Emissions of pollutant loads from combined sewer systems and ...

11 th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008 

Emissions of pollutant loads from combined sewer systems and 

separate sewer systems – Which sewer system is better? 

A. Welker 

Institute of Urban Water Management; University of Kaiserslautern; Paul-Ehrlich-Str. 14, 

67663 Kaiserslautern, Germany; e-mail: awelker@rhrk.uni-kl.de 

ABSTRACT 

Combined sewer systems (CSS) and separate sewer systems (SSS) as well emit significant 

pollution into the receiving waters. Still, an open question is which sewer system has minor 

discharges. In this paper a comparison of annual pollution loads of two representative sewer 

systems is presented. 

With an intensive literature survey representative substance concentration ranges in dry 

weather flow, surface runoff and effluent of waste water treatment plant (WWTP) are stated. 

The selected parameters are two classical wastewater parameters (chemical oxygen demand 

(COD) and ammonium) supplemented by three pollutants (copper, carbamazepine and 

estradiol). Values are used as input for pollution load simulations by using the software 

KOSMO. 

Results presented in this study are annual discharge loads of the main emissions of the 

catchment in the CSS (WWTP effluent and combined sewer overflow (CSO)) and the SSS 

(WWTP effluent and storm sewer outlets (SSO)). 

In summary, a general answer, which sewer system emits higher pollution loads is not 

possible. It is extremely dependent from the considered substance with its specific properties 

and resulting behaviour in the sewer systems. For a first estimation new classes of substances 

are distinguished considering their origin (surface runoff versus dry weather flow) and their 

elimination capacity in the WWTP. 

KEYWORDS 

Combined sewer system, separate sewer system, pollution load simulation, pollutants 

INTRODUCTION 

Discussion about the “right” sewer system has been arising in the last decades and is still 

going on. Generally, both sewer systems and their modifications can discharge significant 

pollutant loads into the receiving waters (see Figure 1). 

In the separate sewer system (SSS) these are the effluents of the waste water treatment plant 

(WWTP) and storm sewer outlets (SSO). 

Main emissions from combined sewer systems (CSS) are the effluents of the WWTP during 

dry weather and during wet weather, whereas emissions during wet weather are often not 

considered properly. Additionally, the CSS discharges combined sewer overflows (CSO). 

Welker 1


separate sewer system 

paved area 

(roof, streets etc.) 

surface runoff 

storm 

sewer 

storm sewer flow 

treatment 

devices 

(if necessary) 

storm sewer outlets (SSO) 

receiving water 

households, 

commerce, 

industry 

dry weather flow 

combined 


combined sewer flow 

CSO 

devices 

(e.g. stormtank) 

combined sewer overflow (CSO) 

combined sewer system 

water supply rainfall water supply 

rainfall 

households, 

commerce, 

industry 

dry weather flow 

sanitary 


inflow wwtp (dw) 

WWTP 

effluent WWTP (dw) 

inflow 

WWTP 

(dw, ww) 

paved area 

(roof, streets etc.) 

surface runoff 

WWTP 

effluent WWTP (dw, ww) 

Figure 1. Pathways and emissions from separate sewer systems and combined sewer systems; 

WWTP: waster water treatment plant; ww: wet weather, dw: dry weather 

A second frequently discussed issue is the question, what are the right substance parameters to 

assess possible impacts of discharges from sewer systems? Up to now, often common 

wastewater parameters (e.g. suspended solids (SS) and chemical oxygen demand (COD)) are 

regarded (de Toffol et al., 2007). The recently released European Community Water 

Framework Directive (WFD) considers further specific pollutants (e.g. heavy metals, 

pesticides, polycyclic aromatic hydrocarbons (PAHs)) as priority substances for classifying 

the chemical status of receiving waters (EC, 2006). In Germany, emissions out of sewer 

systems (SSS and CSS) have already been identified to be a significant source of receiving 

water pollution (Irmer, 2006). As a consequence, it becomes essential to extend the list of 

considered parameters in sewer system discharges. 

Therefore, in this study five substance parameters were selected covering a representative 

range of individual substance properties. These properties are significant for occurrence and 

fate of the substances at relevant emission points out of sewer systems (Welker, 2006). 

In order to assess which sewer system has minor pollution emissions to receiving waters it is 

important to decide which is the right decisive criterion. In this field two possibilities are 

under discussion: (a) long term criteria like annual emitted pollutant loads, which is the 

common procedure in Germany and (b) short term criteria like substance concentration 

values. The focal points of this paper are the relevant emitted pollution loads that are 

calculated with pollution load simulations. 

2 Welker


METHODS 

Selection of pollutant parameters 

In a first step representative substance concentration ranges in dry weather flow, surface 

runoff and effluent of WWTP were stated summarizing reported field data (for details see 

Welker, 2004 and 2007). These values were used as input for pollution load simulations 

described below. The chosen parameters are two classical wastewater parameters (COD and 

ammonium) supplemented by three pollutants (copper (Cu), carbamazepine and estradiol 

(E2)) (see Table 1). 

Copper is a typical surface runoff derived compound that is efficiently removed in WWTPs. 

Carbamazepine is a human pharmaceutical, estradiol is a natural hormone. Both compounds 

derive exclusively from dry weather flows. Still, they have completely different elimination 

rates in the WWTP (estradiol: nearly 95 %; carbamazepine: nearly 0 %). 

Table 1. Substance concentrations in important flow types of the combined sewer system and the 

separate sewer system 

value COD NH4-N Cu carbamazepine 

E2 

[mg/l] [mg/l] [µg/l] [µg/l] [ng/l] 

dry weather flow 441 22 66 0.73 15 

surface runoff 107 1.0 116 < 0.02 * < 0.02 * 

effluent WWTP (dw) 66 2.0 13 0.73 0.8 

effluent WWTP (ww) ** 31 0.7 21 0.21 0.2 

references 1 2-4 2-5 2, 6-13 13-20 

dw: dry weather; ww: wet weather; COD: chemical oxygen demand; NH 4-N: ammonium; E2: 17-β-estradiol; * calculation 

with 50 % of the lowest reported detection limit; ** calculated by a balance 

references: 1: House et al., 1993; 2: Welker, 2004; 3: Brombach et al., 2005; 4: Smullen et al., 1999; 5: Makepeace et al., 

1995; 6: Thornton et al., 2001; 7: Reinhard et al., 2003; 8: Gross et al., 2004; 9: Webb, 2001; 10: Heberer, 2002; 11: Clara et 

al., 2005; 12: Joss et al., 2006; 13: Ternes et al., 2007; 14: Miege et al., 2007; 15: Birkett and Lester, 2003; 16: Spengler et 

al., 2001; 17: ATV-DVWK IG-5.4, 2002; 18: Schlüsener and Bester, 2005; 19: Svenson et al., 2007; 20: Johnson et al., 2005 

Pollution load simulations 

A representative and hypothetical catchment (187 ha) which was previously used for various 

theoretical studies in Germany was chosen for the simulation study (Welker et al., 1999). The 

catchment (9,900 inhabitants) has a total paved area of 98 ha and is drained by a combined 

sewer system including two CSOs and two stormwater tanks (SWT) (for details see Welker, 

2006). Calculations for the separate sewer system were obtained by transferring the chosen 

catchment into a separate sewer system. 

For pollution load simulation the software KOSMO (Kontinuierliche Schmutzfrachtmodellierung) 

has been used (Schmitt, 1993). KOSMO includes models for all relevant 

processes like surface pollution with an exponential accumulation and wash off equation. 

Surface runoff and the corresponding sewer flow were computed with a hydrological method. 

The calculated annual water volumes are illustrated in Table 2. 

Welker 3


Table 2. Calculated annual water volumes for both sewer systems 

[m³/year] combined 

separate 

sewer system sewer system 

surface runoff 570,478 570,478 

urban drainage emissions 189,538 * 570,478 ** 

effluent WWTP wet weather 556,551 - 

effluent WWTP dry weather 1,197,768 1,373,379 

effluent WWTP total 1,754,319 1,373,379 

total 1,943,857 1,943,857 

* combined sewer overflow (CSO); ** storm sewer outlets (SSO) 

RESULTS AND DISCUSSION 

Results come out with annual discharge loads of the main emissions of the two hypothetical 

catchments in the CSS (WWTP effluent and CSO) and the SSS (WWTP effluent and SSO) 

and are illustrated in Figure 2, 3 and 4. 

The results show that pollutant emissions from the two observed sewer systems (CSS versus 

SSS) vary significantly dependent on the considered parameter and its specific properties. 

Simulations for the present state 

Waste water parameters COD and ammonium 

Figure 2 shows the results for the waste water parameters COD and ammonium. 

In the case of ammonium total emissions from both sewer systems are in the same range. 

Since ammonium is predominately derived from the dry weather flow, proportion of 

discharges results by surface runoff (CSS: CSO and in SSS: SSO) in both system is very low. 

Dominant are ammonium discharges from the WWTP during dry weather (grey bars). 

For COD the SSS emits a higher amount than the CSS. In comparison to ammonium COD is 

a significant compound of surface runoff and dry weather flow. Therefore in the SSS the 

proportion of surface water related emissions has a higher relevance and is emitted mostly 

untreated into the receiving waters. This results in high annual loads of COD in the storm 

sewer outlets (black bar). In the CSS these fractions are partly treated in WWTP where the 

elimination rate is very effective. This leads to comparable lower emissions out of the CSO 

(black bar) and therefore reduced total emissions. 

4 Welker


160000 

140000 

120000 

100000 

80000 

60000 

40000 

20000 

0 

WWTP (dw) 

WWTP (ww) 

CSO or SSO 

COD CSS COD SSS NH4 CSS NH4 SSS 

Figure 2. Emitted loads of COD [kg/year] and ammonium (NH4) [kg *10/year] calculated by 

pollution load simulations 

WWTP (dw): effluent WWTP dry weather; WWTP (ww): effluent WWTP wet weather; CSO: combined sewer overflow; SSO: storm sewer outlets; CSS: 

combined sewer system; SSS: separate sewer system 

Selected pollutants copper, carbamazepine and estradiol 

Looking at the results of the three selected pollutants similar effects can be observed (Figure 

3). 

Copper is a dominant component of surface runoff and therefore occurs in significant amounts 

in the surface runoff related compounds in both systems (black bars: CSO and SSO). 

Comparing the sewer systems the CSS is clearly favourable for the receiving waters 

demonstrated by significant lower total annual loads in relation to the SSS. The reasons for 

this behaviour are similar to the effects discussed for the parameter COD (see above). 

Exclusively dry weather flow derived substances show a differing picture. In both sewer 

systems surface runoff related emissions (black bars: CSO and SSO) have a minor importance 

in comparison to copper with the exception of estradiol (see below). 

Comparing estradiol and carbamazepine, the distribution of annual loads especially in the 

CSS is differing. In case of estradiol, the proportion of annual CSO load is significant higher 

than in the case of carbamazepine. This effect is mainly caused by the different elimination 

rate of the substances in the WWTP (estradiol: nearly 95 %; carbamazepine: nearly 0 %). 

Substances like estradiol can be eliminated in a WWTP to a higher extent in comparison to 

the drainage system because the treatment in the WWTP provides retention time of hours to 

days. In contrast, in the sewer system the retention time is often less than 60 min. 

This is not relevant for the compound carbamazepine. Since it is not eliminated neither in the 

drainage system nor in the WWTP total emissions in both sewer system are in the same range. 

Clearly dominant are the emissions of the WWTP during dry weather. For estradiol and 

substances with analogous properties total emissions out of a combined sewer system are 

significant higher than out of a separate sewer system. 

Welker 5

140000 

120000 

100000 

80000 

60000 

40000 

20000 


0 

WWTP dw 

WWTP ww 

CSO or SSO 

Cu CSS Cu SSS carb CSS carb SSS E2 CSS E2 SSS 

Figure 3. Emitted loads of copper (Cu) [g/year], carbamazepine (carb) [mg/10/year] and 

estradiol (E2) [mg*10/year] calculated by pollution load simulations 



Disconnection of paved areas 

A common measure to reduce emissions from urban drainage systems is the disconnection of 

paved areas from the drainage system. This is especially true for low contaminated flow types 

like roof runoff and runoffs from street areas with a low traffic density. These flow types are 

often infiltrated to the groundwater by several devices (e.g. trenches). 

Exclusively by the example of the chosen hypothetical separate sewer system 25 % of the 

paved areas were disconnected, demonstrated in Figure 4 for the surface runoff derived 

parameters COD and copper. This value certainly represents an upper limit in existing 

systems. Within, the annual pollution loads infiltrated into the soil groundwater system are 

illustrated as white bars. 

Results for COD and copper clearly show, that even a significant disconnection of paved 

areas leads to dominant total emissions resulted from the separate sewer system. Therefore, 

also under these conditions, total emissions to the receiving waters from the combined sewer 

system are significant lower in relation to a separate sewer system. 

6 Welker


160000 

140000 

120000 

100000 

80000 

60000 

40000 

20000 

0 

infiltration loads 

WWTP (dw) 

WWTP (ww) 

CSO or SSO 

COD CSS COD SSS Cu CSS Cu SSS 

Figure 4. Emitted loads of COD [kg/year] and copper (Cu) [g/year] calculated by pollution 

load simulations for a disconnection of paved area of 25 % 



Summary of results 

An estimation on the distribution of the emissions from sewer systems and a comparison 

between a combined and a separate sewer system needs information about the predominately 

origin (surface runoff, dry weather flow) and the elimination rate in the WWTP. 

Three classes of substances with certain properties can be distinguished: 

(1) surface runoff derived substances with a high elimination rate in the WWTP 

(copper and other substances e.g. heavy metals or PAHs): Substances with these 

properties generally show significant emissions in both systems from the runoff 

related flows (CSS: CSO versus SSS: SSO). Comparing the sewer systems, the SSS 

emits higher substance loads than the CSS since these substances are eliminated in 

the WWTP to a very high extent and stormwater runoffs are usually not treated 

properly. 

(2) dry weather flow derived substances with a low elimination rate in the WWTP 

as the chosen carbamazepine and other substances e.g. EDTA: Here the effluents out 

of the WWTP, especially during dry weather, are significant. Measures to reduce 

these emissions should therefore concentrate on the WWTP. Differences between 

the sewer systems are not important for this substance group. 

(3) dry weather flow derived substances with a high elimination rate in the WWTP 

as the considered estradiol and other substances e.g. NTA or ibuprofen: In contrast 

to substances from class 1, total emissions from the CSS are much higher than from 

the SSS due to significant emissions at the CSO. This effect is a new aspect for 

exclusively dry weather flow derived substances. 

Welker 7


CONCLUSIONS 

In conclusion, a general answer, which sewer system emits higher pollution loads is not 

possible. It is extremely dependent from the considered substance with its specific properties 

and resulting behaviour in the sewer systems. Therefore it is necessary to define 

representative decisive criteria for receiving water impairment including a definition of the 

relevant substance parameters. 

Results of the pollution load simulation for the combined sewer system show that emissions 

from CSOs are significant for certain substance parameters. This finding applies especially to 

contaminants that are removed efficiently WWTP, such as estradiol. In this cases reduction 

measures should consider discharges out of the WWTP and the CSO. 

Nonetheless, is important to extend the data base on the amount of substances, especially 

organic pollutants in order to verify these first calculated results. These measurements are 

strongly recommended for the combined sewer system and for the CSOs in particular. Beside 

of the introduced substances further compounds (e.g. surfactants, other substances out of 

personal care products, further pharmaceuticals, herbicides, flame retardants) should be taken 

into account. 

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Welker 9

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