Contributions of wastewater, runoff and sewer deposit erosion to wet weather pollutant loads in combined sewer systems

An observatory of urban pollutants was created in Paris for the purpose of assessing the dynamics of wastewater and wet weather flow (WW and WWF) pollutant loads within combined sewers. This observatory is composed of six urban catchments, covering land areas ranging in size from 42 ha to 2581 ha. For a wide array of parameters including total suspended solids (TSS), chemical and biochemical oxygen demand (COD and BOD₅), total organic carbon (TOC), total Kjeldahl nitrogen (TKN), heavy metals (Cu and Zn) and polycyclic aromatic hydrocarbons (PAHs), this article is intended to evaluate the contributions of wastewater, runoff and in-sewer processes to WWF pollutant loads through the use of an entry–exit mass balance approach. To achieve this objective, a total of 16 rain events were sampled on these sites between May 2003 and February 2006. This study has confirmed that at the considered catchment scale (i.e. from 42 ha to 2581 ha) the production and transfer processes associated with WWF pollutant loads do not vary with basin scale. Entry–exit chemical mass balances over all catchments and for a large number of rain events indicate that wastewater constitutes the main source of organic and nitrogenous pollution, while runoff is the predominant source of Zn. For Cu, PAHs and TSS, the calculation underscores the major role played by in-sewer processes, specifically by sediment erosion, as a source of WWF pollution. A significant loss of dissolved metals was also observed during their transfer within the sewer network, likely as a consequence of the adsorption of dissolved metals on TSS and/or on sewer deposits. Moreover, the nature of eroded particles was examined and compared to the various sewer deposits. This comparison has highlighted that such particles exhibit similar organic and PAH contents to those measured in the organic layer, thus suggesting that the deposit eroded during a wet weather period is organic and of a nature comparable to the organic layer. Despite the extent of initial field investigations, no organic deposit was observed to be present on sewer lines within the catchments, which implies that this organic deposit is probably present in another form or to be found elsewhere in the main trunks.     

Contribution of different sources to the pollution of wet weather flows in combined sewers

Experiments performed on "Marais" catchment, in central Paris, aimed to follow up the quality of wet weather flows from the entry to the exit of a combined sewer network. SS, VSS, COD, BOD5, Cd, Cu, Pb, Zn concentrations were measured for an important number of rain events in roof, yard, street runoff, as well as in dry and wet weather flows at the catchment outlet. Mass entry-exit totals, at the scale of the catchment, were calculated over 31 rain events in order to evaluate the contribution of different types of runoff, of sanitary sewage and of sewer sediments to the total wet weather pollutant loads at the catchment outlet. The erosion of in-sewer pollutant stocks was found to be the main source of particles and of organic matter in wet weather flows, whereas heavy metal loads mainly originated from roof runoff, due to the corrosion of metallic roofs. Particles eroded inside the sewer during rain events were found to be quite different from the particles constituting the main part of sewer sediments: they are organic and biodegradable, with rather important settling velocities and seem to accumulate during dry weather periods. A change of the chemical form of heavy metals was noticed during the transport in the sewer and it is suspected that a fraction of the dissolved metals from the runoff is adsorbed on sewer sediments.     

Spatial variability of the characteristics of combined wet weather pollutant loads in Paris

An on-site observatory of urban pollutant loads in combined sewers has been created in Paris in order to investigate wet weather pollutant loads at different spatial scales. This observatory is composed of six urban catchments, covering areas from 41 to 2581ha. For a wide range of parameters including suspended solids (SS), volatile suspended solids (VSS), chemical oxygen demand (COD), biochemical oxygen demand (BOD5), total organic carbon (TOC), total Kjeldahl nitrogen (TKN), metals (Cd, Cu, Pb, Zn), aliphatic hydrocarbons and polycyclic aromatic hydrocarbons (AHs and PAHs), this article serves to evaluate the potential scale effect on wet weather flow (WWF) quality. Although the extensive data set compiled herein has emphasized the high variability in WWF quality from one rain event to the next, no scale effect could be observed for a given rain event on pollutant concentrations, distributions between dissolved and particulate phases, pollutant contents and loads. Such results are of prime importance since they suggest (i) no spatial scale influence on WWF quality for the considered catchments and (ii) similar dominant sources and transfer processes at the various spatial scales.     

Source characterisation and loads of metals and pesticides in urban wet weather discharges

Contaminants in urban wet weather discharges originate from a number of sources such as materials from wet and dry atmospheric deposition, wastewaters, urban surface erosion, traffic-related activities, in-sewer deposits, etc. In the current study, four contributions (rainwater, dry atmospheric deposition, dry weather discharge and catchment surface + possible erosion of in-sewer deposits) to the total concentrations of priority substances have been assessed at the outlet of two urban catchments (one residential catchment with a combined system and one industrial area with a separate stormwater system) for 12 storm events (six for each catchment). Mass balances were calculated for seven metals and four pesticides, as well as for total suspended solids and chemical oxygen demand. The respective contributions of dry and wet atmospheric deposition, wastewater and catchment surface differ for each pollutant type, corresponding to different land use, activities, environments and sewer systems. For most of the pollutants, the catchment surface appears to be the main contribution, with significant storm event variability, excepted for atrazine in one catchment.     

Towards the determination of an optimal scale for stormwater quality management: Micropollutants in a small residential catchment

Stormwater and atmospheric deposits were collected on a small residential urban catchment (0.8 ha) near Paris in order to determine the levels of certain micropollutants (using a preliminary scan of 69 contaminants, followed by a more detailed quantification of PAHs, PCBs, alkylphenols and metals). Atmospheric inputs accounted for only 10%–38% of the stormwater contamination (except for PCBs), thus indicating substantial release within the catchment. On this small upstream catchment however, stormwater contamination is significantly lower than that observed downstream in storm sewers on larger adjacent urban catchments with similar land uses. These results likely stem from cross-contamination activity during transfers inside the sewer system and underscore the advantages of runoff management strategies at the source for controlling stormwater pollutant loads. Moreover, it has been shown that both contamination levels and contaminant speciation evolve with the scale of the catchment, in correlation with a large fraction of dissolved contaminants in upstream runoff, which differs from what has been traditionally assumed for stormwater. Consequently, the choice of treatment device/protocol must be adapted to the management scale as well as to the targeted type of contaminant.     

Production and transport of urban wet weather pollution in combined sewer systems: the “Marais” experimental urban catchment in Paris

An experimental catchment area was set up in the centre of Paris (France) so as to follow up the quality of wet weather flows from the entry to the exit of a combined sewer network. The distinctive characteristic of this site is its location in a town centre and the extent of the equipment used to monitor the water pollution over the whole length of its course through the catchment area. The results obtained show a change in quality between the runoff entering the sewer network and the combined storm water flow at the sewer's outlet, which cannot be explained only by the mixture with domestic wastewater. In particular, an increase was observed in the concentrations of suspended solids (SS), VSS, COD, BOD and Cu, in the proportion of pollutants linked to particles and in the characteristics of the particles. A calculation of the total masses going in and out of the sewer network during a rainfall event shows that the erosion of in-sewer pollution stocks is the main source of particles and of organic matter in wet weather flows, whereas heavy metals loads originated from roof runoff, due to the corrosion of metallic roofs. Particles eroded from the sewer sediments during rain events were found to be quite different from the particles of type A deposits and organic biofilms. Nevertheless, they have mean organic and metallic loads that are of the same order of magnitude as the particles of the organic layer at water sediment interface. A change in the chemical form of heavy metals was noticed during the transport in the sewer and it is suspected that a fraction of the dissolved metals from the runoff is adsorbed on sewer sediments.     

Priority pollutants in wastewater and combined sewer overflow

Implementation of the European Water Framework Directive and its affiliated directives requires Member States to improve their understanding of priority pollutants (PPs) in urban areas and obviously within wastewater systems. As a direct consequence, this study is intended to furnish data on both PP occurrence and the significance of concentrations in wastewater during dry and wet periods within combined sewers. Various sampling sites within the Paris combined sewer network were selected; for each sample, a total of 66 determinants, including metals, polycyclic aromatic hydrocarbons (PAHs), pesticides, organotins, volatile organic compounds, chlorobenzenes, phthalates and alkylphenols, were analysed. A broad range of PPs was observed in wastewater during dry as well as wet weather periods. Of the 66 elements investigated, 33 and 40 priority substances could be observed in raw sewage and wet weather effluent, respectively. As expected, a majority of metals were present in all samples, reflecting their ubiquitous nature. For both periods, chlorobenzenes and most of the pesticides always remained below the limit of quantification, while the majority of other organic pollutants assessed were identified within the microg l(-1) range. As highlighted by the larger number of substances detected in wet weather samples and the significance of their concentrations, runoff via atmospheric inputs and/or surface leaching was found to induce a wider range of PPs (n=40) and lead to higher concentrations of certain metals, PAHs, pesticides and other individual compounds. The data generated during this survey, which constitutes one of the first studies conducted in Europe to report concentrations for a variety of priority substances in wastewater within combined sewers, may be used in the future to identify PPs of potential significance for dry and wet weather periods and targeted for further investigation.     

The use of long-term on-line turbidity measurements for the calculation of urban stormwater pollutant concentrations,loads, pollutographs and intra-event fluxes

This paper presents one of the largest databases on the quality of urban wet weather discharges measured since the development of continuous in-sewer water quality sensors in the late 1990s. Five years of continuous turbidity measurements enabled the validation of 263 and 239 rainfall events, respectively on two experimental catchments in Lyon (France), Chassieu (185 ha separate sewer) and Ecully (245 ha combined sewer). Except for high rainfall events of summer and second half of winter, analysis of database representativeness showed that all seasons were relatively well represented. As a first analysis of the database, traditional tools used in the urban drainage field were applied to assess: i) statistics and analysis of distributions of TSS and COD events loads and event mean concentrations (EMCs) and ii) the correlations between these statistics and events characteristics and iii) M(V) curves describing the intra-event mass distribution. Results showed that: i) EMCs and loads were approximately log-normally distributed, with a clear impact from wastewater contribution in Ecully, ii) EMCs are not correlated with storm event characteristics, whereas loads have shown significant correlation with key storm event variables such as total event volume, rainfall depth, maximum rainfall intensity and discharge and iii) M(V) curves dynamic could be classified in three categories, however with no clear correlation with storm event characteristics. The visual analysis of continuous time series of TSS and COD pollutographs, derived from turbidity time series showed that event pollutographs were highly variable, due to complex interacting processes during and between events, and suggests that further progress in knowledge and modelling of urban wet weather pollutant loads and pollutographs should be based on more detailed analyses of continuous time series rather, than on the traditional single event approach.     

Hydrocarbons and heavy metals in the different sewer deposits in the 'Le Marais' catchment (Paris, France): stocks, distributions and origins

The knowledge of the pollution stored in combined sewers is of prime importance in terms of management of wet weather flow pollution since sewer deposits play a significant role as source of pollution in combined sewer overflows. This work, which focused on the hydrocarbon (aliphatic and aromatic hydrocarbons) and metallic (Fe, Zn, Pb, Cu and Cd) pollution fixed to the different kinds of sewer deposits (gross bed sediment [GBS], organic layer [OL] and biofilm), was performed in order to provide a complete overview of the contaminant storage in the 'Le Marais' combined sewer (Central Paris, France). Firstly, our results have shown that, for all kinds of pollutants, a major part was stored in the GBS (87 to 98%), a lesser part in the OL (2 to 13%) and an insignificant part in the biofilm (<1%). These results demonstrated that the potential contribution of biofilm to wet weather pollution was negligible compared to the OL one. Secondly, the investigation of hydrocarbon fingerprints in each deposit has provided relevant information about contamination origins: (1) aliphatic hydrocarbon distributions were indicative of petroleum input in the GBS and reflected a mixture of biogenic and petroleum inputs in the OL and biofilm, (2) aromatic hydrocarbon distributions suggested an important pyrolytic contamination in all the deposits. Finally, the study of pollutant fingerprints in the different deposits and in the suspended solids going through the collector has shown that: (1) the suspended solids were the major component of OL and biofilm while urban runoff seemed to be the main transport mechanism introducing pollutants in the GBS and (2) the residence times in sewer of OL and biofilm were quite short compared to those for GBS.     

Multivariate analysis for assessing the quality of stormwater from different Urban surfaces of the Patiala city,Punjab (India)

Land use modifications associated with urbanization, such as clearance of vegetation, replacement of previously pervious areas with impervious surfaces and drainage channel modifications, result in increased runoff volumes, which often create flooding hazards and increase pollutant transport. An attempt has been made in the present study to investigate stormwater quality from five different urban sub-watersheds (that differ in land use and development activities) in the city of Patiala, India. The five sub-watersheds have similar geological, topographical and climatic conditions and were chosen to minimize the effect of these characteristics on stormwater quality and quantity. Stormwater samples were collected during six storm events between April 2010 and March 2011 and analyzed for BOD₅, COD, TSS, TDS, Oil and Grease, TKN, Total P, Coliforms and Heavy Metals (Zn, Cd, Ni, Pb, Fe and Cu). Results of the investigation indicate a strong correlation between land use and development activities and the resulting stormwater quality. TSS, COD and Oil and Grease were found to be major pollutants in surface runoff generated from commercial and urbanized catchments (all exceeded the surface water quality standards developed by Central Pollution Control Board, India). The water quality of the smaller residential catchment was better as compared to other catchments. Principal component analysis was investigated to identify linkages between stormwater quality and urban surface types. It was also confirmed through regression analysis that both antecedent dry period and rainfall intensity have telling influence on stormwater quality. Results obtained can provide practical information for improved stormwater management.     

Loss rates of urban biocides can exceed those of agricultural pesticides

Biocides and pesticides are used to control unwanted organisms in urban and agricultural areas. After application, they can be lost to surface waters and impair water quality. Several national consumption studies have shown that urban and agricultural use may be in the same range. It is difficult to judge whether this results in similar loadings of surface waters because there is a lack of sound, comparative studies addressing urban and agricultural losses simultaneously.The aim of this study is thus to relate the biocide and pesticide loads found in surface waters to their respective urban and agricultural usage (loss rates). To simultaneously assess the loss rates, we conducted a comprehensive field study in a catchment of mixed land use on the Swiss Plateau. The study area was divided into four sub-catchments with different degrees of urban and agricultural land use. In addition, we studied the only wastewater treatment plant, a combined sewer overflow and a storm sewer within the area. Rain events were sampled at high temporal resolution from March to November, 2007. Information on agricultural applications was gained from local farmers. For urban uses, consumption estimations were conducted based on statistical and product information.Despite substantially lower amounts used, the measured loads of urban biocides were in the same range as the most widely-used agricultural pesticides. The lower usage was compensated by urban loss rates that were up to ten times higher than agricultural ones (0.6 to 15% for urban, 0.4 to 0.9% for agricultural compounds). For most biocides and pesticides, the loads were controlled by rain events. Besides the rain-controlled losses, some urban-used biocides (e.g. diazinon) showed a continuous load independent of rain events and season. This study demonstrates that in catchments with mixed land use, mitigation strategies have to pay sufficient attention to the urban sources.     

Determination of aliphatic hydrocarbons in urban runoff samples from the "Le Marais" experimental catchment in Paris centre

Aliphatic hydrocarbons were assessed in runoff and waste waters from an urban catchment located in Paris. Runoff were sampled from different types of urban surfaces (11 roofs representing four different covering materials, two courtyards and six streets). Waste water samples were collected at the catchment outlet during dry and wet weather periods as well. This paper gives an overview of the results on the concentration and distribution points of view for both the particulate and the dissolved phases. Results were discussed on the basis of the median. Accordingly, the temporal variability was taken into account. Thus, the concentration medians ranged from 345 to 827, from 297 to 790, and from 393 to 1,359 microg L(-1) in the roof, courtyard and street samples, respectively. The levels found at the catchment outlet during dry and wet weather periods were of the same order of magnitude, i.e. 700 microgL(-1). The particulate phase represented 85% of the total aliphatic hydrocarbon content whatever the sample.     

Assessment of annual pollutant loads in combined sewers from continuous turbidity measurements: Sensitivity to calibration data

This article presents a methodology for assessing annual wet weather Suspended Solids (SS) and Chemical Oxygen Demand (COD) loads in combined sewers, along with the associated uncertainties from continuous turbidity measurements. The proposed method is applied to data from various urban catchments in the cities of Paris and Nantes. The focus here concerns the impact of the number of rain events sampled for calibration (i.e. through establishing linear SS/turbidity or COD/turbidity relationships) on the uncertainty of annual pollutant load assessments. Two calculation methods are investigated, both of which rely on Monte Carlo simulations: random assignment of event-specific calibration relationships to each individual rain event, and the use of an overall relationship built from the entire available data set. Since results indicate a fairly low inter-event variability for calibration relationship parameters, an accurate assessment of pollutant loads can be derived, even when fewer than 10 events are sampled for calibration purposes. For operational applications, these results suggest that turbidity could provide a more precise evaluation of pollutant loads at lower cost than typical sampling methods.     

Spatial variations of storm runoff pollution and their correlation with land-use in a rapidly urbanizing catchment in China

The composition of land use for a rapidly urbanizing catchment is usually heterogeneous, and this may result in significant spatial variations of storm runoff pollution and increase the difficulties of water quality management. The Shiyan Reservoir catchment, a typical rapidly urbanizing area in China, is chosen as a study area, and temporary monitoring sites were set at the downstream of its 6 sub-catchments to synchronously measure rainfall, runoff and water quality during 4 storm events in 2007 and 2009. Due to relatively low frequency monitoring, the IHACRES and exponential pollutant wash-off simulation models are used to interpolate the measured data to compensate for data insufficiency. Three indicators, event pollutant loads per unit area (EPL), event mean concentration (EMC) and pollutant loads transported by the first 50% of runoff volume (FF50), were used to describe the runoff pollution for different pollutants in each sub-catchment during the storm events, and the correlations between runoff pollution spatial variations and land-use patterns were tested by Spearman's rank correlation analysis. The results indicated that similar spatial variation trends were found for different pollutants (EPL or EMC) in light storm events, which strongly correlate with the proportion of residential land use; however, they have different trends in heavy storm events, which correlate with not only the residential land use, but also agricultural and bare land use. And some pairs of pollutants (such as COD/BOD, NH₃-N/TN) might have the similar source because they have strong or moderate positive spatial correlation. Moreover, the first flush intensity (FF50) varies with impervious land areas and different interception ratio of initial storm runoff volume should be adopted in different sub-catchments.     

A survey of heavy metals in municipal wastewater in combined sewer systems during wet and dry weather periods

During wet weather, combined sewer system overflows affect the quality of water in watercourses. For planning overflows, the lowest possible load of priority substances according to Directive 2008/105/EC is crucial and the knowledge of variability in concentrations of elements in the sewer system is necessary. The behaviour of heavy metals in a sewer system was observed in the course of dry weather flow (DWF) and wet weather flow (WWF). We found, from the comparison of concentration medians for the WWF and DWF that during wet weather periods, an increase in the concentrations of As, Cr, Cd, Pb, Mn and Fe occurs in the sewer system and the effect of nonpoint sources manifests itself. Zn, Cu and Ni concentrations decreased during wet weather periods, and Hg concentrations did not significantly change. During the WWF period, a considerable nonhomogeneity of the sewage system was demonstrated.     

On hydraulic and pollution effects of converting combined sewer catchments to separate sewer catchments

The overall objective of this paper is to contribute to the standing debate concerning the advantages of separate sewer systems compared to traditional combined sewer systems. By a case study this investigation reveals that transformation of part of a town from being serviced with combined sewer systems to become serviced with separate sewer systems decreases the volumes of storm water and pollutants diverted to the waste water treatment plant and discharged as combined sewer overflow. This happens at the expense of an increase in volumes of storm water and pollutant loads diverted to local receiving waters when detention ponds are not built-in the new separate sewer systems. It is concluded that consequences can be fatal for receiving waters, if no retention of pollutants is integrated into the system.     

Stochastic modeling of total suspended solids (TSS) in urban areas during rain events

The load of total suspended solids (TSS) is one of the most important parameters for evaluating wet-weather pollution in urban sanitation systems. In fact, pollutants such as heavy metals, polycyclic aromatic hydrocarbons (PAHs), phosphorous and organic compounds are adsorbed onto these particles so that a high TSS load indicates the potential impact on the receiving waters. In this paper, a stochastic model is proposed to estimate the TSS load and its dynamics during rain events. Information on the various simulated processes was extracted from different studies of TSS in urban areas. The model thus predicts the probability of TSS loads arising from combined sewer overflows (CSOs) in combined sewer systems as well as from stormwater in separate sewer systems in addition to the amount of TSS retained in treatment devices in both sewer systems. The results of this TSS model illustrate the potential of the stochastic modeling approach for assessing environmental problems.     

Gross solids from combined sewers in dry weather and storms,elucidating production,storage and social factors

Variation in rates of sanitary hygiene products, toilet tissue and faeces occurring in sewers are presented for dry and wet weather from three steep upstream urban catchments with different economic, age and ethnic profiles. Results show, for example, that total daily solids per capita from the low income and ageing populations are almost twice that from high income or ethnic populations. Relative differences are verified through independent questionnaires. The relationship between solids stored in sewers prior to storms, antecedent dry weather period and the proportion of roof to total catchment area is quantified. A full solids’ flush occurs when storm flows exceed three times the peak dry weather flow. The data presented will assist urban drainage designers in managing pollution caused by the discharge of sewage solids.     

Hydrological controls of pollutant removal from highway surfaces

Sediment and metal loadings to roadside gully-pots (weight/area of catchment/runoff depth) are calculated for two defined sites within a highway catchment and compared with data from other urban studies. The higher metal loadings associated with the more active site are particularly noticeable for Cu but at both sites the relative sizes of the metal loadings reflect the expected metal availabilities.The measured total surface runoff volumes represent between 34 and 83% of the total incoming rainfall volume for the catchment. The possible reasons for these water losses are assessed in terms of surface characteristics, seasonal variations and instrument limitations.Stepwise linear regression analysis of pollutant loadings against five selected hydrological parameters indicates that total runoff volume and storm duration together explain over 90% of the observed variance in Pb, Cd, Mn and sediment loadings. The results demonstrate the lack of importance of antecedent dry period and rainfall intensity in controlling the removal of particulate associated pollutants from this catchment. The derived model is used to predict temporal pollutant removal rates and these are shown to be consistent with those reported in other urban catchment studies.     

A case study of urban water balancing in the partly sewered city of Nablus-East (Palestine) to study wastewater pollution loads and groundwater pollution

Untreated sewage can contribute a significant proportion of urban groundwater recharge, via on-site sanitation facilities and sewer exfiltration. In the West Bank of the Palestinian Territories 94% of sewage is discharged untreated to the ground or surface waters. This has contributed to increasing nitrate concentrations in groundwater, which is the critical water source. In this case study of a drainage catchment from the city of Nablus, a water balance indicates that sewage as a source of groundwater recharge is as much as 50% of total recharge from precipitation, and nitrogen pollutant loads by area are up to 60% as much as those from agriculture. Results suggest that 22% of total wastewater flow directly infiltrates the ground via cesspits and sewer exfiltration.