Mrowiec M.

I-Revues

SESSION 6.2 
NOVATECH 2007  1259 
The improved flow diverter for first flush 
management  
Perfectionnement d'un séparateur de débit pour la gestion du 
premier flot d'orage 
Maciej Mrowiec  
Institute of Environmental Engineering, Czestochowa University of 
Technology, Brzeznicka 60a, 42-200 Czestochowa, Poland 
email : mrowiecm@is.pcz.czest.pl 
RESUME 
La construction d’un réseau d’assainissement semi-séparatif peut être une alternative 
intéressante aux solutions conventionnelles pour gérer le premier flot d’orage. Cet 
exposé présente un séparateur de débit de conception innovante permettant de créer 
un réseau semi-séparatif avec réduction de la surcharge hydraulique de la station 
d’épuration par temps de pluie. Ce séparateur Septurn fonctionne de manière fiable 
sans alimentation énergétique extérieure ni organes de commande sophistiqués et ne 
requiert qu’une maintenance minimum. Des études pratiquées en laboratoire ont 
confirmé la précision élevée du modèle mathématique sur la base duquel le 
séparateur fonctionne. Des simulations hydrodynamiques utilisant le SWMM5 (ondes 
dynamiques) et réalisées pour des d’événements pluvieux unitaires montrent une 
réduction du débit des eaux pluviales envoyé vers la station d’épuration pouvant aller 
jusqu’à 60% tout en maintenant une proportion semblable de la charge totale dirigée 
vers la station d’épuration.  
ABSTRACT 
A conception of the semi-separate sewer system can be an interesting alternative to 
conventional solutions in the field first flush management. Presented in this paper 
innovative construction of the flow diverter allows to create the semi-separate system 
with minimization of a wastewater treatment plant hydraulic overburden during wet 
weather.  Septurn diverter operates reliably without any external power, requires 
minimal maintenance and no complicated controls. Research conducted at laboratory 
scale confirmed the high accuracy of the developed mathematical model of the 
Septurn diverter. Hydrodynamics simulations using the SWMM5 (dynamic wave 
route) made for single rainfall events shows the split reduction of stormwater 
discharge to WWTP can be reduced even to 60% maintaining similar ratio of the total 
load directed to WWTP. 
KEYWORDS:  
First flush, flow diverter, stormwater. 
SESSION 6.2 
1260 NOVATECH 2007  
1 INTRODUCTION 
The stormwater management in Polish cities are still realized by the conventional 
practice – to concentrate runoff and carrying it off a site as quickly as possible 
through storm sewers to the receivers. The majority of the pollutant load is contained 
in the first portion of the event - this phenomenon of a disproportionate delivery of 
pollutants during the initial portion of a storm event is termed a “first flush” (Larson et 
al 1998). Saget et al. have defined an event as having a first flush when 80% of the 
pollutant mass is transported in 30% of the total runoff volume (Saget et al. 1995). 
This was also the definition primarily utilized in work by (Bertrand-Krajewski et al. 
1998). Research by (Saget et al. 1995) investigated events from 17 different 
catchment areas in France and concluded that the first flush phenomenon rarely 
occurred to any significant degree. This study revealed that 20% of the flow contained 
only 20 to 38% of the suspended solids load. In order to contain 80% of the total 
suspended solids loading, 60 to 150 m3/ha active ha of impervious surface would 
need to be collected. Stormwater discharges to receivers can be treated locally 
(oil/grit separators in each outfall) or central when the first flush is transported to 
WWTP making a semi-separate system (Fidala-Szope 1997, Zaizen and Matsumoto 
2005). This conception has not been applied on a wide scale in Poland but in some 
cases may be more effective than treatment  devices on each outfall (maintenance 
costs!). The diversion of first flush is crucial issues in semi-separate system. The ideal 
conception is to intercept the most of contaminants and to minimize the sewage 
quantity transported to WWTP in order to keep an optimal treatment performance.  
Inflow
to receiver
to sanitary
channel
weir
bottom orifice
 
sanitary sewer
storm sewer
river
flow diverter
 
to sanitary
channel
Inflow
to receiver
 
Fig.1. Scheme of the semi-separate sewage system and sample flow diverters: a) diverter with 
weir and bottom orifice (WO), b) cascade diverter 
Application of the semi-separate sewer system is restricted in some cases: 
• when a sanitary and a storm channel cannot be connected due to their elevation 
difference (but usually the foul channels are situated deeper than storm ones)  
• existing WWTP has no enough capacity to treat additional volume of stormwater 
discharged to the sanitary sewers during wet weather - this problem can be 
resolved by a storage tank application. 
Standard flow diverters (fig. 1a,b) have many disadvantages, the main are : 
• cascade diverters operate temporary and not reduce a pollution load when the 
flow rate  is bigger than “first flush“ flow rate (Q0); 
SESSION 6.2 
NOVATECH 2007  1261 
• diverters with transverse weir and sluice orifice (WO) operate continuously but 
cause a hydraulic overload of WWTP during long-lasting rainfalls. 
2 SEPTURN DIVERTER 
Due to abovementioned disadvantages an improved construction of flow diverter 
(fig.2) have been proposed with some initial assumptions:  
• device require no external power (low operational costs),  
• can be constructed in a channel or in a manhole,  
• the flow-rate to the sewer channel (Q0) can be adjusted to required value,  
• the volume of stormwater discharged should be proportional to total impervious 
areas rather than to total inflow volume,  
• total pollutant loads discharged to receiver are comparable to the WO diverter.  
• high concentration of organic solids have been observed in a layer moving just 
above the bed (near-bed solids). 
In proposed construction an outflow to the sanitary channel (2) is realized by the 
bottom orifice. Crucial part of the diverter is the quater turn flap valve that controls the 
flow balance – it has an upper (5) and lower part (4) moved along commonly shared 
axis of rotation. The axis is mounted on casing (3), which is also the transverse weir 
of height hP over the channel bed. The neutral position of the valve close the outflow 
channel to a receiver, so the stormwater is directed to a sanitary sewer.  
1
2
3
4
5
6
 
Fig.2. An isometric view of the Septurn diverter installed in drainage system – all remarks are 
described in the text.  
During initial phase of a rainfall, the inlet channel operate as the storage tank with the 
bottom orifice as flow regulator (fig. 3a) – the outflow-rate through the orifice can be 
calculated using the standard formula. During this phase the heavier solids settle to 
the bottom of the channel. When the depth at the inlet channel achieve the upper 
crest (fig. 3b) of the valve, torque shifts causing a rotation of the flap valve. In effect 
the bottom orifice is closed and the wave of accumulated stormwater flows through 
the weir to a receiver (fig. 3c). When flows in the system have subsided after the 
storm, the valve returns to neutral position. Then the polluted stormwater retained in 
the inlet channel below hP can be passed onward to WWTP (fig. 3d). 
SESSION 6.2 
1262 NOVATECH 2007  
QD
QB=0 QB=0
Q0<QG
Q0=QG
QD
Q0=0
QD QB=QD
inlow 
channel
flap 
valve
to receiver     
to sanitary 
sewer
QB=0
Q0>0
QD=0
casing
a) b)
c) d)
bottom 
orifice
counterweight
 Fig.3. Main operational phases of the Septurn diverter during wet-weather flow.  
 
3 MATHEMATICAL MODEL OF THE SEPTURN DIVERTER 
Mathematical model of the Septurn diverter have been developed based on the 
equilibrium of turning moments from hydrostatic pressures and specific weights of the 
valve. The rotation of the valve for initial angle α (fig.4) is possible when the following 
inequality is satisfied: 
PG G KD D KG G PD D 0 0r P r G r G r P r G⋅ + ⋅ + ⋅ > ⋅ ⋅ + ⋅  
where: 
DG , GG - weight of the upper and lower part of the valve, [N],  
0G  - counterweight (include buoyancy), [N], 
DP , GP  - hydrostatic forces on upper and lower part of the valve, [N], 
PDr , PGr   - arm of hydrostatic forces on upper and lower part of the valve, [m], 
KDr , KGr -  arm of the weights (upper and lower part of the valve), [m], 
0r   - arm of the counterweight, [m], 
β  - angle between the counterweight and horizontal axis, [°], 
The hydrostatic forces on the upper part of valve is proportional to depth at the inlet 
channel. Because the total area of the upper part is larger than for lower one it’s 
possible to precisely define depth hR that causing rotation. 
Counterweight is the crucial part of the device: 
• it is the regulation element allows to adjust the depth corresponding to actuating 
torque, 
• makes possible to return of the valve to neutral position when the water level 
drops to hP. 
SESSION 6.2 
NOVATECH 2007  1263 
 
Return to the neutral position acts under following  condition: 
KD D KG G PD D 0 0r G r G r P r G⋅ + ⋅ < ⋅ ⋅ + ⋅  
where 0G - counterweight (unsubmerged, without buoyancy), [N], 
PG
PD
rPGrPD
G0
α
β
GG
r0
h
GD
 
Fig.4. An scheme for mathematical model of the Septurn diverter.  
 
Theoretical model have been verified on physical model at laboratory-scale – the 
main dimensions of the model: inlet channel diameter 0,15m, crest height 
0,015÷0,045m, weir length 0,14m, max. orifice diameter 0,025m (adjustable). Small 
diameter of the pipe in the model preclude sedimentation processes modeling,  
therefore the verification tests was aimed to quantity aspects (to determine depth hR). 
Some trial test shows a TTS load tends to be split between spill and orifice flow 
similarly as for WO diverter - the total efficiency factor is higher for the Septurn, but 
differences have not exceed 10%. The technical-scale pilot test have to be done in 
order to achieve a credible results. 
 
0,000 0,005 0,010 0,015 0,020 0,025 0,030 0,035 0,040
0,06
0,07
0,08
0,09
0,10
 calculated
 measured
D
ep
th
 h
R
, [
m
] 
counterweight mass, [kg]
 
Fig.5. Laboratory investigations on Septurn diverter: a) model during detention phase b) results 
comparison between measured and calculated values of hR for varying mass of counterweight. 
 
SESSION 6.2 
1264 NOVATECH 2007  
The verification process was developed for variable geometric dimensions of the turn 
valve (width, mass of counterweight etc.). The depth hR have been measured and 
compared to calculation results for 25 cases. Relative differences between the values 
have been on average about 3÷7%. The error is the result of the theoretical model 
assumptions - the mechanical losses on the axis and the velocity head (only 
hydrostatic forces was included) have not been taken into consideration. For the 
engineering purposes such error values can be considered as enough precise.   
4 PERFORMANCE ASSESSMENT OF THE SEPTURN DIVERTER 
The second stage of research was focused on a comparison of the Septurn and WO 
diverter efficiency. The hydrodynamic model (fig.6) of both diverters have been 
simulated in SWMM5 application for dynamic wave routing (solving the full Saint-
Venant equation). The model main dimensions: inlet channel diameter 1200mm, 
slope 0,2%, crest height 0,3m, orifice diameter: 0,3m (WO) and 0,4m (Septurn). Five 
rainfall events measured on the urban watershed (Czestochowa city) had been 
selected to run the simulations – duration from 30 minutes to 4 hours. In order to 
simulate the Septurn operation phases a three control rules have been defined for the 
weir and the orifice (i.e. : Rule R1: if node 2 depth > 1.15 then weir 1 setting = 1 and 
orifice 1 setting = 0) 
 
Fig.6. Scheme of the Septurn diverter in the SWMM5.   
 
The operational performance of the diverters (both Septurn and WO) have been 
expressed using terms (Butler and Davies 2000): 
• flow split factor KF (storm volume directed to sanitary sewer / total storm inflow 
volume) 
• total efficiency factor KE (storm load directed to sanitary sewer/ total storm inflow 
load) 
• treatment factor KT (total efficiency / flow split) 
Simulation results have been presented as outflow hydrographs to sanitary sewer and 
to receiver (fig. 7). On the base of these hydrographs, the total volume of stormwater 
flowing through the orifice/weir was determined. For WO diverter a relationship 
between split factor KF and rainfall duration is linear (from 0,12 for t=30m to 0,22 for 
t=4h), whereas the Septurn diverter is characterized by quasi-constant value (about 
4,2%). The differences between both devices were much higher considering absolute 
values, for 30min rainfall the total volume discharged to sanitary channel was equal to 
91 m3 (Septurn) and 254 m3 (WO) while for 4-hour rainfall was 203 m3 and 926m3 
respectively.  
 
 
SESSION 6.2 
NOVATECH 2007  1265 
  
Fig.7. Hydrographs comparison for 4-hours rainfall: a) WO diverter , b) Septurn diverter 
 
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5
0,00
0,04
0,08
0,12
0,16
0,20
0,24
 WO diverter
 Septurn diverter
Fl
ow
 s
pl
it 
fa
ct
or
 K
F
Rainfall duration [h]
 
Fig.8. Relationship between flow split factor and rainfall duration.  
 
Simulation results clearly show the split factor for the Septurn diverter has 
significantly lower values in comparison to WO separator for equivalent crest height. If 
the total efficiency factor has similar values for both devices (need to be confirmed at 
technical-scale) the treatment factor KT is also expected to achieve significantly better 
values for the Septurn diverter. The final assesstment of a flow diverter should be 
based on long-term simulations rather than single event but the obtained results for 
five different rainfalls suggest that long-term simulations will show similar, positive 
trends for the Septurn diverter.  
The main operational problem is the re-suspension of accumulated solids when the 
valve turns and create a wave of a high turbidity (peak flows) and cause a sediment 
resuspension. This disadvantage can be avoided when the valve is gradually opened. 
SESSION 6.2 
1266 NOVATECH 2007  
5 CONCLUSIONS 
Modern design of drainage systems have to concern with the quantity (i.e. flood 
prevent) as well as quality aspects (i.e. rivers an streams protection) of stormwater 
conveyance. Semi-separate sewage system allows to achieve both targets at 
relatively low investment costs. Presented construction of the flow diverter (Septurn) 
makes possible to reduce volume discharges to WWTPs during wet weather.  
Laboratory-scale tests positively verified the quantity mathematical model of the 
Septurn  - the relative errors were not exceeded 7%. Due to model scale some flow 
conditions (i.e. flow regime, velocity values) in the inflow channel have not been taken 
into consideration, so a larger scale testing is needed in order to evaluate hydraulic 
performance and reliability (i.e. potential blockage problems) of the flow diverter. 
Quantity aspects (TTS load split) are going to be tested at technical-scale on a 1.0 
meter diameter conduit in urban catchment of Czestochowa. 
REFERENCES  
Bertrand-Krajewski, J., Chebbo, G. and Saget, A. (1998). Distribution of Pollutant Mass vs 
Volume in Stormwater Discharges and the First Flush Phenomenon. Water Resources, vol. 
32, No. 8, pp 2341-2356. 
Butler D. and Davies J.W. (2000). Urban drainage. E & FN Spoon, London. 
Fidala-Szope M. (1997), Surface water protection from the discharges of stormwater. Institute of 
Environmental Protection, Warsaw, (text in polish) 
Larson, T., Broch, K. and Anderson, M.R. (1998). “First Flush Effects in an Urban Catchment 
Area in Aalborg.” Water Science Technology, Vol. 37, No. 1, pp 251, 257. 
Lee, J.H., Bang, K.W., Ketchum, L.H., Choe, J.S. and Yu, M.J. (2002). First Flush Analysis of 
Urban Storm Runoff. The Science of the Total Environment, Vol. 293, pp 163-175. 
Saget, A., Chebbo, G. and Desbordes, M. (1995). “Urban Discharges during Wet Weather: What 
Volumes have to be Treated?” Water Science Technology, Vol. 32, No. 1, pp 225-232. 
Zaizen M., Matsumoto Y. (2005). A Proposal of Combined Separate Sewer System (CSSS). 10th 
International Conference on Urban Drainage, Copenhagen, 21-26 August. 


Perfectionnement d'un séparateur de débit pour la gestion du premier flot d'orage

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Perfectionnement d'un séparateur de débit pour la gestion du premier flot d'orage

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