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

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.

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.

Abstract

The aim of the research project

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).

Experimental

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

Table 2 Substrates characteristics.

Table 3. Main results of the research.

Substrates

Effluent

Gas Production Rate

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

5,0

0 20 40 60 80 100 120 140 160 180 200

GP

R, m

3 b/m

3 r

start-up II III IV

SSC

SSC

SSC

SSC

GPR = 0,5384 OLR - 0,3148

R2 = 0,9903

0

0,5

1

1,5

2

2,5

3

3,5

0 2 4 6 8

OLR, kgTVS/m3 d

GP

R, m

3/m

3 d

Results

Digester alkalinity

1000

2000

3000

4000

5000

6000

7000

8000

0 20 40 60 80 100 120 140 160 180 200

mg

CaC

O3/

l

Alkalinity pH 4

Alkalinity ph 6

SSC

SSC SSC

SSC

start-up I II III

Digester pH

4

5

6

7

8

9

0 20 40 60 80 100 120 140 160 180 200

pH

SSC SSC SSC SSC

start-up I II III

Digester VFA

0

50

100

150

200

250

300

350

400

0 20 40 60 80 100 120 140 160 180 200

mg

CO

D/l

SSCSSC

SSC

SSC

start-up I II III

Digester N-NH3

200

400

600

800

1000

1200

1400

1600

1800

0 20 40 60 80 100 120 140 160 180 200

mg

N/l

SSC

SSC

SSC

SSC

start-up I II III

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

Table 1. Operational conditions used in the research.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,66 2,19 3,97 6,18

OLR, kgTVS/m3 d

SG

P, m

3/k

gT

VS

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.

Only during the last experimental run, the higher OLR caused a slight decrease of pH. Anyway the process seems to be not inhibited.

Conclusion

The alkalinity and VFA trends showed not problematical values for the process behaviour.

The main results of this work are:

Run Start-up 1 2 3 T, °C 55.0 54.8 54.5 55.1

HRT (days) 20 18 16 14 OLR(kgTVS/m3r) 0.66 2.19 3.97 6.18

Run Start-up 1 2 3 Sludge

TS, g/Kg 20.9 24.8 30.4 34.6 TVS, g/Kg 13.3 13.9 20.5 21.7

COD, mgO2/l 21942 21734 20178 20742 TKN, mgN/l 1335 865 1595 1873

Ptot, mgP/gTS 20.4 17.5 17.3 8.4 Market-wastes

TS, g/Kg 242.5 241.6 276.4 TVS, g/Kg 192.8 198.4 218.7

COD, mgO2/l 912 920 841 TKN, mgN/g TS 27.3 28.5 32.3 Ptot, mgP/gTS 5.1 7.3 3.9

Run Start-up 1 2 3 Reactor sludge

TS, g/kg 21.3 27.0 42.0 62.1 TVS, g/kg 12.4 17.0 28.9 41.3

COD, mg O2/l 14600 17110 30930 40980 TKN, mgN/l 643 839 1910 2580

Ptot, mgP/gTS 23.0 16.5 12.8 6.0 Stability parameters

pH 7.83 7.90 7.89 7.59 TA(6), mgCaCO3/l 2029 1952 2345 2947 TA(4), mgCaCO3/l 2678 2767 3505 7000

VFA, mgCOD/l 156 70 200 321 NH3/, mgN/l 598 687 1265 1473

Yields GPR, m3/m3 d 0.12 0.85 1.65 3.12

SGP, m3/kgTVS 0.16 0.41 0.42 0.51 SGP sludge, m3/kgTVS 0.16 0.16 0.16 0.16 SGP waste, m3/kgTVS - 0.55 0.52 0.59

CH4, % - - - 67.0 TVS reduction, % 20 50 48 57