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Increased biogas production,
pasteurization and reduced methane
emissions by dry digestion of
dewatered sludge
2014-03-25
Erik Nordell
Development Engineer
Tekniska verken i Linköping AB (public)
Tekniska verken i Linköping AB
• Owns and operates 3 full-scale biogas plants
• Linköping biogas - co-digestion
• Norrköping biogas - co-digestion
• Nykvarnsverket WWTP (Located in Linköping) - sludge
digestion
• Total annual bio-methane production of these
plants are 130 GWh,
whereas 15% of the
production derived from
the WWTP
Nykvarnsverket WWTP: Schematic sketch
7 200 m3/d
Biogas to upgrading
unit (viechle fuel)
7 200 m3/d
Mesophilioc wet
digestion (38°C)
300 t/d
TS = 6%
20 days
(V = 6000 m3)
Dewatering
300 t/d
TS = 3%
Slamlager
Recirculation of reject water for
removal of nitrogen
300 t/d
TS = 3%
270 t/d
TS <1%
28 t/d
TS = 32%
Open ”composting” for 6
months before beeing used
on agriculture land
Background
• 42% of all biogas produced in Sweden (2012) was from
WWTPs (660 GWh)
• Primary- and bio-sludge are the “substrates” at the WWTP
digestion, typically with 50-60% degradation yields due to
short retention time and persistent substances
• Emissions from the storage of
sludge are crucial for the whole
biogas business, a higher degradation
yield is equal to lower emissions
• How can we increase the
biogas production from sludge?
Methane yield – BMP tests
Net production from primary/bio-sludge
Methane production (Nml CH4/gVS)
300
250
200
20% on
VS basis
150
100
Eq. 10%
on w/w
Production from digested sludge
50
0
5
10
Time (days)
15
20
“How can we increase the biogas
production from sludge?”
• Well, increase the retention time by building a postdigestion unit
• However, the post-digestion unit need to be relatively
large to collect as much as possible of the biogas
formed in the post-digester
• At Nykvarn WWTP 3,000-6,000 m3
• Heavy investment cost
Digester
Post-digester
Nykvarnsverket WWTP: Schematic sketch
7 200 m3/d
Biogas to upgrading
unit (vehicle fuel)
7 200 m3/d
Same amount of TS/VS
but only 10% of the w/w
Mesophilic wet
digestion (38°C)
300 t/d
TS = 6%
20 days
(V = 6000 m3)
Dewatering
300 t/d
TS = 3%
Buffer
Recirculation of reject water for
removal of nitrogen
300 t/d
TS = 3%
270 t/d
TS <1%
28 t/d
TS = 32%
Open ”composting” for 6
months before being used
on agriculture land
Dry-digestion of dewatered sludge?
• Digester volume may be decreased with 90%, but the
retention time for the sludge can be the same
• E.g. instead of 6,000 m3, the digester volume can be 600 m3
if the dewatered sludge are used for post-digestion
• Dry-digestion (TS > 20%) technique differ from the normal
wet-digestion (typically 2-6% TS) | mass-transfer is different,
mixing etc.
• A 2nd setup of BMP tests were performed, at 20°C and 38°C
to simulate “optimal” conditions in the current storage as
well as the maximal potential at mesophilic temperature
BMP-test on dewatered sludge from Nykvarnsverket WWTP
Biogas production (m3/y)
20 grader
Reaktorstorlek
1 000
180 000
900
160 000
800
140 000
700
120 000
600
100 000
500
80 000
400
60 000
300
40 000
200
20 000
100
-
0
5
10
15
20
Time (days)
25
30
35
Reactor size - plugflow (m3)
38 grader
200 000
Dry-digestion of dewatered sludge?
• Pasteurization may be applied by using a thermophilic
temperature (52-55 C) to meet the further demand for the
government (“Fosforremissen”) = combo effect!
• Plug-flow reactor (= non mixed reactor) may be used to
guarantee that the sludge are pasteurized
Substrate
Day 1
Digestate
Day 1+HRT
Radial mixing only
How do we proceed form here?
• Swedish Energy Authority (Energimyndigheten) has
granted financial support to the full-scale
investment with 6 mSEK
• This spring, a pilot-scale plug-flow reactor will be
used to evaluate and confirm the pasteurization
effect as well as effect on the viscosity of the final
sludge
Future implementation at Nykvarnsverket WWTP
7 200 m3/d
Biogas to upgrading
unit (vehicle fuel)
7 200 m3/d
Mesophilic wet
digestion (38°C)
300 t/d
TS = 6%
20 days
(V = 6000 m3)
550 m3/d (+7,5%)
Dewatering
300 t/d
TS = 3%
Buffert
Recirculation of reject water for
removal of nitrogen
300 t/d
TS = 3%
270 t/d
TS <1%
28 t/d
TS = 32%
Dry-digestion
(55°C)
36 days
(V = 1 000 m3)
Simultaneously pasteurization
To farming land
Less emissions!
Idé
Conclusion/summary
• By implementing a dry-digestion step to post-digest
the dewatered sludge, the biogas production can be
increased with 8%
• Methane slip from storage will decrease significantly
• Thermophilic temperature will simultaneously
make the post-digestion unit to a
pasteurization step
• Pilot-scale experiment this spring!
Questions or comments?
Contact information
Erik Nordell
Tekniska verken i Linköping AB
Dept. R&D Biogas
erik.nordell@tekniskaverken.se
+46(0)13-30 85 42