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Comparative Performance Analysis of the Wastewater
Treatment Plants for Small Populations
Ana Luísa Martins Dias de Figueiredo Simeão
Under the supervision of Professor Ana Fonseca Galvão
Extended Abstract
July 2014
ABSTRACT
The choice of sustainable wastewater treatment solutions for small populations as always revealed
some challenges.
This study aims to evaluate and compare the performance of twelve Wastewater Treatment Plant
(WWTP) that serve small populations, with less than 2000 inhabitants, using different kinds of systems
(activated sludge, lagoons and constructed wetlands) operating in the region of Algarve, Portugal,
during the years of 2011 and 2012 .
A metabolism model was applied to each WWTP, in order to quantify and evaluate the specific flows
(depending on the volume of treated water, in m3) of pollutants, energy, greenhouse gas (GHG)
emissions, waste and sludge. Afterwards, the operating cost associated to each one of these flows
was calculated considering two environmental costs: the wastewater discharge (TRH) and the GHG
emissions.
Operating costs between 0,53 and 0,014 €/m3 were obtained for the studied WWTP systems. The
energy consumption, when existent, represented 44-98 % of the total operating specific cost. The
activated sludge systems revealed high operating costs and high energy consumption when compared
with the lagoon systems and the constructed wetlands. The constructed wetlands proved to be able to
conduct a proper treatment, with low energy consumption and competitive operating costs.
The environmental contribution costs of TRH and GHG in the total specific cost proved to be
insignificant in the three types of technologies analyzed.
Keywords: environmental costs, performance, operating costs, WWTP
2
1. INTRODUCTION
In 2011, 39% of the Portuguese population
lived in clusters with less than 2000 inhabitants
(INE, 2012), often dispersed in the territory,
which makes the centralized WWTP systems
economically uncompetitive, since they do not
benefit from economies of scale (Galvão and
Matos, 2004). Traditionally, the most used
wastewater treatment technologies for small
clusters in Portugal are the intensive
techniques of activated sludge, trickling filters
and biological disks (Galvão, 2009). However,
extensive processing techniques such as
lagoons and constructed wetlands may be
sustainable and competitive in the wastewater
treatment of small clusters when compared to
conventional activated sludge systems, due to
its low requirements in natural resources as
well as low investment and operation costs
(CE, 2001).
Several techniques have been used to assess
the environmental performance of wastewater
treatment plants (WWTP), with emphasis to
Life Cycle Analysis (LCA) (Machado et al,
2007; Ortiz et al., 2007; Gallego et al., 2008;
Pan et al, 2011; Rodriguez Garcia et al., 2011),
sustainability indicators (Muga and Mihelcic,
2006; Galvão and Matos, 2004; Suriyachan C.
et al., 2009) and performance indicators
(Freire, 2007). Material Flow Analysis (MFA),
which consists in evaluating the metabolism
system by the quantification of input and output
material flows through mass balances, is also
a technique used in the water sector planning
assessment (Jeppson and Hellström, 2002).
Substance Flow Analysis (SFA), a variant of
MFA technique, is also used in the
environmental and economic performance
analysis of the urban wastewater systems
(Benedetti et al, 2008).
The aim of this study is to evaluate and
compare the WWTP performance with different
kinds of treatment systems (activated sludge,
lagoons and constructed wetlands) that serve
less than 2000 inhabitants. The studied WWTP
are in operation in the region of Algarve,
Portugal and a metabolism model was applied
to each WWTP in order to identify and quantify
the mass and energy flows. Subsequently, the
total operating specific costs were calculated
for each facility, i.e., the price in € per m3
of
treated water, taking into account, in
environmental terms, the rate of water
resources (TRH), which reflects the pollutant
load released in the receiving water body, and
greenhouse gas (GHG) emissions. This
performance analysis was applied to the years
of 2011 and 2012.
1.1. Characterization of the studied WWTP
The studied WWTP are located in the Algarve,
south Portugal, whose operation and
management is Águas do Algarve, S.A. (AdA)
responsibility. The general characterization of
the studied systems is presented in Table 1
3
Table 1 – General characterization of the studied WWTP
WWTP Municipality Treatment
system 2
Population
on the
horizon
year of
project
(inhab.)
Flow
rate in
2011
(m3)
Equivalent
population
in 2011
(inhab.)
Flow
rate in
2012
(m3)
Equivalent
population
in 2012
(inhab.)
Alcoutim
Alcoutim
Activated
sludge 868 31925 340 30488 440
Balurcos 4
Activated
sludge under
extended
aeration
421 9522 29 10486 19
Benafim
Loulé
Activated
sludge
(diagonal
aeration)
1000 16766 259 15592 175
Querença 1000 16341 210 7834 93
Ameixial 1000 16327 193 14088 138
Casais
Monchique
Activated
sludge under
extended
aeration
430 6794 83 4506 51
Marmelete 900 15425 174 12856 232
Almádena Lagos
Lagoons -
facultative
and
maturation
pond
1260 46403 479 37491 666
Budens Vila do Bispo
Lagoons -
anaerobic,
facultative
and
maturation
pond
700 48642 379 30502 356
Martinlongo 4
Alcoutim Constructed
wetlands 3
1000 1 49548
257 57057 304
Giões 4 250
1 8625 80 9716 83
Vaqueiros 4 250
1 3732 19 3732 52
1 – Population corresponding to the maximum capacity
2 – There is no sludge stabilization in these facilities
3 – Bed surface area of Martinlongo constructed wetland: 2249,2 m
2; Useful bed surface area of Vaqueiros and Giões
constructed wetlands: 281,2 m2
4 – In these facilities the flow rates are estimated at 80% of the water supplied to the population
2. METHODOLOGY
The methodology developed to evaluate and
analyze the performance of the small WWTP
described in Table 1 is based on the following
steps:
1) Application of a metabolism model,
identifying and quantifying mass and
energy flows of each WWTP. All of the
flows are expressed in terms of annual
specific production/consumption, i.e,
per m3 of treated water
2) Calculation of the specific operating
costs associated to each one of mass
and energy flows previously quantified.
Two environmental externalities were
considered: the TRH and the GHG.
4
2.1 Metabolism model
The metabolism model applied to each WWTP
is shown in Figure 1.
Figure 1 – Metabolism model applied to the WWTP
in study
Pollutants
In this analysis 5 main pollutants are
quantified: Biochemical Oxygen Demand
(BOD5), Chemical Oxygen Demand (COD),
Total Suspended Solids (TSS), Total Nitrogen
(NT) and Total Phosphorus (PT).
These flows represent the annual load present
in the affluent and effluent of each WWTP, and
are expressed in g/m3. The monthly load
values were obtained multiplying the monthly
average flow of treated water by the monthly
average concentration of each pollutant. The
annual load values were then obtained by
summing all the monthly load values.
GHG
These flows correspond to methane (CH4),
nitrous oxide (N2O) and carbon dioxide (CO2)
emissions. These emissions were converted to
CO2 equivalents (CO2e) considering the Global
Warming Potential (GWP) of 21, 310 and 1, for
methane, nitrous oxide and carbon dioxide,
respectively. These flows are expressed in
CO2e/m3.
CH4 and N2O emissions are related to the
emissions released during the wastewater
treatment process and were calculated based
on the Intergovernmental Panel on Climate
Change (IPCC, 2006) and the Portuguese
National Inventory of Greenhouse Gases,
1990-2011 (APA, 2013) methodologies.
These reports do not include calculation
methodology to GHG emissions in constructed
wetlands and in this case, the CH4 emissions
were calculated based on the mean flux of the
fluxes presented in Table 2, fluxes found in
literature.
Table 2 – Fluxes of CH4-C in constructed wetlands
and its average
CH4-C emission in
constructed wetlands
(gCH4-C/m2.year)
Reference
29,57 Mean values –
Mander and Teiter
(2005) 23,04
68,77 Mean value – Søvik
and Kløve (2006)
Mean value=40,46 -
For the activated sludge and lagoon treatment
systems, CH4 emissions were calculated from
the organic mass load received in each WWTP
(BOD5) and the remaining parameters in IPCC
(2006). The values used were assumed as in
APA (2013).
Regarding N2O emissions the methodology
adopted was the one presented in IPCC
(2006). It was therefore considered that part of
the nitrogen is removed during the treatment,
result of nitrification and denitrification
reactions that are favorable to the production
of N2O (N2Otreatment) (Kimochi et al., 1998;
Kampschereur et al., 2009). The emission
factor assumed for the activated sludge
systems during the treatment was 0,00035 kg
N2O-N/kg Naffluent (IPCC, 2006). According to
the CH4 and N2O emission potentials for
wastewater systems described in IPCC (2006),
it was assumed an emission factor of zero for
5
the systems with lagoons. The emission factor
considered for the constructed wetlands
corresponds to the mean value of factors found
in literature, which are presented in Table 3.
Table 3 – Emission factors of N2O – N2Otreatment
– in constructed wetlands and its average
Emission factors in
constructed wetlands
(N2O-N emitted due to the
load of Naffluent)
Reference
0,008
Mean values –
Mander et al. (2008)
1,5
0,47
0,29
0,044 Mean value – Søvik
and Kløve (2006)
Mean value=0,46 -
The nitrogen that is not removed during the
treatment is degraded in the effluent that goes
to the receiving water body (N2Oeffluent). It was
assumed an emission factor of 0,01 kg N2O-
N/kg Neffluent (APA, 2013).
The total nitrous oxide emission corresponds
to the sum of N2Otreatment and N2Oeffluent.
The CO2 emissions quantified in this analysis
are only associated with the emissions
released during the electricity production
required to explore the WWTP during the years
of 2011 and 2012. The calculation of these
emissions consists in the product of the annual
electricity consumption (kWh) and the value of
the specific emission (kgCO2/kWh) in the year
in which it relates to the production (0,29
kgCO2/kWh in 2011 and 0,32 in 2012
kgCO2/kWh.
Energy
This flow represents the electrical energy
consumption in each WWTP and it is
expressed in kWh/m3.
Sludge and waste
Waste and sludge represent the sub products
of the water treatment and are expressed in
kg/m3.
2.2 Operating Costs
The total operating cost of each WWTP
consists on the sum of the specific costs (€ per
m3 of treated water) associated with the
consumption of energy, the production of
sludge and waste, the TRH and the GHG
emissions. The operating costs were
calculated by multiplying the mass or energy
flows previously determined by the
corresponding unit costs. The unit costs for
each component are presented in Table 4.
GHG emission unit costs were assumed as the
average yearly value of the SendeCO2 carbon
rights stock market for 2011 and 2012.
Table 4 – Unit costs for the operating components
Component Units Value
TRH €/kg
Oxidizable matter: 0,13 to
0,20
Phosphorus: 0,06 to 0,21
Nitrogen: 0,04 to 0,07
Electrical
energy
consumption
€/kWh
2011: 0,1262 (Alcoutim
and Balurcos) and 0,1326
(other WWTP)
2012: 0,1369 (Alcoutim
and Balurcos) and 0,1393
(other WWTP)
Sludge
handling €/ton
28 (Benafim)
40 (Casais)
36 (Marmelete)
6
Table 4 – Unit costs for the operating components (cont.)
Component Units Value
Waste
handling €/ton
63,27 (Alcoutim and
Balurcos)
62 (Marmelete and
Casais)
71 (Benafim,
Querença, Ameixial,
Almádena and
Budens)
55 (Constructed
wetlands)
GHG
emissions €/tonCO2e
2011: 12,88
2012: 7,32
3. RESULTS AND DISCUSSION
3.1. Pollutants Removal
The activated sludge facilities showed removal
of BOD5, COD and TSS greater than 90%. In
the lagoons systems there is emphasis to the
BOD5 removal (> 90%) and the COD removal
(> 78%), while the removal of TSS in these
systems is less efficient when compared with
the activated sludge or constructed wetlands
WWTP. Constructed wetlands removal of
BOD5 and COD is above 80%, standing out in
these systems the TSS removal (> 90%).
In all WWTP, nutrient removal is less
satisfactory, especially phosphorus removal,
which is expected since these systems only
performed secondary treatment.
Regarding the legal required discharge
concentration values of CBO5, CQO and TSS
in the effluent, all of the activated sludge and
lagoons systems are operating according to
the required by the licensing authority.
Constructed wetlands are capable of
performing a proper secondary treatment,
according to Decreto-Lei nº 152/97 of 19th
June.
3.2. Specific Consumptions/Productions
The specific consumption (SC) and specific
production (SP) resulting from the metabolism
model applied to each WTTP in the years of
2011 and 2012 are described in Table
Table 5 – Specific Consumption (SC) and Specific Production (SP) of the WWTP in 2011 and 2012
WWTP
2011 2012
SC of
electrical
energy
(kWh/m3)
SP of GHG
(kgCO2e/m3)
SP of
waste
(kg/m3)
SC of
electrical
energy
(kWh/m3)
SP of GHG
(kgCO2e/m3)
SP of
sludge
(kg/m3)
SP of
waste
(kg/m3)
Alcoutim 3,02 1,19 0,027 3
2,21 1,01 - -
Balurcos 4,05 1,45 0,027 3 0,97 0,41 - -
Benafim 1,83 0,92 0,063 1,20 0,63 0,27 N.q. 2
Querença 2,26 0,91 0,046 3,76 1,50 N.c. 1 N.q.
2
Ameixial 0,59 0,44 0,032 0,27 0,30 N.c. 1 N.q.
2
Casais 1,96 0,89 0,007 2,20 1,08 1,78 -
Marmelete 3,49 1,49 0,008 3,12 1,51 1,85 -
Almádena 0,11 0,18 0,021 0,12 0,19 N.c. 1 N.q.
2
Budens 0,00 0,51 0,003 0,00 0,80 N.c. 1 N.q.
2
7
Table 5 – Specific Consumption (SC) and Specific Production (SP) of the WWTP in 2011 and 2012 (cont.)
WWTP
2011 2012
SC of
electrical
energy
(kWh/m3)
SP of GHG
(kgCO2e/m3)
SP of
waste
(kg/m3)
SC of
electrical
energy
(kWh/m3)
SP of GHG
(kgCO2e/m3)
SP of
sludge
(kg/m3)
SP of
waste
(kg/m3)
Martinlongo 0,11 0,25 0,027 3 0,06 0,30 - 0,026
Giões 0,55 0,49 0,027 3 0,31 0,46 - 0,007
Vaqueiros 0,95 0,59 0,027 3
0,78 0,62 - 0,012 1 – N.c.: Not collected in that year
2 – N.q.: Not quantified. Fall-in larger WWTP circuits
3 –
Estimated values based on the inflow
The activated sludge systems support higher
SC of energy when compared to the lagoons
and constructed wetlands systems. In these
systems the removal of organic matter is
promoted by aeration, which represents a
significant portion of the electrical energy
consumption.
As it can be observed in Table , most of the
WWTP decreased the SC of energy from 2011
to 2012, due, in part, to the fact that in 2012
the precipitation was lower compared to 2011.
Additionally, in activated sludge WWTP
aeration levels were reduced to minimum.
Except for the WWTP of Ameixial, the
consumption of electrical energy in the
activated sludge is responsible for the largest
portion of GHG emissions (58-81%). In
lagoons and constructed wetlands systems it
turns out to be the opposite and GHG
emissions resulting from the wastewater
treatment contribute at least 53% of total GHG
emissions, result of reduced energy
consumption, which usually features this type
of technology. This trend is illustrated in Figure
2 for the year 2011, being equivalent in 2012
Figure 2 – Water treatment and electrical energy contribution (%) on SP of GHG of the WWTP studied in 2011
Regarding GHG emissions from wastewater
treatment it appears that there is no big
difference between activated, lagoons and
constructed wetlands systems as it can be
8
seen in Figure 3 for the year of 2011, being
equivalent in 2012. However, the dominant
GHG during the water treatment in most of the
activated sludge WWTP is CH4 in contrary to
what happens in constructed wetlands, where
the dominant gas during the treatment of
wastewater is N2O. In the lagoon systems, the
dominant gas during the wastewater treatment
will depend on whether the system includes an
anaerobic lagoon or not.
Overall, in lagoons and constructed wetlands
there is a SP of GHG emissions lower than
most of activated sludge WWTP.
Figure 3 – SP of GHG of the WWTP studied in 2011, per component
3.3. Operating Costs
With regard to the total specific operating cost,
more than half of the WWTP decreased the
specific operating costs between the years of
2011 and 2012, as it can be seen in Figure 4
and Figure 5. The component that contributes
most to this change is the specific electrical
energy consumption, which represents 44 to
98% of the total specific operating costs
(Figure 6 and Figure 7).
Figure 4 – Total specific operating cost of the WWTP studied in 2011
9
Figure 5 - Total specific operating cost of the WWTP studied in 2012
The analysis of Figure 4 and Figure 5, also
allows concluding that the activated sludge
systems have higher operating costs when
compared to lagoons and constructed
wetlands systems. In activated sludge
systems, the electricity consumption is higher,
which promotes higher specific operating
costs. The WWTP of Ameixial stands out from
among the activated sludge systems, revealing
considerably lower total specific operating
costs than the other facilities that have the
same type of treatment (0,086 €/m3 in 2011
and 0,041€/m3 in 2012).
Figure 6 - Total specific operating cost of the WWTP studied in 2011, per component
10
Figure 7 - Total specific operating cost of the WWTP studied in 2012, per component
Figure 6 and Figure 7 show that the
components related to TRH, GHG emissions
and waste production reveal no significant
contribution in the total specific operating cost
of the studied WWTP during the years of 2011
and 2012. Regarding the contribution of
sludge, more information is needed to be able
to discuss the weight of this component in the
total specific operating cost since it was only
possible to quantify the production of sludge in
the WWTP of Benafim, Casais and Marmelete
in the year of 2012 (Table ).
4. CONCLUSIONS
The performance analysis shows that the
specific consumption of electricity, when
existent, is the element with the highest
contribution to the total specific operating cost.
The results revealed that the TRH and the
GHG have a reduced contribution in the total
specific operating cost. However, the
estimation of GHG has a degree of uncertainty
associated, lacking research and studies that
support the assumptions and the emission
factors used in this analysis.
Overall, the studied lagoons and the
constructed wetlands systems appear to be
competitive in domestic wastewater treatment
for small populations since they show
adequate removal pollutants, reduced energy
consumption and GHG production, with
reduced operating costs when compared to
most of the activated sludge WWTP.
The studied activated sludge systems revealed
that, in the context of small populations, these
systems are not able to take advantage of the
economies of scale, revealing high total
specific operating costs. However, these
systems achieve a higher quality of the final
effluent, which may become relevant if the
effluent quality standards became stricter in
the future.
ACKNOWLEDGMENTS
I would like to thank Águas do Algarve S.A. for
providing the data, especially to Engº Joaquim
Freire.
11
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