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Journal of Water Process Engineering

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Potential of sulfide-based denitrification for municipal wastewatertreatmentAnnelies Van den Hove, Janis E. Baeten, Stijn O. Decru, Eveline I.P. Volcke*Ghent University, Department of Green Chemistry and Technology, Biosystems Control Research Group, Coupure Links 653, 9000 Gent, Belgium

A R T I C L E I N F O

Keywords:Nitrogen removalBiological wastewater treatmentSulfur-oxidizing bacteriaProcess design

A B S T R A C T

Wastewater treatment through anaerobic carbon removal followed by sulfide-based denitrification for nitrogenremoval offers significant advantages over conventional processes in terms of decreased sludge production andreduced energy requirements. While the process has been demonstrated to treat wastewater with very highsulfate concentrations resulting from saline water supply, this contribution assesses the potential of sulfide-baseddenitrification for a broader range of municipal wastewater characteristics. A mass balance-based calculationprocedure was presented to determine the optimal recycle ratio and the corresponding effluent nitrogen con-centrations in a pre-denitrification system. The calculation procedure was first validated with experimental datafrom literature and was then applied to various scenarios. The latter showed that legal effluent requirements maybe achievable only for specific municipal wastewater streams with a low nitrogen concentration and an elevatedsulfate content (>42 g S m 3), e.g. close to the sea, or if a significant amount of biodegradable carbon enters theanoxic reactor to stimulate auxiliary heterotrophic denitrification. A stoichiometric calculation illustrated thateven with external carbon addition to supplement the sulfide-based denitrification, the total sludge productionand aeration energy could still be reduced compared to a scenario with purely heterotrophic denitrification.Finally, it was shown that a broader range of wastewater types could be treated when applying nitrification-denitrification over nitrite instead of over nitrate, due to the lower sulfide and carbon requirements for thereduction of nitrite.

1. Introduction

Anaerobic wastewater treatment for organic carbon removal is awell-proven technology, which results in a low sludge productioncompared to aerobic treatment. Anaerobic treatment of municipalwastewater is especially applied in warmer climates, in countries suchas Brazil and India, but treatment at lower temperatures also lookspromising [1,2]. UASB (Upflow Anaerobic Sludge Blanket) reactors arewidely used for this purpose. They comprise typical operating costssavings of above 50% and investment costs savings of 20–50% com-pared to conventional activated sludge processes [3]. Yet, anaerobictreatment has a lower carbon removal efficiency and therefore mayrequire post-treatment to deal with residual carbon in case of stringenteffluent requirements [3].

Moreover, anaerobic treatment does not remove nitrogen, whichleaves the anaerobic treatment step in the form of ammonium.Additional treatment is thus required to remove both nitrogen and re-sidual carbon from the anaerobic effluent. In case the incoming was-tewater contains sulfate, it is reduced during anaerobic treatment to

sulfide, by sulfate-reducing bacteria (SRB), thereby removing organiccarbon from wastewater. Sulfide production is in many cases unwanted,because it is associated with odour and corrosion, toxicity and a lowermethane yield per unit of degraded organic waste [4]. However, theformation of sulfide during anaerobic wastewater treatment can also bebeneficially applied for nitrogen removal. More specifically, sulfide canbe used by sulfur-oxidizing bacteria (SOB) as an electron donor forsulfide-based denitrification, as an alternative to organic carbon usagein conventional, heterotrophic denitrification [5]. Elemental sulfur is aknown intermediate during sulfide oxidation by SOB, but completeoxidation to sulfate is expected in case of an excess of nitrate, as is thecase for domestic wastewater [6].

The coupling of an anaerobic treatment step for organic carbonremoval and sulfide production with nitrification and sulfide-baseddenitrification results in an integrated sulfur-based carbon and nitrogenremoval process. Integrated sulfur-based carbon and nitrogen removalfrom municipal wastewater has recently been demonstrated on a largescale, to treat municipal wastewater characterized by high sulfateconcentrations (over 150 g SO4

2 -S m−3) resulting from the usage of

https://doi.org/10.1016/j.jwpe.2020.101206Received 16 October 2019; Received in revised form 21 February 2020; Accepted 23 February 2020

⁎ Corresponding author.E-mail address: Eveline.Volcke@UGent.be (E.I.P. Volcke).

Journal of Water Process Engineering 35 (2020) 101206

Available online 13 March 20202214-7144/ © 2020 Elsevier Ltd. All rights reserved.

T

seawater for toilet flushing. This so-called SANI® (Sulfate reductionAutotrophic denitrification Nitrification Integrated) process [5,7] in-volves up to 60–70% reduction in excess sludge compared to a con-ventional wastewater treatment process, taking up 30–40% less volume[7]. This process also promotes saline water supply as an economic andsustainable solution for water scarcity.

In wastewater from municipalities without seawater toilet flushing,sulfate is also commonly present, but in a lower concentration range(8–24 g SO4

2 -S m−3) [8]. Nevertheless, the concentration range of sul-fate in municipal wastewater is typically higher when located closer tothe sea (grab samples indicated concentrations up to 44 g SO4

2 -S m 3 inthe influent of a WWTP close to the sea) or due to brackish (sulfate-rich)groundwater intrusion [9]. Furthermore, the conditions in municipalwastewater treatment are well suited for the application of biologicalsulfate reduction for organic carbon removal, provided that the con-centration of sulfate is sufficiently high (COD/SO < 0.674

2 g COD (g SO42 )−1) [9]. Alternative process configurations

have been proposed in regions where limiting sulfate concentrations areexpected [10]. In some cases experimental measurements revealed thepresence of an unknown sulfur source, which made it possible to re-move more than 75% of the chemical oxygen demand (excluding sul-fide) through anaerobic treatment, thereby extending its beneficialapplication to more locations than initially expected [11]. These find-ings also broaden the potential scope of sulfide-based denitrification aspart of a subsequent nitrogen removal step, which this study focuses on.

This contribution assesses the application potential of sulfide-basednitrogen removal for non-saline municipal wastewater, i.e. withoutseawater toilet flushing, but including the higher sulfate concentrations

encountered in coastal regions. A calculation procedure based on steadystate mass balances for a pre-denitrification system is presented first, tocalculate the optimal recycle flow rate and corresponding lowest ef-fluent nitrogen concentration. This procedure was validated based onexperimental data available in literature. Sulfide-based nitrogen re-moval over nitrite was proposed in this study as an alternative to sul-fide-based nitrogen removal over nitrate for wastewaters with a rela-tively lower incoming sulfate concentration, because of the 40% lesselectron donor usage. For both process options, scenario analysis wasapplied to investigate for which municipal wastewater characteristicssulfide-based denitrification is able to meet the EU Directive nitrogenstandards.

2. Calculation procedure – optimal recycle ratio and effluentnitrogen concentration

2.1. Process configuration for sulfide-based nitrogen removal

The feasibility of nitrogen removal through sulfide-based deni-trification was assessed considering a pre-denitrification system(Fig. 1), analogous to the ones applying conventional, heterotrophicdenitrification. In this configuration, the nitrification reactor is placedafter the denitrification reactor, in order to make the best possible useof the electron donor (organic carbon or sulfide) for denitrification,avoiding its aerobic oxidation [12]. During nitrification, ammoniumpresent in the influent is oxidized with oxygen to nitrate, by nitrifyingbacteria. A recycle stream (QR, m3 d 1) brings nitrate from the ni-trification reactor to the anoxic denitrification reactor. The influent is

List of symbols

N% -removal Total nitrogen removal efficiency %C iorg, Influent biodegradable organics concentration g–COD m 3

C eTKN, Effluent total Kjeldahl nitrogen concentration g N m 3

C iTKN, Influent total Kjeldahl nitrogen concentration g N m 3

Dp,NO2 Denitrification potential for sulfide-based denitrificationover nitrite g N m 3

Dp,NO3 Denitrification potential for sulfide-based denitrificationover nitrate g N m 3

Nc Nitrification capacity g N m 3

Nnlp,NO2 Equivalent nitrite load g N m 3

Nnlp,NO3 Equivalent nitrate load g N m 3

Qe Effluent flow rate m3.d 1

Qi Influent flow rate, m3 d 1

QR Recycle flow rate m d 1

R Recycle ratioRopt Optimal recycle ratioRopt,NO2 Optimal recycle ratio sulfide-based denitrification over

nitriteRopt,NO3 Optimal recycle ratio sulfide-based denitrification over

nitrateSH S i,2 Influent hydrogen sulfide concentration g S m 3

S eNO ,2 Effluent nitrite concentration g N m 3

S eNO ,3 Effluent nitrate concentration g N m 3

SO R,2 Dissolved oxygen concentration in the recycle streamg O2 m 3

YH Heterotrophic yield coefficient on organic substrateg COD (g COD) 1

YSOB Yield coefficient of SOB on sulfide, g COD (g H2S-S) 1

Fig. 1. Process scheme and corresponding catabolic reactions for the sulfide-based nitrogen removal process (SBNR) over nitrate and over nitrite. The numbersbetween brackets refer to the full stoichiometric reactions in the Supplementary Information Table A.1.

A. Van den Hove, et al. Journal of Water Process Engineering 35 (2020) 101206

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assumed not to contain nitrate (nor nitrite). During sulfide-based de-nitrification, nitrate (NO3 ) is converted into nitrogen gas, while sulfideis oxidized into sulfate by sulfide-oxidizing bacteria (SOB). Beside sul-fide, also residual organic carbon remaining after the preceding anae-robic step can be used as electron donor for denitrification. The sulfide-based pre-denitrification and nitrification processes are typically rea-lized in moving bed biofilm reactors (MBBRs) [7]. In this contribution,sulfide-based pre-denitrification over nitrite instead of over nitrate wasconsidered as an alternative process option, implying 40% less electrondonors needed. To this end, ammonium needs to be oxidized to nitriteand consequently nitrite is denitrified to nitrogen gas.

2.2. Denitrification potential

The procedure for sulfide-based nitrogen removal presented in thisstudy was based on the well-established calculation procedure of [13]for conventional nitrogen removal over nitrate in pre-denitrificationsystems. The denitrification potential is defined as the mass of nitrate ornitrite per influent volume that can be denitrified biologically in theanoxic reactor. To which extent nitrate (or nitrite) removal actuallytakes place in the anoxic reactor, depends on the amount of nitrate(nitrite) present, as supplied through the recycle flow [14]. The deni-trification potential over nitrate (Dp,NO3, g N m 3, Eq. (1)) or nitrite(Dp,NO2, g N m 3, Eq. (2)) is defined based on H2S and organics aselectron donors:

= +D S Y C Y· 22.86

· 12.86p H S i i

H,NO ,

SOBorg,3 2 (1)

= +D S Y C Y· 21.71

· 11.71p H S i i

H,NO ,

SOBorg,2 2 (2)

where SH S i,2 is the influent hydrogen sulfide concentration (g S m 3),YSOB the yield coefficient of SOB (g COD (g H2S-S) 1), C iorg, the influentbiodegradable organics concentration (g COD m 3), YH the hetero-trophic yield coefficient (g COD (g COD) 1). The coefficients

Y((2 )/2.86)SOB and Y((2 )/1.71)SOB represent the amount of nitrateand nitrite consumed per gram H2S, respectively, while Y((1 )/2.86)Hand Y((1 )/1.71)H represents the amount of nitrate and nitrite con-sumed per gram COD, respectively (see Fig. 1 and Table A.1 in Sup-plementary Information).

The denitrification potential expresses the amount of nitrate (Eq.(1)) or nitrite (Eq. (2)), which can stoichiometrically be denitrifiedconsidering the available electron donors. No kinetic limitations areconsidered in the anoxic reactor. The complete amount of H2S andorganics is assumed to be available for denitrification. This implies alarge enough anoxic volume to achieve complete consumption withinthe residence time.

In this study, the applicability of sulfide-based denitrification overnitrate was investigated assuming no nitrite accumulation in theaerobic or anoxic reactor. To examine the applicability of sulfide-baseddenitrification over nitrite, only nitrite formation was considered in theaerobic reactor and no nitrate formation, while in the anoxic reactor,denitrification of nitrite to nitrogen gas was considered.

2.3. Equivalent nitrate/nitrite load – optimal recycle ratio

The denitrification potential relates to the amount of electron do-nors that enters the anoxic reactor. The amount of nitrate or nitrite thatwill be actually denitrified depends on the amount of electron acceptorsentering the anoxic reactor through the recycle flow. The total electronacceptor load comprises not only nitrate or nitrite formed in the aerobicreactor, but also oxygen which is unavoidably recycled along. The re-sulting so-called equivalent nitrate load (Nnlp,NO3, g N m 3) or equiva-lent nitrite load (Nnlp,NO2, g N m 3), expressed per volume of influentflow, is defined by Eqs. (3) and (4), respectively:

= +N S RS

R·2.86

·eO R

nlp,NO NO ,,

3 32

(3)

= +N S RS

R·1.71

·eO R

nlp,NO NO ,,

2 22

(4)

where S eNO ,3 (or S eNO ,2 ) denotes the effluent nitrate (or nitrite) con-centration (g N m 3) in the recycle flow, which equals its effluentconcentration. SO R,2 is the dissolved oxygen concentration in the recyclestream (g O2 m 3) and R the recycle ratio between the recycle flow rateand the influent flow rate:

=R QQ

R

i (5)

The optimal recycle ratio is obtained when the equivalent nitrate ornitrite load is equal to the corresponding denitrification potential. Inthis case, all the nitrate or nitrite entering the anoxic reactor will becompletely converted to nitrogen gas through denitrification, so nonitrate or nitrite will be present in the outflow of the anoxic reactor. Asa result, the effluent nitrate or nitrite concentration, denoted by S eNO ,xin general, will be determined by the amount of nitrate or nitrite pro-duced by nitrification:

=+

S NR1e

cNO ,x (6)

in which Nc (g N m 3) denotes the nitrification capacity, which is de-fined as the mass of nitrate or nitrite produced (from the influent TKN)by nitrification per influent volume and which is calculated as thedifference between the influent and effluent TKN concentrations:

=N C Cc i eTKN, TKN, (7)

Nitrogen removal through incorporation in the sludge was reason-ably neglected in Eq. (7). Indeed, while about 20% of the influent ni-trogen ends up in the sludge for activated sludge processes [13], thisfraction will be much lower for the process setup in this study, because(i) most organic carbon has been removed in the preceding anaerobicreactor, (ii) the growth yield of autotrophic denitrifiers is low in com-parison to heterotrophic denitrifiers and (iii) the solids retention time inbiofilm systems is usually long, leading to a low sludge production(Appendix B in Supplementary Information).

Note that the calculation of the effluent nitrate and nitrite con-centrations from Nc, by Eq. (6), takes into account the dilution causedby the recycle flow rate R (Eq. (5)).

The optimal recycle ratios Ropt,NO3 for nitrogen removal over nitrate(Ropt,NO2 for nitrogen removal over nitrite, respectively), are obtainedby setting the equivalent nitrate load (Eq. (3) (or equivalent nitrite loadEq. (4)) equal to the corresponding denitrification potential Eq. (1) (orEq. (2)), substituting the effluent nitrate (or nitrite) concentration byEq. (6) and solving the resulting equation for the recycle ratio:

=

++ + +

RD N S

S

N D S N D SS

( /2.86)2·( /2.86)

( ) ( /2.86)·(2· 2· ( /2.86))2·( /2.86)

p c O R

O R

c p O R c p O R

O R

opt,NO,NO ,

,

,NO2

, ,NO ,

,

33 2

2

3 2 3 2

2

(8)

=

++ + +

RD N S

S

N D S N D SS

( /1.71)2·( /1.71)

( ) ( /1.71)·(2· 2· ( /1.71))2·( /1.71)

p c O R

O R

c p O R c p O R

O R

opt,NO,NO ,

,

,NO2

, ,NO ,

,

22 2

2

2 2 2 2

2

(9)

2.4. Effluent nitrate/nitrite concentration

Three general cases are distinguished depending on the value of therecycle ratio: optimal loading =R Ropt,NOx , underloading <R Ropt,NOx

A. Van den Hove, et al. Journal of Water Process Engineering 35 (2020) 101206

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and overloading >R Ropt,NOx . In what follows, the effluent nitrate (ornitrite) concentrations is calculated for each of these cases.

In case the optimal recycle ratio is applied, the equivalent nitrate ornitrite load is equal to the corresponding denitrification potential, sothe anoxic reactor is optimally loaded, leading to the maximal nitrogenremoval and the minimal effluent nitrate (or nitrite) concentration. Thelatter is obtained by Eq. (6) in which =R Ropt,NO3, Eq. (8) (or

=R Ropt,NO2, Eq. (9)).In case the recycle ratio is lower than the optimum value, the

equivalent nitrate or nitrite load is lower than the corresponding de-nitrification potential. The anoxic reactor is then underloaded withelectron acceptors (recycled nitrate/nitrite and oxygen) coming fromthe aerobic reactor, which will be completely converted. The availableelectron donors (H2S and organics) are not completely consumed, i.e.the denitrification potential is not completely used. Increasing the re-cycle flow rate would increase the nitrogen removal, so the perfor-mance is less than optimal. The effluent nitrate and nitrite concentra-tions are calculated based on Eq. (6), which is valid for recycle ratioslower than or equal to the optimal recycle ratio:

=+

S NR

R R1

forec

NO , opt,NO3 3 (10)

=+

S NR

R R1

forec

NO , opt,NO2 2 (11)

In case the applied recycle ratio would be higher than the optimalone, the equivalent nitrate or nitrite load is higher than the corre-sponding denitrification potential. The anoxic reactor is then over-loaded with electron acceptors coming from the aerobic reactor, so notall recycled nitrate/nitrite can be denitrified. Increasing the recycleflow rate beyond the optimal value leads to decreased denitrification,because the dissolved oxygen input from the aerobic reactor increasesand this is a preferred electron acceptor compared to nitrate/nitrite[15].

The effluent nitrate (or nitrite) concentration is calculated as thedifference between the amount of nitrate (nitrite) formed through ni-trification, added up with the nitrate (nitrite) equivalent of the oxygenconcentration, and the denitrification potential:

= +S NS

R D R R2.86

· fore cO R

pNO ,,

,NO opt,NO32

3 3 (12)

= +S NS

R D R R1.71

· fore cO R

pNO ,,

,NO opt,NO22

2 2 (13)

In case =R Ropt,NO3, Eq. (12) is equivalent with Eq. (10). Analo-gously, in case =R Ropt,NO2, Eq. (13) is equivalent with Eq. (11).

In the extreme case that the recycle ratio becomes so high that thedenitrification potential is completely used up by dissolved oxygencoming from the aerobic reactor, no denitrification can take place. Thisis the case when S R D( /2.86)·O R p, ,NO2 3 or S R D( /1.71)·O R p, ,NO2 2. Theeffluent nitrate/nitrite concentration is then equal to the nitrification

capacity, irrespective of the recycle flow rate, Eqs. (12) and (13) canthus be further generalized to:

= +S NS

R D N R Rmin2.86

· , fore cO R

p cNO ,,

,NO opt,NO32

3 3 (14)

= +S NS

R D N R Rmin1.71

· , fore cO R

p cNO ,,

,NO opt,NO22

2 2 (15)

3. Experimental validation – scenario analysis set-up

The calculation procedure presented in this study was validatedagainst experimental data from literature of a lab-scale sulfur-basedcarbon and nitrogen removal process treating saline wastewater [16].In the latter study, the effluent nitrate concentrations and nitrogen re-moval efficiencies were investigated under different recycle flow ratesbetween the aerobic and anoxic filter in order to determine the optimalrecycle flow rate. In this study, the experimentally obtained values werecompared to the ones obtained from the calculation procedure andconsidering nitrification-denitrification over nitrate with the parametervalues from Table 2, for the average anaerobic effluent TKN and hy-drogen sulfide concentrations from [16] (Table 1). From the latterstudy, the reported amount of organic carbon additionally consumed inthe anoxic filter was taken, assuming that not the complete amount ofanaerobic effluent organic carbon is necessarily biodegradable.

The feasibility of sulfide-based nitrogen removal was assessed for arange of typical municipal wastewater characteristics after anaerobictreatment (Table 1). The hydrogen sulfide-sulfur concentration SH S i,2entering the denitrification reactor was taken from typical SO4

2 con-centrations of municipal wastewater. This assumes a complete reduc-tion of SO4

2 to dissolved H2S in the preceding anaerobic reactor and nogas phase losses due to minimal biogas production and the improvedsolubility of H2S at alkaline conditions, as observed in a pilot-studywith municipal wastewater [11]. A small amount of influent biode-gradable organics C iorg, was assumed as well, based on reported mea-surements of COD in the influent and effluent of sulfide-based deni-trification reactors following anaerobic treatment ([18,16,19,5,7],Appendix C in Supplementary Information). Typical values of C iTKN, inraw municipal wastewater were taken [17].

To calculate the nitrification capacity, a well-functioning nitrifica-tion reactor was assumed, resulting in an effluent TKN concentrationC eTKN, of 2 g N m 3 at all times. In practice, high recycle rates from theaerobic to the anoxic reactor could lead to short-circuiting and (very)low recycle rates may lead to sulfide slipping to the aerobic reactor,where it might inhibit nitrification [20–22]. However, these situationsshould be avoided for optimal operation. In the recycle stream,4.5 g O2 m 3 was assumed to be present, based on measurements in alarge-scale SANI® process [7]. For sulfide-based denitrification overnitrite, a lower oxygen concentration in the recycle stream of3 g O2 m 3 was assumed, based on a study of [23] on nitritation in aMBBR. Table 2 summarizes the applied parameter values.

Table 1Composition of anaerobic effluent, which is fed to a sulfide-based pre-deni-trification process.

Influentcomponent

Referencescenario

Typical range fornon-salinewastewater

Validationscenario [16]

Unit

SH S i2 , 16a 8–24a 76.5 g S m 3

Corg i, 12b 0–24b 4.5 g COD m 3

CTKN i, 60c 30–100c 30 g N m 3

a Ref. [8]; assuming complete reduction of sulfate to sulfide in the anaerobicreactor.

b Based on reported concentrations for the SANI® process (Table C.2 inSupplementary Information).

c Ref. [17].

Table 2Parameter values for sulfide-based nitrogen removal process (SBNR) over ni-trate and over nitrite.

Parameter SBNR over NO3 SBNR over NO2 Unit

CTKN e, 2 2 g N m 3

SO R2, 4.5a 3b g O2 m 3

YH 0.67c 0.67 g COD (g COD) 1

YSOB 0.35d 0.35 g COD (g S) 1

a Ref. [7].b Ref. [23].c Ref. [24].d Ref. [6], after unit conversion.

A. Van den Hove, et al. Journal of Water Process Engineering 35 (2020) 101206

4

According to the EU Directive 98/15/EC, the total effluent nitrogenconcentration of an urban wastewater treatment plant should not ex-ceed 15 g N m 3 for areas with 10,000–100,000 p.e. or 10 g N m 3 forareas with more than 100,000 p.e. and/or have a minimum percentageof reduction, i.e. total nitrogen removal efficiency, of 70–80%. The totalnitrogen removal efficiency N(% -removal) was calculated as the differ-ence between the incoming nitrogen (TKN) concentration and theoutgoing nitrogen concentration (TKN, nitrate or nitrite) relative to theinfluent TKN concentration:

=+ +

NC C S S

C% removal

( )·100i e e e

i

TKN, TKN, NO , NO ,

TKN,

3 2

(16)

These legal requirements were used as criteria to determine whichmunicipal wastewater types could potentially be treated with sulfide-based denitrification. Dedicated operating diagrams that map the ap-plicability of sulfide-based denitrification over nitrate and over nitritein terms of influent hydrogen sulfide and organic carbon concentrationswere set up. The calculation procedure and operating diagrams pre-sented in this study indicate which wastewaters could potentially betreated with sulfide-based nitrogen removal. The procedure is based onthe following assumptions: (i) steady-state mass balances; (ii) completeconversion of sulfate to H2S in the preceding reactor and no loss of H2Sto the gas phase; (iii) no kinetic limitations in the anoxic reactor; (iv) awell-functioning nitrification reactor, ensuring an effluent TKN con-centration as low as 2 g N m 3. These assumptions may entail a some-what overestimation of the reactor performance. The proposed proce-dure should therefore not be seen as a final confirmation of theapplicability of sulfide-based denitrification, but should rather be usedto identify interesting scenarios for further research.

4. Results and discussion

4.1. Validation of the calculation procedure

The effluent nitrate concentrations calculated from Eq. (10) or Eq.(12), depending on the recycle ratio, closely matched the experimen-tally measured data by Tsang et al. [16] for all investigated recycleratios (Fig. 2(a)). The nitrogen removal efficiency was well reproducedfor recycle ratios up to 3, but was overestimated for the highest recycleratio studied by Tsang et al. [16], namely =R 4. Tsang et al. [16] andWang et al. [5], who examined the same lab-scale process, hypothesizedthat this drop in the removal efficiency was due to short circuiting flowsin the nitrification reactor. Such hydraulic constraints associated withscale were not included in the calculation procedure in this study,which assumed a well-functioning nitrification reactor in all cases. As aresult, the calculated optimal recycle flow rate amounted to =R 12opt(Fig. D.1 in Supplementary Information), which is higher than the value( =R 3opt ) found experimentally. Overall, the validation confirms thatthe calculation procedure, despite its associated assumptions, is suitableto identify interesting scenarios for further research.

4.2. Applicability of sulfide-based denitrification over nitrate

4.2.1. Reference scenarioThe calculation procedure was illustrated for the reference scenario

(Table 1), considering nitrogen removal over nitrate. Fig. 3 summarizesthe influence of the recycle ratio on the performance of the deni-trification reactor in terms of the used denitrification potential and theeffluent nitrate concentration.

The optimal recycle ratio for nitrogen removal over nitrate wascalculated from Eq. (8) as =R 0.22opt,NO3 . For recycle ratios betweenzero and 0.22, the anoxic reactor is underloaded with nitrate andoxygen ( <R Ropt,NO3), which implies that a portion of the denitrificationpotential (Dp,NO3) is unused (Fig. 3(a)). As a result, in this zone, an in-creasing recycle rate of nitrate (and oxygen) to the anoxic reactor

results in a decreasing effluent nitrate concentration until a recycleratio of 0.22 is reached.

At the optimal recycle ratio, =R 0.22opt,NO3 , the denitrification po-tential is fully used and its maximum portion is used for nitrate re-moval. In other words, the equivalent nitrate load on the anoxic reactoris equal to the denitrification potential. The recycle ratio correspondingwith the lowest effluent nitrate concentration is termed the optimalrecycle ratio Ropt,NO3 and in this case amounts to 0.22.

For increasing recycle ratios beyond =R 0.22opt,NO3 , more oxygenwill be recycled to the denitrification reactor, which will be pre-ferentially used as an electron acceptor, resulting in a larger fraction ofthe denitrification potential used for oxygen removal, on its turnleading to higher effluent nitrate concentrations. Note that, when in-creasing the recycle ratio to values higher than 6.74, the amount of DOentering the anoxic reactor (S R· /2.86O R,2 ) is even higher than the de-nitrification potential (Dp,NO3). At this point, all electron donors areconsumed with oxygen, while nitrate leaves the process unconverted.No nitrogen removal takes place anymore, which constitutes the limitof applicability of Eq. (12).

The denitrification performance is optimal at Ropt,NO3, but even atthis optimal recycle ratio, the effluent nitrate concentration remains toohigh (48 g N m 3 > 15 g N m 3) and the nitrogen removal efficiency toolow (17% < 70%) to meet the prevailing effluent standards. This is dueto the low optimal recycle rate, which makes that most of the nitrateformed in the aerobic reactor is simply discharged via the effluent. Itcan be concluded that sulfide-based denitrification over nitrate is notfeasible for the reference case, since the legal performance require-ments cannot be met even under the best circumstances ( =R Ropt) dueto physical and (bio)chemical constraints.

4.2.2. Applicability for a range of municipal wastewater characteristicsApart from the reference scenario, for which the effluent nitrate

concentration was too high, the applicability of sulfide-based deni-trification was examined for a wide range of municipal wastewatercharacteristics. Increasing influent H2S and/or organic carbon con-centrations resulted in a higher denitrification potential Dp,NO3 (Eq. (1)).In order to fully use the denitrification potential, more nitrate had to berecycled from the aerobic to the anoxic reactor, resulting in a higheroptimal recycle ratio Ropt,NO3 for increasing influent H2S and influentorganic carbon concentrations (Fig. E.2(b) in Supplementary Informa-tion). As a result, also more nitrate was denitrified in the anoxic reactor

Fig. 2. (a) Effluent nitrate concentration and (b) corresponding nitrogen re-moval efficiency in function of the recycle ratio for sulfide-based denitrificationover nitrate. Comparison between calculation results obtained in this study (fullline) and experimental data of [16] (circular markers).

A. Van den Hove, et al. Journal of Water Process Engineering 35 (2020) 101206

5

and the effluent nitrate concentration decreased. As a result, the ap-plicability of sulfide-based denitrification increased with increasinginfluent H2S and/or influent organic carbon concentrations, Fig. 4(b).

The influent TKN concentrations also affected the optimal recycleratio and the applicability of sulfide-based denitrification. Decreasingthe influent TKN concentration from 60 to 30 g N m 3 led to a lowernitrification capacity Nc. As a consequence, the equivalent nitrate loadwas lower, the optimal recycle ratio was higher (Fig. E.2 inSupplementary Information) and the effluent nitrate concentration waslower, implying a higher nitrogen removal efficiency.

Overall, the applicability of sulfide-based denitrification increasedwith decreasing influent TKN concentrations (Fig. 4). For example,considering an influent TKN concentration of 30 g N m 3 and an in-fluent biodegradable organics concentration of 12 g COD m 3, sulfide-based denitrification over nitrate may be applicable, if the wastewatercontains an influent hydrogen sulfide concentration higher than42 g S m 3 (Fig. 4(a)). For a hydrogen sulfide concentration of44 g S m 3, an effluent nitrate concentration of 6 g N m 3 may beachieved. This is lower than the maximum allowable effluent nitrogenconcentration of 15 g N m 3 (10,000–100,000 p.e.) and 10 g N m 3

(>100,000 p.e.), taking into account the effluent TKN concentration.Also a nitrogen removal efficiency of more than 70% is achieved. Eventhough this influent hydrogen sulfide concentration is higher than theones typically expected for municipal wastewater (Table 1), such highconcentrations are possible for, e.g. a WWTP close to the sea [9]. Fur-thermore, other sulfur sources could still be present in wastewater,leading to higher sulfide concentrations than originating from sulfateonly, a topic that warrants further research [11].

4.2.3. Plant-wide calculationFor lower influent hydrogen sulfide concentrations, more influent

biodegradable organics would be required to have sufficient totalelectron donor for denitrification. For instance, for an influent hy-drogen sulfide concentration of 24 g S m 3 and an influent TKN con-centration of 30 g N m 3 about 100 g bCOD m 3 would need to be sup-plied (Fig. 4(a)). This is higher than typical influent biodegradableorganic carbon concentrations present in anaerobically pre-treated

municipal wastewater (Table 1). In order to supply additional biode-gradable organics to the pre-denitrification system, either the anaerobicpre-treatment could be operated such that a higher amount of biode-gradable organics remains in the anaerobic effluent. Alternatively, ex-ternal organic carbon should be dosed to the anoxic reactor. In anycase, this implies relatively more heterotrophic denitrification, besidessulfide-based denitrification.

A plant-wide calculation was performed comparing scenario (a)based on anaerobic carbon removal and sulfide/heterotrophic deni-trification with a conventional scenario (b) based on carbon and ni-trogen removal in a pre-denitrification configuration (Fig. 5). Thetheoretical plant-wide calculation was based on the assumption ofcomplete sulfate reduction in the anaerobic reactor, and complete sul-fide oxidation to sulfate in the anoxic reactor. This implies no detri-mental influence of residual H2S in the consecutive aerobic reactor. Thecalculations for the sludge production and the oxygen consumption aredetailed in Appendix F, Tables F.4 and F.5. The results show that, evenwhen denitrification relies on organic carbon as an electron donor be-sides sulfide, still plant-wide benefits could be realized in terms of re-duced aeration energy consumption (56% less) and reduced sludgeproduction (30% less). The latter was due to both the low sludge yieldin the anaerobic reactor and the low yield of the SOB in the anoxicreactor.Fig. 3. Influence of the recycle ratio on the (a) percentage of the denitrification

potential used for nitrate removal, for dissolved oxygen removal and unuseddenitrification potential and the (b) effluent nitrate concentration. Results ob-tained for the reference scenario (Table 1), considering sulfide-based deni-trification over nitrate.

Fig. 4. Feasibility to achieve the legal requirements for the total effluent ni-trogen concentration (<15 g N m 3) and total nitrogen removal (>70%) viasulfide-based denitrification over nitrate with an influent TKN concentration of30 g N m 3 (a) and 60 g N m 3 (b). The lines were calculated via Eq. (6) usingthe parameter values shown in Table 2 and an optimal recycle rate, as definedby Eq. (8). The rectangle indicates expected concentrations after anaerobictreatment of typical municipal wastewater without saline water supply(Table 1).

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4.3. Applicability of sulfide-based denitrification over nitrite

A lower amount of electron donors is needed, when realizing sul-fide-based nitrogen removal over nitrite instead of nitrate, broadeningthe application range for sulfide-based denitrification. The denitrifica-tion potential of sulfide-based denitrification over nitrite Dp,NO2 ishigher than the one over nitrate Dp,NO3. This implies that the optimalrecycle ratio Ropt,NO2 for sulfide-based denitrification over nitrite ishigher than the one for sulfide-based denitrification over nitrate andmore nitrogen compounds can be denitrified in the anoxic reactor (Fig.G.3 in Supplementary Information). As a result, the potential of sulfide-based denitrification over nitrite to meet the EU effluent regulations(Fig. 6) is higher than for sulfide-based denitrification over nitrate(Fig. 4).

Sulfide-based denitrification over nitrite was found applicable for aninfluent TKN concentration of 30 g N m 3 and an influent hydrogensulfide concentration at the high end of 24 g S m 3, if the influent bio-degradable organics concentration is higher than 16 g COD m 3. In thisregion, sulfide-based denitrification over nitrite could meet not only atotal effluent nitrogen concentration below 15 g N m 3, but also below10 g N m 3 required for WWTPs larger than 100,000 p.e.

Overall, this study demonstrates the potential of sulfide-based de-nitrification over nitrite for biological nitrogen removal from municipalwastewater with elevated sulfate concentrations. Relatively high sulfateconcentrations in wastewater are encountered particularly in coastalareas, which are typically also most densely populated. For instance,coastal areas made up 40% of the EU territory and hosted 40.8% of theEU population in 2011 [25]. This will become even more important ifsea level rises further due to climate change.

The application of sulfide-based denitrification for nitrogen removalfrom wastewater containing sulfate implies a previous anaerobictreatment step for carbon removal. Anaerobic treatment of municipalwastewater is increasingly advocated for, aiming at energy neutrality oreven net energy production [26,27]. Already proven in warmer cli-mates, the application of anaerobic treatment at lower temperaturesalso looks promising [1,2]. It is clear that the increasing interest foranaerobic treatment of municipal wastewater entails a concomitantincreasing interest in sulfide-based denitrification. Overall, this re-search has shown under which conditions sulfide-based denitrificationhas technical potential. A comparison with alternative process schemes[28] in terms of economics, sustainability etc. is recommended.

5. Conclusions

In this contribution, the applicability of sulfide-based denitrificationfor anaerobically pre-treated municipal wastewater was assessed.

• A procedure to calculate the optimal nitrate recycle ratio and the

resulting effluent nitrogen concentrations, was presented for sulfide-based nitrogen removal in a pre-denitrification configuration.

• The applicability of sulfide-based nitrogen removal was assessed fora wide range of municipal wastewater characteristics. Its applicationpotential increased with increasing hydrogen sulfide and residual

Fig. 5. Plant-wide comparison of municipal wastewater treatment through anaerobic carbon removal and sulfide-based denitrification (scenario (a)) with con-ventional treatment (scenario (b)) in terms of mass balances for biodegradable organic carbon (bCOD), nitrogen and sulfur species.

Fig. 6. Feasibility to achieve the legal requirements for the total effluent ni-trogen concentration (<15 g N m 3) and total nitrogen removal (>70%) viasulfide-based denitrification over nitrite with an influent TKN concentration of30 g N m 3 (a) and 60 g N m 3 (b). The lines were calculated via Eq. (6) usingthe parameter values shown in Table 2 and an optimal recycle rate, as definedby Eq. (9). The rectangle indicates expected concentrations after anaerobictreatment of typical municipal wastewater without saline water supply(Table 1).

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organic carbon concentrations, as well as with decreasing influentTKN concentrations, which were obtained with a higher optimalrecycle ratio.

• Sulfide-based denitrification over nitrate may be applicable, if thewastewater contains an influent hydrogen sulfide concentrationhigher than 42 g S m 3 for a low influent TKN concentration of30 g N m 3 and an influent biodegradable organics concentration of12 g COD m 3.

• For lower influent hydrogen sulfide concentrations or higher in-fluent TKN concentrations, more biodegradable organics need to besupplied to have sufficient total electron donor for denitrification.This implies relatively more heterotrophic denitrification, besidessulfide-based denitrification. Still, plant-wide benefits could berealized in terms of reduced aeration energy consumption and re-duced sludge production.

• The application range for sulfide-based nitrogen removal becamebroader when realizing sulfide-based nitrogen removal over nitriteinstead of nitrate, implying 40% less electron donor needed. For therange of municipal wastewater characteristics considered, sulfide-based denitrification over nitrite was applicable for an influent TKNconcentration of 30 g N m 3 and an influent hydrogen sulfide con-centration at the high end of 24 g S m 3, if the influent biodegrad-able organics concentration is higher than 16 g COD m 3.

Conflict of interest

None declared.

Acknowledgements

The doctoral research work of Janis Baeten has been financiallysupported by the Research Foundation Flanders (FWO), through anFWO Ph.D. fellowship. The doctoral research work of Stijn Decru hasbeen financially supported by a Doctoral Scholarship from the GhentUniversity Special Research Fund and by an FWO ERC runner-up pro-ject for Eveline Volcke.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in theonline version, at https://doi.org/10.1016/j.jwpe.2020.101206.

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