Yves Membrez, EREP SA Aclens (Main Applicant); Dr Jean-Paul Schwitzguébel, Bernard Dubois
Swiss Federal Office of Energy
July 1997 – July 1999
Dominique Descloux, Provimi-Kliba SA Cossonay; Dr. Arthur Wellinger, NOVA Energie GmbH Ettenhausen.
Slaughterhouses, meat-processing industries and large catering services reject huge amounts of organic wastes and effluents. A major proportion of them are amenable to controlled biological treatments, yielding potentially useful and added-value products like organic acids, fertilizers, animal feed, or energy (biogas). If efficient anaerobic bioreactors are used for the treatment of slaughterhouse wastewater, no biological process is available to treat solid wastes, which still contain large quantities of water and organic matter. The aim of the project was to test at laboratory and pilot scale a process based on the anaerobic digestion of the liquid fraction of wastes, whereas the solid residue could be used as animal feed.
The first step in the treatment was to sterilize fresh wastes and to squeeze them in an endless screw filter press. The solid residue, rich in protein and poor in fat, was then conditioned and successfully tested as feed for fattening pigs instead of usual protein ration.
The liquid phase, heavily loaded in suspended organic matter and in fat, was pumped into an anaerobic hybrid bioreactor, operated in upflow mode. This type of bioreactor takes advantage of the properties of a conventional digester in its lower part with those of an anaerobic filter in the upper part, avoids their respective drawbacks, and is appropriate for effluents containing suspended solids. Two bioreactors were thus operated continuously for 9 months at laboratory-scale and one for 5 months at pilot scale on an industrial site. The effects of organic loading rate, hydraulic retention time, shock loads, temperature and pH on reduction of total chemical oxygen demand, production of biogas and methane yield were investigated: 85% of the total chemical oxygen demand was removed at a loading rate higher than 12 kg m-1 d-1 and a short hydraulic retention time of 2 days only. The technical and economical feasibility of an industrial plant treating more than 6’000 t wastes per year was estimated: biogas could be used to provide 15% of the electrical requirement and reduce by 40% fuel oil consumption by the poultry slaughterhouse and meat-processing industry.