Nitrous oxide emissions from three Swedish ... - Svenskt Vatten

Nitrous oxide emissions from three Swedish anaerobic digester
supernatant treatment processes – a comparative study through
full-scale data analysis and mathematical modelling
Erik Lindblom 1,2 , Magnus Arnell 1,3 , Xavier Flores-Alsina 1 , Fredrik Stenström 4,5 ,
David J.I Gustavsson 6 , Ulf Jeppsson 1
1
Div. of Industrial Electrical Engineering and Automation (IEA), Lund University, Sweden
2
Sweco Environment AB, Stockholm, Sweden
3 Urban Water Management, Linköping, Sweden
4
Water and Environmental Engineering, Dept. of Chemical Engineering, Lund University, Sweden
5
VA-ingenjörerna, Örebro, Sweden
6
VA SYD, Malmö, Sweden
Corresponding author: Erik Lindblom, erik.lindblom@sweco.se, +46 (0)702 – 539051.
Abstract for ordinary lecture/paper presentation
Optimal municipal wastewater treatment plant (WWTP) engineering and operation call for
plant-wide process understanding, which can be summarized as mathematical models. Recent
research have shown that some “optimal” strategies, e.g. operation with intermittent aeration
and/or low dissolved oxygen (DO) set-points, might be “sub-optimal” because of the risk for
elevated emissions of the undesired greenhouse gas nitrous oxide (N 2 O). This is possibly due
to lack of knowledge and the inability of WWTP simulators to describe these effects (Flores-
Alsina et al., 2011).
In this study, a biological simulation model that includes N 2 O production in processes treating
supernatant from anaerobic digestion of municipal primary and secondary sludge has been
developed, implemented and validated. An associated physical/hydraulic model describing a
sequencing batch reactor (SBR) has been developed as well. The models are calibrated to
reproduce the set of:
measurements performed by Gustavsson et al. (2011), who investigated a nitritation
only SBR process at Sjölunda WWTP (Malmö, Sweden);
measurements performed by Stenström et al. (2013), who investigated a nitrificationdenitrification
SBR process at Slottshagen WWTP (Norrköping, Sweden); and,
currently unpublished measurements of nitrous oxide emissions from a nitritation-
Anammox process at Hammarby Sjöstad pilot plant (Stockholm, Sweden).
The three case studies involve supernatant treatment processes and all include measurements
of traditional wastewater variables (online and grab samples) and online measurements of
N 2 O (water and gas phase).
The developed SBR model is based on a 10-layer settler model (Takács et al., 1991) extended
to allow: i) variable volume (e.g. during filling, chemical dosage); ii) complete mixing (e.g.
during aeration, mixing); and, iii) biological reactions in all SBR phases.
The biological model was initially based on Hiatt and Grady (2008). This model (ASMN)
extends the well-recognized ASM1 (Henze et al., 2000) with two nitrifying populations:
ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB), using ammonia
(NH 3 ) and nitrous acid (HNO 2 ), respectively, as substrates. Sequential heterotrophic

denitrification of nitrate (NO 3 - ) to nitrogen gas (N 2 ) via nitrite (NO 2 - ), nitric oxide (NO) and
N 2 O is also included. However, the model does not include AOB denitrification which, as
pointed out by both Gustavsson et al. (2011) and Stenström et al. (2013) amongst others,
potentially is the governing process for N 2 O formation in biological WWTPs.
Therefore the hypothesised reactions recently published by Mampaey et al. (2013) were
added. Here AOB are additionally capable of reducing HNO 2 to NO and then to N 2 O through
two possible reaction scenarios. In our final publication, other possible N 2 O production
mechanisms will be evaluated as well.
Figure 1 shows part of the results from Slottshagen WWTP. Figure 1G shows the measured
off-gas N 2 O production and a simulation using the ASMN/Mampaey model. During aeration
the maximum N 2 O emission is reached almost instantly although absence of dissolved N 2 O
from the preceding anoxic phase. Figure 1H shows the dissolved N 2 O concentrations. The
mass of N 2 O-N formed almost equals the mass of denitrified NO 3 -N and when ethanol dosage
begins the N 2 O concentration is immediately reduced. Considering the complexity of the
measurements, the model behaviour satisfactorily describes the measurements.
A: Operational phases of the SBR, Cycle 1 and 2
Filling
Mixing
Ethanol
Aeration
Sedimentation
Decantation
0 2 4 6 8 10 12 14 16
75
50
25
C: NH 4
+ -N [mg N/l]
0
0 2 4 6 8 10 12 14 16
6
4
2
E: NH 3
-N [mg N/l]
0
0 2 4 6 8 10 12 14 16
3
2
1
G: N 2
O-N offgas [kg N/h]
0
0 2 4 6 8 10 12 14 16
9
6
3
120
0
0 2 4 6 8 10 12 14 16
D: (+) NO 2
- -N [mg N/l], (o) NO3
- -N [mg N/l]
80
40
0
0 2 4 6 8 10 12 14 16
30
20
10
B: (-) DO [mg O 2
/l], (--) pH [-]
F: HNO 2
-N [g N/l]
0
0 2 4 6 8 10 12 14 16
15
10
5
H: N 2
O-N water [mg N/l]
0
0 2 4 6 8 10 12 14 16
Figure 1. Results for SBR Cycles 1 and 2 at Slottshagen WWTP. A-B: Input data; C-H: Measured (markers) and simulated
(solid lines) concentrations and mass flows.
Work regarding model validation using data from Sjölunda WWTP is currently being
conducted. The Anammox process model implementation is completed and the validation
effort has been initiated and will be finalized during spring 2013.

The main goal of the paper is to, by the combination of full-scale data and theory, provide
easy accessible information regarding the model development methodology and the important
biological pathways governing N 2 O emissions from supernatant treatment processes. For
practitioners this knowledge is essential since it forms a basis for the appropriate selection of
new process steps and optimal operation of existing plants.
References
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