ANAEROBIC CO-DIGESTION OF ORGANIC FRACTION OF MUNICIPAL

SOLID WASTE AND WASTE ACTIVATED SLUDGE AT DIFFERENT

RATIOS

A. FLOR, L. ARROJA, I. CAPELAEnvironment and Planning Department, Aveiro University,

Campus Universitário 3810-193 Aveiro, Portugal

PORTUGUESE CONCERN

Landfilling is now in Portugal the most common way for solid waste final disposal. But....

Use solid waste management alternatives with lower costs and additional profit.

To accomplish with European directive in the next years (1999/31/EC directive).

there is the need to

In order to

Only about 200 thousand tones/year of OFMSW (Organic fraction of municipal solid waste) were treated biologically in Portugal.

The tighter environmental legislation related to wastewater treatment increase the production of Waste Activated Sludge (WAS) in wastewater treatment plants.

BACKGROUND

A good option to deal with the problem it’s the codigestion of both wastes, a strategy that has been used since the 80s in Denmark with good results in what concerns to:

•better economic feasibility, due to the common use of equipment

•better nutrient balance

•Reduction of the pollutants concentration to c.a. 10-20% of the impute sludge

To study the codigestion of two diferent co-substrates (OFMSW and WAS),

and

two diferent total solid contents (2 and 10% TS) using batch technology

GOAL

FEEDSTOCK• Simulated Portuguese OFMSW was

used throughout all experiments.

METHODS

• The components were crushed with a knife and a food blender

• Centrifuged WAS was obtained from a small domestic wastewater treatment plant

Parameter COD TVS TS Alkalinity

Substrate mg(O2)/Kg(TS) as % of TS ppm mg(CaCO3/Kg(TS)) OFMSW 1 300 90% 246 000 40 000

WAS 1 200 69% 111 000 9 500 Inoculum 900 52% 100 000 7 500

Component Proportion Component Proportion Cabbage 7% Bread 3% Onion 4,5% Fish 2,5% Apple 7,5% Meat 0,5% Potatoes 32% Eggs shell 1% Milted rice 30% Paper 5% Paste 7% Sum 100%

Two series of tests were carried out at 10% and 2% TS reactor content, at 37ºC, shacked at 60 rpm for 15 minutes each hour.

EXPERIMENTAL SETUP

OFMSW/WAS0% 25% 50% 75% 100%

TS content

10%

2%

Alkalinity was added as NaHCO3 and KHCO3 in order to give buffer capacity and process stability.

Nutrient solutions were added to avoid the lack of any chemical constituent.

The performances evaluation was made mainly in what concerns to the removal of total Volatile Solids (TVS) and chemical oxygen demand (COD), specific methane yield (SMY) and volatile fatty acids (VFA) balance.

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50

100

150

200

250

300

350

0 10 20 30 40 50 60 70 80 90 100 110 120 130Days

m3(C

H4)/

Ton

(ST

V) a

dd

ed

Biogas production: Almost al experiments reflect the first stage of

easily biodegradable and a second stage in the degradation of the others compounds

Much higher SMY (c.a. three times) for the 100% OFMSW essay in comparison to the 100% WAS essay at 10% TS at the end.

An increase in SMY can be related with OFMSW proportion.

RESULTS

1.st stage 2.nd stage

100% OFMSW

75% OFMSW

50% OFMSW

0% OFMSW

10% TS

VFA’s values as high as 31 000 mg (acA)/l were observed at 10% TS for the essay with 100% OFMSW.

This VFA’s values never reached a minimum at the end of the essay, although total reactor imbalance was never experienced, when sufficient alkalinity was added but it can be observed a decrease in the TVS removal and SMY kinetics for this essay due to this problems.

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5.000

10.000

15.000

20.000

25.000

30.000

35.000

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Days

VFA

's m

g(A

c)/

l

100% OFMSW

0%

75%

50%

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50

100

150

200

250

300

350

0 10 20 30 40 50 60 70 80 90 100 110 120 130Days

m3 (C

H 4)/

To

n(S

TV

)ad

ded

1.st stage 2.nd stage

100% OFMSW

75% OFMSW50% OFMSW

0% OFMSW

10% TS

The best result of all experiments were given by the 75% OFMSW at 2% TS with a production of about 350 m3(CH4)/Ton(TVS)added at the end of 40 days

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50

100

150

200

250

300

350

0 10 20 30 40 50 60 70 80 90 100 110 120 130Days

m3(C

H4)/

Ton

(ST

V) a

dd

ed

50% OFMSW

75% OFMSW

2% TS

At the end of the 120 days period, there is a direct relation between the increase on the total solids removal and OFMSW proportion,

Except for the essay with 100% OFMSW, which show a TVS reduction smaller than the WAS essay alone.

In what concerns to COD reduction a direct relation is well remarked in al cases

0 50 75 100

SMY (10% TS)10% TS

SMY (2% TS)2% TS

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0,10

0,20

0,30

0,40

0,50

0,60

0,70

TV

S r

em

oval %

& S

MY

l(C

H4)/

g(T

VS

)a

dd

ed

OFMSW %

0 50 75 100

SMY (10% TS)10% TS

SMY (2% TS)2% TS

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0,10

0,20

0,30

0,40

0,50

0,60

0,70

0,80

0,90

CO

Dre

mo

va

l %

& S

MY

l(C

H4)

/g(T

VS

)ad

ded

OFMSW %

SO… The best result was obtained with 75% OFMSW (25% WAS) essays, at 2% TS, with :

• Specific methane yield of about 350 m3(CH4)/Ton (TVS)added

• 70% TVS removal

• 80% COD removal

• No inhibitions without any VFA’s accumulation

0 50 75 100

SMY (10% TS)

10% TSSMY (2% TS)

2% TS

83% COD removal

0,346 SMY

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0,10

0,20

0,30

0,40

0,50

0,60

0,70

0,80

0,90

CO

Dre

mo

va

l % &

SM

Y l(

CH

4)/g

(TV

S) a

dd

ed

OFMSW %

At 2% TS VFA's increased until reached a maximum value much earlier (day 10th) and no imbalance was observed for this essays.

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5.000

10.000

15.000

20.000

25.000

30.000

35.000

0 10 20 30 40 50 60 70 80 90 100 110 120 130Days

VFA

's m

g(A

c)/l

50% OFMSW

75% OFMSW

In both experiments (2% & 10%TS) VFA’a values decreased with increase WAS proportion, meaning that this waste can help to prevent this kind of reactor imbalance.

The results in conjunction with recent findings, point out that the VFA's build up at stable neutral pH are not responsible for process stop.

On the other and, some inhibition occurred, with that increase, as it was observed for the 100% OFMSW essay at 10% TS.

CONCLUSIONS

The codigestion of both substrates in the correct proportion have a synergistic effect on the process, since the adiction of WAS to OFMSW digestion, help to avoid instabilities in the process.

These batch essays findings would also be useful to perform a better start up of continuous and batch anaerobic reactors treating this kind of wastes.

The best startup conditions are at wet digestion (2% TS), for mixtures with 75% OFMSW, witch gave as much as 350 m3(CH4)/Ton(TVS)added, 70% TVS removal and about 80% COD removal, without VFA’s accumulation.

This work, was done under a PhD grant from FCT- Portugal SFRH/BD/1066/2000

and

supported by the project POCTI/CTA/39181/2001