Radiotracer investigation and modelling of an activated sludge system in a pulp and

paper industry

Metali Sarkar1, Vikas Kumar Sangal1, Vijay Kumar Sharma2, Jitendra Samantray2, Haripada
Bhunia1, Pramod Kumar Bajpai1, Anil Kumar3, Anil K. Naithani3 and Harish Jagat Pant2*

1: Department of Chemical Engineering, Thapar University, Patiala-147004, Punjab, India

2: Isotope and Radiation Applications Division, Bhabha Atomic Research Centre, Mumbai-
400085, India
3: Shreyans Paper Ltd., Ahmedgarh, Sangrur, Punjab-148021, India
* Corresponding author: hjpant@barc.gov.in

Abstract: A radiotracer investigation was carried out in an activated sludge system (ASP) of
an effluent treatment plant in a pulp and paper industry. The system consists of an aeration
tank and secondary clarifier connected in series. The primary objective of the investigation
was to investigate hydraulic performance of the ASP. Residence time distributions (RTD) of
wastewater were measured in an aeration tank and a secondary clarifier of the system using
Iodine-131 as radiotracer. The measured RTD data was treated and hydraulic residence times
were estimated. No bypassing and dead volume was found to exist in the aeration tank.
However, the dead volume in the secondary clarifier was estimated to be 4.6%. The treated
curves were further simulated using suitable mathematical models and detailed flow pattern
were deciphered.

Keywords: Activated sludge process, Aeration tank, Clarifier, Settling tank, Radiotracer,
Iodine-131, Residence time distribution, Mean hydraulic residence time, bypassing, dead
volume.

Highlights:

 A radiotracer investigation was carried out in a activated sludge system

 RTDs of aeration tank and secondary clarifier were measured
 Mean hydraulic residence times and dead volumes were determined
 The measured RTDs were simulated using suitable models
1. Introduction
Pulp and paper industry is highly dependent on water for most of its processes. Water is used
in all major stages of the paper production, including preparation of raw materials (e.g.
pulping and bleaching) and operation of various machines. Water is also used for cooling,
materials transport, equipment cleaning, and operation of general facilities and to generate
steam for use in both thermal and mechanical processes as well as on-site electricity
generation. All these processes produce a significant amount of wastewater. The quality and
quantity of wastewater generated vary according to the process adopted by specific industry
(Kamali and Khodaparast, 2015; Thompson et al., 2001). The untreated effluent contains
considerable amount of organics and toxic pollutants, which when discharged in the open
atmosphere or waterbodies causes adverse effect on human being and the environment. The
hostile impact of wastewater produced in pulp and paper industries on aquatic system has
been studied and reported by many researchers, which includes excessive solid accumulation,
eutrophication, oxygen depletion in the receiving bodies and chemical accumulation in
aquatic food web (Afroz and Singh, 2014; Ali and Sreekrishnan, 2001; Andersson et al.,
1988; Owens, 1991). Over the years the growing concern about the environment has led
water intensive industries such as pulp and paper industry to implement necessary treatment
processes to treat wastewater to predefined standards before final disposal in the open
atmosphere.
Various wastewater treatment technologies comprising a combination of primary,
secondary and tertiary treatment processes are applied depending on the degree of treatment
required (Bajpai, 2001). As the wastewater produced in pulp and paper industry contains a
substantial amount of organics, biological treatment becomes an essential part of the
treatment process. Activated sludge process (ASP), developed in the early 1900, remains one
of the most widely used and efficient process for biological treatment of pulp and paper
wastewater (Thompson et al., 2001). ASP involves a mixing tank with suspended microbe
culture that is aerated externally to maintain the aerobic conditions in the tank to breakdown
dissolved organic pollutants in the effluent wastewater into biomass (Peavey et al., 2013;
Tchobanoglous et al., 2003). The aeration system consume around 45% to 75% of the total
energy requirement of the wastewater treatment plant (WWTP) resulting in high operating
cost (Karpinska and Bridgeman, 2016; Rieger et al., 2006), Therefore the proper functioning
of ASP is essential both from environmental and economic point of view.
The efficient operation of an ASP depends upon optimum hydrodynamic parameters
i.e. mean hydraulic residence time, extent of dead volume and flow patterns, and occurrence
of malfunctions such as bypassing, channelling, recirculation, adequate mixing etc.
Measurement and analysis of residence time distribution (RTD) of wastewater in the aeration
tank and the secondary clarifier can provide insight to their hydrodynamic performance.
Conventional tracer like dyes, salts and chemicals for instance rhodamine, LiCl, ammonium
have been used to measure RTD of wastewater and study flow anomalies, mixing
characteristics and evaluation of empirical formulae in activated sludge reactors (Bode and
Seyfried, 1985; Burrows et al., 1999; Gresch et al., 2011; Makinia and Wells, 2005;
Kjellstrand et al., 2005). However, there are many disadvantages associated with the
conventional tracer techniques such as cumbersome and manual offline monitoring, relatively
higher degree of inaccuracy involved etc. All these disadvantages are overcome by the use of
well-established radiotracer techniques (IAEA, 2008; IAEA, 2011). Several authors have
used these techniques for measurement of RTD in various systems in wastewater treatment
plants (Ambrose et al., 1957; Burrows et al., 1999; Debien et al., 2013; Jung et al., 2005;
Kasban et al., 2010; Kumar et al., 2012; Makinia and Wells, 2005; Pant et al., 2000; Pant et
al., 2009; IAEA, 2008; IAEA, 2011; Othman and Kamarudin, 2014). Farooq et al., (2003)
investigated a municipal sewage treatment plant to investigate efficiency of primary clarifier,
aeration tank and secondary clarifier using Bromine-82 as radiotracer. Kim et al., (2005) also
studied the flow characteristics and distribution of solids in secondary clarifier. Although
radiotracer experiments have been performed on wastewater treatment processes but results
obtained cannot be generalized as every single process uses different raw material, design
parameters and operating conditions. The available literature also indicates that activated
sludge process of wastewater treatment in pulp and paper industry has not been studied for
their performance and flow characteristics using radiotracer technique. The present paper
describes measurement and analysis of RTD of wastewater in aeration tank and secondary
clarifier of a wastewater treatment plant based on activated sludge process in a pulp and
paper industry and evaluate the hydraulic performance of the process.

2. Experimental

2.1. Process Description

The schematic diagram of the ASP is shown in in Fig. 1. The activated sludge comprises of
an aeration tank connected in series with a secondary clarifier. The aeration tank was 76 m
long, 18 m wide, 4 m deep with a capacity of 5472 m3. The effluent from a primary clarifier
is fed into the aeration tank at a flow rate of 5.21± 1% m3/min. The tank that contains a
predefined population of active microorganisms suspended in the tank. These
microorganisms use the organic pollutants in the wastewater as a food and convert them into
a biomass at aerobic conditions. Aerobic condition in the tank is maintained by keeping
oxygen concentration between 1.5 - 2.0 mg/l with the help of diffused aeration system fitted
with fine bubble membranes connected to a centrifugal blower. Six mechanical surface
aerators are installed in the tank to homogenize and ensure adequate mixing of wastewater
and air. The secondary clarifier of capacity of 944 m3 is a circular settling tank with a
peripheral feed having diameter of 18 m and height of 3.71 m. The treated water from the
aeration tank flows into the secondary clarifier. The secondary clarifier act as a settling tank
where biomass settles at the bottom and the same is subsequently removed. About 20% of the
flow is recycled back to maintain the mixed liquor suspended solids (MLSS) of 3500 mg/l in
the aeration tank and the remaining waste sludge is removed for further treatment and
ultimate disposal.

2.2. Radiotracer Experiment

The radiotracer experiments were performed in an activated sludge system meant for
wastewater treatment at M/s Shreyans Paper Ltd, Ahemedgarh, India. The experimental setup
is shown in Fig. 1. Stimulus-response approach was used to measure the RTD of the
wastewater in aeration tank and secondary clarifier. Iodine-131 (half-life: 8 days, gamma
energy: 0.36 MeV) as KI supplied by Board of Radiation and Isotope Technology (BRIT)
Mumbai, India,was used as a radiotracer to measure RTD of the wastewater. The radiotracer
(activity: 30 mCi in volume 5 ml) was diluted in about 2 litre of wastewater and
instantaneously injected at the inlet of the aeration tank as shown in Fig. 1. Two different
RTD runs were carried out. In first experiment, the radiotracer was injected at the inlet of the
aeration tank and monitored at the several locations of the aeration tank and the secondary
clarifier. In first run, eight NaI (TI) scintillation detectors D1, D2, D3, D4, D5, D6, D7 and
D8 shown in Figure 1,were used. Whereas, in the second run, the radiotracer was injected at
the inlet of the secondary clarifier and monitored at the inlet, centre and, outlet of the
clarifier. Since monitoring locations were quite far from each other, it was not possible to
connect all the detectors to one common data accusation system (DAS) due to limited length
of the detector-DAS connecting cables. Therefore, the detectors were connected to two
independent DASs. The data acquisition systems were connected to independent laptops and
set to record data points with a sampling time of 300,000 milliseconds. Both the DASs were
switched on to acquire at the same time. The radiotracer was injected after the detectors
acquired sufficient number of data points.

3. Data treatment and analysis

The measured radiotracer concentration data usually contains many undesired influences and
thus needs to be treated before drawing any useful information and modelling the data. The
data treatment involved, zero-shift, background correction, radioactive decay correction, data
filtering and smoothing, tail correction and normalization using a RTD analysis software
(Žitný, 1996, IAEA,2008; Kasban et al., 2010). After treatment the RTD curves were plotted
for the data at the outlets and other locations in the aeration tank and secondary clarifier.
Thus (Levenspiel, 2001):
𝐶(𝑡)
𝐸(𝑡) = ∞ (1)
∫0 𝐶(𝑡)𝑑𝑡
where, C(t): concentration at time t and E(t): normalized RTD curve. For a perfect impulse
injection of radiotracer, the first moment of the RTD curve will directly provide the mean
hydraulic residence time (MHRT) of the wastewater in the system that indicates the average
time spent by the wastewater in the system. Thus (Levenspiel, 2001):
∞
𝑡̅ = 𝑀1 = ∫ 𝑡 𝐸(𝑡)𝑑𝑡 (2)
0

The theoretical MHRT (τ) of the system is the desired hydraulic time considered at the time
of the design of the system at normal operating conditions, and is the ratio of the geometric
volume of the reactor to the total volumetric flow rate fed into the system. Thus (Battaglia et
al., 1993):
𝑉
𝜏= (3)
𝑄𝑜 + 𝑄𝑅

Where, V: volume of the system, Qo: flow rate of waste water into the system, QR: recycle
flow rate of wastewater. Under normal operating conditions, the theoretical and
experimentally measured MHRT should be equal to each other. In case, if the theoretical
MHRT is lesser than the experimental MHRT, this either implies that a fraction of the
radiotracer is held back in the system and is released gradually or the values of flow rates and
volume used for determination of τ are erroneous. Thus, the values of flow rates and volume
need to be rechecked. In case, if the value of experimentally determined MHRT is lesser than
the theoretical MRT, then their exist presence of dead volume i.e. the volume which is not
utilized for flow of wastewater in the system. The presence of the dead volume decreases the
hydraulic efficiency of the system. The extent of dead zone in the system can be estimated by
comparing the experimental and theoretical MHRTs of the system (IAEA, 2008). Thus:

𝑡̅
% 𝑉𝑑 = (1 − ) × 100 (4)
𝜏

The values of dead volumes estimated are given in Table 1.

In order to obtain detailed information about flow structure within the aeration and
secondary clarifier, the measured RTD data was modelled using suitable and representative
mathematical models. Initially many combinations of different models (tank-in-series, axial
dispersion, tank-in-series with backmixing and tank-in-series with dead volumes etc.) were
conceptualized and used for simulating the measured RTD curves (Levenspiel, 2001; Fogler,
2011). After repeated attempts, eventually most suitable and representative models were
zeroed-in and used. The RTD software DTSPRO V 4.21 developed by PROGEPI (Farooq et
al., 2003; Leclerc et al., 1995) was used for modelling the measured RTD data in aeration
tank and secondary clarifier. The software offers the possibility for the users to build any
physically representative model for the system under investigation and subsequently
mathematically simulate the experimentally measured data. The physical representation of
the model used to simulate the RTD data of wastewater measured various systems are shown
in Figs. 2-4. Some of the representative results of the model simulation are shown in Figs. 5-
9.

4. Results and discussion

Usually the radiotracer experiments are carried out with steady state flow conditions i.e. the
feed flow rate is equal to exit flow rate. However, in the present investigation, the activated
sludge system did not strictly operated at the steady state condition as it was not possible to
maintain exactly the same feed rate during the RTD measurements due to inherent
requirement of the production processes in the paper industry. The variation of feed rate with
time during (24 hours) one of the experiments is shown in Fig. 10. It was observed that the
variation in the feed rate was not significant enough to cause any appreciable change or
variation in the measured RTDs. In addition to this the duration of the experiment was long
enough and volume of the system was large to equalize the flow rate due to variations.

4.1. Aeration Tank

The Run 1 was carried out to measure the RTD of the wastewater in the aeration tank. From
the shape of the RTD curve monitored at the outlet of the tank (D5), no specific bypassing
and parallel flow paths were found to exist within the tank. The curve contained three distinct
peaks superimposed on the RTD curve that could be due to undesired minor lumped flow
produced by the mechanical aerators. However, the extent of lumped flow is so low that does
not affect the aeration process. The experimental MHRT of wastewater within the aeration
tank was determined to be 982 min. The volume of the aeration tank was 5472 m3 was and
the volumetric flow rate entering the aeration tank during the experiment was 5.21±0.3
m3/min. In addition to this, a recycle stream from the secondary clarifier also entered the
reactor (Fig. 1) having a flow rate of 1.04 m3/min. Therefore, the theoretical MHRT was
estimated to be 881 min. The values of MHRTs are given in Table 1. Based on the
comparison of theoretical and experimentally measured MHRTs (Equation 4), the dead
volume present within the tank was estimated to be about 3%. The amount of dead volume
present is negligibly small and within the experimental error. This implies that all most entire
geometric volume of the tank isutilized for flow and hence available for the aeration process.
The impulse response i.e. RTD curve measured in Run 1 at the outlet of the aeration
tank (D5) was simulated using a model whose physical representation is shown in Fig. 2. The
model consists of three building blocks i.e. plug flow component (PFR) connected in series
with a tanks-in-series with back-mixing (TISBM) and a recycle stream. The PFR in the
beginning represents initial shift or time taken by radiotracer to appear at the outlet of the
tank without any mixing, whereas the TISBM model represents mixing regime within the
aeration tank. The recycle stream is added as a fraction of outlet stream is recycled back to
the inlet of the tank and the same is represented by a plug flow component. The radiotracer
starts appearing in the recycle stream after about 60 minutes of the injection of the
radiotracer. The proposed model was used to simulate the RTD measured at the outlet of the
tank (D5) and the comparison of experimental and model simulated RTD curves
corresponding to best fit is shown in Fig. 7 The model parameters corresponding to the best
fits were obtained. The MHRTs predicated by the model in the plug flow component in the
beginning, TISBM component (aeration tank) and the plug flow component in the recycle
loop were found to be 10 minutes, 860 minutes and 20 minutes, respectively. The value of
tank number (N) was estimated to be 2 indicating that the aeration tank behaved equivalent to
two ideal continuously stirred tank reactor (CSTR) with back-mixing (flow in backward as
well as forward direction) between them. The back-mixing ratio (α) i.e. ratio of forward to
backward flow was estimated to be 2, which is quite low. For intense back-mixing, the value
of back-mixing ratio tends to be infinity. The MHRT predicted by the model (Fig. 2) is in
good agreement with the experimentally measured MHRT.
In order to know the local mixing patterns the radiotracer concentration was also
measured at two axial locations (D2 and D3) within the aeration tank and are shown in Fig. 5
and Fig. 6 respectively. The RTD curve measured at first axial location (D2) was simulated
using a tank-in-series model with bypassing and the results of model simulation are shown in
Fig. 5. A location D2, a small fraction (3 %) of waste water bypasses the main flow within a
very short time i.e. 3 minutes. The RTD curve measured at second axial location was
simulated using a simple tank in series model and the results of model simulations are shown
in Fig. 6.

4.2. Secondary Clarifier

During the Run 1, the radiotracer concentration was also monitored at the outlet of the
secondary clarifier, but the measured tracer concentration curves at the inlet (outlet of the
aeration tank) and outlet of the clarifier were found to be much dispersed and thus not
considered analysis as they might cause significant errors in the determination of MHRT and
model parameters. Therefore, to characterize the flow of the secondary clarifier, another RTD
experiment (Run 2) was independently carried out. The radiotracer was instantaneously
injected at the inlet of the secondary clarifier and monitored at the outlet of the secondary
clarifier and the treated RTD curve is shown in Fig. 8. From the measured RTD, the MHRT
of wastewater was calculated to be 145 minutes. Since, the volume of the secondary clarifier
was 944 m3 and the total volumetric flow rate entering the secondary clarifier was 6.21±0.3
m3/min. The theoretical mean residence time (MRT) was determined to be 152 min. The
comparison of experimental and model simulated RTDs indicates that about 4.6 % of the
geometric volume of the secondary clarifier is stagnant. This is justified as the sludge
(biomass) settles down at the bottom and reduces the active volume of the clarifier.
Ideally, the secondary clarifier should ideally act as a perfect plug flow reactor with
some degree of axial mixing The RTD curve measured at the outlet of the secondary clarifier
was simulated using a model whose physical representation is shown in Fig. 3. The model
consists of an axial mixing component with recycle loop. The residence time of the
wastewater in the clarifier is much lesser than the residence of waste water in the areation
tank. Thus, the recycle fraction of the radiotracer will not affect the RTD curve of the clarifier
and thus could be neglected for practical purpose. The long tail in the measured RTD curve
indicates presence of stagnant volume at the bottom of the clarifier. The exchange of flow
between the active and stagnant volume leads to the extended tail in the RTD curve. The
model parameter i.e. Peclet number (Pe) corresponding to the best fit was found to be 1.3,
indicating significant degree of backmixing or non-symmetrical dispersion in the clarifier.
This could be due to diffusion of radiotracer into the stagnant volume (sludge) at the bottom
of the clarifier leading to an extended tail in the measured RTD curve. Fig. 8. shows the
comparison of the experimental and model simulated curves corresponding to the optimum
model parameters.

4.3. Activated sludge process

Attempt was also made to model the entire activated sludge process (aeration tank
+secondary clarifier). During the Run 1, the radiotracer was also measured at the outlet of the
clarifier using detector D6. The measure curve provides RTD of the entire ASP and was
independently modelled. The volume of the ASP (aeration tank and secondary clarifier) was
6655 m3 and the total volumetric flow rate entering the aeration tank, including the recycle
stream during the experiment was 6.21±0.3 m3/min. The theoretical MHRT of wastewater in
the entire system was estimated to be 1073 mins (equation 4). However, the experimental
MHRT obtained for the entire system was 1175 minutes (equation 2). As the secondary
clarifier is a settling tank with negligible mixing, the biomass collected at the bottom of the
tank can act as a stagnant zone. A fraction of the radiotracer may get trapped in this stagnant
zone and thus will be unable to flow with the active volume (Kim et al., 2005; Pant et al.,
2012). Also since the role of recycle stream becomes more prominent while studying the
complete ASP, the recirculation of tracer in the system can be the reason of higher value of
experimental MHRT than the theoretical MHRT. Based on the results of the modelling of the
aeration tank and the secondary clarifier, a physically representative model as shown in Fig. 4
was conceptualized and used for modelling of the RTD measured at the outlet of the entire
system (D6). Fig. 9 shows a plot showing comparison of the experimental and model
simulated RTD curves corresponding to the optimum model parameters. The model shows
that the secondary clarifier acts as an axial dispersion component with a Peclet no. of 1.1
which is extremely close to the Peclet no. 1.3 obtained when modelled independently. A
recycle stream was added to the ASP system, with a recycle rate of 20% of the total flow rate.
The model MHRT was found to be 1145 min which is very close to the experimentally
obtained MHRT.

5. Conclusions
Radiotracer investigation was successfully conducted to study the hydraulic behaviour of the
complex activated sludge process in a paper industry in India. The investigation revealed
there were no flow abnormalities such as bypassing and parallel flow paths inside the aeration
tank. The dead volume inside the aeration tank was negligibly small. Therefore, the aeration
tank operates at its designed hydraulic efficiency.The modelling of the measured RTD of the
wastewater in the aeration tank revealed that the hydraulic behaviour of the aeration tank
could be represented by two CSTRs with moderate degree of the backmixing between them.
The representation of aeration tank with two CSTR connected in series with back mixing was
justifiable. In the secondary clarifier was also found to be operating normally without and
significant malfunctioning. A simple axial dispersion model was found suitable to describe
flow behaviour of the secondary clarifier. Based on the priori information and results of the
modelling of the individual systems, a model was proposed for the entire activated sludge
processing system. As the complexity of the system makes it difficult to represent its
behavior with general models present in the literature, the compartment modelling approach
proved to be very effective tool for providing a close representation of the physiochemical
behavior and reasonable model for the process.

Acknowledgement
The work reported in the present paper was carried out as a part of a project (35/14/09/2015-
BRNS/3069) funded by Board of Research in Nuclear Sciences (BRNS), Department of
Atomic Energy (DAE), Mumbai, India. Authors are grateful to the authorities of DAE,
Mumbai for extending financial support and M/s Shreyans Industries Ltd. for providing all
the necessary logistic support for conducting the study in their Effluent Treatment Plant.
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Table 1. Mean hydraulic retention times and dead volumes

Sr. System V (m3) Q τ (Min.) 𝑡̅ (Min.) % Vd

3
No. (m /Min.)
1 Aeration tank 5472 6.2 881 855 3.0
2 Secondary clarifier 944 6.2 152 145 4.6
3 Entire activated sludge 6655 6.2 1073 1175 --
processing system

Fig. 1 Schematic diagram (front view) for radiotracer experiment in ASP

 Tank-in-series with
Tracer Plug Flow Back-mixing
Input component Flow
(R+1)Q (1+αQ Out
)
Q Q
Flow αQ
Input

RQ
Plug Flow Recycle
component

Fig. 2 Conceptual physical model for the aeration tank

Axial
Tracer Dispersion
Input component Flow
Out
(R+1)Q Q
Flow Q
Input
RQ

PFR
Recycle
Stream

Fig. 3 Conceptual physical model for the secondary clarifier

 Tank-in-series with Axial
Tracer Plug Flow Back-mixing Dispersion
Input component component Flow
(R+1)Q Out
Q Q
Flow
Input

RQ
Recycle
Fig. 4 Conceptual physical model for the activated sludge processing system

0.0018
Experimental (MRT=757 Min.)
0.0016 TIS with bypass model
0.0014 (MRT=746 Min., N=1, fB=0.03, tB=3 Min.)
E(t) (Min. )
-1

0.0012
0.0010
0.0008
0.0006
0.0004
0.0002
0.0000
0 1000 2000 3000 4000
Time (Min.)

Fig. 5 RTD curve monitored inside the aeration tank D2 inside the aeration tank
 0.0014 Experimental (MRT = 803 Min.)
TIS Model (MRT = 802 Min., N=1)
0.0012

0.0010
E(t) (Min. )
-1

0.0008

0.0006

0.0004

0.0002

0.0000
0 1000 2000 3000 4000
Time (min)
Fig. 6 RTD curve monitored inside the aeration tank D3 inside the aeration tank

0.0014
Experimental (MRT = 855 mins)
0.0012 TIS with backmixing + plug flow
component in series with recycle
0.0010 (MRT = 858 mins, N = 2,
p= 10 min, r= 20 min)
0.0008
E(t) (min )
-1

0.0006

0.0004

0.0002

0.0000
0 1000 2000 3000 4000 5000
Time (min)
ig. 7 Comparison of experimental (D5) and model simulated RTD curves at the outlet of the
aeration tank (Run 1)
 0.007 Experimental (MRT = 146 mins)
ADM with recycle model
0.006 (MRT = 145 mins, Pe = 1.3,
r = 15 mins)
0.005
E(t) (min-1)

0.004

0.003

0.002

0.001

0.000
0 200 400 600 800 1000 1200
Time (min)

Fig. 8 Experimental and model simulated RTD curves at the outlet of the clarifier (Run 2)

0.0008 Experimental (MRT = 1175 mins)

Model (MRT = 1145 mins)
0.0007 p = 10 mins, N=2,  = 3,Pe = 1.4
ad = 200 mins, r = 20 mins)
0.0006

0.0005
E(t) (min )
-1

0.0004

0.0003

0.0002

0.0001

0.0000
0 1000 2000 3000 4000 5000
Time (min)

Fig. 9 Experimental (D6) and model simulated RTD curves at the outlet of the ASP system
(Run 1)