Combined sewer system versus separate system – a comparison of ecological and economical performance indicators

Abstract

This paper aims at comparing the cost-effectiveness of the two main types of urban drainage systems, that is, the combined sewer system and the separate sewer system, based on the analysis of simulations. The problem of which of the two systems is better was heavily discussed over the years and the answer given to the question was usually: ‘it depends’. In this work, specific impacts are investigated in terms of a cause–effect analysis. The results are subsequently summarized and can help in the choice of the system to be implemented. Despite earlier reasoning, studies on river water quality strongly indicate that the separate system is not always the preferable solution because the polluted runoff from the street, containing e.g. different heavy metals, is discharged directly into the river. This analysis aims to compare the two different sewer systems on the basis of literature data and simulation of specific cases. The results are evaluated, as suggested in the EU-Water Framework Directive, on the basis of different assessment criteria: river water quality and morphology impacts, emissions and costs.

Separate and combined sewer systems: a long-term modelling approach

“…These values were assumed representative of the average conditions for the analysed schemes. This approach is consistent with previous studies (Lau et al 2002;Calabrò & Viviani 2006;De Toffol et al 2007;Fu et al 2008) and can be considered …”

“…The analysis provided the following insights: † A separate sewer system is worse in terms of total pollutant mass discharged into the RWB for both continuous and intermittent discharges. Indeed, separate sewer systems discharge considerable pollutant loads via their overflow structures into the RWB if no stormwater treatment is implemented (De Toffol et al 2007). † Separate and combined sewer system schemes with CSO and STs exhibit a similar behaviour: the total mass discharged into the RWB is almost the same, although the combined scheme discharges a slightly higher mass.…”

“…At the end of the 20th century, a common trend towards separate sewers was established in an effort to better preserve the quality of the RWB by preventing CSO and to allow for easier optimisation of the WWTP (Gagliardi & Viviani 1983;Brombach et al 2005;De Toffol et al 2007). However, recent studies have demonstrated that separate sewers might not be the best solution with respect to the RWB quality state (Paoletti & Sanfilippo 2004;De Toffol et al 2007).…”

“…At the end of the 20th century, a common trend towards separate sewers was established in an effort to better preserve the quality of the RWB by preventing CSO and to allow for easier optimisation of the WWTP (Gagliardi & Viviani 1983;Brombach et al 2005;De Toffol et al 2007). However, recent studies have demonstrated that separate sewers might not be the best solution with respect to the RWB quality state (Paoletti & Sanfilippo 2004;De Toffol et al 2007). Indeed, generally the choice of one system instead of another is driven by the establishment of a compromise between the following opposing requirements: (i) limiting the flow rate diverted to the WWTP in order to limit possible inefficiency resulting from overload of the plant and (ii) attenuating the detrimental effect of the discharged untreated flow.…”

Economic analyses for optimizing the construction of separate sewer in a hybrid sewer system

“…The separate sanitary sewer is specifically designed for domestic wastewater whereas a separate storm sewer is used for stormwater or rainwater (Van der Tak et al 1993). Although the separate sewer is generally more expensive than the combined sewer (Rauch et al 2002;Toffol et al 2007), it may not be the preferable system for protecting river water quality. This is due to the fact that non-point source pollution such as surface runoff from the street that possibly carries various pollutants is discharged directly into the river (Brombach et al 2005;Toffol et al 2007).…”