the "Standard
Methods". Oil was determined by extraction with purified carbon tetrachloride followed
by infrared absorption measurements. Pesticides were extracted with benzene and
analysed by gas chromatography using an electro-capture-detector. Heavy metals were
assayed by atomic absorption measurements on acidified water samples.
RESULTS AND DISCUSSION
The results of the analyses, expressed as the arithmetic mean, standard deviation and
maximum value are presented in Table 1.
Comparison between the three sampling areas
As was found in a previous study, the highways seem to produce a much greater
contamination than the other test areas. If the run-off coefficients are taken into
consideration, this difference is even more pronounced, as is seen in Table 2 showing the
discharges from the three sampling areas. As both the number of cars and the area
covered by roads are on the increase a rapid rise in the contamination from this source
can be expected.
Comparison between pollution from treated sewage and urban storm-water run-off
The amounts of contaminants in biologically and chemically treated sewage are given
in Table 3 together with the corresponding values for the storm-water run-off.
The storm-water run-off contains considerably more of the suspended solids than the
treated sewage. The amounts of BOD, phosphorus and nitrogen are less than in treated
sewage.
In this calculation the arithmetic mean calculated over one year has been used. This
method is valid for treated sewage discharging into the recipient all the year round, but it
should be borne in mind that the rainfall is limited to about 7 per cent of the year (600
hours in Stockholm with 555 mm, of which 120 mm is in the form of snow) and all the
storm-water run-off discharges during this relatively short time.
The 600 hours' contribution by treated sewage and the contribution by the
storm-water overflow, given in Table 4, are calculated on the assumption that 1 per cent
of the total amount of domestic sewage in a year is overflow. In this case the contribution
by the urban storm-water run-off is about 20 times the amount of suspended solids, twice
the amount of BOD and 15 per cent of the amount of phosphorus and nitrogen in the
treated sewage. The storm-water run-off contains a considerably greater quantity of
suspended solids, about the same amount of BOD and nitrogen, and 10 per cent of the
amount of phosphorus in the storm overflow. Since storm overflow from combined
G. Söderlund
Water sampling
An automatic sampler came into use when the rainfall intensity exceeded 1 mm/hour.
Usually one sample was taken every fifteen minutes. The water was shared between four
bottles, one each for general analysis, oil analysis, BOD and bacteriological examination.
The sampler stopped automatically one hour after the rain had ceased. It was loaded with
24 batches each of 4 bottles. Since the study was begun in January, 1969, samples from
about 50 rainfalls have been collected and analysed. The rainfall intensity ranged from 1
to 25 mm/hour. Samples were also taken during the spring thaw.
Analytical methods
Chemical and bacteriological analysis were conducted according to the "Standard
Methods". Oil was determined by extraction with purified carbon tetrachloride followed
by infrared absorption measurements. Pesticides were extracted with benzene and
analysed by gas chromatography using an electro-capture-detector. Heavy metals were
assayed by atomic absorption measurements on acidified water samples.
RESULTS AND DISCUSSION
The results of the analyses, expressed as the arithmetic mean, standard deviation and
maximum value are presented in Table 1.
Comparison between the three sampling areas
As was found in a previous study, the highways seem to produce a much greater
contamination than the other test areas. If the run-off coefficients are taken into
consideration, this difference is even more pronounced, as is seen in Table 2 showing the
discharges from the three sampling areas. As both the number of cars and the area
covered by roads are on the increase a rapid rise in the contamination from this source
can be expected.
Comparison between pollution from treated sewage and urban storm-water run-off
The amounts of contaminants in biologically and chemically treated sewage are given
in Table 3 together with the corresponding values for the storm-water run-off.
The storm-water run-off contains considerably more of the suspended solids than the
treated sewage. The amounts of BOD, phosphorus and nitrogen are less than in treated
sewage.
In this calculation the arithmetic mean calculated over one year has been used. This
method is valid for treated sewage discharging into the recipient all the year round, but it
should be borne in mind that the rainfall is limited to about 7 per cent of the year (600
hours in Stockholm with 555 mm, of which 120 mm is in the form of snow) and all the
storm-water run-off discharges during this relatively short time.
The 600 hours' contribution by treated sewage and the contribution by the
storm-water overflow, given in Table 4, are calculated on the assumption that 1 per cent
of the total amount of domestic sewage in a year is overflow. In this case the contribution
by the urban storm-water run-off is about 20 times the amount of suspended solids, twice
the amount of BOD and 15 per cent of the amount of phosphorus and nitrogen in the
treated sewage. The storm-water run-off contains a considerably greater quantity of
suspended solids, about the same amount of BOD and nitrogen, and 10 per cent of the
amount of phosphorus in the storm overflow. Since storm overflow from combined