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The COD Balance of a WWTP:
Possibilities and Limitations
for Improving the Energy Efficiency
2nd IWA Conference on Holistic
Sludge Management
Malmö, June 9th 2016
Martin Kaleß
Energy content of wastewater
Energy demand of WWTPs
Processes for carbon recovery (examples)
Possibilities and limitations of providing energy
from wastewater by calculation of a COD
balance
Conclusions and outlook
Outline
Energy in wastewater occurs in different forms
Chemically bound energy expressed by COD:
Energy content of wastewater
Authors / reference
COD load
(g COD / (PE * d))
Almeida et al. (1999) 112 (A)
Andreottola et al. (1994) 116 (A, rural) – 120 (A, urban)
ATV DVWK A 131 (DWA 2000) 120 (P)
Hansen et al. (2011) 120 (A)
Hartwig et al. (2010) 120 (A)
Henze (1997) 130 (A)
Jardin (2012) 120 (A)
Jönsson et al. (2005) 134 (A)
Kroiss and Svardal (2009) 110 (A)
Schmidt et al. (2003) 108 (A) / 143 (P)
Stommel (2011) 130 (A) / 175 (P)
A: Average; P: 85-Percentil
Average energy content of municipal
wastewater from literature: 120 g COD/(PE*d)
Assuming 1 kg COD corresponds to 3.5 kWh
(Olsson, 2015), (Mergelmeyer and Kolisch, 2014)
153 kWh/(PE*a) are available in wastewater
Average energy demand of a German WWTP:
34 kWhel/(PE*a)
Energy content of wastewater /
energy demand of its treatment
Average energy content of municipal
wastewater from literature: 120 g COD/(PE*d)
Assuming 1 kg COD corresponds to 3.5 kWh
(Olsson, 2015), (Mergelmeyer and Kolisch, 2014)
153 kWh/(PE*a) are available in wastewater
Average energy demand of a German WWTP:
34 kWhel/(PE*a)
Fate of chemically bound energy
Influent 100%
to bio-
logical
step
70% Effluent 8% Excess
sludge
25%
Primary
clarifyer
Respiration 37%
Digested sludge 28 %
Activated
sludge
process
Biogas 27%
digestion According to Svardal, 2014
r eturn sludge screenings s and
influent
p rimary sludge e xcess sludge
a ctivated
sludge
tank
f inal
sedimentation
tank
screen g rid chamber
and
g rease trap
p rimary
s ettlement
tank
r eceiving
water
Classification of processes for carbon recovery
Carbon recovery from
wastewater
possibilities for carbon
recovery
particulate dissolved
physical sedimentation yes no
sieving yes no
flotation yes no
adsorption yes yes
biological biological
incorporation no yes
chemical precipitation /
flocculation yes partly
Conventional sedimentation processes
remove 30 % of the incoming COD at a retention time
of 0.75 – 1 h (DWA 2000)
Sedimentation performance can only be slightly
improved by prolonging the retention time
Enhancement of sedimentation processes in
primary clarifiers by adding chemicals
Chemically enhanced primary
treatment (CEPT) (1/3)
Laboratory studies within the project are
ongoing
CEPT (2/3)
Results from literature study
CEPT: Removal efficiency (3/3)
Author Total COD (or BOD5)
removal [%]
Conditions
Aiyuk et al. (2004) 73 laboratory scale
Bourke (2000) 55-65 (BOD5) full scale
De Feo et al. (2007) up to 70 laboratory scale
Gerges et al. (2006) 53 laboratory scale
Ismail et al. (2011) 56-61 laboratory scale
Leentvaar et al. (1977) 72 laboratory scale
Muzenda (2012) 58 (BOD5) laboratory scale
Ødegaard (1992) 73 full scale (87 Norwegian
WWTPs)
Screenings usually not considered in terms of
energy
Washing of screenings can help to recover
carbon from screenings
Amount and composition of screenings depend
on the sewer system and the bar spacing of the
screen
Carbon rich wash water can be used for
digestion
Washing of screenings (1/5)
Washing of screenings (2/5)
Washing of screenings (3/5)
Full scale investigations at three German
WWTPs
Record of the hourly amount of raw screenings
within one day
Washing the screenings from 24 hours in
several washings
Determination of the COD load of the wash
water
Comparison of the COD load in the wash water
to the COD load of the influent
Washing of screenings (4/5)
Plant
COD load of the
wash water (kg COD) obtained from
incoming screenings (time span 24h)
COD load (kg COD) of
the WWTP influent
(assumption:
120 g COD / (PE*d))
Ratio (%)
A 11 1,944 0.57
B 10 3,480 0.29
C 17 4,200 0.40
Washing of screenings: results
(5/5)
Faeces easily elutable
COD recovery by treating screenings
excess sludge
from first stage
return sludge
intermediate
settling
excess sludge
from second stage
return sludge
influent
internal recirculation
effluent
high load
stage
low load
stage
final
sedimentation
first stage second stage
Two-stage processes (1/3)
Two-stage processes (2/3)
Advantages
high concentration of substrate in the first stage
two separate biocenoses provide better conditions to
separated microorganisms than one can do
adsorption of dissolved substances in the first stage
less energy consumption (higher load means less
respiration) therefore more energy rich sludge from
first stage
Semi-scale pilot tests were conducted to
quantify COD-elimination of first stage
Two-stage processes: results (3/3)
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0.125 d 0.25 d 0.5 d 1 d 0.125 d 0.25 d 0.5 d 1 d
total dissolved
Eli
min
ati
on
of
CO
D
sludge age tSRT
maximum value
minimum value
75-%-percentile
25-%-percentile
median
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
Eli
min
ati
on
COD balance: possibilities and
limitations (1/2)
Assumptions:
120 g COD / (PE*d)
CEPT: 73 % removal efficiency
8 % still remain in the effluent as in conventional
processes (dissolved inert)
From the remaining 19 % -> 7% are transformed in
the excess sludge (same ratio as in conventional
activated sludge systems)
80 % of the influent COD load could enter the
digestor
COD balance: possibilities and
limitations (2/2)
50 % conversion of sludge into gas
40 % efficiency of combined heat and power
plant
153 kWh/ (PE*a)*0,8*0,5*0,4= 24.5 kWhel/ (PE*a)
Conclusions (1/2)
Holisitic approach combines sludge treatment
as well as sludge providing
Presented numbers will be specified within the
project period
CEPT shows high elimination efficiency for
COD
Washing of screenings has a minor impact on
carbon recovery
Two-stage activated sludge systems may enjoy
comeback due to elimination of dissolved COD
by adsorption
Conclusions (2/2)
COD recovery and transformation into electrical
energy leads to 24.5 kWhel/ (PE*a) in optimistic
case
Compared to 34 kWhel /(PE*a) average energy
consumption on German WWTPs
Outlook
Gap could be closed when considering lower
energy demand due to lower aeration because
of carbon recovery
Denitrification problems may occur and must
be solved by introducing autotrophic
processes
Thank you for your attention!
Acknowledgement: The authors would like to express their sincerest
thanks to the Federal Ministry of Education and Research for funding the
project E-Klär (reference 02WER1319).
Thanks to the project partners
and the co-authors!
Furthers questions: please feel free to ask or send it to