Energy and environmental technology Environment

Posted May 1997

FAO Sustainable Rural Environment and Energy
Working Group on Environmental Aspects of Anaerobic Treatments
Workshop on Anaerobic Conversion for Environmental Protection,
Sanitation and Re-use of Residues
Gent, Belgium
24-27 March, 1997

Abstract: Anaerobic digestion of agroindustrial byproducts and wastes

by R. Braun and R. Steffen
Department of Environmental Biotechnology
Institute for Agobiotechnology
University of Agricultural Sciences
A-3430 Tullin, Konrad Lorenzstrasse, Austria


See also: "Workshop on Anaerobic Conversion for Environmental Protection, Sanitation and Re-use of Residues: Conclusions and recommendations"


CONVENTIONAL RAW MATERIAL processing typically concentrates on high yields of high quality products. Until recently, byproducts and wastes were not considered harmful, thus being sent to landfills, soils and water courses in huge amounts. Although improvements in waste treatment reduced environmental drawbacks, end-of-the-pipe technologies worldwide are still wasting huge amounts of raw materials and energy.

Increasing waste treatment and disposal costs are now gradually forcing waste abatement and byproduct recovery. Nevertheless, the ultimate goal of waste-free (clean) production still seems far away from realization. Although numerous waste and byproduct recovery processes have been introduced, anaerobic digestion has a unique and integrative potential, simultaneously acting as a waste treatment and recovery process. For this reason, numerous applications have been introduced covering all areas of waste recovery in agriculture, industry and communities. Introduction of stricter legislation in landfill standards in the near future will generate additional huge amounts of agro-based waste and byproducts for alternative treatment.

Recent estimations of waste production in the 15 European Union countries resulted in about 1.23 billion tons of organic waste per year. About 90% of the total organic waste stream is derived from agriculture. First estimations reported about 420 agricultural biogas plants in Europe (Hauer, 1993). Latest reports estimate roughly 750 agricultural biogas plants throughout Europe (Holm-Nielsen and Al Seadi, 1997).

Besides manure, many biogas plants are fed with various additional concentrates such as municipal biowaste or organic industrial slops. Cofermentation increasingly is applied also in sewage sludge digesters. Sewage sludge is responsible for less than 2% of the total annual amount of organic waste resulting. Although being state-of-the-art, with numerous applications, precise numbers of municipal sewage sludge digesters are hardly available.

High strength industrial waste waters from the pulp and paper industry, potato and starch processing, sugary refining, soft drink processing, the fermentation industry, etc. usually are pre-treated by anaerobic digestion. Rough estimates count several hundred existing industrial treatment plants throughout Europe. These plants are also used for codigestion of solid agro-based wastes such as press cakes, fibres, husks, peelings and others. In sum, waste waters from industry contribute less than 3% to the overall organic waste stream.

Agro-based organic municipal biowaste (kitchen waste, residual paper, garden waste) counts for less than 4% of the overall waste stream collected. Due to more stringent landfill regulations, source-separated collection of municipal organic biowaste is being gradually introduced in many countries. While composting is widely applied for high dry matter containing wastes, anaerobic digestion turned out to be a good alternative for wet organic wastes. About 30 plants digesting municipal wastes are currently ion operation throughout Europe.

Compared to the huge amount of wastes resulting, obviously only a minor part currently is being treated. Most of the manure collected in agriculture directly is used as fertilizer, often causing severe damage to water courses and agricultural soil. Agroindustrial and agro-based municipal biowastes are still dumped on landfill sites, causing hazardous leachate generation and uncontrolled gaseous emissions.

The reason for the slow introduction of treatment and recycling technologies are manifold. While environmental legislation is gradually improving, economic conditions are still prohibitive. Besides legal and socio-economic conditions, there are still technical reasons for the low acceptance of anaerobic digestion technology. Undefined, respectively contaminated wastes make agricultural applications of the resulting end-products impossible. Process economy often suffers from low local heat demand, respectively inefficient energy exploitation. Finally, changing feedstocks often cause unstable digestion and process failure. For these reasons, anaerobic digestion in many cases is still questioned.

Animal manure, as well as sewage sludge, often is used for codigestion of agroindustrial wastes. Several full-scale installations in Denmark, Sweden, Italy, Germany and Austria are in operation (Braun, 1997). Typically, 5% to 20% of various cosubstrates are fed to codigestion processes.

At the Department of Environmental Biotechnology, systematic pilot plant investigations with slaughterhouse wastes and pharmaceutical wastes based on sewage sludge as well as manure digestion have been undertaken. Similar to sewage sludge, manure-based codigestion of pharmaceutical wastes proved to be advantageously applicable during pilot investigations.

Further efforts have been undertaken to integrate anaerobic digestion into the rendering plant technology. In contrast to the conventional procedure, the liquefied and sterilized residuals are not dried to animal meal but will be fed to anaerobic digestion. Since a sterilized feedstock is used for digestion, problems of BSE should not occur when using the residual anaerobic sludge as a fertilizer on farmland.

The last example to be presented deals with the implementation of biomass (grass, corn, sugar beet) and agro-based byproducts (brewery waste, potato sap, whey, glycerol, sugar wastes) into combined lactic acid and methane fermentation. Applying membrane bioreactors for lactic acid fermentation on pure substrates (whey, extracts), as well as mixed culture fermentations (ensilage) on biomass, the products could be recovered by electrodialysis or supercritical fluid extraction at high yields. The remaining residue was fed to anaerobic digestion.

Anaerobic digestion can contribute substantial solutions both in energy generation as well as in environmentally sound byproduct recovery and waste treatment. Due to its integrative potential, in the long run it could become a key technology acting as a precursor for sustainable development.

Having in mind the high potential of anaerobic digestion comparatively few applications can be found in practice. Although being state-of-the-art in sewage sludge and anaerobic industrial waste water treatment, only a few applications of anaerobic digestion of organic waste reclamation can be found in agriculture.


References

Braun, R. (1997): "Anaerobtechnologie für die mechanisch-biologische Vorbehandlung von Restmüll und Klarschlamm". Studie im Auftrag des österreichischen Bundesministeriums für Umwelt, Jugend und Familie, Stubenbastei 5, A-1010, in Druck

Hauer, I. (1993): "Biogas-, Klargas- und Deponiengaslagen im Praxisbetrieb". ÖKL Landtechnische Schriftenreihe Nr 192. Österr. Kurattorium für Landtechnik, A-1041 Wien

Holm-Nielsen, J.B. and Al Seadi, T. (1997): "The future of biogas in Europe and how to get started". Final report Phase II EU Altener Programme Anex 3/3 Contract Nr. 4.1030/D/95-006. Biomass Institute SUC Niels Bohr Vej 9, DK-6700 Esbjerg



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