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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