1. ANAEROBIC CO-DIGESTION OF ORGANIC FRACTION OF MUNICIPAL SOLID WASTE AND WASTE ACTIVATED SLUDGE AT DIFFERENT RATIOS A. FLOR, L. ARROJA, I. CAPELA Environment and Planning Department, Aveiro University, Campus Universitário 3810-193 Aveiro, Portugal

2.  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

3.  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

4.  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

5.  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

6.  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

7.  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/WAS 0%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.

8.  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

9.  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. 100% OFMSW 0% 75% 50% 1. st stage 2. nd stage 100% OFMSW 75% OFMSW 50% OFMSW 0% OFMSW 10% TS

10.  The best result of all experiments were given by the 75% OFMSW at 2% TS with a production of about 350 m 3 (CH 4 )/Ton(TVS) added at the end of 40 days 50% OFMSW 75% OFMSW 2% TS

11.  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

12.  SO… The best result was obtained with 75% OFMSW (25% WAS) essays, at 2% TS, with : Specific methane yield of about 350 m 3 (CH 4 )/Ton (TVS) added 70% TVS removal 80% COD removal No inhibitions without any VFA’s accumulation

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

14.  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 m 3 (CH 4 )/Ton(TVS) added, 70% TVS removal and about 80% COD removal, without VFA’s accumulation.

15.  This work, was done under a PhD grant from FCT- Portugal SFRH/BD/1066/2000 and  supported by the project POCTI/CTA/39181/2001