Baars, I.

Boogaard, F.

Hof, A.

Kuiper, M.

Langeveld, J.

Liefting, E.

Nijhof, H.

Schilperoort, R.

I-Revues

NOVATECH 2016 
1 
Stormwater management strategies: source control 
versus end of pipe 
Stratégies de gestion de l'eau : contrôle à la source 
versus le "tout-tuyau" 
 
Langeveld, J.G1,2, Liefting, H.J.2, Schilperoort, R.P.S.2, Hof, A.3, 
Baars, I.B.E.3, Nijhof, H.4, Kuiper, M.W4 and Boogaard, F.5 
 
1
 Partners4UrbanWater, Javastraat 104A, 6524 MJ, Nijmegen, The Netherlands 
(Jeroen.langeveld@urbanwater.nl)  
2
 Sanitary engineering, Delft University of Technology, P.O. Box 5048, 2600 GA 
Delft, The Netherlands 
3
 Municipality of Almere, P.O. Box 200, 1300 AE Almere, The Netherlands 
4
 Regional Water Authority Zuiderzeeland, P.O. Box 229, 8200 AE Lelystad, the 
Netherlands 
5
 TAUW, P.O. Box 20748, 1001 NS Amsterdam, The Netherlands 
 
RÉSUMÉ 
Les égouts pluviaux sont connus pour apporter une contribution significative aux charges annuelles de 
polluants dans les plans d’eau où ils se déversent. Les égouts pluviaux de la ville d’Almere rejettent 
les eaux pluviales de 1384 ha de zone imperméable à travers 700 exutoires d’égouts pluviaux dans le 
milieu aquatique récepteur local. Ce milieu aquatique souffre d’eutrophisation et d’une accumulation à 
long terme des niveaux de polluants dans le lit de sédiments. Afin d’être en mesure de choisir la 
stratégie la plus efficace de gestion des eaux pluviales, la municipalité d’Almere et le service des eaux 
Zuiderzeeland ont lancé un important projet de surveillance sur 2 ans afin de mesurer la qualité des 
eaux pluviales et l’impact potentiel du contrôle à la source et des mesures en bout de ligne pour 
diminuer les émissions à travers les exutoires d’égouts pluviaux. Les mesures de contrôle à la source, 
telles que l’élimination des branchements illicites et une fréquence accrue du nettoyage des bouches 
d’égout, se sont avéré les actions les plus efficaces. L’impact potentiel des solutions "tout-tuyau" 
grâce à la décantation s’est révélé très limité, en raison du faible potentiel de décantation des solides 
dans les eaux pluviales d’Almere, aux exutoires d’égouts pluviaux. 
 
ABSTRACT 
Storm sewers are known to significantly contribute to annual pollutant loads to receiving water bodies. 
The storm sewers of the city of Almere discharge the stormwater of 1384 ha of impervious area via 
700 storm sewer outfalls (SSOs) to the local receiving water system. This water system suffers from 
eutrophication and long term build-up of pollutant levels in the sediment bed. In order to be able to 
select the most effective stormwater management strategy, the municipality of Almere and Water 
Authority Zuiderzeeland have launched a 2 year extensive monitoring project to measure the storm 
water quality and the potential impact of source control and end of pipe measures to decrease the 
emission via SSOs. Source control measures, such as removal of illicit connections and increasing the 
cleaning frequency of gully pots showed to be most effective. The potential impact of end of pipe 
solutions based on settling showed to be very limited due to the low settleability of solids in the storm 
water of Almere at the SSOs.  
 
KEYWORDS 
Gully pots, illicit connections, lamella settler, monitoring, storm water quality 
SESSION 
2 
1 INTRODUCTION 
Separate sewer systems are widely applied in economically developed countries. Storm sewers are 
known to contribute significantly to the annual pollutant loads into the receiving waters and many 
cause severe degradation of urban receiving waters (House et al. 1993). In the United States, 
stormwater runoff is a major contributor to pollution of receiving waters (Lee et al. 2007). This requires 
knowledge of local stormwater characteristics to assess the relative contribution of the stormwater 
pollution to the overall load to the receiving waters as well as knowledge of the impact of storm water 
strategies. As storm water characteristics may vary significantly between locations and events (see 
e.g. Langeveld et al., 2012 for a range of values from literature), local knowledge on the stormwater 
quality is required to select the appropriate storm water management strategy. 
In Almere, The Netherlands (as presented during the NOVATECH 2013 and published by Langeveld 
et al., 2014) a very extensive monitoring project has been performed to obtain information on the 
storm water quality and the impact of source control and end of pipe measures to reduce emissions 
from storm sewer outfalls (SSOs). 
This paper shortly describes the set up of the two year monitoring project, the monitoring results and a 
comparison of the effectivity of two source control measures (removal of illicit connections and 
cleaning of gully pots) versus two end of pipe solutions (a lamella settler and a SSO sediment basin). 
2 MATERIAL AND METHODS 
2.1 System description 
Almere (194.500 inhabitants) is a city founded in 1975 after the realisation of the Flevopolder in order 
to accommodate for the increase of population in Amsterdam. In 1995, the municipality counted nearly 
75.000 inhabitants, which had doubled by the year 2000. The main type of sewer system applied in 
Almere is the separate system, with foul sewers discharging to the waste water treatment plant and 
storm sewers discharging via storm sewer outfalls (SSOs) to the receiving waters. Earlier studies 
showed that the quality of the receiving waters is to a large extent determined by nutrient rich seepage 
(Almere is situated 4 m below mean sea level) and the outflow from the SSOs.  
The sewer system of Almere comprises 680 km of storm sewer, 625 km of foul sewer, 700 SSOs and 
1384 ha contributing area. The municipality has divided the catchment into three categories: 
• I. Storm sewers dating before 1985, a period associated with the use of traditional building 
materials; 
• II. Storm sewers dating from 1985, a period associated with the use of more sustainable 
building material; 
• III. Special areas with a high risk of pollution, i.e. industrial zones.  
The municipality of Almere and regional water authority Zuiderzeeland have launched a 
comprehensive research project OSAL (Optimisation Study Almere). The objective of the OSAL 
project is to quantify the emission from SSOs as well as the impact of the SSOs on the receiving water 
quality. The project also looks into strategies/measures to improve the receiving water quality to 
comply with the water quality standards. 
The OSAL project comprises in total 7 pilot projects, figure 1 shows the 5 pilots described in this 
paper. Pilot 1 involves long term monitoring of the storm water quality at 3 SSOs. Pilot 2 analyses the 
impact of the frequency of  gully pot cleaning on the storm water quality at the SSOs. Pilot 3 quantifies 
the impact of illicit connections on the storm water quality at the SSOs. Pilot 4 studies the impact of 
cleaning of the outflow structure of the SSOs on storm water quality. Pilot 5 monitors the efficiency of a 
lamella settler for storm water treatment purposes. In the project, also the quality of ground water in 
the drains connected to the storm sewers and the impact of SSOs on the receiving water quality have 
been studied. However, the latter two are considered beyond the scope of this paper.  
The pilots have been performed at 3 selected locations, see table 1. 
NOVATECH 2016 
3 
Storm sewer Receiving
waters
Gullypot
Pilot 1. Storm water 
quality
Foul sewer ?
Pilot 3. Illicit connections
Pilot 2. Gully
pot cleaning
lamella
settlerPilot 5. storm 
water treatment
SSO sediment basin
Pilot 4. Cleaning SSO 
basin
 
Figure 1. Pilots in OSAL research project 
 
Table 1. Site characteristics 
Location Land use Ha impervious 
area 
connected 
Period of 
development 
Comment 
Baljuwstraat City center  7.7 Before 1985 Category III catchment, 
research location for pilots 1, 2, 
3 and 5  
Sluis Medium density 
residential 
7.0 Before 1985 Category I catchment, research 
location for pilots 1 and 3 
Palembang 
weg 
Medium density 
residential 
5.2 After 1985 Category II catchment, research 
location for pilots 1, 3 and 4 
 
2.2 Monitoring set up and monitoring data 
The monitoring set up at each location comprised a flow sensor, a water level sensor, a rain gauge, a 
automatic sampler and turbidity and electric conductivity sensors just upstream of the SSO. The storm 
water quality has been analysed in the lab per storm event, resulting in an event mean concentration 
(EMC) per event for the parameters listed in table 2. Based on the EMCs, volume weighed Site mean 
concentrations (SMC) have been derived. This paper only discusses the results for zinc and total 
phosphate, being the parameters related to the main water quality issues. 
Table 2. Water quality parameters 
Parameter group Parameter 
General Suspended solids, COD, chloride 
Nutrients  Kjeldahl nitrogen, ammonium, nitrate, phosphate (total P and ortho-P)  
Micro pollutants PAH10, mineral oil 
Metals As, Cd, Cr, Cu, Hg, Ni, Pb, Zn 
Hygienic quality E-Coli 37°C and 44°C 
Treatability Settling velocity 
The monitoring project started in January 2013 and lasted until July 2015. The number of events with 
a good data availability and quality adds to 50 for Baljuwstraat, 38 for Sluis to 31 for Palembangweg.  
SESSION 
4 
3 RESULTS AND DISCUSSION 
3.1 Pilot 1. Storm water quality 
Table 3 shows the storm water quality measured at the 3 locations, compared to literature data. The 
storm water in Almere is, like in Arnhem (Langeveld et al., 2012), relatively clean compared to 
international data. For the parameter zinc, location Palembangweg, shows significantly lower 
concentration levels than the other locations. This confirms the earlier results in Almere, where 
locations build after 1985 showed significantly lower zinc concentrations than locations dating before 
1985 (Langeveld et al., 2014). Based on the average storm quality of the three locations and a nett 
precipitation of 500 mm/a, the total load discharged via all the SSOs of Almere amounts to 680 kg zinc 
and 1.640 kg phosphate per year. 
 
Table 3. Storm water quality, site mean concentration 
Parameter unit Almere 
Baljuwstraat 
Almere 
Sluis 
Almere 
Palembangweg 
Range 3 
locations 
Arnhem* 
Range international 
literature** 
Zn µg/l 82 77 25 76 – 152 146 - 3330 
P-tot mg/l 0,18 0,31 0,42 0,20 – 0,26 0,35 – 3,0 
* Langeveld et al., 2012, ** International data from Bratieres et al. (2008), Salvia-Castellvi et al. (2005), 
Fuchs et al. (2004) and Daligault et al. (1999) 
 
3.2 Pilot 2. The frequency of gully pot cleaning 
The effect of gully pot cleaning has been analysed comparing two cleaning frequencies: 1 times per 
year and 6 times per year. Each time before cleaning, the sediment bed depth was measured in 20 
gully pots and samples have been taken from the sediment for analyses. Cleaning gully pots every 2 
months increases the amount of sediment removed by cleaning the gully pots with nearly a factor 3 
compared to the normal cleaning frequency of 1 times per year. Extrapolating this result for the entire 
city would mean an extra annual removal of 190 tons of sediment or (using the measured 
concentrations of 337 mg Zn/kg solids and 1900 mg P/kg solids) 64 kg zinc and 360 kg phosphate.  
Strikingly, the additional load removed in the gully pots due to an increased frequency of cleaning gully 
pots does not result in cleaner stormwater at the SSO. The stormwater quality monitoring from pilot 1 
has been continued for the second year during which the gully pots have been cleaned once every 
two months. These monitoring results show that the storm water quality did not differ significantly for 
both periods. The SMC calculated for zinc is 5% higher in the year with intensive cleaning and for 
phosphate 10 % lower. This might be due to the monitoring set up, which is designed to take a 
representative sample of the storm water flow, i.e. at a height of 30% of the diameter of the completely 
filled pipe and which does not take rolling bed load into account. Another reason could be the 
influence of the in sewer stocks, which has not been quantified in the monitoring period. 
 
3.3 Pilot 3. Illicit connections 
Before the start of the monitoring period for pilot 1, the three catchments have been analysed for illicit 
connections using distributed temperature sensing (DTS). Only in the Sluis catchment, 8 illicit 
connections have been found and repaired, in the other two catchments no illicit connections have 
been found. Based on the average wastewater production and composition per household, the annual 
load discharged via the 8 illicit connections in Sluis has been calculated. This load has been compared 
to the annual pollutant load discharged via the SSO based on the measured storm water quality in 
pilot 1. The additional load due to illicit connections amounts to +100% of the annual phosphate load 
and to +21% for the annual zinc load for Sluis. Assuming that the results for the three catchments with 
respect to illicit connections are representative for the entire city, removal of all illicit connections would 
reduce the phosphate load with 50 kg zinc and 580 kg phosphate.  
 
NOVATECH 2016 
5 
3.4 Pilot 4. Cleaning of SSO sediment basins 
Nearly 60% of the SSOs in Almere have been provided with a SSO sediment basin. These basins are 
located directly downstream of the SSO and collect the sediment discharged via the SSO. In the 
project, the basins have been cleaned 4 times a year. Before cleaning, the sediment bed height has 
been measured and samples have been taken from the sediment bed. The results show that intensive 
cleaning of all the SSO basins, could result in a removal of 42 tons of sediment annually. With an 
average concentration in the sediment of 150 mg Zn/kg solids and 2000 mg P/kg solids, 6 kg zinc and 
84 kg phosphate can be yearly removed.  
 
3.5 Pilot 5. Storm water treatment with lamella settler 
The efficiency of a lamella settler has been calculated based on the monitoring data of the influent and 
the effluent of the settler. A removal rate of 18% has been found for suspended solids. This low value 
is mainly due to the low settleability of the solids in the storm water in Almere, with only 30% of the 
solids having a higher settling velocity than 1 m/h, which is the design surface loading for the lamella 
settler. Equipping all 700 SSOs with a lamella settler, would result in an annual removal of 49 tons of 
sediment, or 29 kg zinc and 90 kg phosphate. 
 
3.6 Discussion 
Table 4 gives an overview of the results of the 5 pilots, expressed in kg phosphate and zinc per year. It 
is clear that the removal of illicit connections would have the largest influence on the annual emission 
via SSOs, followed by more frequent cleaning of gully pots. More frequent cleaning of SSO sediment 
basins or the application of lamella settlers would only have small influence on the annual emission.  
 
Table 4. Annual load and potential annual load reductions based on results pilots (kg/a) 
Parameter Annual 
emission 
SSOs Almere 
(incl. illicit 
connections) 
‘Normal 
situation’ 
Pilot 1. Annual 
emission SSOs 
Almere 
(excl. illicit 
connections) 
Pilot 2. 
Reduction 
via gully pot 
cleaning  
(6x/year) 
Pilot 3. 
Reduction via 
illicit 
connection 
removal 
Pilot 4. 
Reduction 
via SSO 
basin 
cleaning 
(4x/year) 
Pilot 5. 
Reduction 
via lamella 
settler 
Zn (kg/a) 730 680 64 50 6 29 
Ptot (kg/a) 2.220 1.640 360 580 84 90 
 
4 CONCLUSIONS 
The two year extensive monitoring campaign in Almere has resulted in an extensive dataset. This 
extended abstract shows a small proportion of the results. The presented results show that storm 
water in the Netherlands is relatively clean compared to international literature. This might be due to 
the low gradient of the catchments and storm sewers discharging below the water table of the 
receiving waters, typically resulting in low flow velocities. 
Based on the results for zinc and phospate it is concluded that: 
• Illicit connections are the most influencing factor with respect to the annual pollutant load 
• Additional cleaning of gully pots results in a significant removal of pollutants, however, this did 
not result in measurable improved storm water quality within one year. 
• End of pipe solutions, such as sediment basins and lamella settlers can only have a limited 
influence on the annual pollutant load 
• The researched source control options are more effective than the end of pipe solutions 
 
SESSION 
6 
LIST OF REFERENCES 
2000/60/EC. Directive of the European Parliament and of the Council establishing a framework for Community 
action in the field of water policy (2000/60/EC). 23 October 2000; http://europa.eu.int/eur-lex/. 
Bratieres, K., Fletcher, T. D., Deletic, A., & Zinger, Y. 2008. Nutrient and sediment removal by stormwater 
biofilters: A large-scale design optimisation study. Water Research, 42(14), 3930-3940.  
Daligault, A., Meaudre, D., Arnault, D and Duc, V. 1999. Stormwater and lamella settlers: efficiency and reality. 
Water Science and Technology 39(2) 93-101 
Fuchs, S., Brombach, H. and Weiß, G. 2004. New database on urban runoff pollution. Proceedings of 
NOVATECH 2004, Lyon, France, pp. 145-152.  
House, M.A., Ellis, J.B., Herricks, E.E., Hvitved-Jacobsen, Seager, J., Lijklema, L., Aalderink, H. and Clifforde, I.T. 
1993. Urban drainage – Impacts on receiving water quality. Water Science and Technology, 27(12), 117-158. 
Langeveld, J.G, Boogaard, F., Liefting, H.J., Schilperoort, R.P.S., Hof, A. , Nijhof, H., de Ridder, A.C. and Kuiper, 
M.W. 2014. Selection of monitoring locations for storm water quality assessment. Water Science and 
Technology 69 (12), pp. 2397-2406 
Langeveld, J.G., Liefting, H.J., Boogaard, F.C. 2012. Uncertainties of stormwater characteristics and removal 
rates of stormwater treatment facilities: Implications for stormwater handling. Water Research 46 (2012) 6868 
– 6880 
Lee, H., Swamikannu, X., Radulescu, D., Kim, S.-j., Stenstrom, M.K. 2007. Design of stormwater monitoring 
programs Water Research 41 (18), 4186-4196 
Salvia-Castellvi, M., Iffly, J.F., Guignard, C. and Pfister, L. 2005. Characterisation of urban stormwater runoff from 
separated sewer overflows in Luxembourg. 10th Int. Conf. on Urban Drainage, Copenhagen/Denmark, 21-26 
August 2005 


Stratégies de gestion de l'eau : contrôle à la source versus le "tout-tuyau"

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Stratégies de gestion de l'eau : contrôle à la source versus le "tout-tuyau"

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