618 © IWA Publishing 2011 Water Science & Technology | 64.3 | 2011

Urine nitrification and sewer discharge to realize in-sewer

denitrification to simplify sewage treatment in Hong Kong

F. Jiang, Y. Chen, H. R. Mackey, G. H. Chen and M. C. M. van Loosdrecht

ABSTRACT

The chemically enhanced primary treatment works in Hong Kong will be upgraded for biological

nitrogen removal. This study proposed a novel approach to waive the upgrading by urine source-

separation, onsite nitrification and discharge of nitrified urine into sewers to achieve in-sewer

denitrification. Human urine was collected and a lab-scale experiment for full urine nitrification was

conducted. The results showed that full nitrification was achieved with alkaline addition. Simulation

of nitrified urine discharge into an 8-km pressure main in Hong Kong was conducted with a quasi-2D

dynamic sewer model developed from a previously calibrated sewer biofilm model. It was assumed

that 70% of the residents’ urine was collected and fully nitrified on-site. The simulation results

revealed that the proposed approach is effective in removal of nitrogen within the sewer, which

decreases ammonia-N at the sewer outlet to a level required for secondary effluent discharge in

Hong Kong.

doi: 10.2166/wst.2011.491

F. JiangSchool of Chemistry and Environment,South China Normal University,Guangzhou,China

F. JiangY. ChenH. R. MackeyG. H. Chen (corresponding author)Department of Civil and Environmental Engineering,The Hong Kong University of Science and

Technology,Clear Water Bay, Kowloon,Hong Kong,ChinaE-mail: ceghchen@ust.hk

M. C. M. van LoosdrechtDepartment of Biotechnology,Delft University of Technology,Julianalaan 67, NL-2628 BC Delft,The Netherlands

Key words | denitrification, human urine, nitrification, sewer modeling

INTRODUCTION

Chemically enhanced primary treatment (CEPT) treats more

than 50% of sewage in Hong Kong. Although CEPT canremove 80% of suspended solids (SS), only a small fractionof ammonia and soluble organic matter are removed.

Hence, upgrade of CEPT works to secondary treatment sys-tems is urgently needed in Hong Kong. This faces two majorchallenges: (1) no adequate land available and (2) requiring

extra carbon for denitrification since most of particulatechemical oxygen demand (COD) is removed by CEPT.Meanwhile, odor problems occur in some force mains andCEPT works in Hong Kong due to seawater toilet flushing,

though the problems are trivial compared to 30% watersaving from the saline supply (Chen ). In order toexplore a holistic solution for sustainable sewage treatment

in Hong Kong, we propose to collect source-separatedurine, nitrify it onsite and then discharge to sewers to mini-mize upgrade requirements and abate sewage odor

problems. As over 80% nitrogen of the municipal sewagecomes from urine (Fittschen & Hahn ), various benefitsof urine separation have been confirmed, including ability toprovide peak shaving and lower flush water requirements

(Rossi et al. ), opportunities for upstream P recovery(Guest et al. ), lower energy use, and increased capacity

of centralized nutrient removal plants (Wilsenach & van

Loosdrecht ). Urine collection could be cost effectivein Hong Kong where high-rise buildings dominate the city.Urine treatment can be nitrified to nitrite partly when

alkali is not added (Udert et al. a; Wilsenach et al.) and it is possible to utilize the sewers as active bio-reactors (Hvitved-Jacobsen et al. ). Based on these

background studies, this paper is to address the followingtwo technical issues: (1) can we achieve full nitrification ofcollected urine? and (2) how much nitrogen and COD canbe removed after nitrified urine is discharged. As an explora-

tory study, we focus on the possibility of full nitrification ofurine in a sequencing batch reactor and feasibility of nitri-fied-urine-sewer-discharge (NUSD) through sewer process

modeling in an ideal case. Many issues still exist such asstruvite scaling, odor, acceptance, maintenance, cost andtechnical reliability and operation of onsite nitrification,

phosphorus recovery etc. Part of these will be addressed inseparate papers in the future due to page limitations. It isadmitted that the technology is still in its infancy and, asmore studies and a move to mass production occurs, such

issues are likely to, and in many instances are, beingresolved (Lienert et al. ; Larsen et al. ).

619 F. Jiang et al. | Urine nitrification and sewer discharge simplification in Hong Kong Water Science & Technology | 64.3 | 2011

METHODS AND MATERIALS

Urine nitrification

An investigation of full urine nitrification was conductedwith a lab-scale sequencing batch reactor (SBR) by control-ling pH using diluted urine. Urine dilution was initially 12

times with a final dilution of 3.5, representing a dilutionclose to what may be expected from flushing. The SBRhad a working volume of 3 L, an internal diameter of 15

cm, and the exchange ratio was one third. The lab-scaleSBR is shown in Figure 1. The cycle was divided into 10 haeration, 1.5 h settling and 0.5 h decanting where feeding

was divided into three equally spaced periods of 1 hduring the aeration period. This feeding strategy ensuredminimal inhibitory effects from free nitrous acid (FNA)

and free ammonia (FA). The feeding strategy also representsa semi-continuous addition of urine as would occur in anapartment block. 0.75 M Na2CO3 was dosed into the reactorto control the pH between 7.3 and 7.6 after the initial pH

decrease. Temperature was controlled at 25 WC whilst aera-tion was through a porous stone diffuser keeping DOabove 1.5 mg/L. The reactor was seeded with activated

sludge from a local leachate treatment activated sludgeplant, which was then cultivated on a synthetic ammoniasolution for three months, with an initial MLSS concen-

tration of 1,240 mg/L.Urine was collected from a group of five students at the

Hong Kong University of Science & Technology (HKUST)campus in a portable non-flush toilet. The daily collected

urine volume was 0.8–3 L. Urine was collected and storedat room temperature prior to dilution with tap water foruse as influent. The chemical composition of the urine was

analyzed and shown in Table 1. Values for total nitrogen(TN) and COD were roughly two thirds of other reported

Figure 1 | The lab-scale SBR for urine nitrification.

data from European studies (Ciba-Geigy ; Kirchmann &

Pettersson ), which may be attributable to the differentdiet in Hong Kong. Regular analysis of reactor performancewas carried out including COD, TN, NH3þNH4

þ, NO2�,

NO3�, and SS in accordance with the Standard Methods

(APHA ). The studywas carried out for a period of 90 days.

Sewer modeling

The sewer model was developed from a dynamic sewer bio-film model proposed by Jiang et al. (), because biofilmplays an important role in pollutant transformation insewers (Chen et al. ). The previous sewer biofilm

model is a one-dimensional model (1D) on the directionvertical to the sewer wall, and capable of simulating thepollutants transformation and transport, oxidation, nitrifi-

cation, denitrification, sulfate reduction and sulfideoxidation in both the bulk water phase and sewer biofilm.The 1D sewer biofilm model has been calibrated and vali-

dated with typical sewers in Hong Kong (Jiang et al., ). To simulate the nitrogen and COD removalalong the sewer, the previous sewer biofilm model shallbe extended. Hence, a quasi-2D dynamic sewer model

was developed. The concept of the quasi-2D sewer modelis demonstrated in Figure 2. The sewer pipe is regardedas a series of biofilm reactors. To simulate the substance

transport and dispersion along the sewer directionEquation (1) is applied.

@c@t

þ u@c@x

¼ Kx@2c@x2

�X

ρðcÞ ð1Þ

where c is the concentration of a certain substance (g/m3);Kx is the dispersion coefficient (m2/s); and

PρðcÞ is the

source or sink by physical, chemical and biological

Table 1 | Composition of collected urine

Parameter pH TN NH4þþNH3 COD Cl

Unit mg-N/L mg-N/L mg/L mg/L

Fresh 6.4 5,980 374 6,700 5,077

Hydrolyzed 9.2 5,900 5,380 6,700 5,077

620 F. Jiang et al. | Urine nitrification and sewer discharge simplification in Hong Kong Water Science & Technology | 64.3 | 2011

transformations in the bulk water, and the diffusion fluxfrom or into the biofilm (g/m3/s).

The equation matrix,P

ρðcÞ involves a biochemical pro-cess model to describe the pollutant transformations. Thebiochemical process model is the same as that used in the

1D sewer biofilm model, which is shown in Table 2. Becauseof the page limitation, the validated parameters and thenotation in Table 2 will not be elaborated in this paper,

but can be found in Jiang et al. (). The biochemical kin-etic equations developed from the Activated Sludge ModelNo. 3 (ASM3, Gujer et al. ) are adopted in this study.

This is because ASM3 has been approved as capable ofsimulating sewer processes (Huisman & Gujer ).Hence, the main process related to nitrogen removal andcorrespoding carbon oxidation is heterotrophic denitrifica-

tion, with rate equations similar to those of ASM3, asshown in Table 2. The stoichiometric parameters inTable 2 as x and y can be calculated from COD and nitrogen

balance respectively, using the conservation matrix.Since the previous sewer biofilm model has been proven

to be capable of simulating the pollutant transformation and

transport in both sewer biofilm and bulk water phases in asingle pipe segment (Chen ), the proposed quasi-2Dsewer model extended from the sewer biofilm model could

be a study tool to investigate the impacts of NUSD. In our

Figure 2 | Schematic diagram of the quasi-2D sewer model.

recent study, the quasi-2D sewer model has been applied

in a real pressured sewer in Hong Kong with good simu-lation results (Chen ). Hence, the quasi-2D sewermodel was applied in this study to investigate the effect of

in-sewer denitrification by discharge of the source-separatedand nitrified urine to be collected from the residential area.

RESULTS AND DISCUSSION

Urine nitrification

Complete nitrification and high organic removal wereachieved in the SBR (Figure 3), although initial removalfor the first month was slow, possibly due to the biomass

having difficulty hydrolyzing the urea. At the final influentconcentration (dilution of 3.5 times) 89.2% COD and99.6% ammonium nitrogen (standard deviations of 1.1%

and 0.24%) were removed at a COD and TKN loadingrate of 1.2 kg-COD/m3/day and 1.1 kg-N/m3/day, respect-ively. These removal efficiencies were calculated from theinfluent and effluent concentrations of COD and ammo-

nia-N in the final 20 days. During this period there was anaverage total nitrogen loss of 6.5% across the reactor cycle.Nitrifying biomass reached 9% of the total biomass, deter-

mined by OUR batch tests, with an MLSS of 7,600 mg/Lmaintained by weekly wastage. The reactor sludge formeddense and thick sludge flocs with about 10% granules

(approximate diameter 1 mm, Figure 4), resulting in an SVI5of 40 g/ml. This was an interesting observation as two criticalparameters: critical settling velocity and air-flow velocity

(Beun et al. ) were well below reported values (0.43 m/h

Table 2 | Stoichiometric and conservation matrix of the biochemical processes

Variable Index (i) 1 2 3 4 5 6 7 8 9 10 11 12 13

Modelcomponent

SO SF SA SNH SNO SSO4 SH2S XI XS XH XSTO XA XSRB Rate Equation

Unit (g m�3) COD COD COD N N S S COD COD COD COD COD COD

k Stoichiometric Matrix(vk,j)

1 Hydrolysis x1 y1 �1KH � Xs=XH

Kx þ XS=XH�XH

2 Aerobic storageof solublesubstrate (SS)

x2 �SF/(SFþ SA) �SA/(SFþ SA) y2 YSTO,O2

kSTO � SO

KOþSO� SF þ SA

KSþSF þ SA�XH

3 Anoxic storage ofSS

�SF/(SFþ SA) �SA/(SFþ SA) y3 x3 YSTO,NOx

kSTO�ηNO � KO

KOþSO� SNO

KNOþSNO

� SF þ SA

KSþSF þ SA� XH

4 Aerobic growthofheterotrophicbacteria (XH)

x4 y4 1 �1/YH,O2

μH �SO

KOþSO� SNH

KNHþSNH

� XSTO=XH

KSTOþXSTO=XH�XH

5 Anoxic growth ofXH

y5 x5 1 �1/YH,NOx

ηNO�μH � KO

KOþSO� SNO

KNOþSNO�

SNH

KNHþSNH� XSTO=XH

KSTOþXSTO=XH�XH

6 Aerobic end.respiration forXH

x6 y6 fXI �1bH;O2 � SO

KOþSO� XH

7 Anoxic end.respiration forXH

y7 x7 fXI �1bH;NO � KO

KOþSO� SNO

KNOþSNO�XH

8 Aerobicrespiration forstorageproduct (XSTO)

x8 �1bSTO;O2 � SO

KOþSO� XSTO

9 Anoxicrespiration forXSTO

x9 �1bSTO;NO � KO

KOþSO� SNO

KNOþSNO�XSTO

10 Nitrification x10 y10 1/YA 1μA � SO

KA;OþSO� SNH

KA;NHþSNH� XA

(continued)

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Table 2 | continued

Variable Index (i) 1 2 3 4 5 6 7 8 9 10 11 12 13

11 Aerobic end.respiration fornitrifiers (XA)

x11 y11 fXI �1bA;O2 � SO

KO þ SO�XA

12 Anoxic end.respiration forXA

y12 x12 fXI �1bA;NO � KO

KOþSO� SNO

KNOþSNO� XA

13 SRB growth �1 y13 � x13 x13 YSRB

μSRBKSRB;O

KSRB;OþSO

SSO4

KSO4þSSO4

SA

KSA þ SAXSRB

14 SRB decay y14 1 �1 bSRBXSRB

15 Heterotrophsinactivation

y15 1 �1kina

KS

KSþSFþSAXH

16 Storage productinactivation

1 �1kina

KS

KSþSFþSAXSTO

17 Autotrophsinactivation

y17 1 �1kina

KO

KOþSOXA

18 SRB inactivation y18 1 �1 kinaSO

KSRB;OþSOXSRB

19 Sulfide oxidation x19 1 �1kSO � SO

0:1 � SH2S

20 Fermentation �1 1 y20q fe

KO

KO þ SO

KNO

KNO þ SNO

SF

K feþSFXH

21 Reaeration (forgravity sewer)

1 kLa � ðSO;sat�SOÞ

ConservationMatrix

1 COD(l1,j) �1 1 �4.57 2 1 1 1 1 1 1

2 Nitrogen(l2,j) iNSf 1 1 iNXI iNXS iNBM iNBM iNBM

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Figure 3 | (a) Fate of nitrogen in the SBR reactor (influent NOx-N was negligible). (b) Soluble COD removal in the SBR.

Figure 4 | SEM image of the granules and microscopic images of the sludge flocs (magnification: 10×).

623 F. Jiang et al. | Urine nitrification and sewer discharge simplification in Hong Kong Water Science & Technology | 64.3 | 2011

and 0.11 m/min, respectively). Further study on the for-mation mechanism is underway.

In the study urine provided only 41.2% of the alkalinityrequired for complete nitrification, less than the theoreticalvalue of 50% (Udert et al. b). This may be attributable

to the maintenance of a pH above 7.3 or to some alkalinityconsumption from the conversion of organic acids.

Modeling result

Complementing the successful study of urine nitrificationwhere the nitrogen of the separated urine could be comple-tely converted into NOx-N without inhibition, the discharge

of nitrified urine to achieve in-sewer denitrification wassimulated. A simulation case study on a district area of

624 F. Jiang et al. | Urine nitrification and sewer discharge simplification in Hong Kong Water Science & Technology | 64.3 | 2011

75,000 residents, generating 36,000 m3/day sewage, was

conducted. The pressure main of 8 km sends the sewage toa CEPT works with a mean HRT in the sewer of around 5 h.

Under an assumption of 70% urine collection and full

nitrification of collected urine, the total nitrified nitrogenaccounts for 56% of the total nitrogen in the sewage, becauseurine contains 80% of the nitrogen in sewage (Fittschen &Hahn ). The average influent sewage quality for simu-

lation was proposed, as summarized in Table 3. Theseinfluent pollutant concentrations are the typical values forraw sewage in Hong Kong (Tam et al. ; Chen ). As

this study focused on the nitrogen removal in the sewer, itwas assumed that the influent COD concentrations in thesewer were the same with or without urine nitrification.

Because the sewage quality varies significantly from daytimeto nighttime, a 24-h variation of the influent quality was givenbased on the typical variation measured in a real sewer ofHong Kong (Chen & Leung ). The COD fractions and

other parameters were the same as that used in the sewer bio-film modeling work (Jiang et al. ).

Table 3 | The average influent quality adopted in this simulation work

Parameter TCOD SCOD NH3-N TKN NOx-N TN

Unit mg/L mg/L mg-N/L mg-N/L mg-N/L mg-N/L

Influent withoutnitrified urinedosing

500 55 22 40 0 40

Influent withnitrified urinedosing

500 55 0 18 22 40

Note: TCOD and SCOD stand for total COD and soluble COD.

Figure 5 | Simulated 24-h average concentrations of NH3-N, NOx-N and TN in sewer influent a

Both the pollutant transformation before and after

NUSD were simulated and the main simulation results arecompared in Figures 5–7. By averaging the 24-h variationof the influent and effluent NH3-N, NOx-N and TN concen-

trations, the removal efficiencies of nitrogen in the sewerwere determined. Compared with the situation withoutNUSD, the in-sewer N removal efficiency significantlyincreased after applying NUSD, as shown in Figure 5. The

average ammonium nitrogen level of the sewer effluentdecreased to 2.5 mg-N/L. Figure 6 compares the 24-h vari-ations of the effluent NH3-N concentration in the sewer

with and without NUSD. It shows that most of the NH3-N

nd effluent with and without nitrified-urine-sewer-discharged.

Figure 6 | Simulated 24-h variation of effluent NH3-N concentration with and without

NUSD in the sewer.

625 F. Jiang et al. | Urine nitrification and sewer discharge simplification in Hong Kong Water Science & Technology | 64.3 | 2011

will be collected and removed from the sewer under urine

separation and on-site nitrification. The effluent sewagequality meets the discharge standard of ammonium nitrogenbelow 5 mg-N/L at all times. Injecting the nitrified urine into

the pressured sewer leads to a notable increase in the influ-ent NOx-N concentration in the sewer, as shown in Figure 7(the long dash line). Under the effect of heterotrophic deni-trification in the sewer, 60.8% NOx-N is removed, which

results in a TN removal efficiency of 33.7% during the 5-hsewage conveyance in the sewer. The removal rate of nitro-gen in this case study is calculated to be 2.7 mg-N/L/h. It

should be noted that the predicted nitrogen removal isonly attributed to heterotrophic denitrification in the sewerin this study. The potential of autotrophic denitrification

by sulfide oxidation could enhance the nitrogen removalrate in sewers. Most of the remaining nitrogen in thesewer effluent was organic nitrogen, especially the particu-late organic nitrogen, which could be removed as part of

SS by CEPT.Hence, an integration of urine nitrification, in-sewer

treatment and CEPT can provide a space-saving solution

for improving the CEPT’s effluent quality without upgrad-ing. Since the investment in the separate urine collectioncan be better staged over time than the construction of a sec-

ondary treatment process, a financial advantage could beachieved. The cost estimation for the case will be conductedto investigate the financial feasibility of applying this tech-

nique in Hong Kong in our further study.

Figure 7 | Simulated 24-h variation of NOx-N concentration of the sewer influent and

effluent with NUSD.

DISCUSSION

There are three key points of the nitrified-urine-sewer-

discharge technique: urine collection, on-site nitrification,and denitrification in sewers. Hong Kong is a densely popu-lated city; hence urine collection will be more efficient thanmany other countries investigating urine separation, for

instance in Europe. At this stage no local acceptance studieshave been conducted so no further discussion of urine col-lection is made in this study. The study shows the source-

separated urine can be effectively nitrified in a SBR reactorwith alkalinity addition. In the future application of NUSDin Hong Kong, on-site urine nitrification could be made in

a deep shaft SBR attached to the tall building wall, whichwould help increase the nitrification efficiency with highpressure and high oxygen transfer rate. This study is cur-

rently conducted in our lab, and future results will bereported separately.

In addition to the model results, increases in sewer nitro-gen removal could be possible. While the NUSD is found to

be able to achieve significant in-sewer nitrogen removal inthis study, it is mainly due to the effect of heterotrophic deni-trification in this case, because heterotrophic denitrification

is the only process for NOx-N reduction in the proposed bio-chemical model. However, in a saline sewer, such as in HongKong, the N removal rate could be enlarged because the

NOx-N can be reduced not only by heterotrophic but alsoautotrophic denitrification. In our up-to-the-minute study,we injected calcium nitrate into a real pressure sewer to aNOx-N level of 10–50 mg-N/L to simulate the effect of

NUSD. After the sewer conveyance, it was found that allthe NOx-N has been completely removed. The results indi-cate a maximum nitrogen removal rate of ∼10 mg-N/L/h,

which is significantly higher than that estimated in thispaper. It is most likely attributed to the simultaneous auto-trophic and heterotrophic denitrification in the sewer. It

implies that, while the investigated effect of NUSD in thispaper is appropriate for the sewer conveying normal munici-pal sewage, the nitrogen removal rate could be significantly

higher for the sewer conveying saline sewage. This interest-ing finding could promote the application of NUSD inHong Kong, and will be investigated in the further study.

CONCLUSIONS

Full nitrification of source-separated urine was realized in a

lab-scale SBR. Simulation of the discharge of nitrified urineinto the pressure main induced effective in-sewer

626 F. Jiang et al. | Urine nitrification and sewer discharge simplification in Hong Kong Water Science & Technology | 64.3 | 2011

denitrification, enabling the sewage quality at the sewer

outlet to satisfy the discharge standard if 70% of the humanurine is to be source-separated, collected and nitrified fullyon site. This study demonstrated the feasibility and benefits

of NUSD on nitrogen removal in a densely populatedcity like Hong Kong. Under the further studies of urinenitrification in deep shaft SBR and autotrophic and hetero-trophic denitrification in saline sewer, urine source

separation, nitrification and sewer-discharge would becomea promising technology to remove nitrogen in Hong Kong.

ACKNOWLEDGEMENT

The authors wish to thank the financial support from the

Hong Kong Research Grant Council for the urine nitrifica-tion work (611607), the National Natural ScienceFoundation of China (50808088), and Guangdong Natural

Science Foundation (8451063101001185).

REFERENCES

American Public Health Association (APHA) StandardMethods for the Examination of Water and Wastewater, 21stedition. American Public Health Association, New York.

Beun, J. J., Hendriks, A., van Loosdrecht, M. C. M., Morgenroth, E.,Wilderer, P. A. & Heijnen, J. J. Aerobic granulation in asequencing batch reactor.Water Research 33 (10), 2283–2290.

Chen, G. H. Sewer Biofilm Modeling for Sulfide Formation InSewers. A Technical Report submitted to Drainage ServicesDepartment of the HKSAR Government via HKUSTRDC.

Chen, G. H. Sewer Biofilm Modeling for Sulfide Formation inSewers. A final report submitted to Drainage ServicesDepartment of Hong Kong. HKUST RDC, Hong Kong.

Chen, G. H. & Leung, D. H. W. Utilization of oxygen in asanitary gravity sewer. Water Research 34 (15), 3813–3821.

Chen, G. H., Leung, D. H. W. & Hung, J. C. Biofilm in thesediment phase of a sanitary gravity sewer. Water Research37 (11), 2784–2788.

Ciba-Geigy Wissenschaftliche Tabellen Geigy; TeilbandKorperflussigkeiten (Geigy Scientific Tables; Volume BodyFluids), 8th edition. Basel, Germany.

Fittschen, I. & Hahn, H. H. Characterization of themunicipal wastewater part human urine and a preliminarycomparison with liquid cattle excretion. Water Science andTechnology 38 (6), 9–16.

Guest, J. S., Skerlos, S. J., Barnard, J. L., Beck, M. B., Daigger, G. T.,Hilger, H., Jackson, S. J., Karvazy, K., Kelly, L., Macpherson,L., Mihelcic, J. R., Pramanik, A., Raskin, L., van Loosdrecht,M. C. M., Yeh, D. & Love, N. G. A new planning and

design paradigm to achieve sustainable resource recoveryfrom wastewater. Environmental Science and Technology 43(16), 6126–6130.

Gujer, W., Henze, M., Mino, T. & van Loosdrecht, M. C. M. Activated sludge model No.3. Water Science and Technology39 (1), 183–193.

Huisman, J. L. & Gujer, W. Modelling wastewatertransformation in sewers based on ASM3. Water Science andTechnology 45 (6), 51–60.

Hvitved-Jacobsen, T., Vollertsen, J. & Matos, J. S. The seweras a bioreactor – a dry weather approach. Water Science andTechnology 45 (3), 11–24.

Jiang, F., Leung, D. H. W., Li, S. Y., Lin, G. S. & Chen, G. H. A new method for determination of parameters in sewerpollutant transformation process model. EnvironmentalTechnology 28 (11), 1217–1225.

Jiang, F., Leung, D. H. W., Li, S. Y., Chen, G. H., Okabe, S. & vanLoosdrecht, M. C. M. A biofilm model for prediction ofpollutant transformation in sewers. Water Research 43 (13),3187–3198.

Kirchmann, H. & Pettersson, S. Human urine; Chemicalcomposition and fertilizer use efficiency. Fertilizer Research40, 149–154.

Larsen, T. A., Alder, A. C., Eggen, R. I. L., Maurer, M. & Lienert, J. Source separation: will we see a paradigm shift inwastewater handling? Environmental Science andTechnology 43 (16), 6121–6125.

Lienert, J., Thiemann, K., Kaufmann-Hayoz, R. & Larsen, T. A. Young users accept NoMix toilets – a questionnairesurvey on urine source separating toilets in a college inSwitzerland. Water Science and Technology 54 (11–12),403–412.

Rossi, L., Lienert, J. & Larsen, T. A. Real-life efficiency ofurine source separation. Journal of EnvironmentalManagement 90 (5), 1909–1917.

Tam, L. S., Tang, T.W., Lau,G.N., Sharma,K. R.&Chen,G.H. A pilot study for wastewater reclamation and reuse withMBR/RO and MF/RO systems.Desalination 202 (1–3),106–113.

Udert, K. M., Fux, C., Münster, M., Larsen, T. A., Siegrist, H. &Gujer, W. a Nitrification and autotrophic denitrificationof source-separated urine. Water Science and Technology 48(1), 119–130.

Udert, K. M., Larsen, T. A., Biebow, M. & Gujer, W. b Ureahydrolysis and precipitation dynamics in a urine-collectingsystem. Water Research 37 (11), 2571–2582.

Wilsenach, J. A. & van Loosdrecht, M. C. M. Effects ofseparate urine collection on advanced nutrient removalprocesses. Environmental Science and Technology 38 (4),1208–1215.

Wilsenach, J. A., van Bragt,W. P.M., de Been, P.& van Loosdrecht,M. C. M. Evaluation of separate urine collection andtreatment to augment existing wastewater treatment works.Water Science and Technology, 52 (4), 71–80.

First received 17 July 2010; accepted in revised form 8 December 2010