THERMOPHILIC ANAEROBIC CO-DIGESTION OF AGRO-WASTE AND WASTE
ACTIVATED SLUDGE: INFLUENCE OF THE ORGANIC LOADING RATE.
Pavan P. 1, Bolzonella D. 2, Mata-Alvarez J. 3, Cecchi F. 2
1
Department of Environmental Sciences, University of Venice. Calle Larga S. Marta 2137, 30123 Venice, Italy.
2 Department of Science and Technology, University of Verona. Strada Le Grazie,15 - 37134 Verona, Italy.
3 Department of Chemical Engineering. University of Barcelona. Martí i Franquès 1, plta.6; 08028 Barcelona
Abstract
The alkalinity and VFA trends showed not problematical values
for the process behaviour.
Digester alkalinity
8000
Digester VFA
SSC
350
300
mgCOD/l
6000
5000
SSC
SSC
4000
SSC
200
2000
50
1000
0
20
40
60
start-up
80
100
120
I
140
160
180
SSC
150
100
0
SSC
250
3000
SSC
0
200
20
40
60
80
start-up
III
II
100
I
120
140
160
II
180
200
III
Digester N-NH3
Digester pH
1800
9
SSC
SSC
SSC
SS
C
1600
SSC
8
1400
1200
pH
mgN/l
7
6
The aim of the research project
400
SSC
Alkalinity pH 4
Alkalinity ph 6
7000
mgCaCO3/l
This poster deals with the thermophilic anaerobic co-digestion of
solid agro-waste (mainly fruit and vegetable residuals from
markets) and waste activated sludge originated from the biological
treatment of municipal wastewater. A pilot scale digestor of 200 l
working volume was used. Treating only wasted sludge, the initial
Organic Loading Rate (OLR) was 0,7 kgTVS/m3 d; then it was
increased up to 4 and then to 6 kgTVS/m3. Increasing the amount
of OFMSW in the feeding, the gas production rate (GPR) raised from
0.12 to 3,12 m3/m3 d and the Specific Gas Production (SGP) from
0.16 to 0.51 m3/kgTVS. Regarding the process behaviour, its
stability was showed also in critical operatif conditions. Considering
the option of retrofitting the anaerobic digesters of the existing
sludge line in WWTPs, the obtained data clearly show the process
feasibility.
Results
SSC
SSC
1000
SSC
800
600
5
400
The work carried out is part of a research
considered in the VI framework program
financially supported by the European
Union, the “Cropgen” project (contract n.
SES6-CT-2004-502824).
The main aim of the project is to firmly establish biogas technology
as an economically attractive energy production process for both on
and off-site use. In particular this work investigates the thermophilic
co-digestion of WAS and market waste, also in order to evaluate its
feasibilty on existing WWPT so to exploit the already avaiable
structures existing in wastewater treatment plants operating within
the UE.
200
4
0
20
1
54.8
18
2.19
2
54.5
16
3.97
80
100
I
120
140
II
160
180
200
0
20
40
60
start-up
III
80
Run
Start-up
1
Reactor sludge
21.3
27.0
12.4
17.0
14600
17110
643
839
23.0
16.5
Stability parameters
7.83
7.90
2029
1952
2678
2767
156
70
598
687
Yields
0.12
0.85
0.16
0.41
0.16
0.16
0.55
20
50
TS, g/kg
TVS, g/kg
COD, mg O2/l
TKN, mgN/l
Ptot, mgP/gTS
pH
TA(6), mgCaCO3/l
TA(4), mgCaCO3/l
VFA, mgCOD/l
NH3/, mgN/l
GPR, m3/m3 d
SGP, m3/kgTVS
SGP sludge, m3/kgTVS
SGP waste, m3/kgTVS
CH4, %
TVS reduction, %
Substrates
140
160
180
200
III
2
3
42.0
28.9
30930
1910
12.8
62.1
41.3
40980
2580
6.0
7.89
2345
3505
200
1265
7.59
2947
7000
321
1473
1.65
0.42
0.16
0.52
48
3.12
0.51
0.16
0.59
67.0
57
Gas Production Rate
3,5
5,0
GPR = 0,5384 OLR - 0,3148
R2 = 0,9903
3
4,5
GPR, m3/m3 d
SSC
4,0
3
3
GPR, m b/m r
3,5
3,0
2,5
SSC
2,0
2,5
2
1,5
1
SSC
1,5
1,0
0,5
SSC
0,5
0
0,0
20
start-up
40
60
80
II
100
120
III
140
160
180
0
200
2
While GPR shows a linear trends
increasing OLR, SGP reached a
plateau when OLR approached to
6 kgTVS/m3 d. This was supposed
to be the process limit for these
conditions
Conclusion
4
6
8
OLR, kgTVS/m3 d
IV
0,6
0,5
SGP, m3/kgTVS
0
Effluent
120
II
Table 3. Main results of the research.
3
55.1
14
6.18
The used digester was a CSTR
type, fed semicontinuously
once a day and maintained at
55 ± 1 °C. The substrates
were acquired daily from the
wastewater treatment plant
of Treviso (sludge) and from
the
municipal
collection
from fruit and vegetables
markets
100
I
Only during the last experimental run, the higher OLR
caused a slight decrease of pH. Anyway the process seems
to be not inhibited.
Table 1. Operational conditions used in the research.
Start-up
55.0
20
0.66
60
start-up
Experimental
Run
T, °C
HRT (days)
OLR(kgTVS/m3r)
40
0,4
0,3
0,2
0,1
0
0,66
2,19
3,97
6,18
OLR, kgTVS/m3 d
The main results of this work are:
Table 2 Substrates characteristics.
Run
Start-up
TS, g/Kg
TVS, g/Kg
COD, mgO2/l
TKN, mgN/l
Ptot, mgP/gTS
20.9
13.3
21942
1335
20.4
TS, g/Kg
TVS, g/Kg
COD, mgO2/l
TKN, mgN/g TS
Ptot, mgP/gTS
1
Sludge
24.8
13.9
21734
865
17.5
Market-wastes
242.5
192.8
912
27.3
5.1
2
3
30.4
20.5
20178
1595
17.3
34.6
21.7
20742
1873
8.4
241.6
198.4
920
28.5
7.3
276.4
218.7
841
32.3
3.9
Biogas production (GPR) increase linearly with OLR, while
the specific production reached a plateau when OLR
approached to 6 kgTVS/m3d. This could be the process limit
for these conditions;
The OLR range from 2 to 6 kgTVS/m3 d was investigated,
observing a completely stable situation in all the conditions
studied. pH and alkalinity values were always in typical
ranges. The addition of increasing amount of waste in the
feed lead to an improvement in buffer capacity of the
digester (from 2700 to 7000 mg CaCO3/l);
The comparison with mesophilic process shows that, at
very low OLR (<1 kgTVS/m3 d), there are no big difference
between the two processes. However, things may be
different when higher loading rate are applied. Other
experimental work has to be done to investigate this aspect.