Modeling sewage lagoons in the Arctic
Yehuda Kleiner, Boris Tartakovsky, Datong Song, Andrew Colombo, Qianpu Wang
National Research Council of Canada
ARTEK 2016 - Sanitation in cold climate regions
ARTEK 2016 - Sanitation in cold climate regions, Sisimiut, Greenland, April 12-14
Freeze/thaw cycle
After decanting (September)
2
Freeze/thaw cycle
Before freezing (October)
3
Freeze/thaw cycle
Begin freezing (October - November)
4
Freeze/thaw cycle
Early winter (November - December)
5
Freeze/thaw cycle
Mid winter (December - January)
6
Freeze/thaw cycle
Late winter (February - March)
7
Freeze/thaw cycle
Spring (April - May)
8
Freeze/thaw cycle
End of summer (August - September)
9
Freeze/thaw cycle
After decanting (September)
10
Pond Inlet (Nunavut) lagoon
Max depth about 4m
Mean depth about 2.5m
Capacity about 130K m3
Pond Inlet
lagoon (mid-Nov. 2015)
12
Chemical oxygen demand (COD) fractions
(ASM1-based)
Total
COD
Biodegradable
COD
Soluble
Nonbiodegradable
COD
Particulate
Soluble
Aerobic
Heterotrophs
Particulate
Active biomass
COD
Autotrophs
Anaerobes
Aerobic vs anaerobic conditions
• Boundaries fuzzy and change continuously (light, temperature, liquid
properties).
• Ice cap largely eliminates aerobic activity.
• Zero biological activity in ice is assumed (anaerobic activity possible in
winter, given sufficient lagoon depth).
Aerobic phase
Anoxic phase
DO > 1 mg/L
0.1 < DO < 1 mg/L
Anaerobic phase
14
DO < 0.1 mg/L
Dynamic processes – conceptual representation
Aerobic
growth
Anoxic
growth
Autotrophic and
Heterotrophic biomass
Heterotrophic biomass
Anaerobic biomass (methanogens)
Hydrolysis of
entrapped
particulate
organic matter
15
Anaerobic
growth
Decay
(death)
Material balances and kinetic equations
(Multiplicative Monod-like kinetics used)
∂SS
1
1
1
= − µmax,H M 2 M 8h X B,H − µmax,H M 2ηg I8 M 9 X B,H −
µmax,AN M 2,an I8 X B,AN
∂t
YH
YH
YAN
144424443 1444424444
3 14444244443
Aerobic growth of
heterotrophic biomass
Anoxic growth of
heterotrophic biomass
Anaerobic growth of
anaerobic particulate biomass
q in
+ kh ksat M 8h X B,H + kh ksatηh I 8 M 9 X B,H +
SS,in − SS )
(
14
4244
3 1442443 14
V 4244
3
Hydrolysis of entrapped
particulate organics
Where (e.g.,)
SS
M2 =
K S + SS
M 8h =
SO
K O,H + SO
Hydrolysis of entrapped
organic nitrogen
flow term
Ss – Biodegradable soluble substrate
XB,H – Particulate heterotrophic biomass
XB,A – Particulate autotrophic biomass
XB,AN – Particulate anaerobic biomass
Substrate saturation constant
Interim simplifying assumptions (COMSOL)
• Uniform horizontal layers (length of reactor does not matter).
• Thin ice cap (305 days - modelled as zero oxygen penetration) no freezing
through depth.
• Settling of solids not modelled (yet).
o 25% of solids are suspended (75% settle) towards decanting
o All biomass is settled
o tCOD in effluent = soluble COD + suspended solids COD
• COD to BOD5 ratio ~ 2:1
• Algae bloom not modeled (yet) – affecting oxygen penetration, solids, etc.
• Intermittent inflow (weekly batch inflows).
• Decanting 80% fluid volume over 14 days.
Assumed raw sewage characteristics
parameter
Total COD
Symbol
CODTotal
value
unit
1025
mg/L
Soluble COD
SS,in
340
mg/L
COD in particulates (solids)
X S,in
400
mg/L
COD of active autotrophic biomass
X B,A,in
70
mg/L
COD of active anaerobic biomass
X B,AN,in
20
mg/L
COD of active heterotrophic biomass
X B,H,in
70
mg/L
25
mg N /L
COD in total nitrogen (nitrite, nitrate,
ammonium, organic)
Non-biodegradable soluble COD
S I,in
50
mg/L
Non-biodegradable particulate COD
X I,in
50
mg/L
Key model parameters
Units
Our value
(~5oC)
Range in
literature
Maximum specific growth rate for heterotrophs
day-1
1~4
0.6 ~ 13.2
Heterotrophic decay coefficient
day-1
0.15
0.05 ~ 1.6
Maximum specific growth rate for anaerobes
day-1
0.05 ~ 0.2
0.025 ~ 0.5
Anaerobic decay coefficient
day-1
0.001
Maximum specific growth rate for autotrophs
day-1
0.2
0.2 ~ 1.0
Autotrophic decay coefficient
day-1
0.05
0.05 ~ 0.2
Hydrolysis rate (gr biodegradable to soluble COD)
day-1
1.0
1.0 ~ 3.0
Parameter
Model results: Hydrolysis (only)
Assumed zero degradation (aerobic or anaerobic) activity
COD [mg/L]
800
Total CODs
600
Soluble COD
400
Solid CODs
200
Estimated effluent COD based on Dalhousie’s Ragush et al. (2015)
0
0
90
180
270
360
Time (days)
C.M. Ragush et al. / Performance of municipal waste stabilization ponds in the Canadian Arctic Ecological
Engineering 83 (2015) 413–421
Model results: Aerobic degradation only
Assumptions:
a) 25% of solids are suspended (75% settle) towards decanting
b) All biomass is settled
c) tCOD in effluent = soluble COD + suspended solids COD
Effluent tCOD [mg/L]
800
600
Total COD
Heterotrophic µmax= 1
Heterotrophic µmax= 2
Heterotrophic µmax= 4
400
Soluble COD
200
0
0
90
180
Time (days)
270
360
Model results: Anaerobic degradation only
Effluent tCOD [mg/L]
800
600
400
Anaerobic µmax= 0.05
200
Anaerobic µmax= 0.1
Anaerobic µmax= 0.2
0
0
90
180
Time (day)
270
360
Model results: Aerobic + Anaerobic (simultaneous)
degradation
200 mg/L COD ~ 100 mg/L BOD5 (approx. present in Pond Inlet lagoon)
Effluent tCOD [mg/L]
800
600
400
Aerobic µmax= 1 day-1
Anaerobic µmax= 0.05
200
Anaerobic µmax= 0.1
Anaerobic µmax= 0.2
Total nitrogen
0
0
90
180
Time (days)
270
360
Model results: Oxygen concentration
Day 352 (just before decanting).
• Very narrow layer of oxygen penetration.
• Wind shear and waves not modeled (would work to increase penetration).
• Settling of solids and biomass also not modeled (would work to increase oxygen
penetration – less COD to degrade – more oxygen available.
Conclusions
Short summer period and low oxygen transfer rate limit aerobic activity. Lagoon
likely acts as a primary settler and anaerobic digester.
Hydrolysis leads to increased soluble COD concentration during the summer.
Anaerobic activity contributes to COD degradation. It is active year-round in
unfrozen liquid and is un-inhibited due to low oxygen penetration.
Simultaneous aerobic and anaerobic degradation appears to explain observed
experimental data with reasonable magnitude of kinetic parameters.
Deep primary lagoon and shallow secondary lagoon or lagoon with variable
depth (deep at the beginning and shallow towards the end) could improve
lagoon performance
To be addressed in future model development:
Settling of solids
Wind shear and waves
Algae bloom
Freezing depth