Vaes G.

I-Revues

SESSION 2.1 
NOVATECH 2007  303 
LIBRA : prioritizing investments in combined 
sewer systems  
LIBRA : planifier les investissements pour les systèmes 
d'assainissement unitaires 
 
Van den Broeck S.*, Vaes G.** 
*  Vlaamse MilieuMaatschappij, Bondgenotenlaan 140, 3000 Leuven, 
s.vandenbroeck@vmm.be 
** HydroScan, Tiensevest 26/4, 3000 Leuven, guido.vaes@hydroscan.be 
 
 
RESUME 
Pour établir des plans d’investissements, le plus grand problème est la comparaison 
plus ou moins objective des projets qui sont souvent d’une nature complètement 
différente. LIBRA est un instrument qui est développé pour évaluer des projets 
d’assainissement concernant leur impact. Cet instrument est basé sur le 
comportement hydrologique des systèmes d’assainissement (unitaires) en utilisant 
les résultats des simulations continus et à long terme avec un logiciel simplifié.  
LIBRA utilise l’information et les mesures disponibles et des paramètres faciles à 
estimer.  Tous les résultats sont exprimés dans la même unité finale.  L’utilisation 
pratique de LIBRA pendant deux années de plans d’investissements prouve qu’il est 
possible de comparer l’impact de projets complètement différents d’une manière 
standardisée et d’établir des classifications réalistes comme aide à la décision qui 
soutient la stratégie de l’agence environnementale. 
 
ABSTRACT 
 
When making up investment plans for urban drainage and waste water treatment, 
a major problem is the comparison of projects of completely different kind and this in 
a more or less objective way. The LIBRA tool has been built for the evaluation of 
projects on urban drainage and treatment with respect to their impact.  This tool is 
based on the hydrological behaviour of (combined) sewer systems using the results 
of continuous long term simulations with a reservoir model.  LIBRA uses readily 
available data and measurements. The required data for the analysis of projects are 
general parameters that can relatively easily be estimated.  All results are expressed 
in the same final unit. The practical use of this tool during two years of investment 
planning shows that it is possible to compare the impact of completely different types 
of projects in an standardised way and to make up realistic rankings as decision 
support for investment plans, which support the policy of the environmental agency. 
 
KEYWORDS 
combined sewer overflows, investments, optimisation, reservoir model, source 
control, waste water treatment 
SESSION 2.1 
304 NOVATECH 2007  
1 INTRODUCTION 
 
As Belgium is a federal state the regions (i.e. Flanders, Wallonia, Brussels) are fully 
responsible for all aspects concerning the environment.  Concerning waste water 
treatment in Flanders, one has to distinguish between the local and the supra-local 
level. The local level is responsible for the collection of the discharges of the 
households in sewers. The supra-local level is responsible for the transport of 
the collected wastewater via main sewers and for the treatment of the wastewater 
at the Urban Waste Water Treatment Plants (UWWTP’s). Both for the local and 
the supra-local level substantial investments are needed in new infrastructure and 
in the optimization of existing infrastructure in the context of the Water Framework 
Directive.  The major part of the sewer systems in Flanders are combined sewer 
systems, although the last decades priority is given to source control and separate 
sewer systems. 
 
2 DEFINITION OF THE PROBLEM 
 
Concerning the Flemish Region, the Flemish Environment Agency (VMM) is 
responsible for drawing up the investment plans for the supra-local level and for the 
assignment of subsidies to the local authorities.  A major problem in this task is how 
to compare investments with a very different nature.  
For example how can we compare :  
• a project that aims to raise the capacity of a UWWTP 
• a project that aims to build extra storage capacity in the collecting system 
• a project that aims to disconnect non-polluted waters (e.g. runoff water from 
fields or drainages) from the sewerage 
• a project that aims to connect  the households formerly treated by septic tanks or 
other individual treatment 
• … 
In order to draw up the most effective investment plan a balanced comparison is 
crucial.  Moreover, this comparison must be based on readily available data and 
measurements and the results must be expressed in the same units.  A similar 
approach was already worked out for the evaluation of the exploitation of supra-local 
transport and treatment of combined waste water using performance indicators based 
on readily available measurements and expressing the results in untreated Inhabitant 
Equivalents (Vaes et al., 2005). 
 
3 THE SOLUTION 
 
To solve this problem the Flemish Environment Agency has developed a tool called 
LIBRA (Latin for balance).  
SESSION 2.1 
NOVATECH 2007  305 
The preconditions for this tool were  : 
• Stimulating remediation at the source of the problem instead of end-of-pipe 
• Taylor-made approach for each collecting system (no strong generalisations) 
• Take Combined Sewer Overflows (CSO’s) into account in a balanced way 
 
The LIBRA tool is based on the reservoir model Remuli (developed at the 
K.U.Leuven) (figure 1) (Vaes, 1999) which takes into account  : 
• The storage capacity 
• The capacity of the UWWTP 
• The connected load (inhabitants and industrial) 
• The connected surface (runoff) 
• The measured dry weather flow (Vaes et al., 2005) 
• The parameters of the project 
The scientific foundations of the model were worked out by HydroScan, based on the 
reservoir model Remuli (Vaes, 1999).  This is a conceptual model that runs with 
continuous long term simulations (100 years).  In this way a good estimation of the 
overall performance of the CSO’s on the long term can be assessed as well as the 
variation of inflow in the treatment plant.   
 
 
Figure 1 : Schematic overview of the Remuli modelling system (Vaes, 1999). 
 
SESSION 2.1 
306 NOVATECH 2007  
With this Remuli model the effect of different measures on the overflow emissions 
(frequency, volume) and on the treated volumes can be assessed for a wide range of 
wet weather conditions.  Figure 2 represents the variation of overflow frequencies 
for a wide range of specific storage volumes and overcapacities (= (capacity – DWA) 
/ contributing area), which are the results of the Remuli model. A similar relationship 
is used for the overflow volumes (figure 3). 
 
The LIBRA tool makes a volume balance for the system before and after a project 
based on the simulations of the reservoir model.  The results are expressed in 
Inhabitant Equivalents (IE) that on the one side go to the treatment plant and on the 
other hand are spilled at the CSO’s based on the ratio of volumes.  An Inhabitant 
Equivalent corresponds to the waste water production of one person per year. 
Also the effect at the input side is taken into account, e.g. how much more pollution is 
taken up by the system in order to transport it to the treatment plant, how much 
rainfall runoff is disconnected through source control measures, ...  This input aspect 
is important to support the source control strategy of the Flemish government. 
 
 
Figure 2 : Overflow frequencies calculated with a single reservoir model with a constant 
throughflow and a concentration time of 60 minutes for the Belgian rainfall (Vaes, 1999). 
 
SESSION 2.1 
NOVATECH 2007  307 
 
Figure 3 : Overflow volumes calculated with a single reservoir model with a constant throughflow 
and a concentration time of 60 minutes for the Belgian rainfall (Vaes, 1999). 
 
By using this model, it is possible to compare projects of different type in different 
drainage systems in a balanced way.  The possible projects that can be evaluated 
are : 
• Extra pollutant loads connected (households or industrial) 
• Disconnection of infiltration flow or pervious areas 
• Disconnection of impervious area (source control, separate systems) 
• Additional UWWTP capacity (treatment plant upgrade) 
• Additional storage in the combined sewer system (new pipes or storage tanks) 
In figure 4 an example of using the LIBRA tool is shown. 
 
Additional to the basic calculation of the mean impact, an extra tool was incorporated 
to estimate also the uncertainty on the result.  For the main input parameter a range 
can be specified in which the mean value is situated.  Using Monte Carlo simulations 
the uncertainty on the result can be calculated.  If this is done for different input 
parameters separately, the most sensitive input parameters can be determined and 
a better assessment of the mean value and the possible variation of this input 
parameter can be carried out in order to make the final result less uncertain. 
 
 
SESSION 2.1 
308 NOVATECH 2007  
LIBRA : Calculation tool for the assessment of the Inhabitant Equivalent of a project
project name
number UWWTP 166
name treatment area Lier
general parameters mean values
runoff coefficient impervious 0.8
runoff coefficient pervious 0.3
reduction pervious to impervious 0.375
theoretical impervious area 85 m²/IE
mean rainfall 760 mm/year
theoretical dry weather flow 150 l/day/IE
system storage 7.0 mm
parameters for the UWWTP and area
number of inhabitants 25250 inhabitants
industrial discharge 0 m³/day 
industrial load 0 IE
real dry weather flow 12000 m³/day
capacity UWWTP 25000 m³/day
equivalent impervious area 0 m²
storage 0 m3
theoretical dry weather flow 3788 m³/day
estimated equivalent impervious area 2146250 m²
estimated overcapacity 0.252 mm/h
specific storage 7.0 mm
correction overflow 79.85%
specific throughflow 95.38%
specific overflow volume 20.15%
estimated overflow frequency 21.1 days
project parameters
How much load is theoretically extra connected ? 2500 inhabitants
0 m³/day 
How much impervious area is disconnected ? 0 m2
How much pervious area is disconnected ? 0 m2
How much extra throughflow capacity is created ? 3500 m³/day 
How much extra storage is build ? 1500 m3
calculations for the project
predicted dry weather flow 12375 m³/day 103.13%
predicted equivalent impervious area 2358750 m² 109.90%
predicted overcapacity 0.285 mm/h 112.86%
predicted mean storage 7.0000 mm 100.00%
predicted correction factor overflow 81.46% 102.02%
predicted throughflow 95.53% 100.17%
predicted specific overflow volume 18.54% 91.99%
predicted overflow frequency 19.6 days 92.54%
global result
Calculated equivalent inhabitants for the project
1200 IE 4.75%
2428 IE 9.62%
-56 IE -0.22%
total Inhabitant Equivalent 3572 IE 14.15%
effect on overflow
relative effect on the input
How much parasitic flow (e.g. infiltration) is disconnected ?
theoretical values
measured values
calculated values
effect on throughflow to UWWTP
 
Figure 4 : Example of the use of the LIBRA tool. 
SESSION 2.1 
NOVATECH 2007  309 
4 THE RESULTS 
4.1 Theoretical test 
 
Before using this methodology in drawing up the investment plans, sensitivity tests 
were carried out to have a decent view on the behaviour and extrapolation limits of 
the model. From these tests we have learned that LIBRA meets the initial 
preconditions and the results correspond with the expectations and the policy. 
 
4.2 Practical use 
 
This model has, until now, been used as assistance tool in two supra-local annual 
investment plans for the Flemish Region (approximately 150 projects were 
calculated). For all the projects that are applied for the impact is calculated and the 
projects are ranked.  Apart from the results of the LIBRA model, practical 
considerations (combination with projects of other departments, social and 
economical impact, …) also play a role in the drawing up of the investment 
programme. 
 
Some examples of practical application (the results are expressed in IE and as 
a percentage of the global number of IE connected to this UWWTP) : 
• Project 1 : extra connections + upgrade UWWTP 
o extension of UWWTP with 3500 m3/day 
o extra load of 2500 inhabitants connected 
o 1500 m3 extra storage in new pipes 
Global result (see also figure 4) :  
1200 IE 4.75%
2428 IE 9.62%
-56 IE -0.22%
total Inhabitant Equivalent 3572 IE 14.15%
effect on overflow
relative effect on the input
effect on throughflow to UWWTP
 
• Project 2 : separate system in stead of combined 
o extra load of 100 inhabitants connected 
o 1000 m3/day parasitic flow disconnected 
o 10 ha impervious area disconnected 
o decrease of storage in the pipes with 100 m3 
Global result (same basic data as for project 1 in figure 4) :  
1936 IE 7.67%
197 IE 0.78%
569 IE 2.25%
total Inhabitant Equivalent 2702 IE 10.70%
effect on overflow
relative effect on the input
effect on throughflow to UWWTP
 
The first subresult “relative effect on the input” concerns the impact on the input to the 
drainage system, e.g. source control measures.  The second subresult “effect on 
throughflow” concerns the difference in treated volumes in the UWWTP.  The third 
subresult “effect on overflow” concerns the difference in spilled volumes at the CSO’s. 
SESSION 2.1 
310 NOVATECH 2007  
5 CONCLUSIONS 
 
The LIBRA tool has been built for the evaluation of projects with respect to their 
impact.  This tool is based on the hydrological behaviour of (combined) sewer 
systems using the results of continuous long term simulations with the reservoir 
model Remuli.  The input parameters for the tool are readily available data and 
measurements and the results are for all types of projects expressed in the same 
units, i.e. Inhabitant Equivalents. This tool makes it possible to compare the impact of 
completely different types of projects and to make up rankings as decision support for 
investment plans. 
 
LIST OF REFERENCES  
Vaes G. (1999). The influence of rainfall and model simplification on combined sewer system 
design. PhD, K.U.Leuven, Belgium. 
Vaes G., Swartenbroekx P., Provost F., Van den Broeck S. & Bourgoing L. (2005). Performance 
indicators for waste water treatment and urban drainage system. 10th International 
Conference on Urban Drainage, Copenhagen, Denmark, August 2005. 
Vaes G., Willems P. & Berlamont J. (2005). Filter method for infiltration flow quantification. 
10th International Conference on Urban Drainage, Copenhagen, Denmark, August 2005. 


LIBRA : Planifier les investissements pour les systèmes d'assainissement unitaires

http://documents.irevues.inist.fr/bitstream/2042/25238/1/0303_261vandenbroeck.pdf

LIBRA : Planifier les investissements pour les systèmes d'assainissement unitaires

Abstract

Similar works

Full text

Available Versions

I-Revues