Review Article Greenhouse Gases Emissions from Wastewater...

Review ArticleGreenhouse Gases Emissions from Wastewater TreatmentPlants Minimization Treatment and Prevention

J L Campos1 D Valenzuela-Heredia1 A Pedrouso2 A Val del Riacuteo2

M Belmonte34 and A Mosquera-Corral2

1Facultad de Ingenierıa y Ciencias Universidad Adolfo Ibanez Avenida Padre Hurtado 750 2520000 Vina del Mar Chile2Department of Chemical Engineering School of Engineering University of Santiago de Compostela Rua Lope Gomez de Marzoa sn15782 Santiago de Compostela Spain3Department of Environment Faculty of Engineering University of Playa Ancha Avenida Leopoldo Carvallo 2702340000 Valparaıso Chile4School of Biochemical Engineering Pontifical Catholic University of Valparaıso Avenida Brasil 2085 2340000 Valparaıso Chile

Correspondence should be addressed to J L Campos jluiscamposuaicl

Received 28 December 2015 Revised 29 March 2016 Accepted 5 April 2016

Academic Editor Claudio Di Iaconi

Copyright copy 2016 J L Campos et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

The operation of wastewater treatment plants results in direct emissions from the biological processes of greenhouse gases (GHG)such as carbon dioxide (CO

2) methane (CH

4) and nitrous oxide (N

2O) as well as indirect emissions resulting from energy

generation In this study three possible ways to reduce these emissions are discussed and analyzed (1) minimization throughthe change of operational conditions (2) treatment of the gaseous streams and (3) prevention by applying new configurations andprocesses to remove both organic matter and pollutants In currentWWTPs to modify the operational conditions of existing unitsreveals itself as possibly the most economical way to decrease N

2O and CO

2emissions without deterioration of effluent quality

Nowadays the treatment of the gaseous streams containing the GHG seems to be a not suitable option due to the high capital costsof systems involved to capture and clean them The change of WWTP configuration by using microalgae or partial nitritation-Anammox processes to remove ammonia from wastewater instead of conventional nitrification-denitrification processes cansignificantly reduce the GHG emissions and the energy consumed However the area required in the case of microalgae systemsand the current lack of information about stability of partial nitritation-Anammox processes operating in the main stream of theWWTP are factors to be considered

1 Introduction

In the past years most efforts to improve wastewater treat-ment plants (WWTPs) performance have been focused onobtaining a good effluent quality [1ndash5] However nowadaysnew challenges are under consideration oriented to ensurethe sustainability of WWTPs in terms of their economicfeasibility and environmental impact Energy consumptionand greenhouse gases (GHG) emissions are among theaspects that have become key-factors concerning the overallperformance of the WWTPs [6 7] Recent studies haveidentified the WWTPs as potential sources of anthropogenicGHG emissions contributing to climate change and airpollution [8ndash10] WWTPs produce carbon dioxide (CO

2)

methane (CH4) and nitrous oxide (N

2O) during the biolog-

ical wastewater treatment processes and CO2is also emitted

during the production of the energy required for the plantoperation The CO

2released due to the energy demand can

be directly reduced by enhancing the energy efficiency of theWWTPs In this way both the reduction of environmentalimpacts and the decrease of treatment costs by enhancing theenergy savings can be accomplished simultaneously

With regard to each GHG source the N2O emitted is

generated by nitrification and denitrification processes usedto remove nitrogenous compounds from wastewater Itsproduction occurs mainly in the activated sludge units (90)while the remaining 10 comes from the grit and sludgestorage tanks [11] N

2O gas is an intermediate of biological

Hindawi Publishing CorporationJournal of ChemistryVolume 2016 Article ID 3796352 12 pageshttpdxdoiorg10115520163796352

2 Journal of Chemistry

NO

NO

Autotrophic nitrification

Heterotrophic denitrification

Organicmatter

Organicmatter

Organicmatter

Organicmatter

Nitrifierdenitrification

Hydroxylamineoxidation

Chemicalhydroxylamineoxidation

N2O

N2O

N2ON2O

O2 O2 O2NH2OH N2NH4

+ NO2

minus NO2

minusNO3

minus

Figure 1 Biological and chemical pathways of N2O production in the nitrification and denitrification processes

processes such as heterotrophic denitrification and nitrifica-tion (Figure 1) It is formed during denitrification operatedat low pH values and toxic compounds or low dissolvedoxygen (DO) concentrations are present in themedia [12ndash14]Nitrifying bacteria are able to produce N

2O under aerobic or

anoxic [15] conditions In anoxic conditions both ammonia-and nitrite-oxidizing bacteria are able to produce it whileonly ammonia-oxidizing bacteria do it in aerobic conditionsIn the latter case the production is stimulated by the presenceof low DO concentrations and presence of nitrite (NO

2

minus) ororganic matter in the liquid media [12 13] Nitrous oxide canbe produced also from chemical reactions taking place in thepresence of hydroxylamine and nitrite [16]

In practice nitrous oxide is emitted in the WWTPpredominantly in the aerobic tank [17] However the con-tribution of the anoxic and aerobic reactors to this pro-duction remains still unclear since it can be produced inthe anoxic stage and be subsequently stripped to the gasphase in the aerated compartment [18] Ammonia-oxidizingbacteria have been identified as the main N

2O producers

while heterotrophic denitrifying bacteria contribution is onlyrelevant when nitrite andor oxygen are present in theanoxic stage [19] According to Tallec et al [20] undercommon operational conditions the N

2O production occurs

mainly via denitrification by nitrifying bacteria Howeverhydroxylamine oxidation pathway can be the main processresponsible for the production of N

2O emissions at high

ammonia (NH4

+) and lownitrite concentrationswhen a highmetabolic activity of ammonia-oxidizing bacteria is present(at 2 to 3mg O

2L) [19]

With regard to CH4emissions Daelman et al [21]

found out that about 1 of the incoming chemical oxygendemand (COD) to theWWTPs was emitted as methaneThisamount exceeds the amount of carbon dioxide emission thatwas avoided by utilizing the produced biogas in anaerobicdigestion The main sources of methane detected by theseauthors were related to the sludge line units where anaerobicdigestion is carried out the primary sludge thickener thecentrifuge the exhaust gas of the cogeneration plant thebuffer tank for the digested sludge and the storage tank forthe dewatered sludge These units contribute to around 72of methane emissions of the WWTPs while the remainingemissions come from the biological reactors and can be

mainly attributed to the CH4dissolved in the wastewater

which is not totally removed by the biological systemResearch works of Yver Kwok et al [22] and Oshita et al[23] also showed that most of the methane emissions fromWWTPs are closely related to processes involved in the sludgeline

With respect to CO2its production is attributed to two

main factors biological treatment process and electricityconsumption In the main stream of the WWTP the organiccarbon of wastewater is either incorporated into biomass oroxidized to CO

2 In the sludge line it is converted mainly

to CO2and CH

4during anaerobic digestion and finally

methane is oxidized to CO2during biogas combustion

In recent literature the emissions of GHG from theconventional configurations ofWWTPswere determined butthe analysis of the possible alternatives to minimize theseemissions is generally not done [24ndash28] On the other handmost of the papers studying the application of new processesto remove pollutants from wastewaters are mainly focusedon the energy savings [29ndash32] and only few of them alsogive an environmental evaluation [33ndash36] Therefore themain objective of this work is to provide an overview of allpossible ways to reduce GHG emissions from WWTPs Forthat two possible scenarios are discussed and analyzed (1)to maintain the present scheme of operation of the WWTPand to modify the operational conditions (minimization) orto implement capture and treatment units for the gaseousstreams (treatment) and (2) to change the scheme of oper-ation and to implement new processes which produce lowerGHG emissions than the existing ones (prevention)

2 Minimization of GHG Emissions

Perhaps the most efficient way in terms of costs to reduceGHG emissions is to modify the operational conditions ofWWTPs units but this is not always possible due to theoperational limitations of the installed units In the followingsections some recommendations about the possible actionsto put in practice to operateWWPTs in order to reduce GHGemissions are provided

21 N2O Production Data obtained from the operation of

full-scale WWTPs show a wide range of values for the

Journal of Chemistry 3

fraction of nitrogen that is emitted as N2O (0ndash146 of the

nitrogen load) [12] Such large variation can be related tothe different operational conditions imposed in the studiedWWTPs Having this in mind decreasing the amounts ofN2O emitted from activated sludge processes presents a great

potential for improvement by avoiding those operationalconditions identified as responsible for its production Someidentified conditions are (i) low dissolved oxygen concen-tration in the nitrification and the presence of oxygen indenitrification stages (ii) high nitrite concentrations in bothnitrification and denitrification stages (iii) low CODN ratioin the denitrification stage (iv) sudden shifts of pH anddissolved oxygen and ammonia and nitrite concentrationsand (v) transient anoxic and aerobic conditions [12 13]

Therefore to minimize N2O emissions biological waste-

water treatment plants should be operated at high solidretention times (SRT) to maintain low ammonia and nitriteconcentrations in the media Furthermore large bioreactorvolumes are recommended to dispose of systems able tobuffer loadings and reduce the risk of transient oxygendepletion N

2Oemissions can be also reduced if nitrous oxide

stripping by aeration is limited since microorganisms wouldhave more time to consume it [37]

22 CH4Production CH

4emissions can be minimized if

thickening sludge tanks and sludge disposal tanks are coveredto avoid gas leakages and their emissions are captured byhoods which could be burnt with excess biogas in a torch [21]Besides the methane produced in the plant itself methanealso enters the plant from outside via the influent since itcontains CH

4that has been formed in the sewerThemethane

load was estimated as 1 of the influent COD load and ismainly oxidized in the activated sludge tanks (80) whichcould be exploited as a means to further decrease methaneemissions from wastewater treatment [21]

23 CO2Production Organicmatter oxidation in the biolog-

ical reactors and combustion of CH4are responsible for the

direct CO2emissions while indirect emissions are attributed

to the energy consumption of the WWTP [26] The SRTapplied to the biological reactor is a key operational factorthat affects these emissions The operation of the activatedsludge system at high values of SRT promotes endogenousrespiration of biomass which increases the amount of CODoxidized to CO

2and decreases the overall sludge production

This decrease of sludge production implies a decrease ofthe methane production and therefore a decrease of theCO2emissions associated with its combustion [38] Both

tendencies counteract each other and the addition valueof both quantities remains almost constant Furthermorethe decrease of the SRT also involves an increase of theenergy efficiency of the WWTP and therefore a decreaseof indirect CO

2emissions Therefore CO

2emissions should

be minimized by applying the shortest SRT value as possiblewithout negatively affecting the effluent quality

The effect of SRT on the overall CO2emissions of a

conventional WWTP can be quantified by performing massand energy balances according to themethodology describedby Campos et al [39] and using the parameters given in

Table 1 Values assigned to the parameters used to estimate CO2

emissions

Parameter Units ReferenceCO2emissions from

energy consumption 0391 kgCO2kWsdoth

[40]CO2emissions from

COD oxidation 008 kgCO2kg CODlowast

CO2emissions from

CH4combustion 35 kgCO

2Nm3 CH

4

lowastlowast

lowastEstimated taking into account an elemental composition of C243H396O forthe biodegradable fractions of the COD [41] lowastlowastCalculated from stoichiom-etry and ideal gas law

0

01

02

03

04

05

10 302015SRT (d)

(kg

CO2m

3

ww

)

CO2 emissions from the aerobic reactorCO2 emissions from CH4 combustionCO2 emissions from energy consumptionTotal CO2 emissions

Figure 2 CO2emissions estimated for a conventional WWTP

operated at different SRT values

Table 1The SRT values tested ranged from 10 to 30 d in orderto guarantee a stable nitrification Results showed that anincrease of the SRT from 10 to 30 days supposed an increaseof 76 of the CO

2emissions (Figure 2)

3 Treatment

A second possible option to reduce GHG emissions fromWWTPs is to capture and treat them An important numberof technologies are available to destroy or capture N

2O CH

4

and CO2from industrial gaseous streams but there is still

a need for the development of efficient low-cost abatementtechnologies to treat gaseous streams from WWTPs On theother hand the capital costs required to cover the differenttanks and capture GHG emissions are relatively high [47]

31 N2O Removal Traditional technologies such as selec-

tive catalytic reduction and selective noncatalytic reductionare currently used to control NOx emissions from powerplants [48ndash50] However both processes require operatingat high temperatures or using catalysts which revert to highinstallation and maintenance costs These total costs becomeprohibitive in large-scale facilities treating air flows contain-ing low-to-moderate concentrations of NOx [51] Recently


many different bioprocesses using nitrifying and denitrifyingbacteria or microalgae have been developed to control NOxgas emissions Technologies based on the denitrificationprocess have been successfully used to remove N

2O with

efficiencies of 75ndash99 [51ndash53] However the low aqueoussolubility of this greenhouse gas limits the mass transfer ratefrom the air flow to the liquid phase and therefore highhydraulic retention times (HRT) are required to achieve highN2O removal efficiencies These long applied HRT result in

large bioscrubber (or biofilter) volumes with the subsequentincrease in capital costs [53] Another alternative is to collectthe outlet gaseous stream from the top of the nitrifying unitcontaining N

2O and use it as oxidizer to burn the methane

produced in the anaerobic sludge digester [54]

32 CH4Removal Biological technologies to remove CH

4

from waste gaseous emissions based on biofilter systemshave been studied since the early 1990s although they are notyet consolidated at industrial scale [55 56] Several biologicalprocesses are capable of oxidizing methane into CO

2(1mol

to 1mol) which allow reducing the total GHG emissions interms of CO

2equivalents since the warming factor of CO

2

is lower than that of methane In aerobic conditions CH4

is oxidized by methanotrophic bacteria in the presence ofoxygen Another option relies on the application of anaerobicconditions and exploitation of the activity of bacteria andarchaea to oxidize CH

4using sulfate nitrite nitrate Mn+4

or Fe+3 as electron acceptors [57]

CH4+ SO4

minus2997888rarr HCO

3

minus+HSminus +H

2O

Δ119866 = minus166 kJmol(1)

3CH4+ 8NO

2

minus+ 8H+ 997888rarr 3CO

2+ 4N2+ 10H

2O


5CH4+ 8NO

3


2+ 4N2+ 14H

2O


CH4+ 4MnO

2+ 7H+ 997888rarr HCO

3

minus+ 4Mn+2 + 5H

2O


CH4+ 8Fe (OH)

3+ 15H+

997888rarr HCO3

minus+ 8Fe+2 + 21H

2O

Δ119866 = minus270 kJmol

(5)

As in the case of the N2O gas the low solubility of CH

4

implies the necessity to operate the biofilters at high residencetime values (2ndash30minutes) [58] For these reasons nowadaysthe interest has moved to remove the CH

4directly from

the liquid phase before it is stripped to the atmosphere Toaccomplish this removal the anaerobic methane oxidation iscoupled to a denitrification process ((2)-(3)) which uses themethane as electron donor In this case methane and bothnitrite and nitrate are removed from wastewater [59] Fur-thermore not only is the GHG removed but also the electron

donor requirements for the denitrification processes dimin-ish reducing the costs of potential addition of external carbonsource

After the biological processes the methane remainingin the exhausted gaseous stream can be submitted to apostcombustion process [60]

33 CO2Removal For the CO

2gas removal extensive

research has been carried out on the study of its captureby chemical or physical sorption and membrane separationprocesses from power cycles and industrial processes [61 62]However the application of these technologies is generallyassociated with high capital and operating costs and thegeneration of waste streams For these reasons nowadays thecultivation of microalgae is being considered as an attractivealternative for CO

2gas sequestration InWWTPsmicroalgae

can be used for precombustion CO2capture as an economic

way for biogas purification [63] or for postcombustion CO2

capture in order to maximize the microalgae production fortheir use as biofertilizer [64] or as substrate to increase biogasproduction [65]

4 Prevention

Most of the efforts to improve WWTPs performance arebeing focused on economic aspects related to energy con-sumption reduction minimization of sludge production andmaximization of the amount and quality of biogas generatedTo face these topics is important not only in terms ofoperational costs but also in terms of environmental impactssince it allows reducing direct and indirect GHG emissions[9 66 67]

Nowadays only around 35ndash45 of the energy con-tained in the raw wastewater as organic compounds isconverted into CH

4during anaerobic digestion of primary

and secondary sludge The remaining part is wasted underaerobic conditions due to the use of conventional nitrifica-tion and denitrification processes to remove nitrogen andorganic matter simultaneously [68] An alternative is to applyautotrophic processes to remove nitrogen such as thosebased on the combination of the partial nitrification plusAnammox processes or the use of microalgae and even theapplication of biochemical processes In thisway both organicmatter and nitrogen compounds can be removed in separatedprocesses as the former is not required for denitrification butdirected to the anaerobic digestion for biogas productionThus oxygen requirements are minimized while methaneproduction is maximized [69ndash71]

41 Application of Partial Nitritation and Anammox Pro-cesses to Remove Ammonia In principle according to themetabolism of Anammox bacteria these are not directlyinvolved in the production of N

2O [72] and therefore the

application of the Anammox process in the WWTPs insteadof the conventional nitrification-denitrification processes isexpected to reduce N

2O emissions However in practice

during the operation of full-scale Anammox reactors treatingthe reject water from sludge anaerobic digesters N

2O emis-

sions have been detected and accounted for up to 06 of


the converted nitrogen [46] This value is much higher thanthe percentages previously measured in lab-scale Anammoxenriched reactors fed with synthetic media of 003ndash01[73 74] For this reason the results at full scale can beattributed to the presence of nitrifying bacteria entering theAnammox reactor in the stream coming from the previouspartial nitrification unit [74]

Furthermore Kampschreur et al [46] measured also N2O

emissions in a partial nitritation full-scale reactor attributedto denitrification carried out by ammonia-oxidizing bacteriawhich corresponded to the 17 of the inlet nitrogen load[18] From the previous results obtained from full-scalesystems about 23 of the nitrogen load can be converted toN2O in nitritation-Anammox systems In these conditions a

two-stage partial nitritation-Anammox process appears as anonsuitable alternative to reduce N

2O emissions in WWTPs

[66 67]Emitted percentages can be reduced down to 08ndash12 if

a one-reactor nitritation-Anammox system is used [43 75ndash77] At full scale this configuration is the most applied onefor the treatment of reject water from the sludge line [78]For this reason all the latest studies of the application at themain stream of the Anammox based processes have beencarried out in single-stage systems However this kind ofsystems must be operated at low dissolved oxygen concen-trations to maintain the balance between ammonia oxidationand Anammox rates and therefore the achieved nitrogenremoval rates are relatively low [68] However to operateat low dissolved oxygen levels promotes the developmentof nitrite-oxidizing bacteria and favours the oxidation ofammonia to nitrate instead of its desired conversion to N

2

[78] Due to this difficulty in avoiding the activity of thenitrite-oxidizing bacteria a change of concept has occurredand most of the research in course to implement the Anam-mox process at the main stream is focused on the two-stagereactor configuration In this way operational strategies toavoid the development of nitrite-oxidizing bacteria withoutaffecting the Anammox bacteria can be evaluated [68]This might imply that the emissions of N

2O would hamper

the practical application of the partial nitritation-Anammoxprocess from the energy-saving and cost-effective point ofview Nevertheless since the total amount of N

2O emission

from the partial nitrification unit is correlated to the nitriteconcentration present N

2O emissions about 01 of the

inlet nitrogen load are expected considering a nitritationunit operated for the treatment of the main stream (20ndash25mg NO

2

minus-NL) [44] Until now data of N2O emissions

from Anammox systems in operation in the main streamconditions are not available in the literature although if theentrance of nitrifying bacteria inside the Anammox system isminimized the expected emissions would be limited to 01of the inlet nitrogen load and mainly due to the presence ofheterotrophic denitrifying bacteria [45 79] This means thatthe partial nitritation-Anammox system treating the mainstream would emit in total around 02 of the inlet nitrogenload as N

2O

Taking into account the fact that WWTPs with nitrogenremoval carried out by nitrification-denitrification processeshave a median emission factor of 001 kg N

2O-Nkg Ninfluent

[42] meaning that 06 of the inlet nitrogen is convertedinto N

2O the application of partial nitritation-Anammox

processes in both sludge line (20 of the total nitrogen loadwith a conversion of 08 into N

2O) and main stream (80

of the total nitrogen loadwith a conversion of 02 intoN2O)

will signify an important decrease of the N2O emissions

42 CANDO Process Recently Scherson et al [54] intro-duced a new N removal process called CANDO (CoupledAerobic-anoxic Nitrous Decomposition Operation) whichinvolves three steps (1) biological conversion of NH

4

+ toNO2

minus (2) biological or chemical partial anoxic reduction ofNO2

minus to N2O and (3) N

2O conversion to N

2with energy

recoveryThen from steps (1) and (2) ammonia is converted to

N2O which is used in step (3) as a cooxidant for CH

4

combustion or decomposed over a metal oxide catalyst torecover energyThe end product of the reaction is the N

2The

innovation consists of utilizing N2O as a renewable energy

source and reducing the requirements of organic matterwhich is consumed during denitrification Combustion ofCH4with N

2O releases roughly 30 more heat than using

O2((6) and (7)) and mitigates the release of N

2O to the

atmosphere

CH4+ 4N2O 997888rarr CO

2+ 2H2O + 4N

2

Δ119867 = 1219 kJmol CH4

(6)

CH4+ 2O2997888rarr CO

2+ 2H2O

Δ119867 = 890 kJmol CH4

(7)

Steps (1) and (3) of the CANDO process have been alreadyapplied at full scale while step (2) is still under study [5471 80] In these research works two ways of producingnitrous oxide from nitrite are proposed (1) abiotic reductionby Fe(II) with conversions over 90 and (2) partial het-erotrophic denitrification (62 of NO

2

minus converted to N2O)

43 Application of Microalgae One of the main operat-ing costs of conventional activated sludge systems wherenitrogen removal takes place is associated with the largeaeration requirements Alternative systems like those basedon microalgae are being considered as potential substi-tutes In these systems nitrogen is removed via assimila-tion for biomass growth without oxygen consumption (8)[81] decreasing energy requirements Moreover a low N

2O

production is expected (0005 kg N2O-Nkg Napplied) if

microalgae are used to remove nitrogen [82]

106CO2+ 236H

2O + 16NH

4

++HPO

4

minus2+ light

997888rarr C106

H181

O45N16P + 118O

2+ 171H

2O + 14H+

(8)

When microalgae are applied for wastewater treatment culti-vation the process is generally carried out in open racewayponds since the capital costs of these systems are lowerthan those of photobioreactors [83] These microalgae pondsoccupy large land areas which limits their use to rural areas


Heterotrophicbacteria Microalgae

Organicmatter

Light

Ammonia

CO2

O2

Figure 3 Interactions of a mix culture containing heterotrophicbacteria and microalgae

Another disadvantage of the microalgae application relieson the poor settling properties of the microalgae whichimplies the use of coagulants and flocculants for separationfrom the treated wastewater [84] For this reason a novelapproach consisting in the use of algal-bacterial cocultureshas received significant attention in recent years as well Inthis way the bacterial population would profit from the O

2

produced by algae reducing the aeration requirements oftreatment processes and at the same time greenhouse gasemissions aremitigated by theCO

2consumption during algal

photosynthesis (Figure 3) According to (8) microalgae pro-duce 17 kgO

2kgNremovedThen for typical urbanwastewater

the amount of oxygen produced by microalgae would behigher than the O

2amount needed to remove organic matter

by the activity of the heterotrophic biomass [85] Thereforeboth organic matter and nitrogen could be simultaneouslyremoved in an open raceway pond without oxygen externalsupply In addition challenges associated with the highenergy requirements for algal biomass harvesting might beovercome by means of the better settleability properties ofthe algal-bacterial coculture Su et al [86] demonstrated thatan algal-bacterial coculture is able not only to achieve highCOD and nutrient removal efficiencies but also to settlecompletely over 20 minutes They also argued that the sharesof algae and sludge inoculated in the pond have an influenceon the nutrient removal efficiency and settleability and theyidentify the ratio value of 1 5 (algaesludge by weight) as thatproviding the biomass with the best settleability

5 Case Studies

In order to quantify the potential reduction of GHG emis-sions due to the implementation of new processes inWWTPs(prevention strategy) five different configurations were eval-uated for comparison purposes

Case A A conventional activated sludge system was used as abase case performing the nitrification-denitrification processto remove both organic matter and nitrogen (Figure 4)The operational conditions of this system were SRT of 15 dhydraulic retention time (HRT) of 12 h internal recycle ratioof 3 external recycle ratio of 1 and aerobic volume percentageof 45 In the primary settler a particulate COD removalefficiency of 45 was assumed Primary and secondary

sludge are treated in an anaerobic digester (SRT 30 d) in orderto produce biogas used in a cogeneration unit and reducethe amount of sludge generatedThe following cases take thisone as a base case and only the descriptions of modificationsapplied to this configuration are included

Case B The activated sludge system was substituted by anaerobic reactor operated at a SRT of 2 d to remove organicmatter and to maximize the sludge generation in order toincreasemethane production followed by a partial nitritationand anAnammox reactor to remove nitrogen In this case theprimary settler is intended for removing all the particulateCOD to promote the anaerobic route of organic matterNitrogen is treated from the return sludge stream by a 1-stagepartial nitritation-Anammox reactor

Case C A CANDO system comprising a partial nitrificationand a partial denitrifying reactor is implemented in the sludgeline In this case organic matter separated in the primarysludge is increased 20 since the nitrogen load applied to theactivated sludge system is decreased in the same percentageIn the sludge line an acidogenic reactor is used to provideorganic matter to the partial denitrifying reactor ProducedN2O is used instead of O

2 to burn methane

Case D The activated sludge system was substituted by anaerobic reactor operated at a SRT of 2 d to remove organicmatter followed by a high rate microalgae pond operated ata HRT of 6 days to remove nitrogen

Case E A high ratemicroalgae pond (SRT andHRTof 6 days)where the microalgae remove the nitrogen and provide theoxygen required for the heterotrophic bacteria to oxidize theorganic matter was evaluated in substitution of the activatedsludge system

Mass and energy balances were performed by using Excelspreadsheets according to the methodology described inCampos et al [39] Finally the emissions of CO

2 CH4

and N2O were estimated considering the results obtained

from the mass balances and the parameters given in Tables1 and 2 From these values the global GHG emissions ofeach configuration expressed as kg CO

2m3 of wastewater

treated were calculated taking into account the greenhousegas production impact factors of 21 and 310 for CH

4andN

2O

respectivelyResults obtained from the calculations indicate that

systems using microalgae to remove nitrogen are the mostsuitable systems to reduce GHG emissions during wastewatertreatment (Figure 5) This fact is mainly due to the highamount of CO

2captured by the microalgae together with the

contribution of three other factors (1) the improvement oftheWWTP energy efficiency in CaseD since the applicationof microalgae to remove nitrogen does not require thepresence of organic matter most of it can be converted intomethane while in Case E oxygen generated by microalgaeallows an important energy saving in terms of aeration (2)the very low emissions of N

2O observed in the high rate

microalgae ponds and (3) the additional biogas productiondue to the anaerobic digestion of the generated microalgae


Case APrimarysettlerInfluent Effluent

Thickening tankPrimary sludge

Sludge digester

Thickening tankSecondary sludge

Anoxic Aerobic

Dehydrationsystem

Dehydratedsludge

SecondarysettlerOxygen

Biogas

Gas engine cogeneration system

Energy

Water lineSludge lineGas line



Case B

AerobicPrimary settler

Influent Effluent


Sludge digester


Dehydrationsystem

Dehydratedsludge

Secondarysettler

Nitritation

Nitritation-

Anammox

Biogas


Energy

Case C

PrimarysettlerInfluent Effluent


Acidogenicreactor


Anoxic Aerobic

Dehydrationsystem

Dehydratedsludge


Biogas


Energy

NitritationDenitritation

Methanogenicreactor

Organicmatter

Anammox

N2O

SRT 2d

Figure 4 Continued




Case D

Case E



Sludge digesterBiogas


Aerobic

Dehydrationsystem

Dehydratedsludge

Secondarysettler

High ratemicroalgae pond

Secondarysettler


Energy

Primary settlerInfluent Effluent


Sludge digester


Dehydrationsystem

Dehydratedsludge

Secondarysettler

High rate microalgae pond

Biogas


Atmospheric

Energy

CO2

CO2

CO2

Atmospheric CO2

SRT 2d

Figure 4 Layout of the five WWTPs configurations evaluated in terms of GHG emissions

Table 2 Values assigned to the parameters used to estimate GHG emissions for the case studies

Parameter Units ReferenceCH4emissions from leakages 085 of COD treated + 13 of CH

4burntlowast [21]

N2O emissions from nitrification-denitrification

units 05 of the nitrogen treated [42]

N2O emissions from 1-stage partial

nitritation-Anammox reactors (sludge line) 08 of the nitrogen treated [43]

N2O emissions from PN reactor (main stream) 01 of the nitrogen treated [44]

N2O emissions from Anammox reactor (main

stream) 01 of the nitrogen treated [45]

N2O emissions from CANDO process 34 of the nitrogen treated + 13 of N

2O burntlowastlowast [21 46]

lowastTaking into account the fact that CH4 leakage from the cogeneration engine is 15 of the CH4 emissions lowastlowastTaking into account the fact that all the ammoniapresent in the wastewater is converted into nitrite in the partial nitrification reactor and supposing a leakage of the N2O from the cogeneration engine similarto that of the CH4


minus06

minus04

minus02

0

02

04

06

08

Case A Case ECase DCase CCase B

(kg

CO2m

3

ww

)

CO2 emissions from aerobic reactor

CO2 emissions from algae growth

CO2 emissions from CH4 combustion

CO2 emissions from CH4 leakages

CO2 emissions from energy consumption

Total CO2 emissionsCO2 emissions from N2O generated

Figure 5 GHG emissions (expressed as kg CO2 equivalentm3 of

wastewater treated) of different WWTPs configurations

When the partial nitritation and Anammox processes areused to remove ammonia instead of conventional nitri-fication and denitrification processes the WWTP energyefficiency is also improved which also causes a decrease ofGHG emissions However this decrease is considerably lowerthan that obtained by microalgae systems

The WWTP configuration based on the application ofthe CANDO process (Case C) has associated GHG emissionshigher than those of the conventional system This can beattributed to the increase of N

2O emissions due to the

implementation of a partial nitrification reactor in the sludgeline and the leakage of nitrous oxide expected in the exhaustgas On the other hand in this process organic matter is usedto denitrify nitrite intoN

2O decreasingmethane production

while N2O generated can be used to oxidize only around 8

ofmethaneThose factors limit the energy efficiency improve-ment achieved by the application of CANDO process

Nowadays there are several technologies already impl-emented at full scale to perform partial nitrification-Ana-mmox processes in the sludge line [87 88] However inspite of the recent advances their implementation at themain stream is still a challenge due to the strict control ofoperational conditions needed to maintain the stability ofthe partial nitrification process [89] The use of microalgaesystems to remove nitrogen from domestic wastewater is afeasible option when enough land is available since this kindof systems would require about ten times the area necessaryfor activated sludge systems [90] The CANDO process canbe used to improve energy efficiency of WWTPs but canonly be applied to the sludge line Moreover the need of thedevelopment of a reliable technology for its implementationat full scale and its negative environmental impact make itnot as attractive as the partial nitrification-Anammox andmicroalgae systems

6 Conclusions

Minimization N2O and CO

2emissions can be decreased by

a good control of the operational conditions of the activatedsludge system CH

4emissions can be minimized if emissions

from the different units of the sludge line are captured byhoods and burnt together with the biogas generated in thesludge anaerobic digester N

2O emissions will dependmainly

on the operational conditions (NO2

minus and O2concentrations)

of the reactor systems

Treatment Nowadays most of the technologies availableto remove GHG are expensive or even not suitable to beapplied to gaseous streams of theWWTPs Biological systemstreatment has low operating costs but their capital costs arehigh due to their size The correct selection of the process tobe installed in the plant will provide the best results as it isthe case of the partial nitritation-Anammox process which isfeasible in two units applied in the main stream of the plantbut not for the treatment of the sludge line

Prevention The configuration of the next generation ofWWTPs shouldmaximize the anaerobic pathway for organicmatter removal and the use of microalgae if enough areais available or partial nitritation-Anammox processes toremove ammonia

Competing Interests

The authors declare that they have no competing interests

Acknowledgments

This work was supported by FONDECYT 1150285 (Chile)and Postdoctoral FONDECYT 3140276 (Chile) and by theSpanish Government through FISHPOL (CTQ2014-55021-R)and GRANDSEA (CTM2014-55397-JIN) projects cofundedby FEDER The authors A Pedrouso A Val del Rıoand A Mosquera-Corral belong to the Galician Competi-tive Research Group GRC 2013-032 program cofunded byFEDER

References

[1] N Bolong A F Ismail M R Salim and T Matsuura ldquoAreview of the effects of emerging contaminants in wastewaterand options for their removalrdquo Desalination vol 238 no 1ndash3pp 229ndash246 2009

[2] L Zanetti N Frison E Nota M Tomizioli D Bolzonella andF Fatone ldquoProgress in real-time control applied to biologicalnitrogen removal from wastewater A short-reviewrdquo Desalina-tion vol 286 pp 1ndash7 2012

[3] W Luo F I Hai W E Price et al ldquoHigh retention membranebioreactors challenges and opportunitiesrdquo Bioresource Technol-ogy vol 167 pp 539ndash546 2014

[4] A Santos W Ma and S J Judd ldquoMembrane bioreactors twodecades of research and implementationrdquoDesalination vol 273no 1 pp 148ndash154 2011


[5] Q Zhang J Hu and D J Lee ldquoAerobic granular processescurrent research trendsrdquo Bioresource Technology vol 210 pp74ndash80 2016

[6] W Mo and Q Zhang ldquoEnergy-nutrients-water nexus inte-grated resource recovery in municipal wastewater treatmentplantsrdquo Journal of EnvironmentalManagement vol 127 pp 255ndash267 2013

[7] L Yerushalmi O Ashrafi and F Haghighat ldquoReductions ingreenhouse gas (GHG) generation and energy consumptionin wastewater treatment plantsrdquoWater Science and Technologyvol 67 no 5 pp 1159ndash1164 2013

[8] M Bani Shahabadi L Yerushalmi and F Haghighat ldquoImpactof process design on greenhouse gas (GHG) generation bywastewater treatment plantsrdquoWater Research vol 43 no 10 pp2679ndash2687 2009

[9] T A Larsen ldquoCO2-neutral wastewater treatment plants or

robust climate-friendly wastewater management A systemsperspectiverdquoWater Research vol 87 pp 513ndash521 2015

[10] C Sweetapple G Fu and D Butler ldquoIdentifying sensitivesources and key control handles for the reduction of greenhousegas emissions from wastewater treatmentrdquoWater Research vol62 pp 249ndash259 2014

[11] P Czepiel P Crill and R Harriss ldquoNitrous oxide emissionsfrom municipal wastewater treatmentrdquo Environmental Scienceamp Technology vol 29 no 9 pp 2352ndash2356 1995

[12] M J Kampschreur H Temmink R Kleerebezem M S MJetten and M C M van Loosdrecht ldquoNitrous oxide emissionduringwastewater treatmentrdquoWater Research vol 43 no 17 pp4093ndash4103 2009

[13] Y Law L Ye Y Pan andZ Yuan ldquoNitrous oxide emissions fromwastewater treatment processesrdquo Philosophical Transactions ofthe Royal Society B Biological Sciences vol 367 no 1593 pp1265ndash1277 2012

[14] J Desloover S E Vlaeminck P Clauwaert W Verstraete andN Boon ldquoStrategies to mitigate N

2O emissions from biological

nitrogen removal systemsrdquo Current Opinion in Biotechnologyvol 23 no 3 pp 474ndash482 2012

[15] J L Campos B Arrojo J R Vazquez-Padın A Mosquera-Corral and R Mendez ldquoN

2O production by nitrifying biomass

under anoxic and aerobic conditionsrdquoApplied Biochemistry andBiotechnology vol 152 no 2 pp 189ndash198 2009

[16] A Soler-Jofra B Stevens M Hoekstra et al ldquoImportance ofabiotic hydroxylamine conversion on nitrous oxide emissionsduring nitritation of reject waterrdquoChemical Engineering Journalvol 287 pp 720ndash726 2016

[17] J H Ahn S Kim H Park B Rahm K Pagilla and K Chan-dran ldquoN

2O emissions from activated sludge processes 2008-

2009 results of a national monitoring survey in the UnitedStatesrdquo Environmental Science amp Technology vol 44 no 12 pp4505ndash4511 2010

[18] C M Castro-Barros M R J Daelman K E Mampaey M CM van Loosdrecht and E I P Volcke ldquoEffect of aeration regimeon N2O emission from partial nitritation-anammox in a full-

scale granular sludge reactorrdquoWater Research vol 68 pp 793ndash803 2015

[19] PWunderlin J Mohn A Joss L Emmenegger and H SiegristldquoMechanisms of N

2O production in biological wastewater

treatment under nitrifying and denitrifying conditionsrdquo WaterResearch vol 46 no 4 pp 1027ndash1037 2012

[20] G Tallec J Garnier G Billen and M Gousailles ldquoNitrousoxide emissions from secondary activated sludge in nitrifying

conditions of urban wastewater treatment plants effect ofoxygenation levelrdquo Water Research vol 40 no 15 pp 2972ndash2980 2006

[21] M R J Daelman E M van Voorthuizen U G J M vanDongen E I P Volcke and M C M van Loosdrecht ldquoMeth-ane emission during municipal wastewater treatmentrdquo WaterResearch vol 46 no 11 pp 3657ndash3670 2012

[22] C E Yver Kwok DMuller C Caldow et al ldquoMethane emissionestimates using chamber and tracer release experiments for amunicipal waste water treatment plantrdquo Atmospheric Measure-ment Techniques vol 8 no 7 pp 2853ndash2867 2015

[23] K Oshita T Okumura M Takaoka T Fujimori L Appelsand R Dewil ldquoMethane and nitrous oxide emissions followinganaerobic digestion of sludge in Japanese sewage treatmentfacilitiesrdquo Bioresource Technology vol 171 no 1 pp 175ndash1812014

[24] D Gupta and S K Singh ldquoGreenhouse gas emissions fromwastewater treatment plants a case study of Noidardquo Journal ofWater Sustainability vol 2 no 2 pp 131ndash139 2012

[25] H Yoshida J Moslashnster and C Scheutz ldquoPlant-integratedmeasurement of greenhouse gas emissions from a municipalwastewater treatment plantrdquo Water Research vol 61 pp 108ndash118 2014

[26] D Kyung M Kim J Chang andW Lee ldquoEstimation of green-house gas emissions from a hybrid wastewater treatment plantrdquoJournal of Cleaner Production vol 95 pp 117ndash123 2015

[27] M Molinos-Senante F Hernandez-Sancho M Mocholı-Arceand R Sala-Garrido ldquoEconomic and environmental perfor-mance of wastewater treatment plants potential reductions ingreenhouse gases emissionsrdquo Resource and Energy Economicsvol 38 pp 125ndash140 2014

[28] A Rodriguez-Caballero I Aymerich M Poch and M PijuanldquoEvaluation of process conditions triggering emissions of green-house gases from a biological wastewater treatment systemrdquoScience of the Total Environment vol 493 pp 384ndash391 2014

[29] H Bozkurt M C van Loosdrecht K V Gernaey and G SinldquoOptimal WWTP process selection for treatment of domesticwastewatermdasha realistic full-scale retrofitting studyrdquo ChemicalEngineering Journal vol 286 pp 447ndash458 2016

[30] A Mahdy L Mendez M Ballesteros and C Gonzalez-Fernan-dez ldquoAlgaculture integration in conventional wastewater treat-ment plants anaerobic digestion comparison of primary andsecondary sludge with microalgae biomassrdquo Bioresource Tech-nology vol 184 pp 236ndash244 2015

[31] Y D Scherson and C S Criddle ldquoRecovery of freshwaterfrom wastewater upgrading process configurations to maxi-mize energy recovery and minimize residualsrdquo EnvironmentalScience and Technology vol 48 no 15 pp 8420ndash8432 2014

[32] W Dai X Xu B Liu and F Yang ldquoToward energy-neutralwastewater treatment a membrane combined process of anaer-obic digestion and nitritation-anammox for biogas recoveryand nitrogen removalrdquo Chemical Engineering Journal vol 279pp 725ndash734 2015

[33] T Schaubroeck H De Clippeleir N Weissenbacher et alldquoEnvironmental sustainability of an energy self-sufficientsewage treatment plant improvements through DEMON andco-digestionrdquoWater Research vol 74 pp 166ndash179 2015

[34] A B Bisinella de FariaM Sperandio AAhmadi and L Tiruta-Barna ldquoEvaluation of new alternatives in wastewater treatmentplants based on dynamic modelling and life cycle assessment(DM-LCA)rdquoWater Research vol 84 pp 99ndash111 2015


[35] X Hao R Liu and X Huang ldquoEvaluation of the potential foroperating carbon neutral WWTPs in Chinardquo Water Researchvol 87 pp 424ndash431 2015

[36] M Hauck F A Maalcke-Luesken M S Jetten and M AHuijbregts ldquoRemoving nitrogen from wastewater with sidestream anammox what are the trade-offs between environmen-tal impactsrdquoResources Conservation and Recycling vol 107 pp212ndash219 2016

[37] Y Law P Lant and Z Yuan ldquoThe effect of pH on N2O

production under aerobic conditions in a partial nitritationsystemrdquoWater Research vol 45 no 18 pp 5934ndash5944 2011

[38] H Ge D J Batstone and J Keller ldquoOperating aerobic wastewa-ter treatment at very short sludge ages enables treatment andenergy recovery through anaerobic sludge digestionrdquo WaterResearch vol 47 no 17 pp 6546ndash6557 2013

[39] J L Campos A Mosquera-Corral A Val del Rıo et al ldquoEnergyand resources recovery in wastewater treatment plantsrdquo inEnvironmental Science amp Engineering vol 9 of Environmentaland Energy Management Ethics Laws and Policies pp 60ndash78Studium Press 2015

[40] P Atkins D Colbourne M Dieryckx et al ldquoMethologiesrdquo inSafeguarding the Ozone Layer and the Global Climate SystemIssues Related to Hydrofluorocarbons and Perfluorocarbons BMetz L Kuijpers S Solomon et al Eds Cambridge UniversityPress Cambridge UK 2005

[41] I Takacs and P A Vanrolleghem ldquoElemental balances inactivated sludge modellingrdquo in Proceedings of the InternationalWater Association Congress (IWA rsquo06) Beijing China 2006

[42] J Foley D de Haas Z Yuan and P Lant ldquoNitrous oxidegeneration in full-scale biological nutrient removal wastewatertreatment plantsrdquo Water Research vol 44 no 3 pp 831ndash8442010

[43] S Wyffels P Boeckx K Pynaert et al ldquoNitrogen removal fromsludge reject water by a two-stage oxygen-limited autotrophicnitrification denitrification processrdquoWater Science and Technol-ogy vol 49 no 5-6 pp 57ndash64 2004

[44] C M Castro-Barros A Rodrıguez-Caballero E I P VolckeandM Pijuan ldquoEffect of nitrite on the N

2O andNOproduction

on the nitrification of low-strength ammonium wastewaterrdquoChemical Engineering Journal vol 287 pp 269ndash276 2016

[45] S OkabeMOshiki Y Takahashi andH Satoh ldquoN2Oemission

from a partial nitrification-anammox process and identificationof a key biological process of N

2O emission from anammox

granulesrdquoWater Research vol 45 no 19 pp 6461ndash6470 2011[46] M J Kampschreur W R L van der Star H A Wielders J

W Mulder M S M Jetten and M C M van LoosdrechtldquoDynamics of nitric oxide and nitrous oxide emission duringfull-scale reject water treatmentrdquoWater Research vol 42 no 3pp 812ndash826 2008

[47] M-S Chou and W-H Cheng ldquoGaseous emissions and controlin wastewater treatment plantsrdquo Environmental EngineeringScience vol 22 no 5 pp 591ndash600 2005

[48] K Skalska J S Miller and S Ledakowicz ldquoTrends in NOxabatement a reviewrdquo Science of the Total Environment vol 408no 19 pp 3976ndash3989 2010

[49] M Konsolakis ldquoRecent advances on nitrous oxide (N2O)

decomposition over non-noble-metal oxide catalysts catalyticperformance mechanistic considerations and surface chem-istry aspectsrdquo ACS Catalysis vol 5 no 11 pp 6397ndash6421 2015

[50] F Kapteijn J Rodriguez-Mirasol and J A Moulijn ldquoHet-erogeneous catalytic decomposition of nitrous oxiderdquo AppliedCatalysis B Environmental vol 9 no 1ndash4 pp 25ndash64 1996

[51] Y JinMCVeiga andCKennes ldquoBioprocesses for the removalof nitrogen oxides from polluted airrdquo Journal of ChemicalTechnology amp Biotechnology vol 80 no 5 pp 483ndash494 2005

[52] O D Frutos I A Arvelo R Perez G Quijano and R MunozldquoContinuous nitrous oxide abatement in a novel denitrifyingoff-gas bioscrubberrdquo Applied Microbiology and Biotechnologyvol 99 no 8 pp 3695ndash3706 2015

[53] O D Frutos G Quijano R Perez and R Munoz ldquoSimul-taneous biological nitrous oxide abatement and wastewatertreatment in a denitrifying off-gas bioscrubberrdquo Chemical Engi-neering Journal vol 288 pp 28ndash37 2016

[54] Y D Scherson G F Wells S-G Woo et al ldquoNitrogen removalwith energy recovery through N

2O decompositionrdquo Energy amp

Environmental Science vol 6 no 1 pp 241ndash248 2013[55] J Nikiema R Brzezinski and M Heitz ldquoElimination of

methane generated from landfills by biofiltration a reviewrdquoReviews in Environmental Science and BioTechnology vol 6 no4 pp 261ndash284 2007

[56] M F M Abushammala N E A Basri D Irwan and M KYounes ldquoMethane oxidation in landfill cover soils a reviewrdquoAsian Journal of Atmospheric Environment vol 8 no 1 pp 1ndash14 2014

[57] M Cui A Ma H Qi X Zhuang and G Zhuang ldquoAnaerobicoxidation of methane an lsquoactiversquo microbial processrdquoMicrobiol-ogyOpen vol 4 no 1 pp 1ndash11 2015

[58] M Veillette M Girard P Viens R Brzezinski and M HeitzldquoFunction and limits of biofilters for the removal of methane inexhaust gases from the pig industryrdquo Applied Microbiology andBiotechnology vol 94 no 3 pp 601ndash611 2012

[59] J Zhu Q Wang M Yuan et al ldquoMicrobiology and potentialapplications of aerobic methane oxidation coupled to denitrifi-cation (AME-D) process a reviewrdquoWater Research vol 90 pp203ndash215 2016

[60] M Kumar G Rattan and R Prasad ldquoCatalytic abatement ofmethane emission from CNG vehicles an overviewrdquo CanadianChemical Transactions vol 3 no 4 pp 381ndash409 2015

[61] AAOlajire ldquoCO2capture and separation technologies for end-

of-pipe applicationsmdasha reviewrdquo Energy vol 35 no 6 pp 2610ndash2628 2010

[62] M K Mondal H K Balsora and P Varshney ldquoProgressand trends in CO

2captureseparation technologies a reviewrdquo

Energy vol 46 no 1 pp 431ndash441 2012[63] L Meier R Perez L Azocar M Rivas and D Jeison ldquoPho-

tosynthetic CO2uptake by microalgae an attractive tool for

biogas upgradingrdquo Biomass and Bioenergy vol 73 pp 102ndash1092015

[64] RWang B Peng and K Huang ldquoThe research progress of CO2

sequestration by algal bio-fertilizer in Chinardquo Journal of CO2

Utilization vol 11 pp 67ndash70 2015[65] M Debowski M Zielinski A Grala and M Dudek ldquoAlgae

biomass as an alternative substrate in biogas productiontechnologiesmdashreviewrdquo Renewable and Sustainable EnergyReviews vol 27 pp 596ndash604 2013

[66] H Gao Y D Scherson and G F Wells ldquoTowards energyneutral wastewater treatmentmethodology and state of the artrdquoEnvironmental Sciences Processes amp Impacts vol 16 no 6 pp1223ndash1246 2014

[67] D Kim J D Bowen and E C Ozelkan ldquoOptimizationof wastewater treatment plant operation for greenhouse gasmitigationrdquo Journal of Environmental Management vol 163 pp39ndash48 2015


[68] N Morales A Val del Rıo J R Vazquez-Padın R MendezA Mosquera-Corral and J L Campos ldquoIntegration of theAnammox process to the rejection water and main stream linesof WWTPsrdquo Chemosphere vol 140 pp 99ndash105 2015

[69] H Siegrist D Salzgeber J Eugster and A Joss ldquoAnammoxbringsWWTP closer to energy autarky due to increased biogasproduction and reduced aeration energy for N-removalrdquoWaterScience and Technology vol 57 no 3 pp 383ndash388 2008

[70] R Khiewwijit H Temmink H Rijnaarts and K J KeesmanldquoEnergy and nutrient recovery for municipal wastewater treat-ment how to design a feasible plant layoutrdquo EnvironmentalModelling amp Software vol 68 pp 156ndash165 2015

[71] Y D Scherson S-G Woo and C S Criddle ldquoProductionof nitrous oxide from anaerobic digester centrate and itsuse as a co-oxidant of biogas to enhance energy recoveryrdquoEnvironmental Science and Technology vol 48 no 10 pp 5612ndash5619 2014

[72] B KartalMMM Kuypers G Lavik et al ldquoAnammox bacteriadisguised as denitrifiers nitrate reduction to dinitrogen gas vianitrite and ammoniumrdquo Environmental Microbiology vol 9 no3 pp 635ndash642 2007

[73] M Strous J J Heijnen J G Kuenen and M S M Jetten ldquoThesequencing batch reactor as a powerful tool for the study ofslowly growing anaerobic ammonium-oxidizing microorgan-ismsrdquoAppliedMicrobiology and Biotechnology vol 50 no 5 pp589ndash596 1998

[74] S Wyffels P Boeckx K Pynaert W Verstraete and O VanCleemput ldquoSustained nitrite accumulation in a membrane-assisted bioreactor (MBR) for the treatment of ammonium-richwastewaterrdquo Journal of Chemical Technology amp Biotechnologyvol 78 no 4 pp 412ndash419 2003

[75] M J Kampschreur R Poldermans R Kleerebezem et alldquoEmission of nitrous oxide and nitric oxide from a full-scalesingle-stage nitritation-anammox reactorrdquo Water Science andTechnology vol 60 no 12 pp 3211ndash3217 2009

[76] J Yang J Trela E Plaza and K Tjus ldquoN2O emissions from a

one stage partial nitrificationanammox process in moving bedbiofilm reactorsrdquo Water Science and Technology vol 68 no 1pp 144ndash152 2013

[77] S Lackner E M Gilbert S E Vlaeminck A Joss H Horn andM C M van Loosdrecht ldquoFull-scale partial nitritationana-mmox experiencesmdashan application surveyrdquo Water Researchvol 55 pp 292ndash303 2014

[78] B Ma S Wang S Cao et al ldquoBiological nitrogen removal fromsewage via anammox recent advancesrdquo Bioresource Technologyvol 200 pp 981ndash990 2016

[79] T Muangthong-on and C Wantawin ldquoEvaluation of N2O pro-

duction from anaerobic ammonium oxidation (Anammox) atdifferent influent ammonia to nitrite ratiosrdquo Energy Procediavol 9 pp 7ndash14 2011

[80] J Myung Z Wang T Yuan et al ldquoProduction of nitrous oxidefrom nitrite in stable type II methanotrophic enrichmentsrdquoEnvironmental Science and Technology vol 49 no 18 pp10969ndash10975 2015

[81] J B K Park and R J Craggs ldquoNutrient removal in wastewatertreatment high rate algal ponds with carbon dioxide additionrdquoWater Science and Technology vol 63 no 8 pp 1758ndash1764 2011

[82] C Alcantara R Munoz Z Norvill M Plouviez and BGuieysse ldquoNitrous oxide emissions from high rate algal pondstreating domestic wastewaterrdquo Bioresource Technology vol 177pp 110ndash117 2015

[83] A I Barros A L Goncalves M Simoes and J C M PiresldquoHarvesting techniques applied tomicroalgae a reviewrdquoRenew-able and Sustainable Energy Reviews vol 41 pp 1489ndash15002015

[84] B D Shoener I M Bradley R D Cusick and J S GuestldquoEnergy positive domestic wastewater treatment the rolesof anaerobic and phototrophic technologiesrdquo EnvironmentalSciences Processes amp Impacts vol 16 no 6 pp 1204ndash1222 2014

[85] N C Boelee H Temmink M Janssen C J N Buisman andR H Wijffels ldquoScenario analysis of nutrient removal frommunicipal wastewater by microalgal biofilmsrdquoWater vol 4 no2 pp 460ndash473 2012

[86] Y Su A Mennerich and B Urban ldquoSynergistic cooperationbetween wastewater-born algae and activated sludge for waste-water treatment influence of algae and sludge inoculationratiosrdquo Bioresource Technology vol 105 pp 67ndash73 2012

[87] W Abma C Schultz J M Mulder et al ldquoThe advance ofanammoxrdquoWater21 vol 36 pp 36ndash37 2007

[88] S Lackner E M Gilbert S E Vlaeminck A Joss H Horn andM C M van Loosdrecht ldquoFull-scale partial nitritationana-mmox experiences an application surveyrdquoWater Research vol55 pp 292ndash303 2014

[89] A Malovanyy J Trela and E Plaza ldquoMainstream wastewatertreatment in integrated fixed film activated sludge (IFAS)reactor by partial nitritationanammox processrdquo BioresourceTechnology vol 198 pp 478ndash487 2015

[90] R J Craggs S Heubeck T J Lundquist and J R BenemannldquoAlgal biofuels from wastewater treatment high rate algalpondsrdquo Water Science and Technology vol 63 no 4 pp 660ndash665 2011

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Inorganic ChemistryInternational Journal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy


Carbohydrate Chemistry

International Journal of


Journal of

Chemistry


Advances in

Physical Chemistry

Hindawi Publishing Corporationhttpwwwhindawicom

Analytical Methods in Chemistry

Journal of

Volume 2014

Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

SpectroscopyInternational Journal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Medicinal ChemistryInternational Journal of


Chromatography Research International


Applied ChemistryJournal of



Theoretical ChemistryJournal of


Journal of

Spectroscopy

Analytical ChemistryInternational Journal of


Journal of


Quantum Chemistry


Organic Chemistry International

ElectrochemistryInternational Journal of



CatalystsJournal of


NO

NO

Autotrophic nitrification

Heterotrophic denitrification

Organicmatter

Organicmatter

Organicmatter

Organicmatter

Nitrifierdenitrification

Hydroxylamineoxidation

Chemicalhydroxylamineoxidation

N2O

N2O

N2ON2O

O2 O2 O2NH2OH N2NH4

+ NO2

minus NO2

minusNO3

minus

Figure 1 Biological and chemical pathways of N2O production in the nitrification and denitrification processes

processes such as heterotrophic denitrification and nitrifica-tion (Figure 1) It is formed during denitrification operatedat low pH values and toxic compounds or low dissolvedoxygen (DO) concentrations are present in themedia [12ndash14]Nitrifying bacteria are able to produce N

2O under aerobic or

anoxic [15] conditions In anoxic conditions both ammonia-and nitrite-oxidizing bacteria are able to produce it whileonly ammonia-oxidizing bacteria do it in aerobic conditionsIn the latter case the production is stimulated by the presenceof low DO concentrations and presence of nitrite (NO

2

minus) ororganic matter in the liquid media [12 13] Nitrous oxide canbe produced also from chemical reactions taking place in thepresence of hydroxylamine and nitrite [16]

In practice nitrous oxide is emitted in the WWTPpredominantly in the aerobic tank [17] However the con-tribution of the anoxic and aerobic reactors to this pro-duction remains still unclear since it can be produced inthe anoxic stage and be subsequently stripped to the gasphase in the aerated compartment [18] Ammonia-oxidizingbacteria have been identified as the main N

2O producers

while heterotrophic denitrifying bacteria contribution is onlyrelevant when nitrite andor oxygen are present in theanoxic stage [19] According to Tallec et al [20] undercommon operational conditions the N

2O production occurs

mainly via denitrification by nitrifying bacteria Howeverhydroxylamine oxidation pathway can be the main processresponsible for the production of N

2O emissions at high

ammonia (NH4

+) and lownitrite concentrationswhen a highmetabolic activity of ammonia-oxidizing bacteria is present(at 2 to 3mg O

2L) [19]

With regard to CH4emissions Daelman et al [21]

found out that about 1 of the incoming chemical oxygendemand (COD) to theWWTPs was emitted as methaneThisamount exceeds the amount of carbon dioxide emission thatwas avoided by utilizing the produced biogas in anaerobicdigestion The main sources of methane detected by theseauthors were related to the sludge line units where anaerobicdigestion is carried out the primary sludge thickener thecentrifuge the exhaust gas of the cogeneration plant thebuffer tank for the digested sludge and the storage tank forthe dewatered sludge These units contribute to around 72of methane emissions of the WWTPs while the remainingemissions come from the biological reactors and can be

mainly attributed to the CH4dissolved in the wastewater

which is not totally removed by the biological systemResearch works of Yver Kwok et al [22] and Oshita et al[23] also showed that most of the methane emissions fromWWTPs are closely related to processes involved in the sludgeline

With respect to CO2its production is attributed to two

main factors biological treatment process and electricityconsumption In the main stream of the WWTP the organiccarbon of wastewater is either incorporated into biomass oroxidized to CO

2 In the sludge line it is converted mainly

to CO2and CH

4during anaerobic digestion and finally

methane is oxidized to CO2during biogas combustion

In recent literature the emissions of GHG from theconventional configurations ofWWTPswere determined butthe analysis of the possible alternatives to minimize theseemissions is generally not done [24ndash28] On the other handmost of the papers studying the application of new processesto remove pollutants from wastewaters are mainly focusedon the energy savings [29ndash32] and only few of them alsogive an environmental evaluation [33ndash36] Therefore themain objective of this work is to provide an overview of allpossible ways to reduce GHG emissions from WWTPs Forthat two possible scenarios are discussed and analyzed (1)to maintain the present scheme of operation of the WWTPand to modify the operational conditions (minimization) orto implement capture and treatment units for the gaseousstreams (treatment) and (2) to change the scheme of oper-ation and to implement new processes which produce lowerGHG emissions than the existing ones (prevention)

2 Minimization of GHG Emissions

Perhaps the most efficient way in terms of costs to reduceGHG emissions is to modify the operational conditions ofWWTPs units but this is not always possible due to theoperational limitations of the installed units In the followingsections some recommendations about the possible actionsto put in practice to operateWWPTs in order to reduce GHGemissions are provided

21 N2O Production Data obtained from the operation of

full-scale WWTPs show a wide range of values for the









22 CH4Production CH













2emissions should





emissions





CO2emissions from


2Nm3 CH

4

lowastlowast


0

01

02

03

04

05

10 302015SRT (d)

(kg

CO2m

3

ww

)






3 Treatment


2O CH

4







2O with






4


2(1mol



2





CH4+ SO4


3

minus+HSminus +H

2O


3CH4+ 8NO

2


2+ 4N2+ 10H

2O


5CH4+ 8NO

3


2+ 4N2+ 14H

2O


CH4+ 4MnO

2+ 7H+ 997888rarr HCO

3

minus+ 4Mn+2 + 5H

2O


CH4+ 8Fe (OH)

3+ 15H+

997888rarr HCO3

minus+ 8Fe+2 + 21H

2O


(5)


4


4directly from











4 Prevention










2O emis-










2


2O would hamper


2O emission




2



2O


2O-Nkg Ninfluent








4

+ toNO2



2O conversion to N

2with energy



4


2The





2O to the

atmosphere


2+ 2H2O + 4N

2

Δ119867 = 1219 kJmol CH4

(6)


2+ 2H2O

Δ119867 = 890 kJmol CH4

(7)


2



2O



106CO2+ 236H

2O + 16NH

4

++HPO

4

minus2+ light

997888rarr C106

H181

O45N16P + 118O

2+ 171H

2O + 14H+

(8)




Organicmatter

Light

Ammonia

CO2

O2



2








5 Case Studies






2 to burn methane




2 CH4



2m3 of wastewater


4andN

2O










Sludge digester


Anoxic Aerobic

Dehydrationsystem

Dehydratedsludge


Biogas


Energy




Case B


Influent Effluent


Sludge digester


Dehydrationsystem

Dehydratedsludge

Secondarysettler

Nitritation

Nitritation-

Anammox

Biogas


Energy

Case C



Acidogenicreactor


Anoxic Aerobic

Dehydrationsystem

Dehydratedsludge


Biogas


Energy


Methanogenicreactor

Organicmatter

Anammox

N2O

SRT 2d

Figure 4 Continued




Case D

Case E





Aerobic

Dehydrationsystem

Dehydratedsludge

Secondarysettler


Secondarysettler


Energy



Sludge digester


Dehydrationsystem

Dehydratedsludge

Secondarysettler


Biogas


Atmospheric

Energy

CO2

CO2

CO2

Atmospheric CO2

SRT 2d




4burntlowast [21]












minus06

minus04

minus02

0

02

04

06

08


(kg

CO2m

3

ww

)

















6 Conclusions












Competing Interests


Acknowledgments


References



























































2O andNOproduction








































































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of









22 CH4Production CH













2emissions should





emissions





CO2emissions from


2Nm3 CH

4

lowastlowast


0

01

02

03

04

05

10 302015SRT (d)

(kg

CO2m

3

ww

)






3 Treatment


2O CH

4







2O with






4


2(1mol



2





CH4+ SO4


3

minus+HSminus +H

2O


3CH4+ 8NO

2


2+ 4N2+ 10H

2O


5CH4+ 8NO

3


2+ 4N2+ 14H

2O


CH4+ 4MnO

2+ 7H+ 997888rarr HCO

3

minus+ 4Mn+2 + 5H

2O


CH4+ 8Fe (OH)

3+ 15H+

997888rarr HCO3

minus+ 8Fe+2 + 21H

2O


(5)


4


4directly from











4 Prevention










2O emis-










2


2O would hamper


2O emission




2



2O


2O-Nkg Ninfluent








4

+ toNO2



2O conversion to N

2with energy



4


2The





2O to the

atmosphere


2+ 2H2O + 4N

2

Δ119867 = 1219 kJmol CH4

(6)


2+ 2H2O

Δ119867 = 890 kJmol CH4

(7)


2



2O



106CO2+ 236H

2O + 16NH

4

++HPO

4

minus2+ light

997888rarr C106

H181

O45N16P + 118O

2+ 171H

2O + 14H+

(8)




Organicmatter

Light

Ammonia

CO2

O2



2








5 Case Studies






2 to burn methane




2 CH4



2m3 of wastewater


4andN

2O










Sludge digester


Anoxic Aerobic

Dehydrationsystem

Dehydratedsludge


Biogas


Energy




Case B


Influent Effluent


Sludge digester


Dehydrationsystem

Dehydratedsludge

Secondarysettler

Nitritation

Nitritation-

Anammox

Biogas


Energy

Case C



Acidogenicreactor


Anoxic Aerobic

Dehydrationsystem

Dehydratedsludge


Biogas


Energy


Methanogenicreactor

Organicmatter

Anammox

N2O

SRT 2d

Figure 4 Continued




Case D

Case E





Aerobic

Dehydrationsystem

Dehydratedsludge

Secondarysettler


Secondarysettler


Energy



Sludge digester


Dehydrationsystem

Dehydratedsludge

Secondarysettler


Biogas


Atmospheric

Energy

CO2

CO2

CO2

Atmospheric CO2

SRT 2d




4burntlowast [21]












minus06

minus04

minus02

0

02

04

06

08


(kg

CO2m

3

ww

)

















6 Conclusions












Competing Interests


Acknowledgments


References



























































2O andNOproduction








































































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of



2O with






4


2(1mol



2





CH4+ SO4


3

minus+HSminus +H

2O


3CH4+ 8NO

2


2+ 4N2+ 10H

2O


5CH4+ 8NO

3


2+ 4N2+ 14H

2O


CH4+ 4MnO

2+ 7H+ 997888rarr HCO

3

minus+ 4Mn+2 + 5H

2O


CH4+ 8Fe (OH)

3+ 15H+

997888rarr HCO3

minus+ 8Fe+2 + 21H

2O


(5)


4


4directly from











4 Prevention










2O emis-










2


2O would hamper


2O emission




2



2O


2O-Nkg Ninfluent








4

+ toNO2



2O conversion to N

2with energy



4


2The





2O to the

atmosphere


2+ 2H2O + 4N

2

Δ119867 = 1219 kJmol CH4

(6)


2+ 2H2O

Δ119867 = 890 kJmol CH4

(7)


2



2O



106CO2+ 236H

2O + 16NH

4

++HPO

4

minus2+ light

997888rarr C106

H181

O45N16P + 118O

2+ 171H

2O + 14H+

(8)




Organicmatter

Light

Ammonia

CO2

O2



2








5 Case Studies






2 to burn methane




2 CH4



2m3 of wastewater


4andN

2O










Sludge digester


Anoxic Aerobic

Dehydrationsystem

Dehydratedsludge


Biogas


Energy




Case B


Influent Effluent


Sludge digester


Dehydrationsystem

Dehydratedsludge

Secondarysettler

Nitritation

Nitritation-

Anammox

Biogas


Energy

Case C



Acidogenicreactor


Anoxic Aerobic

Dehydrationsystem

Dehydratedsludge


Biogas


Energy


Methanogenicreactor

Organicmatter

Anammox

N2O

SRT 2d

Figure 4 Continued




Case D

Case E





Aerobic

Dehydrationsystem

Dehydratedsludge

Secondarysettler


Secondarysettler


Energy



Sludge digester


Dehydrationsystem

Dehydratedsludge

Secondarysettler


Biogas


Atmospheric

Energy

CO2

CO2

CO2

Atmospheric CO2

SRT 2d




4burntlowast [21]












minus06

minus04

minus02

0

02

04

06

08


(kg

CO2m

3

ww

)

















6 Conclusions












Competing Interests


Acknowledgments


References



























































2O andNOproduction








































































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of









2


2O would hamper


2O emission




2



2O


2O-Nkg Ninfluent








4

+ toNO2



2O conversion to N

2with energy



4


2The





2O to the

atmosphere


2+ 2H2O + 4N

2

Δ119867 = 1219 kJmol CH4

(6)


2+ 2H2O

Δ119867 = 890 kJmol CH4

(7)


2



2O



106CO2+ 236H

2O + 16NH

4

++HPO

4

minus2+ light

997888rarr C106

H181

O45N16P + 118O

2+ 171H

2O + 14H+

(8)




Organicmatter

Light

Ammonia

CO2

O2



2








5 Case Studies






2 to burn methane




2 CH4



2m3 of wastewater


4andN

2O










Sludge digester


Anoxic Aerobic

Dehydrationsystem

Dehydratedsludge


Biogas


Energy




Case B


Influent Effluent


Sludge digester


Dehydrationsystem

Dehydratedsludge

Secondarysettler

Nitritation

Nitritation-

Anammox

Biogas


Energy

Case C



Acidogenicreactor


Anoxic Aerobic

Dehydrationsystem

Dehydratedsludge


Biogas


Energy


Methanogenicreactor

Organicmatter

Anammox

N2O

SRT 2d

Figure 4 Continued




Case D

Case E





Aerobic

Dehydrationsystem

Dehydratedsludge

Secondarysettler


Secondarysettler


Energy



Sludge digester


Dehydrationsystem

Dehydratedsludge

Secondarysettler


Biogas


Atmospheric

Energy

CO2

CO2

CO2

Atmospheric CO2

SRT 2d




4burntlowast [21]












minus06

minus04

minus02

0

02

04

06

08


(kg

CO2m

3

ww

)

















6 Conclusions












Competing Interests


Acknowledgments


References



























































2O andNOproduction








































































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of



Organicmatter

Light

Ammonia

CO2

O2



2








5 Case Studies






2 to burn methane




2 CH4



2m3 of wastewater


4andN

2O










Sludge digester


Anoxic Aerobic

Dehydrationsystem

Dehydratedsludge


Biogas


Energy




Case B


Influent Effluent


Sludge digester


Dehydrationsystem

Dehydratedsludge

Secondarysettler

Nitritation

Nitritation-

Anammox

Biogas


Energy

Case C



Acidogenicreactor


Anoxic Aerobic

Dehydrationsystem

Dehydratedsludge


Biogas


Energy


Methanogenicreactor

Organicmatter

Anammox

N2O

SRT 2d

Figure 4 Continued




Case D

Case E





Aerobic

Dehydrationsystem

Dehydratedsludge

Secondarysettler


Secondarysettler


Energy



Sludge digester


Dehydrationsystem

Dehydratedsludge

Secondarysettler


Biogas


Atmospheric

Energy

CO2

CO2

CO2

Atmospheric CO2

SRT 2d




4burntlowast [21]












minus06

minus04

minus02

0

02

04

06

08


(kg

CO2m

3

ww

)

















6 Conclusions












Competing Interests


Acknowledgments


References



























































2O andNOproduction








































































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of




Sludge digester


Anoxic Aerobic

Dehydrationsystem

Dehydratedsludge


Biogas


Energy




Case B


Influent Effluent


Sludge digester


Dehydrationsystem

Dehydratedsludge

Secondarysettler

Nitritation

Nitritation-

Anammox

Biogas


Energy

Case C



Acidogenicreactor


Anoxic Aerobic

Dehydrationsystem

Dehydratedsludge


Biogas


Energy


Methanogenicreactor

Organicmatter

Anammox

N2O

SRT 2d

Figure 4 Continued




Case D

Case E





Aerobic

Dehydrationsystem

Dehydratedsludge

Secondarysettler


Secondarysettler


Energy



Sludge digester


Dehydrationsystem

Dehydratedsludge

Secondarysettler


Biogas


Atmospheric

Energy

CO2

CO2

CO2

Atmospheric CO2

SRT 2d




4burntlowast [21]












minus06

minus04

minus02

0

02

04

06

08


(kg

CO2m

3

ww

)

















6 Conclusions












Competing Interests


Acknowledgments


References



























































2O andNOproduction








































































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of




Case D

Case E





Aerobic

Dehydrationsystem

Dehydratedsludge

Secondarysettler


Secondarysettler


Energy



Sludge digester


Dehydrationsystem

Dehydratedsludge

Secondarysettler


Biogas


Atmospheric

Energy

CO2

CO2

CO2

Atmospheric CO2

SRT 2d




4burntlowast [21]












minus06

minus04

minus02

0

02

04

06

08


(kg

CO2m

3

ww

)

















6 Conclusions












Competing Interests


Acknowledgments


References



























































2O andNOproduction








































































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


minus06

minus04

minus02

0

02

04

06

08


(kg

CO2m

3

ww

)

















6 Conclusions












Competing Interests


Acknowledgments


References



























































2O andNOproduction








































































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of























































2O andNOproduction








































































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of




































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of

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