evaluation of the environmental implications to include structural changes in a wastewater treatment...
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Evaluation of the environmental implications toinclude structural changes in a wastewatertreatment plantNuria Vidal,1 Manel Poch,1 Eugenia Martı2 and Ignasi Rodrıguez-Roda1*1Laboratori d’Enginyeria Quımica i Ambiental, Universitat de Girona, Campus Montilivi s/n, 17071 Girona, Spain2Centre d’Estudis Avancats de Blanes, CSIC, Camı de Santa Barbara s/n, 17300 Blanes, Spain
Abstract: The environmental implications of including structural changes in a wastewater treatment
plant to decrease effluent concentrations of nitrogen were evaluated in this study. Environmental
effects from these structural changes were assessed by using the Life Cycle Assessment theoretical
framework. The wastewater treatment plant selected as a reference scenario had an activated sludge
configuration. The Ludzack–Ettinger and Oxidation Ditch configurations were selected as modifi-
cations of the reference scenario. Results from this study show that the inclusion of nitrogen removal
mechanisms in the configuration of the plant reduces the effect of the plant on the eutrophication, but
simultaneously increases the effect on the consumption of abiotic resources, global warming,
acidification and human toxicity. These general trends, however, vary depending on the configuration
selected to remove nitrogen. Taking all the impacts together, the Oxidation Ditch configuration would
cause less environmental impact than the Ludzack–Ettinger configuration, given the characteristics of
the selected scenarios.
# 2002 Society of Chemical Industry
Keywords: environmental; biotechnology; wastewater; nitrogen; life cycle assessment; eutrophication
NotationBOD5 Biochemical Oxygen Demand (gm�3)
Cij Potential contribution of a given emission jto the environmental impact i (kgequivalent)
COD Chemical Oxygen Demand (gm�3)
DO Dissolved Oxygen concentration (gm�3)
Ej Emission or resource consumption (gt�1
wastewater)
F/M Sludge loading (kg BOD5kg�1 MLVSS)
MLVSS Mixed Liquor Volatile Suspended Solids
(gm�3)
SRT Sludge Retention Time (d)
SS Suspended Solids (gm�3)
TKN Total Kjeldhal Nitrogen (gm�3)
Wij Weighting factor of a given emission j to the
environmental impact i
1 INTRODUCTIONEnvironmental preservation has recently become a key
issue in society. Human activities are major contribut-
ing factors to environmental degradation. Conse-
quently, environmental legislation is increasingly
restrictive in terms of emissions from human activities.
For instance, the European Water Act (91/271/EC)
establishes restrictive thresholds on the concentrations
of wastewater emissions. To cope with these laws, over
the last decades there has been an exponential increase
in the implementation of end-of-pipe technologies.
Whilst these technologies have greatly contributed to
ameliorating environmental quality, they all have
environmental side-effects, such as emissions to the
atmosphere, use of natural resources (either renewable
or not), energy consumption, and generation of by-
products.1 Additionally, some of the existing end-of-
pipe technologies need to be restructured to fulfil the
requirements of the new legislation.
The objective of this study was to evaluate the
environmental implications derived from including
changes in one of these end-of-pipe technologies to
comply with the actual legislation. In particular, we
studied a wastewater treatment plant (WWTP) which
had to be modified to include biological nitrogen
removal. Environmental effects from these structural
changes were assessed by using the Life Cycle
Assessment theoretical framework.
Life Cycle Assessment (LCA) is a tool that helps to
identify the overall environmental effects associated
with the whole life of a product or process.2 This
assessment is done by a systematic four-step procedure
(Received 9 August 2001; revised version received 5 April 2002; accepted 11 April 2002)
* Correspondence to: Ignasi Rodrıguez-Roda, Laboratori d’Enginyeria Quımica i Ambiental, Universitat de Girona, Campus Montilivi s/n,17071 Girona, SpainContract/grant sponsor: Catalonian Water AgencyContract/grant sponsor: Spanish MCyT; contract/grant number: DP1-0665-C02-01
# 2002 Society of Chemical Industry. J Chem Technol Biotechnol 0268–2575/2002/$30.00 1206
Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 77:1206–1211 (online: 2002)DOI: 10.1002/jctb.674
which includes: goal definition and scope, inventory,
environmental impact assessment, and interpretation.
Although this tool was first developed for manufac-
tured products or materials,3 this conceptual approach
has been adapted to evaluate environmental side-
effects from the operation of a WWTP. Some previous
recent studies have used a similar approach in the same
domain. Bengtsson et al4 used LCA to facilitate the
municipalities decision-making regarding choice of
technologies for wastewater systems. In other studies,
the use of LCA was more focussed on the evaluation of
the operation of a WWTP from a holistic perspective.
Clauson-Kaas5 optimised the operation of a WWTP
from Denmark using LCA methodology. On the other
hand, Mels et al6 identified more sustainable sewage
treatment scenarios based on structural changes in the
physical–chemical pre-treatment. This study intro-
duces the use of the LCA approach in a rather different
perspective. Structural changes in the configuration of
a WWTP to decrease effluent concentrations of
nitrogen were compared, not only from the opera-
tional perspective, but also from the perspective of
their environmental side-effects.
2 DESCRIPTION OF THE STUDY SCENARIOSThe WWTP selected as a reference scenario had an
activated sludge configuration. The wastewater line
consisted of three consecutive treatment units: a
primary settler, a complete mixed aeration tank (the
bioreactor), and a secondary settler (Fig 1(a)). It also
presented an external sludge return from the second-
ary settler. The by-product of the process (the sludge)
was concentrated in two different units (gravity and
flotation), and centrifuged before being disposed of
(Fig 1(a)). This plant was designed to remove organic
matter and suspended solids.
To decrease the concentration of nitrogen in the
effluent, two alternative configurations that implied
structural changes in the reference scenario were
considered: the Ludzack–Ettinger7,8 and the Oxida-
tion Ditch.7,8 These configurations (hereafter referred
to as modified scenarios) are the most commonly
implemented processes to simultaneously remove
organic matter and nitrogen in the region where this
project was conducted (Catalonia, NE Spain).
The Ludzack–Ettinger configuration consists of an
anoxic tank to denitrify nitrate followed by a second
aeration tank to nitrify the ammonium, with internal
and external sludge return (Fig 1(b)). The Oxidation
Ditch configuration consists of an oval aerated tank
where the wastewater and the activated sludge are
pumped around. This type of reactor has an external
sludge return and is adaptable for carbon oxidation,
nitrification and denitrification (Fig 1(c)). In this
study, comparisons were made between the LCA
results from these modified scenarios, and then
contrasted with those from the reference scenario.
The influent characteristics were assumed to be
equal for the three scenarios, using the influent average
values for WWTP in the Catalonia region. The
Organic matter concentrations were 550gm�3
(measured as COD) and 350gm�3 (measured as
BOD5), suspended solids concentration was 150gm�3
(SS), and nitrogen concentration was 35gm�3
(measured as TKN). The selected influent flow rate
was 3800m3d�1. Values for the sludge loading, the
sludge retention time and the dissolved oxygen set-
points in the bioreactor were obtained from the
literature (Table 1).7,8
3 METHODOLOGYA Life Cycle Assessment theoretical framework was
used to assess the changes in environmental effects due
to the structural modifications in the bioreactor. The
study domain was delimited to evaluate the impacts
produced only by the operation of the plants, and no
considerations were given to the energy and natural
resources needed to build the treatment plants. This
assumption was based on results from Tillman et al9
who found that the environmental impact of the
construction phase of a WWTP does not differ much
between the different scenarios.
For each of the three scenarios, the emissions and
the use of natural resources due to their operation were
evaluated. Emissions and consumption of energy from
the plant operation were obtained from the results of
simulations. Other researchers have also used this
approach in LCA analysis, since it is shown to be
Figure 1. Description of the study scenarios: reference scenario (activatedsludge configuration (a)) and the two modified (Ludzack–Ettinger (b) andOxidation Ditch configurations (c)).
J Chem Technol Biotechnol 77:1206–1211 (online: 2002) 1207
Environmental implications of a wastewater treatment plant
particularly useful in cases where there is a lack of
information about the post-treatment data.10 The
IAWPRC Activated Sludge Model No 111 and a ten-
layer one-dimensional settler model12 were used to
simulate the biological reactions and the settling
process, respectively. The simulations were carried
out using default kinetics and settling parameters.
The Spanish energy and transport database from
Life Cycle Analysis Inventory Tool13 was used to
estimate the resource consumption to produce the
energy required for operation of the plant. This
database was also used to estimate the use of resources
and the generation of emissions associated with the
transportation of sludge from the facility to the nearest
landfill (ie diesel consumption). For this study, the
distance considered between the facility and the
landfill was 30km for a two-way journey.
Values for each emission and resource consumption
were referred to one tonne of wastewater treated. In
addition, the emissions and resources were sorted into
different groups according to their potential impact on
the environment. These impacts were chosen from the
suggested environmental impacts listed by Heijungs.14
Once this classification was done, the potential
contribution (Cij) of a given emission or resource
consumed (j) to each environmental impact (i) was
quantified. The potential contribution was calculated
as follows:15
Cij ¼ EjWij
where Ej is the emission or resource consumption and
Wij is the weighting factor.15 The total potential effect
of the plant operation and sludge transport of a given
environmental impact (Ci) is the sum of each potential
contribution (Cij).
Finally, the values obtained for each environmental
impact were normalised to facilitate the comparison
among the different impacts of the three scenarios.
Normalisation factors were referred as to West Europe
contribution.14,16–19
4 RESULTS4.1 Inventory and quantification of resources usedand emissions producedThe natural resources considered in this study were
natural gas, crude oil, coal hard, and uranium. All of
them were consumed to produce energy for the plant
operation, whereas natural gas and crude oil were also
consumed to produce the diesel to transport the
sludge. Consumption values for all these resources
were higher for the Ludzack–Ettinger configuration
than for the reference scenario (Table 2). In contrast,
the values from the Oxidation Ditch configuration did
not differ significantly from the reference scenario
(Table 2). The fraction of resources consumed due to
sludge transportation was lower than that due to the
water treatment process in all the scenarios studied.
Percentage consumption due to transport for natural
gas was between 1.5 and 2.5, and for crude oil was
between 10 and 15. Major differences in resources
consumption among scenarios were associated with
the water treatment process (Table 2).
Table 1. Design parameters selected for thereference scenario, and the modified scenarios:Ludzack–Ettinger and Oxidation Ditch
Scenarios F/M SRT
Anoxic
zone
Aeration
zone
Reference scenario 0.6 5 – 1.5
Modified scenarios Ludzack–Ettinger 0.4 7 0 2
Oxidation Ditch 0.13 20 0 2
F/M: Sludge loading (kg BOD5kg�1 MLVSS), SRT: sludge retention time (d), DO: dissolved oxygen set-
points (gm�3).
Table 2. Consumption of energy and resourcesfrom the operation of the three scenarios
Reference
scenario
Modified scenarios
Ludzack–Ettinger Oxidation Ditch
Electrical energy 0.8 1.2 0.9
Resource consumption for water treatment
Coal hard 26 38 28.5
Uranium 20.5�10�4 30�10�4 22.5�10�4
Natural gas 0.75 1.10 0.82
Crude oil 4.3 6.3 4.8
Resource consumption for transport
Natural gas 1.7�10�2 1.6�10�2 1.5�10�2
Crude oil 0.77 0.70 0.66
Energy units are in MJ t�1 wastewater treated, and units for resources are in g t�1 wastewater treated.
Consumption of electrical energy is only for the water treatment process. Consumption of resources
from the operation is differentiated between resources consumed in the water treatment process and
those consumed in the sludge transportation.
1208 J Chem Technol Biotechnol 77:1206–1211 (online: 2002)
N Vidal et al
The inventory of emissions produced was sorted
according to the main environmental medium where
they were discharged (ie atmosphere and water).
These emissions are generated (a) from the energy
and diesel production processes, (b) from the waste-
water treatment process, and (c) from the sludge
transportation. The fractions of emissions from sludge
transportation are estimated based on the amount of
sludge produced in each scenario (1500kgd�1 in the
reference scenario, 1300kgd�1 in the Ludzack–Ettin-
ger, and 1270kgd�1 in the Oxidation Ditch). Sludge
water content was the same for all scenarios (80%)
because they all use the same dewatering process.
Table 3 shows the list of emissions considered in this
study and the values obtained from the different
scenarios. As with the consumption of resources, the
Ludzack–Ettinger configuration showed highest
values for most of the emissions to the atmosphere,
except for the CO2 produced by the microorganisms
metabolism in which the Oxidation Ditch had the
highest value (Table 3). Sludge transportation mostly
contributes to the NOx atmospheric emissions and its
contribution to the water emissions is almost negli-
gible, except for phenols (Table 3).
The scenarios studied also differed in terms of the
emissions to water through the plant effluent, in
particular for the nitrogen values. The two modified
scenarios had lower nitrogen emissions (TKNþNO3�)
than the reference scenario (Table 3). Nevertheless,
nitrate emissions were the highest in the Ludzack–
Ettinger configuration (Table 3). We want to empha-
sise that the main objective of the alternative config-
urations was to reduce the nitrogen content in the
effluent to fit the requirements of the Catalonian
legislation (total nitrogen <15gm3, Directive
91/271/EC). According to this assumption, effluent
concentrations of nitrogen in the two alternative
configurations were below this legal threshold. Under
these conditions, the high NO3� emissions obtained in
the simulations of the Ludzack–Ettinger configuration
indicate that there was only a partial denitrification of
the produced nitrate in the plant.
4.2 Assessment of environmental impactsAll resources consumptions and emissions were sorted
into five environmental impact categories: abiotic
depletion, eutrophication, global warming, acidifica-
tion, and human toxicity. Each category included all
the resources and emissions that could contribute to it
to some degree. Therefore, a given emission or
consumption of a resource has been considered in
several of these categories (Table 4).
Among the factors studied, crude oil and natural gas
are the ones that contribute to the greatest extent (ie
have a higher weight) to the abiotic depletion
impact.16 NH3 (either released to the atmosphere or
to the water) and NH4þ are the major contributing
factors to the impact of eutrophication.14 Similarly,
CO2 (from either biotic or abiotic processes), NH3
released to the atmosphere, and NOx are the most
important factors influencing the impacts of global
warming,17 acidification18 and human toxicity,19
respectively.
Values from the quantification of each environ-
mental impact for each of the three scenarios are
shown in Table 5. The most significant difference
between the reference scenario and the two modified
scenarios is the reduction of the eutrophication impact
in the latter cases. The eutrophication impact was
reduced by 68% in the Ludzack–Ettinger configura-
tion and 75% in the Oxidation Ditch configuration
with respect to the reference scenario. Nevertheless,
the modified scenarios present an increase in the
magnitude of the rest of the environmental impacts (ie
Table 3. Emissions to the atmosphere and water by the operation of the wastewater treatment plant under the three scenarios Values are given in total g t�1
wastewater treated and as percentage of total emissions due to the sludge transportation.
Environmental
medium Emissions
Modified scenarios
Reference scenario Ludzack–Ettinger Oxidation Ditch
Total
(g t�1)
Transport
(% of total)
Total
(g t�1)
Transport
(% of total)
Total
(g t�1)
Transport
(% of total)
Atmosphere
NH3 1.4�10�6 0 2.0�10�6 0 1.5�10�6 0
NOx 0.11 39.1 0.13 28.7 0.11 33.4
CO2 88.7 2.9 128.4 1.9 96.7 2.3
CO2(bio) 40.8 0 57.8 0 91.2 0
N2O 23�10�4 5.9 34�10�4 3.7 25�10�4 4.6
CH4 0.21 0.4 0.30 0.3 0.23 0.3
SO2 0.13 3.8 0.19 2.4 0.14 3.0
Water
COD 51.2 0 48.9 0 48.1 0
NO3� 2.9 0 9.4 0 2.33 0
NH4þ 21.3 0 2 0 2.37 0
TKN 22.6 0 3 0 3.1 0
NH3 13�10�7 0 19�10�7 0 14�10�7 0
Phenol 1.3�10�5 93.1 1.3�10�5 89.4 1.2�10�5 91.3
J Chem Technol Biotechnol 77:1206–1211 (online: 2002) 1209
Environmental implications of a wastewater treatment plant
negative effect). When comparing these negative
effects between the two modified scenarios, the
Ludzack–Ettinger showed higher relative contribu-
tions to the environmental impacts (about 30%) than
the Oxidation Ditch (<10%), except in the case of
global warming where values are similar.
Results from the normalised impact values show
that eutrophication is the major environmental impact
caused by all three scenarios (Fig 2). Abiotic depletion
and global warming are also relevant, but their impact
is lower than that of eutrophication. Impacts on
acidification and human toxicity are almost negligible.
5 DISCUSSIONWWTP are designed to minimise the direct impact of
sewage into recipient ecosystems; however, their
operation is not devoid of environmental side-effects.
These effects are mostly due to the use of natural
resources as well as the generation of emissions. In this
study, the application of the LCA methodology has
allowed not only the identification of these side-effects,
but their quantification. Moreover, this methodology
permitted comparison of different WWTP configura-
tions on the basis of their associated environmental
effects. Results from this study show that major
differences among wastewater treatment configura-
tions are associated with the impacts caused by their
water treatment process rather than by the transporta-
tion of sludge.
Among all the possible side-effects caused by the
operation of the three scenarios studied, the impact on
eutrophication is the largest one. Nevertheless, results
from the LCA reveal that impacts on abiotic depletion
and global warming may also be relevant.
Results from this study show that the inclusion of
nitrogen removal mechanisms in the configuration of
the plant reduces the effect of the plant on eutrophica-
tion. Not so obviously, the results also show that the
decrease in the eutrophication impact is somehow
counterbalanced by a simultaneous increase in effects
on the rest of the environmental impacts studied.
Modified scenarios required more oxygen to reduce
the nitrogen load (ie enhance the nitrification–deni-
trification process). Consequently, their energy re-
quirements are larger than in the reference scenario,
and thus the use of natural resources, and the
emissions, increase. These general trends, however,
vary depending on the selected configuration to
remove nitrogen, as shown by the differences between
the two modified scenarios. Taking all the impacts
together, the Oxidation Ditch configuration would
cause less environmental impacts than the Ludzack–
Table 4. Classification of the emissionsand resources consumed to differenttypes of environmental impacts (Ej) andweighting factors (Wj) for each of them
Abiotic depletion
(kg antimony eq)
Eutrophication
(kg PO43� eq)
Global
warming
(kg CO2 eq)
Acidification
(kg SO2 eq)
Human toxicity
(kg 1,4-
dichlorobzene eq)
Ej Wj Ej Wj Ej Wj Ej Wj Ej Wj
Crude oil 0.0201 NH3 (atm) 0.35 CO2 (atm) 1 NH3 (atm) 1.88 NOx(atm) 1.2
Natural gas 0.0187 NH3 (water) 0.35 CH4 (atm) 56 SOx(atm) 1 NH3 (atm) 0.1
Coal hard 0.0134 NH4þ
(water) 0.33 N2O(atm) 280 NOx(atm) 0.7 SOx(atm) 0.096
Uranium 0.0028 NOx(atm) 0.13 Phenol(water) 0.049
NO3�
(water) 0.1
COD(water) 0.02
Table 5. Quantification ofenvironmental impacts
Environmental impact
Reference
scenario
Modified scenario
Ludzack–Ettinger
Oxidation
Ditch
Abiotic depletion (kg antimony eq) 0.467 0.688 0.508
Eutrophication (kg PO4 eq) 8.46 2.69 2.09
Global warming (kg CO2 eq) 142 207 202
Acidification (kg SO2 eq) 0.2 0.29 0.21
Human toxicity (kg 1,4-dichlorobenzene eq) 0.132 0.18 0.13
Figure 2. Results from the normalised impact values.
1210 J Chem Technol Biotechnol 77:1206–1211 (online: 2002)
N Vidal et al
Ettinger configuration, given the characteristics of the
selected scenarios. The Oxidation Ditch is the most
commonly implemented configuration in the Catalo-
nian region for those plants with a similar capacity to
the one studied. In this particular case it seems that the
theoretical and the real scenarios converge. Never-
theless, information from LCA studies applied in this
field is subject to economical, political, and social
constraints and, thus, although it provides valuable
information it can only be used as a complementary
tool to reach a final decision.
Nowadays evaluation of environmental side-effects
from operating systems is still not a key issue for
decision-makers. However, there is an increasing
sensitivity for environmental issues at social level,
which can act as a driving force to include this kind of
environmental assessment tool in the decision-making
process.
ACKNOWLEDGEMENTSThis study was supported by a grant from the
Catalonian Water Agency and the Spanish MCyT
projects (DPI-0665-C02-01). Nuria Vidal acknowl-
edges a pre-doctoral TDOC grant.
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