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.

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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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