Citation: Beck, D.; Thamsen, P.U.
Development of Sewage Pumps with
Numerical and Experimental
Support. Int. J. Turbomach. Propuls.
Power 2023,8, 18. https://doi.org/
10.3390/ijtpp8020018
Academic Editors: János Gábor Vad,
Csaba Horváth and Tamás Benedek
Received: 13 February 2023
Revised: 3 May 2023
Accepted: 21 May 2023
Published: 2 June 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY-NC-ND) license
(https://creativecommons.org/
licenses/by-nc-nd/4.0/).
Turbomachinery
Propulsion and Power
International Journal of
Article
Development of Sewage Pumps with Numerical and
Experimental Support
David Beck * and Paul Uwe Thamsen
Chair of Fluid System Dynamics, Faculty of Mechanical Engineering and Transport Systems,
Technische Universität Berlin, 10623 Berlin, Germany
*Correspondence: [email protected]; Tel.: +49-30-314-70941
Abstract:
Especially in the field of sewage pumps, the design of radial impellers focuses not only on
maximum efficiency but also on functionality in terms of susceptibility to clogging by fibrous media.
In general, the efficiency of sewage impellers is significantly lower than that of clear water impellers.
These sewage impellers are designed with a low number of blades to ensure that fibrous media
can be pumped. This paper describes the methodology of an optimisation for a sewage impeller.
The optimisation is carried out on a semi-open two-channel impeller as an example. Therefore, a
new impeller is designed for a given volute casing. Based on a basic design for given boundary
conditions, the impeller is verified by means of numerical simulation. The manufactured impeller
is then tested on the test rig to verify the simulation. With regard to the optical investigations, the
clogging behaviour of the impeller is specifically improved over three different modifications in order
to finally present an impeller with good efficiency and a low clogging tendency.
Keywords: sewage impeller; CFD; optical measurement
1. Introduction
Sewage pumps face increased operational problems due to clogging, for example,
clogging within the impeller or the volute casing, according to increased contamination
of solids in sewage. This represents the main reason for downtime, wear and manual
labour in wastewater plants [
1
,
2
]. These solids, which are mainly responsible for pump
clogging, are often tear-resistant fibrous materials, for example wet wipes. Figure 1shows
the components of a pump blockage, where tear-resistant fibrous materials are the main
component for the pump blockage, which were found in [3].
Figure 1. Constituents of pump blockage [3].
In contrast to freshwater pumps, impellers with a low number of blades or special
impeller shapes are deliberately used in sewage pumps to handle solids in sewage as
efficiently as possible. Typical impellers for sewage conveyance are closed and semi-open
Int. J. Turbomach. Propuls. Power 2023,8, 18. https://doi.org/10.3390/ijtpp8020018 https://www.mdpi.com/journal/ijtpp
Int. J. Turbomach. Propuls. Power 2023,8, 18 2 of 13
multi-channel impellers, closed and semi-open single-channel impellers, as well as special
forms such as the vortex impeller [4].
Up to now, pumps have been evaluated according to DIN EN ISO 9906 only with
regard to their clear water values. There is no general criterion about the susceptibility to
clogging for pumping sewage containing solids [5].
Compared to the described classical approach, the functional performance test has
already been introduced in [
6
], which tests both the efficiency
η
and the clogging suscepti-
bility of pumps.
In this paper, the methodology of an optimization for a sewage impeller is described.
As an example, a semi-open two-channel impeller is designed. By means of simulation and
functional performance tests on the test rig, a hydraulic system is to be developed, which,
in addition to good efficiency, should have a low susceptibility to clogging. A classification
of the susceptibility to clogging for the semi-open two-channel impeller is carried out using
the evaluation criteria of the long-time functional performance test.
2. Materials and Methods
2.1. Basic Design
The new impeller is designed as a semi-open two-channel impeller. In contrast to
closed impellers, which have both a hub and a shroud, semi-open impellers do not have a
rotating shroud. For this impeller, the shroud is installed stationary in the housing, which
creates a small gap between the impeller and the volute casing. This gap shall assist the
discharge of added fibrous materials.
The impeller should be designed as a radial impeller and be within a range for the
specific speed of n
q
= 30
. . .
50, which is typical for radial sewage impellers [
7
]. The specific
speed n
q
is calculated according to [
8
] as follows, using the speed n(1/min), the flow Q
(m3/s) and the head H(m):
nq=n√Q
H3/4 (1)
The design point for this impeller is given in normalised form, as the main focus in
this paper is on the methodology of optimisation. The design point is continuously given
for the flow with Qdesign = 1.0; analogously, the head results in Hdesign = 1.0.
One requirement of the impeller is the maximum variability for changing the leading-
edge geometry of the blades. Previous investigations have shown that impellers with
classic leading edges often tend to increase clogging in two-channel impellers. The aim
for this impeller is to be able to incorporate a 3D-printed geometry to connect the leading
edges of the impeller, so-called connected leading edges (CLE), with maximum variability
for the basic impeller. The classic impeller with two leading edges and a possible design of
CLE are shown schematically in Figure 2.
The upper part of Figure 2shows a classic approach for a semi-open two-channel
impeller. The lower part of the figure shows two variants of a possible meridional contour
for CLE: (A) shows a convex shape, which is intended to follow the course of the blade
widths of the original blades. (B) shows a meridional contour for CLE that is flat and
parallel to the suction side of the pump.
For the basic design of the impeller, the shape of two leading edges is chosen according
to the upper part of Figure 2. The impeller is designed for the maximum specific speed
according to the said range with a specific speed of n
q,design
= 50, since a shift of the best
efficiency point (BEP) due to the CLE in the direction of smaller flows is expected. This shift
of the best efficiency point towards smaller flows is caused, among other things, by the fact
that the effective area of the impeller suction side due to a decreasing inlet diameter as well
as the blade inlet angle
β1
are reduced. Accordingly, a steeper characteristic curve is to be
expected for the impeller with CLE compared to the impeller with two leading edges [4].
Int. J. Turbomach. Propuls. Power 2023,8, 18 3 of 13
Figure 2.
Meridional section and plan view for the semi-open two-channel impeller with two leading
edges (top) and for the CLE (bottom), with a convex shape (A) as well as a flat shape (B).
The impeller is designed for a given sewage volute. Previous investigations of this
volute casing have shown that it is particularly resistant to clogging. This is necessary in
order to evaluate the impeller in terms of susceptibility to clogging. Clogging in the volute
casing has a negative influence on the general susceptibility of the hydraulic and thus on
the measurement result, because it is only possible to measure the total amount of wet
wipes within the pump after the end of the test.
The spiral contour of the volute casing is circular. The built-in stationary shroud is
used for all measurements.
The theoretical throttle curve is calculated for the impeller according to [
8
]. However,
this is only a reference and is verified by the subsequent simulation and the measurement
results of the tests.
2.2. Numerical Support
The impeller with two leading edges is simulated before manufacturing to evaluate
the design.
For the numerical simulation, the software ANSYS CFX is used. For the impeller,
a structured mesh is used. The volute is meshed as an unstructured mesh. The setup
including the mesh is shown in Figure 3. Details on the mesh qualities are shown in Table 1.
Figure 3. Mesh used for simulation.
Int. J. Turbomach. Propuls. Power 2023,8, 18 4 of 13
Table 1. Mesh statistics.
Part Elements
Impeller 827,772
Volute 864,356
The simulation is calculated in a stationary manner in order to shorten the calculation
time. The shear stress transport model (SST) is chosen as the turbulence model in order to
generate good resolution and convergence both in near-wall areas and free streams [9].
Some simplifications are applied for the simulation: The impeller, which is designed
as a semi-open two-channel impeller, is simulated without the sidewall gap, as primarily
the simulation is used for estimating the general design. The same applies to the gap
between the impeller and the stationary shroud, as the size of the gap has an influence on
the discharge of the wet wipes, which must be determined experimentally. Likewise, disk
friction losses are not represented in their entirety by the setup.
Accordingly, the focus of the simulation is on verifying the basic geometry of the
impeller. It can be assumed that the simulated results are better than the manufactured
impeller due to the neglected losses. This applies in particular to the efficiencies at higher
volume flows, as the hydraulic losses for higher volume flows have greater influence.
Ten operating points are simulated. Of these, the design point as well as three oper-
ating points to smaller flows compared to the design point and six points to higher flows
compared to the design point are simulated.
Since the impeller is a semi-open two-channel impeller simulated without a gap, the
shroud is assumed to be a counter-rotating wall. In addition, the shaft-hub connection of
the hub is assumed to be a free-slip wall, as this does not exist in the prototype, so that the
described CLE can be installed.
2.3. Impeller Prototyping
The basic impeller with two leading edges is made of cast iron, the only design change
compared to the simulation is the shaft–hub connection. The impeller is manufactured
in such a way that a 3D-printed attachment can be placed on the central shaft screw to
connect the leading edges.
The impeller manufactured from cast iron is shown in Figure 4.
Figure 4. Modification 0—manufactured impeller.
The attachments are made from plastic polylactic acid (PLA) using a 3D printer,
as this manufacturing is relatively inexpensive and fast concerning design changes. In
addition, previous tests have shown that PLA exhibits sufficient strength when used with
artificial sewage.
Int. J. Turbomach. Propuls. Power 2023,8, 18 5 of 13
2.4. Test Procedure
The test rig for functional performance of the Chair of Fluid System Dynamics at
the Technische Universität Berlin has two tanks, the wastewater tank (WWT) and the
freshwater tank (FWT), each with a volume of 4 m3.
Two different clogging tests can be carried out on the test rig using tear-resistant wet
wipes—the functional performance test and the long-time functional performance test. The
test rig has pinch valves, which make it possible to switch between the different tests and
shut off the pump. In addition, a filter is installed in the FWT, which is decoupled from the
system and removes all residual fibres in the test rig. The results of this paper are generated
with the long-time functional performance test. Tear-resistant wet wipes according to the
contamination classes in Table 2are used for the tests.
Table 2. Contamination classes.
Contamination Class Wipes/m3
Clear water (CW) 0
Low contamination (L25) 25
Medium contamination (L50) 50
High contamination (L100) 100
Figure 5shows the wet wipes used for the experiment. These have the dimensions of
22 ×30 cm and a surface weight of 60 g/m2.
Figure 5. Wet wipes used for tests.
Generally, for both tests, the pumps are tested at their BEP, as well as at 80% of the
BEP in partial load and at 120% of the BEP in overload. A total of nine measurements per
impeller are thus carried out on the test bench.
Long-time functional performance test:
The long-time functional performance test describes the 60 min pumping of the
artificial wastewater in a loop from the WWT to the WWT, shown in Figure 6. In the
process, the measured values of the head, the flow and the electrical power are recorded
every second. The efficiency
η
is determined using these data. After the test, the pump is
shut off and opened, and the remaining wet wipes are documented and removed. From
this, the degree of long-time functional performance (D
LTF
) is determined according to
Equation (2):
DLTF =1
2
ηtest,0–60min[−]
ηCW,OP[−]+1
2
mW,total[g]−mW,pump[g]
mW,total[g](2)
Here, the first part of the D
LTF
represents the ratio of the averaged efficiency over the
60-min measurement period
(ηtest,0–60min)
to the clear water efficiency at the corresponding
operating point
(ηCW,OP)
. The second term of the D
LTF
represents the ratio of the difference
Int. J. Turbomach. Propuls. Power 2023,8, 18 6 of 13
between the total amount of wipes supplied (m
W,total
) and the wipes remaining in the pump
(mW,pump) to the total amount of wipes supplied.
AD
LTF
of 0 indicates that the impeller is completely clogged, and conveying is no
longer possible, while a value of 1 describes clog-free operation. Based on the tests already
carried out, a D
LTF
of at least 0.7 is required for the prototype, as it has been shown that
impellers above a value of 0.7 exhibit good clogging behaviour and can handle the added
fibrous material well.
Figure 6. Long-time functional test cycle.
2.5. Optical Access
The test rig has an optical access on the suction side, which is similar to the optical
access presented in [
10
]. By means of an endoscope, an LED light ring and a high-speed
camera, it is possible to obtain closer insights at the impeller inlet during operation. Figure 7
shows the installed access.
Figure 7. Test setup for optical access.
As an additional measurement method, this approach provides an opportunity to
describe the interaction between the flow and the wet wipes during operation and to
make targeted changes in the geometry from the results. For these tests, the camera
records at 1000 frames per second (fps) over a period of 20 seconds with a resolution of
768 ×768 pixels
. The test cycle is analogous to the long-time functional performance test,
but the contamination class is different. For these tests, ten wet wipes per m
3
of water are
used, as it has been shown that with this quantity, initial clogging behaviour can be easily
seen, while there are not too many wipes to impair the optimal measurement.
Int. J. Turbomach. Propuls. Power 2023,8, 18 7 of 13
The endoscope has a diameter of 10 mm and a viewing direction of 45
◦
. Due to
the resulting installation position, this angle ensures that the wipes do not clog on the
endoscope and thus a uniform supply of the wipes into the inlet of the pump is possible.
The field of view is 88
◦
. The LED light ring consists of eight individual LEDs, which
provide a luminous flux of 5595 lumen each.
3. Results and Discussion
3.1. Results—Simulation
The simulation results in relation to the theoretical design curve are shown in Figure 8.
For the simulation results, only the throttle curves are compared with each other, as an
efficiency estimate was not carried out.
Figure 8. Modification 0—characteristic curves.
The results of the simulation show the same trend of the design characteristic. The
simulated throttle curve shows a slightly higher level and is superior to all calculated
operating points of the design throttle curve. This is due, among other things, to the
simplifications of the simulation described above. This can also be said about the efficiency,
for which a value of
ηBEP,CFD
= 79.5% is achieved in the simulation. However, it can be seen
that the BEP of the hydraulic with QBEP,CFD = 1.2 is slightly larger than the design point.
Overall, this comparison shows that the design is well represented by the simulation
with regard to its design characteristic. Due to the simplified assumptions, the efficiency
reaches a higher value in the simulation than is to be expected in the measurement. Es-
pecially not all hydraulic losses are taken into account by the simplifications. Since these
increase with higher flows, a shift of the optimal efficiency is to be expected with smaller
flows. Likewise, a reduced efficiency at the BEP of the manufactured impeller can be assumed.
3.2. Results—Modification 0
Figure 8additionally shows the measurement of the characteristic curves of the
impeller with two leading edges on the test bench in relation to the simulation results as
well as the design throttle curve.
The measured characteristic curves show the same trend of the simulation around the
design point. Especially in the direction of smaller flows of the design point, the simulated
and the measured throttle curves show an almost identical trend.
For strong partial load and overload, the measured throttle curve deviates from the
design throttle curve. However, the design throttle curve only represents a trend and is
mainly to be assumed for the range of best efficiency.
The maximum efficiency of the measured impeller is around 5% lower than the best
efficiency value of the simulation. The BEP of the impeller is thus at a flow of Q
BEP,0
= 0.9
Int. J. Turbomach. Propuls. Power 2023,8, 18 8 of 13
with an efficiency value of
ηBEP,0
= 73.9%. This deviation is again due to the simplified
assumptions of the simulation. This BEP results in a specific speed of n
q,0
= 48, which,
taking into account the boundary conditions for the specific speed, provides a good starting
point for the further process of inserting a printed part for CLE.
To obtain an impression of the susceptibility to clogging of the basic impeller, the
long-time functional performance test with medium contamination (L50) is carried out for
the impeller in Modification 0.
The impeller clogs with a result of D
LTF,0
= 0.32. The amount of wet wipes removed
from the impeller at the end of the test corresponds to 72% of the amount of wet wipes
supplied. The clogging has accumulated at the leading edges during the test and has
grown from there. The clogging inside the impeller after the end of the long-time functional
performance test can be seen in Figure 9.
Figure 9. Modification 0—clogging after long-time functional test (BEP, L50).
This impeller geometry is not able to discharge the wet wipes. Due to the high
susceptibility to clogging, no further tests of the impeller are carried out.
3.3. Results—Optimization
Modification 1: For the first modification, a printed part for the CLE with a strongly
convex shape in the meridional section as shown in Figure 2, example (A), is used, which
follows the width progression of the two blades. The characteristic curves of modification 1
compared to modification 0 are shown in Figure 10.
Figure 10. Modification 1—characteristic curves.
The characteristic curves of Modification 1 show that the BEP shifts towards smaller
flows due to the CLE. The optimum efficiency of
ηBEP,1
= 68.1% is achieved for a flow of
Q
BEP,1
= 0.7. The resulting specific speed is n
q,1
= 37 and is therefore within the specified limits.
Int. J. Turbomach. Propuls. Power 2023,8, 18 9 of 13
The long-time functional performance test with low contamination (L25) at the BEP
shows a poor clogging result with a D
LTF,1
= 0.19, which is insufficient. Overall, 91% of
310 g of supplied wet wipes are absorbed. A conveying of the wipes does not seem possible
due to this result of the long-time functional performance test.
The result of the long-time functional test suggests that conveying of the wet wipes
cannot take place with high numbers of wet wipes arriving in the impeller at the same
time, which clogs the impeller. In addition, the convex shape of the CLE extends very far
forward into the inlet that the wipes cannot be discharged via the gap between the impeller
and the volute casing as soon as the clogging has reached a certain degree.
In Figure 11 on the left-hand side, it is shown that the accumulation of the wet
wipes extends into the suction pipe. As soon as the wipes are lying on the CLE and are not
conveyed, the clogging grows in the direction of the suction pipe. In this case, self-cleansing
of the impeller can no longer be expected.
Figure 11.
Modification 1—clogging after long-time functional test (BEP, L25) (
left
) and optical
investigations (right).
The optical measurement shows the extent to which the wet wipes interact with the
impeller. The investigation has shown that single wet wipes lay loose on the printed part
for the CLE but are conveyed within a few revolutions. A screenshot from the optical
investigation is shown in Figure 11 on the right-hand side.
The impeller has suitable free-flushing characteristics for the arrival of single wet
wipes, as the investigation has shown. Nevertheless, it can be said that the impeller is no
longer able to flush off the wet wipes for higher incoming concentrations of wet wipes.
Modification 2: The second modification has a printed part for the CLE that is plane
and parallel to the inlet of the pump, according to shape (B) in Figure 2. This is intended
to prevent the flow from decreasing due to throttling on the suction side in the inlet pipe
because of wet wipes, as can be seen for Modification 1.
While looking at the characteristic curves in Figure 12, the maximum efficiency of
ηBEP,2
= 69.1% is reached at a flow of Q
BEP,2
= 0.7. As a result, the impeller achieves a
specific speed of n
q,2
= 35. The throttle curve of Modification 2 shows a similar behaviour
to Modification 1.
The long-time functional performance test for Modification 2 results in a D
LTF,2
= 0.43
for the BEP with low contamination (L25). In total, 64% of 309 g of supplied wet wipes are
absorbed, which shows a significant improvement in clogging compared to Modification 1.
The clogging inside the impeller after the end of the long-time functional performance
test can be seen in Figure 13 on the left-hand side.
Contrary to Modification 1, the wipes do not grow into the suction pipe and do not
throttle the incoming flow. The impeller appears to have a degree of saturation, which,
once reached, cannot accommodate any further wipes. This is particularly evident from
the fact that a significantly lower proportion of wet wipes are absorbed compared with
Modification 1.
Int. J. Turbomach. Propuls. Power 2023,8, 18 10 of 13
Figure 12. Modification 2—characteristic curves.
Figure 13.
Modification 2—clogging after long-time functional test (BEP, L25) (
left
) and optical
investigations (right).
By means of optical observations, it can be seen in Figure 13 on the right-hand side,
that the wet wipes, which are sucked in centrally, also hit the centre of the CLE analogously
to Modification 1, where they accumulate. In addition, the clogging on the CLE increases
between the beginning of the test and the end of the test, which could be detected through
the optical investigations. The geometry of the impeller is not able to discharge the wet
wipes. Likewise, the wet wipes cannot be loosened by the impact of further incoming wet
wipes. Accordingly, this flat contour is also not optimal for conveying wet wipes and tends
to clog, even if the clogging result is improved compared to Modification 1.
Altogether, the modification is not able to convey wet wipes even in small concentra-
tions, even if this modification shows a minor saturation of wet wipes and therefore has a
higher DLTF.
Modification 3: Another printed part is manufactured for the third modification, which
combines the good conveying characteristics of Modification 1 with the advantages of the
axial length of Modification 2 concerning the maximum amount of wet wipes absorbed.
The third modification shows a significantly smaller convex shape compared to Modi-
fication 1 to reduce the axial length and maintain the conveyance characteristics. Analogous
to the first two modifications, Figure 14 shows the characteristic curves of Modification 3.
Modification 3 has its BEP as well as the other two modifications at Q
BEP,3
= 0.7. The
best efficiency for this modification is
ηBEP,3
= 64.5%. This modification results in a specific
speed of nq,3= 37.
Int. J. Turbomach. Propuls. Power 2023,8, 18 11 of 13
Figure 14. Modification 3—characteristic curves.
A long-time functional performance test with low contamination (L25) is also carried
out for this modification. The impeller achieved a D
LTF,3
= 0.98. At the end of the mea-
surement, the impeller had not absorbed any wet wipes from the 299 g supplied. The
missing 2% of the D
LTF
also can be attributed concerning Equation (2) to slight hydraulic
dips during the test, which occur due to brief blockages within the test caused by the wet
wipes. However, the impeller manages to continuously discharge the wet wipes.
The impeller manages to convey the wet wipes into the channels continuously over
the measurement period for this type of CLE.
Table 3shows the results for the long-time functional performance tests for all nine
measurement setups:
Table 3. Measurement results: DLTF for Modification 3 for three operating points.
Q/QBEP L25 L50 L100
0.8 0.97 0.99 0.94
1.0 0.98 0.91 0.94
1.2 0.74 0.90 0.74
No further modification is necessary, as the impeller exceeds the required degree of
long-time functional performance of 0.7 for all measurement points and thereby has the
best clogging behaviour of all modifications.
3.4. Comparison of Results
Overall, the optimisation shows a considerable improvement in the D
LTF
and thus the
susceptibility to clogging, as can be seen in Figure 15.
Modification 0 shows a poor clogging result for the medium contamination with two
leading edges. By using the CLE, the first modification also achieves an insufficient result
in the long-time functional performance test. This is improved by targeted changes in the
geometry through the detection of the weak points by means of optical tests as well as the
long-time functional test.
The second modification still shows a high susceptibility to clogging with wet wipes,
even though the DLTF is improved a lot.
Through renewed optimization based on the findings of the optical tests and the test
results of both previous modifications, a modification is tested that delivers a clog-free
result for the low contamination. Likewise, the required D
LTF
of over 0.7 will be achieved
for all contaminations, in the BEP, in partial load and in overload.
Int. J. Turbomach. Propuls. Power 2023,8, 18 12 of 13
Figure 15. Overview over DLTF for tested modifications for Q/QBEP = 1.0.
4. Conclusions
The use of simulations for a calculated sewage impeller in combination with a test
rig to test the susceptibility to clogging has shown that impellers for sewage pumps can
be designed and optimised according to specific criteria. The use of optical access is
particularly noteworthy to specifically address the interaction between the wet wipes and
the impeller and for making specific design changes.
Author Contributions:
Conceptualization, D.B. and P.U.T.; methodology, D.B. and P.U.T.; software,
D.B.; validation, D.B. and P.U.T.; formal analysis, D.B.; investigation, D.B.; resources, D.B. and P.U.T.;
data curation, D.B.; writing—original draft preparation, D.B.; writing—review and editing, P.U.T.;
visualization, D.B.; supervision, P.U.T.; project administration, P.U.T. All authors have read and
agreed to the published version of the manuscript.
Funding: This research received no external funding.
Data Availability Statement:
The data presented in this study are available on request from the
corresponding author. The data are not publicly available due to the fact that it is standardized data
that only supports the methodology of optimizing sewage impellers at the test bench in general.
Conflicts of Interest: The authors declare no conflict of interest.
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