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Origins and characteristics of urban wet weather pollution in combined sewer systems: The experimental urban catchment “Le Marais” in Paris
~
Pergamon
T~ch.
Sci. Vol. 37. No. I. pp. 35-43.1998. =Wal. 1998 IAwQ. Published by Elsevier ScienceLId Printedin GrealBritain.
PH: S0273-1223(97)OO753-1
0273-1223/98 $19'00 + 0-00
ORIGINS AND CHARACTERISTICS OF URBAN WET WEATHER POLLUTION IN COMBINED SEWER SYSTEMS: THE EXPERIMENTAL URBAN CATCHMENT "LE MARAIS" IN PARIS Marie-Christine Gromaire-Mertz, Ghassan Chebbo and Mohamed Saad CERGRENE. EcoleNationale des Pontset Chaussees, 6-8 avo Blaise Pascal. Cite DescartesChampssur Marne, 77455Marne/a Val/ee Cedex 2. France
ABSTRAcr An experimental urban catchment has been created in the centre of Paris. in order to obtain a description of the pollution of urban wet weather flows at different levels of the combined sewer system. and to estimate the contribution of runoff. waste water and sewer sediments to this pollution. Twenty-two rainfall events were studied from May to October 1996. Dry weather flow was monitored for one week. Roof. street and yard runoff. total flow at the catchment outlet and waste water were analysed for S5. V5S. COD and BODS. on both total and dissolved fraction. Results show an evolution in the characteristics of wet weather flow from up to downstream : concentrations increase from the catchment entry to the outlet, as well as the proportion of particle-bound pollulants and the part of organic matter. A first evaluation of the different sources of pollution establishes that a major part of wet weather flow pollution originates from inside the combined sewer. probably through erosion of sewer sediments. © t 998 IAWQ. Published by Elsevier Science Ltd
KEYWORDS Combined sewer; field experiments; pollution; urban runoff; sewer sediments; waste water.
INTRODucrION Previous research (Bachoc, 1992; Chebbo, 1992; Saget, 1994) showed that urban wet weather flow pollution is far worse in combined sewers then in storm sewers. Average suspended solid concentrations calculated Over a year are 50% higher in combined sewer discharges (240 to 670 mgll) than in storm sewer discharges (160 to 460 mg/l). For BOD5 , the average concentration in combined sewer discharges (90 to 270 mg/l) is more than twice that of storm sewers (13 to 130 mg/l). Combined wet weather flows are generally more loaded in organic matter: the VSSISS ratio is from 10% to 36% for storm sewers, whereas it is from 24% to 55% in combined sewers . Moreover, suspended solids from combined sewers have lower and more varying settling velocities than those from storm sewers.
3S
36
M.-C. GROMAlRE·MERTZ et al.
As shown by Krejci et al. (1987), Bachoc (1992) and Chebbo (1992), the presence of waste water alone does not explain these differences which are mainly linked to the erosion of sewer sediments. These authors gave a first appraisal for the contribution of runoff, waste water and sewer sediments to the pollution of combined sewer discharges. Their evaluations were based on measurements at the outlets of both combined and separate sewers, not on simultaneous measurements at different levels of the same catchment. Results were given either at the scale of the year or for very few rain events. Thus, the pollution from these different sources has not been characterised and information available on the variability of these contributions depending on rain event characteristics is scarce. Because of this, a new research program was started at CERGRENE in 1994, financed by the Water Agencies, the municipality of Paris, the French Ministries of Research and of Public Works and the lie de France county. It aims to fit out an experimental urban catchment provided with combined sewers, in order to study the pollution of wet weather flows at different levels of the water cycle. This will enable a better understanding of the characteristics of the pollution transported in combined sewers and the evaluation of the contribution of different sources to this pollution for a large number of rainfall events. This article describes the experimental methods used for our research and the first results obtained. EXPERIMENTAL METHODS The experimental catchment The experimental catchment "Paris-Le Marais" is situated in central Paris, in an old residential district with small businesses . Its small size (42 hectares) makes it possible to visit it entirely and to describe it in detail. Roofs represent 54.4% of the catchment area and streets 22.4%. The last 23% are taken by yards, gardens and squares. The mean slope of the catchment is of 0.84%. The storm runoff-coefficient is about 0.78. The population is dense: 295 inhabitantslhectare. This catchment is drained by a combined, Parisian type, sewerage system. All sewers are man entry and equipped with non selective gullies. The sewerage system includes three ovoid trunks with sidewalks, with a total length of 1.8 krn, and about fifty egg-shaped collectors with a total length of 5.8 km. The mean slope of the collectors is of 0.8% whereas the slope of the trunks is less then 0.1%. Street gutters are swept every day. Sidewalks and gutters are washed two to five times a week with a pressurised water jet. Most streets are sucked up with a dust-collector every day, excepted on week ends, with a dust collector car.
0: c:afdulIenf DUtlet
s: .treet runoff.ampUni
Y: yard nmofl' .ampliD&
R: rooCrunoff'.ampUne
G: niDlallles Figure 1.Location andmap of theexperimental catchment with sampling sites.
Urban wet weather pollut ion on combined sewer systems
37
SampHni sites and eQuipment The experimental catchment was provided with equipment allowing us to characterise and to quantify the pollution of surface runoff at the entry of the sewerage system and the pollution of the total flow at the catchment outlet during dry and wet weather. Sampling sites for street runoff were chosen using a statistical survey. All streets in the catchment could be classified into seven groups, depending on features relative to the accumulation and the wash-off of pollutants (size, road traffic, frequenting, commercial activities , slope, covering ...). Four gully holes, situated at street comers were monitored (Figure I). Each one collects runoff from two streets belong ing to two different groups , all in all eight streets belonging to seven different groups. For financial reasons we studied only 3 streets out of the 7 at each rain event. Equipping a street gully hole (Figure 2) consisted of separating waters collected on each side of the gully by a wall, canalising the water, removing coarse solids (larger than some centimetres), measuring the flow and taking flow weighted samples. The flow is measured either with a tipping bucket flow meter or with a triangular weir. Two automatic vacuometric water samplers are used at each gully: the first one with 24 bottles of I litre in order to establish pollutographs, the second one with a 25 litres container in order to measure settling velocities. These water samplers are driven both by the flow meter and a conductivity sensor : sampling starts when a flow with a conductivity less then 450 IlS/cm2 is measured in the gully (which stops samples of street washing waters being taken), sampling rate is flow proportional. Sep .... ion ",.11 .nd • •'".1 ....__..
SEWE R
GULLY CO
NECTION
"." EJ S.mpl. "
Sepuation and wat er
• • nllizin,
~:=:~=r::d;:;;:::7\
S iphon Condu c.ivilV se nior
Figure 2. Street runoff monitoring equipment.
Roof runoff is sampled from four roofs (Figure I) with different coverings: zinc, slate, interlocking clay tiles and flat clay tiles, which correspond to the main types of covering in our catchment. These samples are taken in the roof gutters and collected in 100 litres containers . Three yards (Figure I) were filled out with automatic water samplers . They have been chosen so as to be representative of our catchment. One is paved with stones, the second one is concreted and planted with trees, the last is permeable with grass and gravel covering . At the outlet of the catchment (Figure 1), flow is monitored with an "Ultraflux" flow meter and recorded every two minutes. This apparatus uses a time of flight ultrasonic system to measure in-sewer velocity on
38
M.-C. GROMAIRE-MERlZ et al.
four horizontal lines in the cross section. The water level is measured both with an ultrasonic and a pressure sensor. Flow weighted samples are taken with two automatic vacuometric water samplers, driven by the flow meter and started when the water level exceeds the maximum dry weather level. The first sampler contains 24 bottles of 2.8 litres each. It is used to establish pollutographs. The second sampler takes a mean sample with a volume up to 70 Iitres, it is used for settling velocities. Rainfall is measured on the catchment with two tipping bucket raingauges. Measurements Table I indicates the type of measurements performed for each kind of sampling site. Table I. Kind of measurementsperformed on the different experimental sites Roofs
Yards
mean concentrations
mean concentrations
Streets hydrograph polJutographs mean concentrations settlmz velocities
Outlet hydrograph pollutographs
mean concentrations settling velocities
All samples are analysed for suspended solids (SS), volatile suspended solids (VSS), chemical oxygen demand (COD). biological oxygen demand (BOD5), heavy metals (lead. zinc. cadmium) and hydrocarbons. both on the total and on the dissolved fraction (after filtration at 0.7 11m). For street runoff and for the total flow these analyses are also performed on the different classes of settling velocities. Flow from yards and roofs is calculated theoretically. This paper presents results for SS. COD, BOD5, VSS event mean concentrations and for the distribution between dissolved and solid fractions. RESULTS AND DISCUSSION Characterisation of waste water Dry weather flow has been characterised at the outlet of the catchment for six different days of the week. between the 15th and the 27th July 1996. Each of these days, 24 hourly mean samples were taken and analysed for SS, VSS. COD and BOD5. The total daily flow on the catchment represents 565 l/head/day, which is very high compared to the figure of 100 to 300 l/head/day given by Saget and Chebbo (1996). This high rate of water production can be explained by the importance of street washing, business (restaurants, trade. offices) and the probable contribution of parasite waters. Daily figures for the pollutant load of waste water are those usual in France (Table 2). Table 2. Mean load for waste water
55 Paris-Le Marais Degremont (1989), Chebbo (1992), Chebbo et at (1995)
Il!!headlday 70
ml!!1 125
70 to 90
100to 500
BOD5 mg/I ' !!Iheadldav 84 150 60 to 80
100 to 500
COD mg/I CODIBOD5 194 290 2t02,5
VSS %
83
200101000 65 to 85
Urban wet weather pollution on comb ined sewer system s
39
The particulate fraction represents 70% of the daily COD and BOD5 load. This percentage is higher at the daily peak flow (80%) and less important at night when up to 50% of the pollut ion is in the dissolved fraction. As illustrated in Figure 3, concentrations and flow rate show a well marked daily pattern . Maximum values are reached between 10 and I I in the morning, with a smaller peak in the afternoon. Slight differences are measured between weekdays and weekends: the pollution load is 15% lower during the weekend and flow decreases by 6% on Sunday only. Suspended solids , COD, BOD5 and VSS variations are correlated to the daily flow variations, but concentrations vary in a wider range (23 to 225 mgIJ for SS, i.e. from 1 to 10) than flow (0.047 to O. I24 m 3/s , i.e. I to 3). This phenomenon can be partly linked to a dilution of domestic sewage with infiltration water, the rate of dilution depend ing on the sewage flow. It probably also involves the settling of particles at night when the energy of the flow is too low for their transportation and the erosion of sewer sediments during peak flows (Verbanck , 1990). Yet, differences in the qua lity of sewage produced at different times of the day , due for instance to professional activities, are not excluded. Dry weather flow and SS concentration
.
500 400
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Figure 3. Daily variation of 55 con centrations and n ow du ring dry weather.
Characterisation of wet weather pollution
Characteristics of the studied rain events. Rain events have been defined as rainfalls with a minimum depth of I mm and separated by at least 30 minutes without any rain recording. Between May and October 1996, we studied twenty-two rain events with a wide range of rain depth and intensities (Table 3). Table 3. Rain event characteristics minimum maximum median rain depth (rnm) 2 14,6 5,6 mean intensity (mm/h) 1,4 42 4,7 maximum intensity (mm/h) * 2,7 180 21,2 rain duration (h) 00:10 07:30 00:45 dry weather period (days) 0,03 30 0,9 *: calculatedon the duration between two successivetippings
Mean samples were collected for almost all these events from the outlet , one street and four roofs . For some of these twenty-two rain events (Table 4), samples were also collected from two other streets and three yards . For roof runoff, very closed rainfalls were collected in the same samp le.
40
M.-C. GROMAIRE-MERTZ et al.
Table 4. Numberof rainfall events studied for each sampling site Roof runoff Industrialtiles I Flal tiles I Zinc 18 I 15 I 18
I Siale I
18
Yard runoff Street runoff Concrete I Stone I Gravel Street 1 I Street 2 I Street 3 20 I 3 I 6 I 4 I 5 9
Outlet
21
Event mean concentrations (Table 5). For all types of runoff. BODS concentrations were found to be very low whereas 55 and COD concentrations were very variable from one event to another. The part of volatile solids (12 to 70%) was far more important than what is usually admitted for surface runoff (10% for street solids according to Sartor et al.• 1974; Butler. 1992; Xanthopoulos and Hahn. 1993). Table S. Event mean concentrations measuredon the different samplingsites 5S mg/I minimum- mean maximum value 56 industrial tiles 710211 81075 37 Roof fiat tiles 71013\ 46 runoff zinc 8 t091 27 slate Yard runoff Street runoff Outlet
concrete
pavement gravel Street 1 Street 2 Street 3
31 to 70 II to 38 32 to 490 57 to 497 41 to 206 10 to 181 105 to 559
45 24 20/ 242 78 79 307
CODm-gfJ minimum- mean maximum value 9 to 120 32 38 15t091 49 910111 510198 34 59 to 182 29 to 71 42 to 211 12410964 S6 to 171 2S to 94 123 to 736
/23 43 89 377 /0/
59 428
BODS ml!l1 minimum- mean maximum value 5 3 to \3 4to 22 6 4 to 31 7 3 to 42 7 13to 47 6 to 16 8 to 27 28 to 160 16t032 14 to 20 67 to 296
27 /0
/8 82 24 17 /81
%VSS minimum- mean maximum value 28 t4 to 44 43 32 to 66 43 33 to 59 37 28 to 62 60 to 77 75 44 to 70 62 12 to 27 J7 49 to 70 59 41 to 67 56 41 to 61 47 53 to 72
66
Roof runoff was usually little concentrated. but for some events 5S concentrations up to 210 mg/l and COD concentrations up to 198 mg/I were measured. Good linear relations could be established (Table 6) between S5 concentrations from roof runoff and the following rain event characteristics: dry weather duration (OW), average rain intensity (Iav), maximum rain intensity (lmax) and rain duration (D). There was no significant variation from one roof to another for SS. COD. BODS. Table 6. Multiple linear regression between roof runoff and rainfall characteristics Type of roof Industrial clay tiles Flat clay tiles Zinc Slate
R1
Multiple linear regression 0,86 rSS1= 1,16 OW +4.76 lay - 38 77 0 + 0.13 Imax - 0.92 0.91 rSS]= 1.05 OW -0,72 Iav - 120700+019 Imax + 38,13 085 IISS]= 1,02 OW + 3.90 lav- 78.85 0 + 0.05 Imax + 14.45 0.76 [SS)= 0.72 OW + 3.24 Iav- 59.39 D - 0.16 Imax +15.02
For yard and street runoff, results were very different from one site to another. depending on the land use. Ground wash-off on the gravel covered yard lead to high SS concentrations with low VSS rates. Differences between the concreted and the paved yard are linked to the presence of trees and birds on the concreted yard and to the difference of cleaning practices. Street 2 and 3 gave similar results but extremely high concentrations (500 mg/l for SS. 960 mg/l for COD and 160 mgll for BODS) were measured on street I, which is a small and crowded street with a lot of snack bars. On this site. correlation between SS concentrations and rainfall characteristics gave poor results. Nevertheless. the pollution load seems to grow with the dry weather period (R = 0.49). Surprisingly. no correlation was found between SS concentration in street runoff and the period since the last street washing.
Urban wet weather pollution on combined sewer systems
41
Concentrations measured at the outlet of the catchment are similar to those reported by Saget and Chebbo (1996) for several combined sewers in France. They vary with rainfall characteristics but also with the time at which the rainfall occurred: lower values are measured at night when waste water is scarce. For diurnal rain events, 55 concentrations were found to be correlated to the rain depth (R -0.75), the rain duration (R -0.62) and the previous dry weather period (R 0.46).
=
=
=
It should be observed that event mean concentrations measured at the catchment outlet were usually equivalent or superior to that of waste waters at the same time of the day, whereas runoff was in average far less concentrated. This supposes a contribution of sewer sediments.
Distribution between dissolved and particle-bound pollutants. Table 7 gives the distribution between particulate and dissolved COD and BOD5 loads. At the catchment outlet, 70 to 90% of the total load is linked to particles, which corroborates results from previous studies (Chebbo, 1992). For runoff the contribution of the dissolved fraction is much more important and increases from streets to yards and roofs. Yet the distribution between dissolved and particulate fractions was very inconstant in the runoff and variations of the particulate fraction from 30 to 80% from one sampling site or rain event to another have not been explained. The evolution of the particulate fraction from the sewer entry to the catchment outlet can be explained both by physical (erosion of sewer sediments) and chemical (adsorption of pollutants on particles) reactions inside the sewer system. Table7. Distribution between dissolved and particle-bound pollution loads %
Roof runoff Yardrunoff Streetrunoff Outlet Ori~ins
minimum 34 38
24 72
particulate COD mean maximum 58 86 56 83 68 88 83 92
%
minimum 17 37 50
71
particulate BOD5 maximum mean 48 76 57 82 66 93 82 91
of wet weather pollution
We can identify three sources of pollution in combined wet weather flows: urban runoff from streets, yards and roofs, waste water and sewer sediments. The contribution of these sources has been evaluated for five rainfall events, for which we had results on almost all sites. Characteristics of these rain events are given in . The following method was used: pollution mass in runoff (MPR): - for each rain event, the average of the concentrations measured on the four experimental roofs was assigned to the total roof surface; and the average of the concentrations measured on the three experimental streets was assigned to the total surface of streets; - as for these rain events no measurement was available for yard runoff, an average concentration has been calculated on all rain events for which we had measurements and assigned to the whole surface of yards; - the volume of runoff was calculated using theoretical runoff coefficients. It is probably overestimated, as it is superior to the difference between the total flow at the outlet and volume calculated for waste waters; mass of pollution from waste waters (MPW): waste water concentrations and volume used for the calculation are those measured at the catchment outlet during dry weather, at the same period of the year, for the same day of the week and the same time of the day; total mass of pollution (MPT): its calculation was based on flow and quality measurement at the catchment outlet; mass of pollution from sewer sediments (MPS): MPS MPT - MPR - MPW
=
42
M.-C. GROMAIRE-MERTl et al.
Results given in Table 7 show a very high contribution of sewer sediments. For these five events 40 to 60% of SS and COD loads originate from inside the sewer and this contribution is rather higher for BODS and VSS. Runoff contributes approximately for 30% of the SS and COD loads. about 20% of the VSS loads and less than 20% of BODS loads. The contribution of waste water depends on the rainfall: it is negligible at night or for short and heavy rain events. but can be important for long and light rainfalls. This contribution is higher for BODS and COD than for SS. Table 8. Contributionof thedifferent sources of pollution for5 rain events (rough estimation) Rain event characteristics Beginning Duration time mn 05107/1996 05:00 190+90 1010811996 18:00 180 11/0811996 17:30 13 1210811996 04:00 45 19109/1996 10:30 435 Date
lav mmlh 4.5 4.7 35.3 5.6 1.8
% of contribution of to the total SS load Waste Yards Roofs Streets Sewer waters sediments 5% 28% 6% 15% 47% 4% 9% 10% 17% 59% 7% 7% 10% 10% 66% 10% 6% 23% 10% 50% 3% 37% 3% 11% 45%
As our first hypothesis may have lead us to underestimate the pollution from runoff. our second approach was to evaluate the maximum value for the contribution of this source. For each kind of runoff, the maximum of the concentrations measured on our different sampling sites was attributed to the whole drainage surface. Even this way. more then 20% of SS, COD, VS and more then 40% of BODS was found to originate from the sewer. Similar results were obtained by Krejci et al. (1987) and by Bachoc (1992). On a small catchment (12.7 ha) and for four rain events Krejci calculated that 59% of suspended solids came from inside the sewer. 20% resulting from slimes and 39% from sewer deposits. Bachoc evaluated a contribution of sewer sediment from 30 to 45% of SS load for three rain events at the outlet of trunk 13 in Marseilles. On the scale of the year. Chebbo (1992) estimated the contribution of the sewer to 20% only. whereas runoff was found to be the main source for SS and COD (about 50% of the total load) and waste water to bring 55% of the BOD5. CONCLUSION This research underlines the importanceof combined sewers as physical and chemical reactors. An evolution of the characteristics of wet weather flows has been noticed from runoff to the catchment outlet. Concentrations measured during rain events at the catchment outlet are equivalent to those of waste water and much higher to the average runoff concentration. The particulate fraction is far more important at the catchment outlet than in runoff and these particles are more loaded with organic matter. First results show that a main part of the wet weather pollution originates from the combined sewer itself. which is an important source of particles and of organic matter. Yet. these results have to be confirmed by a wider number of rain events. Mass balance sheet calculations are going to be improved in the next month. The variability of runoff concentration at the scale of the catchment is going to be evaluated more precisely and taken into account in our mass balance. REFERENCES Bachoc, A. (1992). Le transfert des solides enreseaux d'assainissement unitaires. Ph.D. Thesis. Institut National Polytechnique de Toulouse. Toulouse. France. 281p + appendices. Butler. D., Thedchanamoorthy, S.and Payne. J. A. (1992). Aspects of surface sediment characteristics onanurban catchment in London. War. Sci. Tech., 25(8).13-19. Chebbo, G. (1992). Solides des rejets urbains par temps de pluie: caractl!risation et traitabilite. Ph.D. Thesis, Ecole Nationale des Ponts etChaussees, Paris. France. 41 Op + appendices. Chebbo, G., Mouchel, J. M.• Saget, A. and Gousailles, M. (1995). Lapollution des rejetsurbains par temps de pluie: flux. nature etimpacts. TSM. 11. 796-806. Degrl!mont (1989). Mimento techniquede l'eau: Paris. Lavoisier: Technique et Documentation. 1459p.
Urban wet weather pollution on combined sewer systems
43
Krejci. V., Dauber. L.• Novak, B. and Gujer, W. (1987). Contribution of different sources to pollutant loads in combined sewers. Proc. 4th lnt. Conf. on Urban Storm Drainage, Lausanne. Switzerland. Aug 31-Sept4. 34-39. Saget, A. (1994). Base de donnees sur la qualitc! des rejets urbains de temps de pluie: distribution de la pollution rejetee, dimension des ouvrages d'interception. Ph.D. Thesis, Ecole Nationale des Ponts et Chaussees, Paris. France. 333p. Sagel, A. and Chebbo , G. (1996). Pollution loads of urban wet weather discharges. Preprint 7th IntConf. on Urban Storm Drainage. Hannover. Germany , Sept 9-13, 61-66. Sartor. J. D., Boyd. G. B. and Argady. F. J. (1974). Water pollution aspects of street surface contaminants. J. Water pollution Control Fed.• 49(3). 458-467. Verbanck, M. A. (1990). Sewer sediment and its relation with the quality characteristics of combined sewer flows. Wat Sci. Tech.. 22(10111).247-257. Xanthopoulos. C. and Hahn. H. (1993). Anthropogenic pollutants wash-off from street surfaces. Preprini 6th lra.Conf. on Urban Storm Drainage, Niagara Falls, Ontario, Canada, Sept 12-17. 417-422.
Pergamon
T~ch.
Sci. Vol. 37. No. I. pp. 35-43.1998. =Wal. 1998 IAwQ. Published by Elsevier ScienceLId Printedin GrealBritain.
PH: S0273-1223(97)OO753-1
0273-1223/98 $19'00 + 0-00
ORIGINS AND CHARACTERISTICS OF URBAN WET WEATHER POLLUTION IN COMBINED SEWER SYSTEMS: THE EXPERIMENTAL URBAN CATCHMENT "LE MARAIS" IN PARIS Marie-Christine Gromaire-Mertz, Ghassan Chebbo and Mohamed Saad CERGRENE. EcoleNationale des Pontset Chaussees, 6-8 avo Blaise Pascal. Cite DescartesChampssur Marne, 77455Marne/a Val/ee Cedex 2. France
ABSTRAcr An experimental urban catchment has been created in the centre of Paris. in order to obtain a description of the pollution of urban wet weather flows at different levels of the combined sewer system. and to estimate the contribution of runoff. waste water and sewer sediments to this pollution. Twenty-two rainfall events were studied from May to October 1996. Dry weather flow was monitored for one week. Roof. street and yard runoff. total flow at the catchment outlet and waste water were analysed for S5. V5S. COD and BODS. on both total and dissolved fraction. Results show an evolution in the characteristics of wet weather flow from up to downstream : concentrations increase from the catchment entry to the outlet, as well as the proportion of particle-bound pollulants and the part of organic matter. A first evaluation of the different sources of pollution establishes that a major part of wet weather flow pollution originates from inside the combined sewer. probably through erosion of sewer sediments. © t 998 IAWQ. Published by Elsevier Science Ltd
KEYWORDS Combined sewer; field experiments; pollution; urban runoff; sewer sediments; waste water.
INTRODucrION Previous research (Bachoc, 1992; Chebbo, 1992; Saget, 1994) showed that urban wet weather flow pollution is far worse in combined sewers then in storm sewers. Average suspended solid concentrations calculated Over a year are 50% higher in combined sewer discharges (240 to 670 mgll) than in storm sewer discharges (160 to 460 mg/l). For BOD5 , the average concentration in combined sewer discharges (90 to 270 mg/l) is more than twice that of storm sewers (13 to 130 mg/l). Combined wet weather flows are generally more loaded in organic matter: the VSSISS ratio is from 10% to 36% for storm sewers, whereas it is from 24% to 55% in combined sewers . Moreover, suspended solids from combined sewers have lower and more varying settling velocities than those from storm sewers.
3S
36
M.-C. GROMAlRE·MERTZ et al.
As shown by Krejci et al. (1987), Bachoc (1992) and Chebbo (1992), the presence of waste water alone does not explain these differences which are mainly linked to the erosion of sewer sediments. These authors gave a first appraisal for the contribution of runoff, waste water and sewer sediments to the pollution of combined sewer discharges. Their evaluations were based on measurements at the outlets of both combined and separate sewers, not on simultaneous measurements at different levels of the same catchment. Results were given either at the scale of the year or for very few rain events. Thus, the pollution from these different sources has not been characterised and information available on the variability of these contributions depending on rain event characteristics is scarce. Because of this, a new research program was started at CERGRENE in 1994, financed by the Water Agencies, the municipality of Paris, the French Ministries of Research and of Public Works and the lie de France county. It aims to fit out an experimental urban catchment provided with combined sewers, in order to study the pollution of wet weather flows at different levels of the water cycle. This will enable a better understanding of the characteristics of the pollution transported in combined sewers and the evaluation of the contribution of different sources to this pollution for a large number of rainfall events. This article describes the experimental methods used for our research and the first results obtained. EXPERIMENTAL METHODS The experimental catchment The experimental catchment "Paris-Le Marais" is situated in central Paris, in an old residential district with small businesses . Its small size (42 hectares) makes it possible to visit it entirely and to describe it in detail. Roofs represent 54.4% of the catchment area and streets 22.4%. The last 23% are taken by yards, gardens and squares. The mean slope of the catchment is of 0.84%. The storm runoff-coefficient is about 0.78. The population is dense: 295 inhabitantslhectare. This catchment is drained by a combined, Parisian type, sewerage system. All sewers are man entry and equipped with non selective gullies. The sewerage system includes three ovoid trunks with sidewalks, with a total length of 1.8 krn, and about fifty egg-shaped collectors with a total length of 5.8 km. The mean slope of the collectors is of 0.8% whereas the slope of the trunks is less then 0.1%. Street gutters are swept every day. Sidewalks and gutters are washed two to five times a week with a pressurised water jet. Most streets are sucked up with a dust-collector every day, excepted on week ends, with a dust collector car.
0: c:afdulIenf DUtlet
s: .treet runoff.ampUni
Y: yard nmofl' .ampliD&
R: rooCrunoff'.ampUne
G: niDlallles Figure 1.Location andmap of theexperimental catchment with sampling sites.
Urban wet weather pollut ion on combined sewer systems
37
SampHni sites and eQuipment The experimental catchment was provided with equipment allowing us to characterise and to quantify the pollution of surface runoff at the entry of the sewerage system and the pollution of the total flow at the catchment outlet during dry and wet weather. Sampling sites for street runoff were chosen using a statistical survey. All streets in the catchment could be classified into seven groups, depending on features relative to the accumulation and the wash-off of pollutants (size, road traffic, frequenting, commercial activities , slope, covering ...). Four gully holes, situated at street comers were monitored (Figure I). Each one collects runoff from two streets belong ing to two different groups , all in all eight streets belonging to seven different groups. For financial reasons we studied only 3 streets out of the 7 at each rain event. Equipping a street gully hole (Figure 2) consisted of separating waters collected on each side of the gully by a wall, canalising the water, removing coarse solids (larger than some centimetres), measuring the flow and taking flow weighted samples. The flow is measured either with a tipping bucket flow meter or with a triangular weir. Two automatic vacuometric water samplers are used at each gully: the first one with 24 bottles of I litre in order to establish pollutographs, the second one with a 25 litres container in order to measure settling velocities. These water samplers are driven both by the flow meter and a conductivity sensor : sampling starts when a flow with a conductivity less then 450 IlS/cm2 is measured in the gully (which stops samples of street washing waters being taken), sampling rate is flow proportional. Sep .... ion ",.11 .nd • •'".1 ....__..
SEWE R
GULLY CO
NECTION
"." EJ S.mpl. "
Sepuation and wat er
• • nllizin,
~:=:~=r::d;:;;:::7\
S iphon Condu c.ivilV se nior
Figure 2. Street runoff monitoring equipment.
Roof runoff is sampled from four roofs (Figure I) with different coverings: zinc, slate, interlocking clay tiles and flat clay tiles, which correspond to the main types of covering in our catchment. These samples are taken in the roof gutters and collected in 100 litres containers . Three yards (Figure I) were filled out with automatic water samplers . They have been chosen so as to be representative of our catchment. One is paved with stones, the second one is concreted and planted with trees, the last is permeable with grass and gravel covering . At the outlet of the catchment (Figure 1), flow is monitored with an "Ultraflux" flow meter and recorded every two minutes. This apparatus uses a time of flight ultrasonic system to measure in-sewer velocity on
38
M.-C. GROMAIRE-MERlZ et al.
four horizontal lines in the cross section. The water level is measured both with an ultrasonic and a pressure sensor. Flow weighted samples are taken with two automatic vacuometric water samplers, driven by the flow meter and started when the water level exceeds the maximum dry weather level. The first sampler contains 24 bottles of 2.8 litres each. It is used to establish pollutographs. The second sampler takes a mean sample with a volume up to 70 Iitres, it is used for settling velocities. Rainfall is measured on the catchment with two tipping bucket raingauges. Measurements Table I indicates the type of measurements performed for each kind of sampling site. Table I. Kind of measurementsperformed on the different experimental sites Roofs
Yards
mean concentrations
mean concentrations
Streets hydrograph polJutographs mean concentrations settlmz velocities
Outlet hydrograph pollutographs
mean concentrations settling velocities
All samples are analysed for suspended solids (SS), volatile suspended solids (VSS), chemical oxygen demand (COD). biological oxygen demand (BOD5), heavy metals (lead. zinc. cadmium) and hydrocarbons. both on the total and on the dissolved fraction (after filtration at 0.7 11m). For street runoff and for the total flow these analyses are also performed on the different classes of settling velocities. Flow from yards and roofs is calculated theoretically. This paper presents results for SS. COD, BOD5, VSS event mean concentrations and for the distribution between dissolved and solid fractions. RESULTS AND DISCUSSION Characterisation of waste water Dry weather flow has been characterised at the outlet of the catchment for six different days of the week. between the 15th and the 27th July 1996. Each of these days, 24 hourly mean samples were taken and analysed for SS, VSS. COD and BOD5. The total daily flow on the catchment represents 565 l/head/day, which is very high compared to the figure of 100 to 300 l/head/day given by Saget and Chebbo (1996). This high rate of water production can be explained by the importance of street washing, business (restaurants, trade. offices) and the probable contribution of parasite waters. Daily figures for the pollutant load of waste water are those usual in France (Table 2). Table 2. Mean load for waste water
55 Paris-Le Marais Degremont (1989), Chebbo (1992), Chebbo et at (1995)
Il!!headlday 70
ml!!1 125
70 to 90
100to 500
BOD5 mg/I ' !!Iheadldav 84 150 60 to 80
100 to 500
COD mg/I CODIBOD5 194 290 2t02,5
VSS %
83
200101000 65 to 85
Urban wet weather pollution on comb ined sewer system s
39
The particulate fraction represents 70% of the daily COD and BOD5 load. This percentage is higher at the daily peak flow (80%) and less important at night when up to 50% of the pollut ion is in the dissolved fraction. As illustrated in Figure 3, concentrations and flow rate show a well marked daily pattern . Maximum values are reached between 10 and I I in the morning, with a smaller peak in the afternoon. Slight differences are measured between weekdays and weekends: the pollution load is 15% lower during the weekend and flow decreases by 6% on Sunday only. Suspended solids , COD, BOD5 and VSS variations are correlated to the daily flow variations, but concentrations vary in a wider range (23 to 225 mgIJ for SS, i.e. from 1 to 10) than flow (0.047 to O. I24 m 3/s , i.e. I to 3). This phenomenon can be partly linked to a dilution of domestic sewage with infiltration water, the rate of dilution depend ing on the sewage flow. It probably also involves the settling of particles at night when the energy of the flow is too low for their transportation and the erosion of sewer sediments during peak flows (Verbanck , 1990). Yet, differences in the qua lity of sewage produced at different times of the day , due for instance to professional activities, are not excluded. Dry weather flow and SS concentration
.
500 400
. : " . .. . . .. ..'.
"
... , , ..
0.15 .,
.
:
-.»
'
~ 300
. !'
e
gj 200 100
,
~
~
I'" ~
o
a
0
v
~ ~
...
C"l
g
N
/. III
..,s
~
,... g CD
01
s
... . '. .
.
V l\ V I"'"
~
_I"'"
l\
-0.05
...... ... ... ,...... ... s s g g s C"l
01
III
... ... Time[r'rom m ldnfght) CD
: : :-.::
0
- - minimum SS - - maximum SS • - - meanSS 0.05 ~ maximum flow ~ . ··· .. meanflow 0.00 ii: minimum flow 0.10
N
..,
CD
CD
-0.10 C"l
Cii
N
0 N
~
s
g
Figure 3. Daily variation of 55 con centrations and n ow du ring dry weather.
Characterisation of wet weather pollution
Characteristics of the studied rain events. Rain events have been defined as rainfalls with a minimum depth of I mm and separated by at least 30 minutes without any rain recording. Between May and October 1996, we studied twenty-two rain events with a wide range of rain depth and intensities (Table 3). Table 3. Rain event characteristics minimum maximum median rain depth (rnm) 2 14,6 5,6 mean intensity (mm/h) 1,4 42 4,7 maximum intensity (mm/h) * 2,7 180 21,2 rain duration (h) 00:10 07:30 00:45 dry weather period (days) 0,03 30 0,9 *: calculatedon the duration between two successivetippings
Mean samples were collected for almost all these events from the outlet , one street and four roofs . For some of these twenty-two rain events (Table 4), samples were also collected from two other streets and three yards . For roof runoff, very closed rainfalls were collected in the same samp le.
40
M.-C. GROMAIRE-MERTZ et al.
Table 4. Numberof rainfall events studied for each sampling site Roof runoff Industrialtiles I Flal tiles I Zinc 18 I 15 I 18
I Siale I
18
Yard runoff Street runoff Concrete I Stone I Gravel Street 1 I Street 2 I Street 3 20 I 3 I 6 I 4 I 5 9
Outlet
21
Event mean concentrations (Table 5). For all types of runoff. BODS concentrations were found to be very low whereas 55 and COD concentrations were very variable from one event to another. The part of volatile solids (12 to 70%) was far more important than what is usually admitted for surface runoff (10% for street solids according to Sartor et al.• 1974; Butler. 1992; Xanthopoulos and Hahn. 1993). Table S. Event mean concentrations measuredon the different samplingsites 5S mg/I minimum- mean maximum value 56 industrial tiles 710211 81075 37 Roof fiat tiles 71013\ 46 runoff zinc 8 t091 27 slate Yard runoff Street runoff Outlet
concrete
pavement gravel Street 1 Street 2 Street 3
31 to 70 II to 38 32 to 490 57 to 497 41 to 206 10 to 181 105 to 559
45 24 20/ 242 78 79 307
CODm-gfJ minimum- mean maximum value 9 to 120 32 38 15t091 49 910111 510198 34 59 to 182 29 to 71 42 to 211 12410964 S6 to 171 2S to 94 123 to 736
/23 43 89 377 /0/
59 428
BODS ml!l1 minimum- mean maximum value 5 3 to \3 4to 22 6 4 to 31 7 3 to 42 7 13to 47 6 to 16 8 to 27 28 to 160 16t032 14 to 20 67 to 296
27 /0
/8 82 24 17 /81
%VSS minimum- mean maximum value 28 t4 to 44 43 32 to 66 43 33 to 59 37 28 to 62 60 to 77 75 44 to 70 62 12 to 27 J7 49 to 70 59 41 to 67 56 41 to 61 47 53 to 72
66
Roof runoff was usually little concentrated. but for some events 5S concentrations up to 210 mg/l and COD concentrations up to 198 mg/I were measured. Good linear relations could be established (Table 6) between S5 concentrations from roof runoff and the following rain event characteristics: dry weather duration (OW), average rain intensity (Iav), maximum rain intensity (lmax) and rain duration (D). There was no significant variation from one roof to another for SS. COD. BODS. Table 6. Multiple linear regression between roof runoff and rainfall characteristics Type of roof Industrial clay tiles Flat clay tiles Zinc Slate
R1
Multiple linear regression 0,86 rSS1= 1,16 OW +4.76 lay - 38 77 0 + 0.13 Imax - 0.92 0.91 rSS]= 1.05 OW -0,72 Iav - 120700+019 Imax + 38,13 085 IISS]= 1,02 OW + 3.90 lav- 78.85 0 + 0.05 Imax + 14.45 0.76 [SS)= 0.72 OW + 3.24 Iav- 59.39 D - 0.16 Imax +15.02
For yard and street runoff, results were very different from one site to another. depending on the land use. Ground wash-off on the gravel covered yard lead to high SS concentrations with low VSS rates. Differences between the concreted and the paved yard are linked to the presence of trees and birds on the concreted yard and to the difference of cleaning practices. Street 2 and 3 gave similar results but extremely high concentrations (500 mg/l for SS. 960 mg/l for COD and 160 mgll for BODS) were measured on street I, which is a small and crowded street with a lot of snack bars. On this site. correlation between SS concentrations and rainfall characteristics gave poor results. Nevertheless. the pollution load seems to grow with the dry weather period (R = 0.49). Surprisingly. no correlation was found between SS concentration in street runoff and the period since the last street washing.
Urban wet weather pollution on combined sewer systems
41
Concentrations measured at the outlet of the catchment are similar to those reported by Saget and Chebbo (1996) for several combined sewers in France. They vary with rainfall characteristics but also with the time at which the rainfall occurred: lower values are measured at night when waste water is scarce. For diurnal rain events, 55 concentrations were found to be correlated to the rain depth (R -0.75), the rain duration (R -0.62) and the previous dry weather period (R 0.46).
=
=
=
It should be observed that event mean concentrations measured at the catchment outlet were usually equivalent or superior to that of waste waters at the same time of the day, whereas runoff was in average far less concentrated. This supposes a contribution of sewer sediments.
Distribution between dissolved and particle-bound pollutants. Table 7 gives the distribution between particulate and dissolved COD and BOD5 loads. At the catchment outlet, 70 to 90% of the total load is linked to particles, which corroborates results from previous studies (Chebbo, 1992). For runoff the contribution of the dissolved fraction is much more important and increases from streets to yards and roofs. Yet the distribution between dissolved and particulate fractions was very inconstant in the runoff and variations of the particulate fraction from 30 to 80% from one sampling site or rain event to another have not been explained. The evolution of the particulate fraction from the sewer entry to the catchment outlet can be explained both by physical (erosion of sewer sediments) and chemical (adsorption of pollutants on particles) reactions inside the sewer system. Table7. Distribution between dissolved and particle-bound pollution loads %
Roof runoff Yardrunoff Streetrunoff Outlet Ori~ins
minimum 34 38
24 72
particulate COD mean maximum 58 86 56 83 68 88 83 92
%
minimum 17 37 50
71
particulate BOD5 maximum mean 48 76 57 82 66 93 82 91
of wet weather pollution
We can identify three sources of pollution in combined wet weather flows: urban runoff from streets, yards and roofs, waste water and sewer sediments. The contribution of these sources has been evaluated for five rainfall events, for which we had results on almost all sites. Characteristics of these rain events are given in . The following method was used: pollution mass in runoff (MPR): - for each rain event, the average of the concentrations measured on the four experimental roofs was assigned to the total roof surface; and the average of the concentrations measured on the three experimental streets was assigned to the total surface of streets; - as for these rain events no measurement was available for yard runoff, an average concentration has been calculated on all rain events for which we had measurements and assigned to the whole surface of yards; - the volume of runoff was calculated using theoretical runoff coefficients. It is probably overestimated, as it is superior to the difference between the total flow at the outlet and volume calculated for waste waters; mass of pollution from waste waters (MPW): waste water concentrations and volume used for the calculation are those measured at the catchment outlet during dry weather, at the same period of the year, for the same day of the week and the same time of the day; total mass of pollution (MPT): its calculation was based on flow and quality measurement at the catchment outlet; mass of pollution from sewer sediments (MPS): MPS MPT - MPR - MPW
=
42
M.-C. GROMAIRE-MERTl et al.
Results given in Table 7 show a very high contribution of sewer sediments. For these five events 40 to 60% of SS and COD loads originate from inside the sewer and this contribution is rather higher for BODS and VSS. Runoff contributes approximately for 30% of the SS and COD loads. about 20% of the VSS loads and less than 20% of BODS loads. The contribution of waste water depends on the rainfall: it is negligible at night or for short and heavy rain events. but can be important for long and light rainfalls. This contribution is higher for BODS and COD than for SS. Table 8. Contributionof thedifferent sources of pollution for5 rain events (rough estimation) Rain event characteristics Beginning Duration time mn 05107/1996 05:00 190+90 1010811996 18:00 180 11/0811996 17:30 13 1210811996 04:00 45 19109/1996 10:30 435 Date
lav mmlh 4.5 4.7 35.3 5.6 1.8
% of contribution of to the total SS load Waste Yards Roofs Streets Sewer waters sediments 5% 28% 6% 15% 47% 4% 9% 10% 17% 59% 7% 7% 10% 10% 66% 10% 6% 23% 10% 50% 3% 37% 3% 11% 45%
As our first hypothesis may have lead us to underestimate the pollution from runoff. our second approach was to evaluate the maximum value for the contribution of this source. For each kind of runoff, the maximum of the concentrations measured on our different sampling sites was attributed to the whole drainage surface. Even this way. more then 20% of SS, COD, VS and more then 40% of BODS was found to originate from the sewer. Similar results were obtained by Krejci et al. (1987) and by Bachoc (1992). On a small catchment (12.7 ha) and for four rain events Krejci calculated that 59% of suspended solids came from inside the sewer. 20% resulting from slimes and 39% from sewer deposits. Bachoc evaluated a contribution of sewer sediment from 30 to 45% of SS load for three rain events at the outlet of trunk 13 in Marseilles. On the scale of the year. Chebbo (1992) estimated the contribution of the sewer to 20% only. whereas runoff was found to be the main source for SS and COD (about 50% of the total load) and waste water to bring 55% of the BOD5. CONCLUSION This research underlines the importanceof combined sewers as physical and chemical reactors. An evolution of the characteristics of wet weather flows has been noticed from runoff to the catchment outlet. Concentrations measured during rain events at the catchment outlet are equivalent to those of waste water and much higher to the average runoff concentration. The particulate fraction is far more important at the catchment outlet than in runoff and these particles are more loaded with organic matter. First results show that a main part of the wet weather pollution originates from the combined sewer itself. which is an important source of particles and of organic matter. Yet. these results have to be confirmed by a wider number of rain events. Mass balance sheet calculations are going to be improved in the next month. The variability of runoff concentration at the scale of the catchment is going to be evaluated more precisely and taken into account in our mass balance. REFERENCES Bachoc, A. (1992). Le transfert des solides enreseaux d'assainissement unitaires. Ph.D. Thesis. Institut National Polytechnique de Toulouse. Toulouse. France. 281p + appendices. Butler. D., Thedchanamoorthy, S.and Payne. J. A. (1992). Aspects of surface sediment characteristics onanurban catchment in London. War. Sci. Tech., 25(8).13-19. Chebbo, G. (1992). Solides des rejets urbains par temps de pluie: caractl!risation et traitabilite. Ph.D. Thesis, Ecole Nationale des Ponts etChaussees, Paris. France. 41 Op + appendices. Chebbo, G., Mouchel, J. M.• Saget, A. and Gousailles, M. (1995). Lapollution des rejetsurbains par temps de pluie: flux. nature etimpacts. TSM. 11. 796-806. Degrl!mont (1989). Mimento techniquede l'eau: Paris. Lavoisier: Technique et Documentation. 1459p.
Urban wet weather pollution on combined sewer systems
43
Krejci. V., Dauber. L.• Novak, B. and Gujer, W. (1987). Contribution of different sources to pollutant loads in combined sewers. Proc. 4th lnt. Conf. on Urban Storm Drainage, Lausanne. Switzerland. Aug 31-Sept4. 34-39. Saget, A. (1994). Base de donnees sur la qualitc! des rejets urbains de temps de pluie: distribution de la pollution rejetee, dimension des ouvrages d'interception. Ph.D. Thesis, Ecole Nationale des Ponts et Chaussees, Paris. France. 333p. Sagel, A. and Chebbo , G. (1996). Pollution loads of urban wet weather discharges. Preprint 7th IntConf. on Urban Storm Drainage. Hannover. Germany , Sept 9-13, 61-66. Sartor. J. D., Boyd. G. B. and Argady. F. J. (1974). Water pollution aspects of street surface contaminants. J. Water pollution Control Fed.• 49(3). 458-467. Verbanck, M. A. (1990). Sewer sediment and its relation with the quality characteristics of combined sewer flows. Wat Sci. Tech.. 22(10111).247-257. Xanthopoulos. C. and Hahn. H. (1993). Anthropogenic pollutants wash-off from street surfaces. Preprini 6th lra.Conf. on Urban Storm Drainage, Niagara Falls, Ontario, Canada, Sept 12-17. 417-422.