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Combined anaerobic and aerobic digestion for increasedsolids reduction and nitrogen removal

John T. Novak a,*, Sarita Banjade a, Sudhir N. Murthy b

aDepartment of Civil & Environmental Engineering, Virginia Tech, Blacksburg, VA 24061, United StatesbDC Water & Sewer Authority, Washington DC 20032, United States

a r t i c l e i n f o

Article history:

Received 5 April 2010

Received in revised form

13 July 2010

Accepted 10 August 2010

Available online 17 August 2010

Keywords:

Sludge treatment

Nitrogen removal

Anaerobic digestion

Aerobic digestion

* Corresponding author. Tel.: þ1 540 231 613E-mail address: jtnov@vt.edu (J.T. Novak)

0043-1354/$ e see front matter ª 2010 Elsevdoi:10.1016/j.watres.2010.08.014

a b s t r a c t

A unique sludge digestion system consisting of anaerobic digestion followed by aerobic

digestion and then a recycle step where thickened sludge from the aerobic digester was

recirculated back to the anaerobic unit was studied to determine the impact on volatile

solids (VS) reduction and nitrogen removal. It was found that the combined anaerobic/

aerobic/anaerobic (ANA/AER/ANA) system provided 70% VS reduction compared to 50% for

conventional mesophilic anaerobic digestion with a 20 day SRT and 62% for combined

anaerobic/aerobic (ANA/AER) digestion with a 15 day anaerobic and a 5 day aerobic SRT.

Total Kjeldahl nitrogen (TKN) removal for the ANA/AER/ANA system was 70% for sludge

wasted from the aerobic unit and 43.7% when wasted from the anaerobic unit. TKN

removal was 64.5% for the ANA/AER system.

ª 2010 Elsevier Ltd. All rights reserved.

1. Introduction reduction and coliform destruction with the combined system

Minimization of sludge generated fromwastewater treatment

plants is of interest because of the cost, health concerns and

environmental factors associated with the transport and

disposal of biosolids. Under 40 CFR 503 Part (b) sludge reuse

and disposal regulations (U.S EPA, 1992), specific levels of

treatment of sludge are required for pathogen deactivation

prior to land application. One of the major processes for

achieving acceptability for land application is anaerobic

digestion. While this process is effective for reducing patho-

gens and destroying organic matter, solids reduction above

50% is often difficult to achieve. For this reason, advanced

digestion processes have gained interest in recent years.

Combined anaerobic and aerobic digestion has been

investigated by several researchers. Pagilla et al., (2000)

investigated the use of a thermophilic aerobic pretreatment

stage prior to anaerobic digestion and found improved solids

2; fax: þ1 540 231 7916.ier Ltd. All rights reserve

compared to anaerobic digestion alone. They also saw better

dewatering properties for the pretreated sludges. Akunna

et al., (1994) investigated the combined anaerobiceaerobic

treatment of synthetic wastewater and found that the COD in

anaerobic effluent from an upflow filter could be degraded an

additional 30% by aerobic treatment. Subramanian et al.,

(2007) conducted batch aerobic digestion studies for anaero-

bically digested sludges’ and found that, in addition to

increased solids reduction, sludge dewatering properties

improved.

Recent research in our laboratory has focused on combined

anaerobic/aerobic sludge digestion. There are many advan-

tages to combined anaerobic/aerobic sludge digestion and

these include better solids reduction, improved sludge dew-

atering properties and reduction of nitrogen (Kumar et al.,

2006). The combination of both an anaerobic and an aerobic

step seems to provide for additional solids reduction that is

d.

Fig. 1 e Combined ANA/AER/ANA with sludge wastage

from either the anaerobic or aerobic unit.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 6 1 8e6 2 4 619

not possible by one digestion mode alone (Park et al., 2006).

Typically, conventional anaerobic digestion is followed by an

aerobic step as short as 3 days to gain an additional 10% VS

destruction and removal of up to 90% ammonia nitrogen

(Kumar et al., 2006).

It was thought that by adding an additional anaerobic

sequence to the anaerobic/aerobic process, additional solids

destruction could occur without additional tankage volume.

Solids from the aerobic digester were thickened, the liquid

discharged and the concentrated solids recycled back to the

anaerobic unit. The addition of the recycled sludge back to the

anaerobic unit does not increase the reactor size but does

require a separate thickening and recycle step. Solids can be

wasted from either the anaerobic or the aerobic reactor.

The goal of this study was to determine if a combined

anaerobic/aerobic/anaerobic digestion system could provide

additional solids destruction beyond conventional single

stage mesophilic digestion and combined anaerobic/aerobic

digestion and to determine the impact on nitrogen removal.

The combined anaerobic-aerobic-anaerobic system was

operated so that sludge could be wasted from either the

aerobic unit or the anaerobic unit.

1.1. Objectives

The specific objectives of this study were:

a) To determine the impact of combined anaerobic/aerobic/

anaerobic digestion on anaerobic digestion efficiency as

measured by volatile solids and COD reduction.

b) To determine the effect of combined anaerobic/aerobic/

anaerobic digestion on nitrogen removal.

c) To determine the best location for sludgewastage from the

ANA/AER/ANA system, the anaerobic unit or the aerobic

unit.

Table 1 e SRTs of the anaerobic and aerobic digesters inthe systems.

System SystemSRT(days)

AnaerobicSRT(days)

AerobicSRT(days)

Conventional MAD 20 20 e

Sequential ANA/AER 20 15 5

ANA/AER/ANA e anaerobic waste 35 15 5

ANA/AER/ANA e aerobic waste 35 15 2.5

2. Materials and methods

2.1. Research approach

Three separate digestion combinations were run. A conven-

tional mesophilic digester with an SRT of 20 days, a combined

anaerobic/aerobic system with an anaerobic SRT of 15 days

and an aerobic stage of 5 days and an anaerobic/aerobic/

anaerobic system. All of the units were set up in a 35 �Cconstant temperature room. The anaerobic units were at 35 �Cand the aerobic units were at 32e34 �C due to cooling from

aeration.

Flow diagrams for the ANA/AER/ANA systems are shown

in Fig. 1 with sludge wastage from either the anaerobic unit or

the aerobic unit. The control anaerobic digester was a single

unit, identical to the digesters used for the anaerobic phase of

the combined ANA/AER/ANA studies. The digesters used for

the ANA/AER study were the same digesters used for the

combined ANA/AER/ANA study, but these were operated

without a recycle step from the aerobic to the anaerobic

digester.

The SRT for the systems is shown in Table 1. The units

were operated in a similar manner with regard to the sludge

volumes in the reactors and the sludge feed volume and

wastage. However, since 2 L of sludge was fed to the aerobic

unit when wastage was from the aerobic unit, the SRT in the

aerobic digester was half that of the system that wasted

sludge from the anaerobic unit.

Plastic, egg-shaped fermenters supplied by Hobby

Beverage Equipment Company, were used as anaerobic

digesters. Mixing was by gas recirculation from top to bottom

through a port provided at the bottom of the reactor and an

outlet drilled into the top. For the aerobic digesters, 9.5 L glass

digesters, approximately 21 cm diameter with a narrow screw

cap top (Fisher Scientific) were used. Bubble diffusers were

used for maximum oxygen transfer and a compressor was

used to supply oxygen. The dissolved oxygen concentration in

the aerobic reactor varied from zero immediately after feeding

to 2.5e3.0mg/L just prior to feeding. Distilledwater was added

each day to counter any water loss due to evaporation in the

aerobic reactor.

All the reactors were slug fed once per day. Typically,

sludge was withdrawn from the reactor and then feeding

followed. For the anaerobic/aerobic system, sludge was first

removed from the aerobic reactor for testing and wastage and

then sludge was removed from the anaerobic reactor and fed

to the aerobic reactor. The anaerobic reactor then received the

raw sludge feed. For the anaerobic/aerobic/anaerobic systems,

sludge to be recycled back to the anaerobic reactor was

removed, centrifuged using a lab centrifuge, the centrate

wasted, and the solids combined with raw sludge and fed to

the anaerobic reactor.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 6 1 8e6 2 4620

For mixing in the anaerobic digesters, peristaltic pumps

(Cole Parmer 6e600 rpm)were used to recirculate gas from the

headspace to the bottom of the digesters. The pumps were

operated at 50% of their maximum speed. To ensure greater

mixing of the digesters before and after feeding, gas recircu-

lation in the digesters was increased by increasing the speed

of the pumps to 100% for 10 min before sampling and also for

10 min after feeding.

The anaerobic digesters were seeded with mesophilic

anaerobically digested sludge taken from Pepper’s Ferry

Regional Wastewater Treatment Facility, Radford, Virginia,

USA. The digesters were monitored for steady-state. Steady-

state was determined to have been reached when the VS

reduction and gas production showed less than 5% variation.

Once steady-state occurred, complete sampling and analysis

took place.

The feed for the anaerobic digester was a mixture of

primary and secondary sludge (gravity thickened sludge and

air flotation thickened waste activated sludge). The sludge

was supplied weekly by DCWASA Blue Plains Advanced

Wastewater Treatment Facility and shipped overnight. Total

solids percentages of both the sludges’ were measured and

a mixture of 1:1 by weight of the sludges with a total solid

percentage of 5% was made by dilution. The sludge was

blended and was stored in a 4 �C room until used. Tomaintain

the SRT of both the anaerobic and aerobic digesters, constant

volume was maintained and same amount of sludge was fed

and wasted daily from the digesters. The daily biogas

production by the anaerobic digesters was measured using

a RebelTM wet-tip gas flow meters.

2.2. Analytical methods

Liquid samples were analyzed for total solids (TS), volatile

solids (VS), pH, total Kjeldahl nitrogen (TKN), and ammonium

(NH3-N), according to Standard Methods (APHA, 1998). An ORP

probe (Model 96-78-BN) was used to measure oxida-

tionereduction potential of the aerobic digesters.

The oxidationereduction potential (ORP) was measured

using an ORP probe (Model 96-78-BN).

40

45

50

55

60

0 25 50 75 100 125 150 175 200

)%(

noitcudeR

sdiloSelitalo

V

Time (days after reaching steady state)

MAD (20d SRT) Average VSR 20d SRT (50%)

Fig. 2 e Volatile solids removal data for the control

digester.

3. Results and discussion

3.1. Performance of the digesters

Lab-scale anaerobic and aerobic digestion systems were

operated to determine the performance of anaerobic digestion

followed by aerobic digestion and aerobic/anaerobic diges-

tion. The analyses were performed after determination of

steady-state conditions andwere evaluated bymonitoring pH,

biogas production and solids removal. Different performance

parameters such as volatile solids destruction, COD removal,

nitrogen removal, biogas production and VFA production and

destruction were measured. VFA and biogas data are not

included in this paper, but were consistent with the volatile

solids removal data. All the digesters performed well during

the steady-state phases.

3.1.1. Volatile solids reductionA mixture of primary and secondary sludge in a ratio of 1 to 1

by dry solids from the DC Water and Sewer Authority was

anaerobically digested in a constant temperature room at

35 �C to determine the volatile solids reduction. The single

stage mesophilic (control) digester data operated at a 20 day

SRT is shown in Fig. 2. It can be seen that the average volatile

solids reduction (VSR) was 50% over a period of approximately

6 months of operation. Variations were expected because of

variations in the feed solids.

The VSR for the ANA/AER system is shown in Fig. 3. It can

be seen that the VSR for this system was approximately 62%.

The overall SRT for the combined ANA/AER digester was 20

days, the same as for the single stage conventional digester,

with the aerobic portion being 5 days.

The two systems operated as ANA/AER/ANA provided

a VSR of approximately 70% (Fig. 3) and the removal of waste

sludge from either the anaerobic reactor or the aerobic reactor

did not appear to make a difference in the VSR. However, as

shown in Table 1, whenwastage was from the aerobic reactor,

the aerobic SRT was 2.5 days compared to an SRT of 5 days

when wastage was from the anaerobic reactor. The overall

SRT for the systemwas 35 days. The higher SRT resulted from

the thickening and recirculation of solids from the aerobic

reactor back to the anaerobic reactor. The SRT was calculated

assuming that the centrate from the solid/liquid separation

process for the recycle of solids from the aerobic unit to the

anaerobic unit did not contain any solids.

COD removals were similar to the VS removals for all the

systems (data not shown). Total solids removals were lower

than the VS removal. The MAD has a TS removal of 42%, the

ANA/AER TS removal was 54%, the ANA/AER/ANA with

wastage from the anaerobic unit was 64% and the ANA/AER/

ANA with wastage from the aerobic unit was 63%.

Two factors are thought to account for the increased solids

removal by the combined ANA/AER/ANA. First, the SRT

increased from 20 to 35 days. Second, by combining an addi-

tional anaerobic step, aerobically digested sludgewas exposed

to an additional degradation cycle where biopolymer that was

generated by aerobic growth could be further degraded

anaerobically. Novak et al., 2003 suggested that some fractions

of biological floc could be degraded only aerobically while

0

1000

2000

3000

4000

5000

6000

7000

Operating Time (days)

)L/

N-

gm

(N

KT

0

20

40

60

80

100

la

vo

me

RN

KT

%

TKN in Anaerobic Digester

TKN in Aerobic Digester

% TKN Removal

0 605040302010

Fig. 5 e TKN in the reactors for the ANA/AER system.

40

45

50

55

60

65

70

75

0 20 40 60 80 100 120

)%

( n

oit

cu

de

R R

SV

Steady State Operating Time (days)

Ana/Aer/Ana-Ana wastage

Ana/AerAna-Aer wastage

Ana/Aer

Fig. 3 e Volatile solids removal data for combined digestion

systems.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 6 1 8e6 2 4 621

others could only be degraded anaerobically. It is thought that

some of the additional aerobic floc generated in the aerobic

digester was degradable anaerobically so by recycling sludge

from the aerobic digester back to the anaerobic digester, more

solids would be destroyed.

3.1.2. Nitrogen removalOf interest to this study was the effect of the combined

digestion processes on nitrogen removal. Kumar et al. (2006)

reported that combined anaerobic/aerobic digestion could

remove up to 90% of the ammonia nitrogen by nitrification/

denitrification. Kumar et al. (2006) operated laboratory

digesters in the same manner as this study with sludge being

fed to the digestion units once per day. They showed that

immediately after feeding anaerobically digested sludge into

the aerobic digester, denitrification took place over several

hours. When the readily degradable organic material was

depleted, the dissolved oxygen increased and nitrification

occurred. Kumar et al. (2006) measured nitrite and nitrate in

the aerobic digesters and found little nitrate, suggesting that

the high temperature of the reactors (32e34 �C) of the reactors

slowed nitrite oxidation. A similar process was thought to

occur in this study.

Data for nitrogen speciation in the control reactor is shown

in Fig. 4. It can be seen in Fig. 5 that the TKN was approxi-

mately 3000 mg/L entering the control digester and the

ammonia content was about 20% of the TKN. After anaerobic

digestion, the ammonia content increased to about 50% of the

0

500

1000

1500

2000

2500

0 50 100 150 200

Nitro

gen

(m

g/L

)

Operating Time (days)

TKN in

TKN out

NH3 in

NH3 out

Fig. 4 e Nitrogen Speciation in the Control Anaerobic

Digester.

TKN and as expected, the TKN did not change. The only

change was that organic nitrogen was converted to ammonia.

For the ANA/AER system, it can be seen in Fig. 5 that the

removal of TKN was approximately 65% in the aerobic

digester. In Fig. 6, the ANA/AER system is shown to remove in

excess of 80% of the ammonia. This combined digestion

system has many advantage over conventional anaerobic

digestion including additional VS reduction and a high degree

of ammonia removal. Therefore, the impact of recycle streams

from sludge dewatering processes would be reduced if

a combined anaerobic/aerobic system is used for digestion.

When an additional anaerobic step is added, as shown in

Fig. 3, additional solids reduction occurred, even though the

complete reactor system was no larger than the anaerobic/

aerobic system. However, the fate of nitrogen is of interest and

the reactor selected for wastage determined the overall

nitrogen removal. In Fig. 7, the fate of TKN in the system with

wastage from the anaerobic system is presented. The nitrogen

data is presented in terms of gm/day instead of mg/L because

the recycle affects the concentration in the reactors. It can be

seen in Fig. 7 that the overall TKN removal is slightly above

40%. However, it can also be seen that the TKN in the aerobic

digester is low relative to the influent concentration, but

because the wastage is from the anaerobic reactor, removal of

TKN is less than 50%.

In comparison, if removal is from the aerobic reactor

(Fig. 8), the TKN removal increases to 70%. This is to be

expected because when sludge is removed from the anaerobic

0

500

1000

1500

2000

2500

Operating Time (days)

)L/

N-

gm

(ai

no

mm

A

0

10

20

30

40

50

60

70

80

90

100

la

vo

me

Rai

no

mm

A%

Ammonia in Anaerobic Digester

Ammonia in Aerobic Digester

% Ammonia Removal

Fig. 6 e Ammonia in the reactors for the ANA/AER system.

0

500

1000

1500

2000

2500

3000

3500

4000

4500

Operating Time (days)

)d

ee

f f

o L/

d/g

m(

ne

go

rti

N

Influent TKN (mg/d/L)

Effluent TKN (mg/d/L)

Influent Amm (mg/d/L)

Effluent Amm (mg/d/L)

0 10080604020

Fig. 9 e Influent TKN and ammonia from the systemwith

wastage fromtheanaerobic reactor inunits ofmg/d/L of feed.

0

1000

2000

3000

4000

5000

6000

Operating Time (days)

)y

ad/

N-

gm

(N

KT

0

20

40

60

80

100

la

vo

me

RN

KT

%

TKN in feed

TKN in Ana Dig

TKN in Aer Dig

% TKN Removal

0 80604020 100

Fig. 7 e TKN removal from ANA/AER/ANA system with

sludge wastage from the anaerobic reactor.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 6 1 8e6 2 4622

digester, feed sludge that has not undergone aerobic digestion

is part of the material that is wasted. When sludge is wasted

from the aerobic digester, all sludge in the waste stream has

undergone some degree of aeration. In addition, more TKN is

converted to ammonia andmore of the ammonia is converted

to nitrogen gas.

It can also be seen from the data in Figs. 7 and 8 that the

concentration of TKN in the aerobic reactor is lower in the

system which has wastage from the anaerobic reactor while

the TKN is higher in the system where wastage occurs from

the aerobic reactor. In Table 1, it can be seen that the SRT in

the aerobic reactor is lower for the system which undergoes

wastage from the aerobic system. It may be that additional

TKN removal could occur if the aerobic reactor was operated

at 5 days instead of 2.5 days. No attempt was made to assess

the impact of varying SRT’s for the systems that were studied

in this research. The approach in this study was to make the

overall volume of the reactors the same, although the

combined systems used two reactors instead of one.

The data in Figs. 9 and 10 also provide for a comparison of

the wastage from the two systems. In Fig. 9, the nitrogen data

is shown for wastage from the anaerobic unit and in Fig. 10,

nitrogen in the wastage from the aerobic unit can be seen. The

major difference in is the effluent TKN and effluent ammonia.

Recent data from out lab suggests that the removal of

ammonia in the aerobic digester can be improved by

0

1000

2000

3000

4000

5000

6000

7000

Operating Time (days)

)y

ad/

gm

(N

KT

0

20

40

60

80

100

la

vo

me

RN

KT

%

TKN in feed

TKN in Aer Dig

% TKN Removal

0 10 3020 40

Fig. 8 e TKN removal from ANA/AER/ANA system with

sludge wastage from the aerobic reactor.

continuous feeding and by cycling air on and off. Ammonia

concentrations of 100e150mg/L have been obtained using this

approach. If the ammonia removal process was optimized,

TKN concentrations of 600e700 mg/L could be expected and

this would increase the overall TKN removal from the system

with wastage from the aerobic digester to 80% andmost of the

remaining TKNwould be relatively non-biodegradable organic

nitrogen. Nitrate and nitrite were measured several times in

the aerobic reactor just before feeding from the anaerobic unit

andwere always below the detection limit of 0.1mg/Lwhich is

consistent with the data of Kumar et al., 2006.

The oxidationereduction potential (ORP) over one cycle of

the aerobic digester was measured and the data are shown in

Table 2. These data are for the system with wastage from the

aerobic unit. It can be seen from the data that the ORP declines

for the first 7 h after feeding and then increases to amaximum

at 24 h when feeding again takes place. Nitrogen data indicate

that ammonia is converted to nitrite and then to nitrogen gas.

Nitrate production is minimal. The data in Table 2 also indi-

cate that readily degradable organic matter enters the aerobic

digester and is rapidly degraded over the first 4e8 h.

The unique characteristics of the combined anaerobic/

aerobic system provided optimal conditions for both nitrogen

0

500

1000

1500

2000

2500

3000

3500

4000

4500

Operating Time (days)

)d

ee

ff

oL

/d

/g

m(

ne

go

rt

iN

Influent TKN (mg/d/L)

Effluent TKN (mg/d/L)

Influent Amm (mg/d/L)

Efflent Amm (mg/d/L)

0 10 20 30

Fig. 10 e Influent TKN and ammonia from the system with

wastage from the aerobic reactor in units of mg/d/L of feed.

Table 2 e Oxidationereduction potential in the aerobicdigester after feeding from the anaerobic digester.

Time after Feeding (h) ORP (mV)

0 240

1 �114

2 �173

3 �200

4 �270

5 �288

6 �287

7 �87

8 �25

9 �58

17 15

18 20

19 24

20 40

21 83

22 188

23 238

24 230

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 6 1 8e6 2 4 623

and solids removal. With regard to nitrogen removal, the

increased solids destruction resulted in more conversion of

organic nitrogen to ammonia. The ammonia was oxidized to

nitrite effectively in the aerobic digester, but conversion to

nitratewas slowed by the operational temperature of 32e34 �Cwhich has been shown to limit nitrite oxidation (Hellinga

et al., 1998). It is thought that combined nitrification/denitri-

fication occurs in the aerobic digester in the 6e8 h after the

feeding cycle when the ORP drops to less than �100 mv (Zeng

et al., 2004).

3.2. Summary

Anaerobic digesters were operated using three anaerobic/

aerobic combinations and these were compared to a conven-

tional mesophilic anaerobic digester. The parameters of

interestwere volatile solids reduction and nitrogen removal. It

was found that conventional anaerobic digestionwith a 20 day

SRT resulted in 50%VS reduction for sludge from theDCWater

and Sewer Authority. When a combined anaerobic/aerobic

digestion system was used, the Vs destruction increased to

62%. When a recycle step was added in which sludge from the

aerobic digester was concentrated and returned to the anaer-

obic unit, VS destruction increased to 70%.

TKN removal in the anaerobic/aerobic unit was approxi-

mately 65%. For the ANA/AER/ANA systems, nitrogen removal

depended on the unit fromwhich sludgewaswasted. If sludge

was wasted from the anaerobic unit, TKN removal was

approximately 45%, but if wastage was from the aerobic unit,

it was approximately 70%. Data from other studies suggest

that nitrogen removal can be improved in the ANA/AER/ANA

system by increasing the SRT in the aerobic unit, providing

continuous feeding and cycling air on and off. Additional

research is needed to optimize this process.

Overall, the combination of ANA/AER/ANA digestion

appears to be a cost effective approach to achieve high VS

destruction and effective ammonia removal from digested

sludge. In addition, data from Banjade, (2008) indicates that

the sludge dewatering properties for this system are better

than for conventional anaerobic digestion and odors are

greatly reduced.

4. Conclusions

This study was conducted to compare the performance of

combined anaerobic/aerobic digestion systems to conven-

tional mesophilic anaerobic digestion. In particular, it was of

interest to evaluate a digestion subsystem in which anaerobic

digestion was followed by aerobic digestion and then some of

the aerobically digested sludge was thickened and recycled

back to the anaerobic unit. Based on the data collected, the

following conclusions are drawn:

1. Combined anaerobic/aerobic digestion increased VS

reduction from 50% to 62% using the same overall SRT and

the conventional digester. TKN removal was approximately

65%.

2. For the combined anaerobic/aerobic/anaerobic digestion

system, VS destruction was 70% compared to the conven-

tional digester at 50%.

3. For the ANA/AER/ANA system with wastage from the anaer-

obic unit, TKN removalwas approximately 45%but increased

to 70%when sludge was wasted from the aerobic unit.

Acknowledgements

Support for this study was provided by the District of

ColumbiaWater and Sewer Authority. The assistance of Chris

Wilson and Charan Tanneru with laboratory operation and

analysis is gratefully acknowledged.

r e f e r e n c e s

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Banjade, S., 2008. Anaerobic/Aerobic Digestion for EnhancedSolids and Nitrogen Removal. M.S. Thesis. Virginia PolytechnicInstitute & State University, Blacksburg, VA, USA.

Hellinga, C., Mulder, J.W., van Loosdrecht, M.C.M., Schellen, A.A.J.C., 1998. The SHARON process: an innovative method fornitrogen removal from ammonium-rich waste water. WaterSci. Technol. 37 (9), 135e142.

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