(organic waste composting at SAB, Austria)

Flashback: Compost is a substance made of partly decayed organic material; when free of contaminants, it is an excellent to soil fertilizer as it increases its humus content. Its main components originate from plant materials (e.g., grass clippings, leaves, peat, etc.), manure, and soil. In comparison to other fertilizers of a particular organic or inorganic composition, that are added to the soil to replace or increase its nutrient contents, compost does already reveal a balanced "nutrients" mixture. Organic fertilizers include animal and green manure, fish and bone meal, guano (seabird excrement, that have a very narrow C:N-ratio). Soil micro-organisms further decompose this material so that "liberated" nutrients can readily be taken up by the hair roots of a plant. Most fertilizers contain a class of ingredients termed "major nutrients", these include nitrogen (N), phosphorus (P), and potassium (K) in proportions required by major crops. Properly used, fertilizers increase crop yields and can compensate for the ongoing decrease in farmland (be it for land, urbanisation, or over-population). In that regard, compost does represent an ideal soil additive. In this aspect, composting of un-contaminated houshold (kitschen) material should be favoured over inorganic fertilization. The application of inorganic fertilizers tend to suppress in-soil nitrogen-fixing bacteria and trigger a vicious cycle in which more and more fertilizer will be needed in the future. Furthermore, excess nitrogen fertilizer application is easily wash off from farms into lakes and streams where they promote algal blooms, overgrowth of aquatic fauna and flora and ultimately cause eutrophication.

Modern food industry with its vast spectrum of pre-processed foodstuffs, and in combination with the packaging industry, contribute substantially to the daily pile of rubbish. Besides being poor in nutritional value, processed and packaged food maintains an unhealthy pool of consumer that sooner or later pose a heavy burden onto a nations health care system. Switching back to fresh fruits and vegetables and simulateneously decreasing the currently existing overload of meat-consuming habits, not only boosts people immune systems, make them less prone to infections, and ultimately improving the quality of life, but also represents a serious respect for nature while promoting ecological thinking in daily life. In this regard, what can be better than reusing the organic leftover's generated during and after a day of kitchen activity? Returning this organic fraction into the production cycle is just that what our ancestors did before the onset of our industrial revolution. In addition, separating organic matter from the other "rubbish" fractions not only reduces the total amount of solid waste, it also lowers the total organic carbon content (TOC) that would other wise enter a landfill or an incinerator.
Since most of us prefer an urban life-style, composting one's own kitchen material is limited to restricted number of households. Therefore, composting in an urbanised setting can only be achieved by providing some kind of infrastructure.

Collection: In Salzburg (AUT) organic waste collected at household level. Collection at the site of consumption guarantees a quality product of supreme purity. This purity though can only be achieved, when every individual contributing to the system act in a responsible manner by avoiding deliberate contamination. The containers filled with organic debris have to be emptied at regular intervals to avoid any mass outbreaks of "airborne" (mediated by insects) soil forming organisms - especially during the hot summer months. To slow down any fermentation processes taking place within the container, it is recommended to lower the level of humidity by wrapping a days' portion of organic material in newspaper before disposing it into the bin.

Collection of biodegradables

Preselection: Before the organic fraction is fed into a shredder, the material is checked manually for any major disturbing constituents as well as major contaminants.
It is probably not avoidable that a certain amount of inorganic matter is present, although one should keep in mind that it is up to the individual household to raise or maintain a preset quality of biodegradable matter. After the shredding process, the organic matter can be processed in various ways (either aerobically or an-aerobically).

Manual screening

Processing: A common way to decompose organic matter is achieved by exposing it to elevated temperatures. Bacteria mediated anaerobic fermentation - contrary to aerobic fermentation facilitate decomposition. Aerobic fermentation requires constant reshuffling and aeration of the material involved. Anaerobic fermentation is done in two steps and involves a bioreactor and an aerobic tunnel composter. The biogenic material fed into the bioreactor stays there for about three weeks at temperatures well above 55C. It is suggested that anaerobic fermentation is best used in very humid and cool climatic zones where rain and low outside temperatures could affect this process (such as in Salzburg, Austria). Under these circumstances, the entire process is executed in fermentation silos under controlled conditions where parameters can be accurately set.
A common byproduct in anaerobic fermentation is biogas that is utilized to cover the thermo-electrical requirements of the compost processing plant.
Þ For gas processing, see also the landfil-section.


An aerobic tunnel composting process follows the anaerobic step. For about 36 hours, the rotting material is thoroughly mixed by a whirling current of air. This treatment makes sure that any oxygen-requiring residual components are converted into inorganic matter. Finally, for another 3 weeks, the semi-composted material is stored for the post-composting procedure. Periodic turnover guarantees that all of the compost will undergo the desired final transitions to make good humus.

Post composting

After that 3 week period, the organic material y reveals its typically brownish-black colour indicating that it has decomposed substantially. At this stage the material has reached humus status and can be classified as such; i.e. "humus is characterized as organic matter that decayed to a relatively stable, amorphous state. It is an important component of fertile soil, it affects physical properties such as soil structure, water retention, and increases resistance to erosion". Now the humus can be safely used for re-greening denuded areas, in agriculture, or to enrich existing soils.

In more arid climatic zones (as in the east of Austria, Vienna), aerobic fermentation can take place under the open sky. Even though outside temperatures may fall below zero, the core temperature of a pile may well stay at 70C. This temperature is sufficient to kill any bacterial and protozoic activity that may be of concern harm. To obtain a more or less homogenous end product, aerobic fermentation requires wide spaces and regular turnover rates. Under special climatic conditions fouling smells in uncovered aereobic fermentation plants do occur and can be the cause of complaints of nearby communities, thus limiting aerobic fermentation to selected sites only.

Aerobic fermentation

References: Abfallsentsorgungs-Konzept der SBGer Abfallbeseitigung (1998); A-5101 Bergheim - AT
Leitbild der SBGer Abfallbeseitigung; Leitbild (1997); A-5101 Bergheim - AT
For additional information visit the SAB web site or any of the others

Intro / Paper / Glass / Plastic / Metal / Compost / Toxic / Residual / Sewage / Landfill