Biogas an all-rounder

An Economic Cycle: The Fermentation of Biogenic Waste

“Waste is a raw material in the wrong place”. This is the motto of the German waste management industry. Around 62 percent of German municipal waste is recycled. In the case of organic and green waste, the recycling quota is even 100 percent. Most of this waste is composted, and the resulting product replaces peat from moors as an excellent fertiliser and soil improver. However, moist, biogenic waste also contains a great deal of energy, which can be exploited in the form of biogas produced by anaerobic digestion.

Complex organic waste fractions

The separate collection of biogenic waste started around 25 years ago in Germany. Since then, the utilisation rate of organic waste has increased dramatically: Today, 80 percent of the population participate in a collection system for household organic waste. Around four million metric tons of old bread, leftovers, kitchen scraps, rotten and inedible fruit and vegetables are collected through this scheme. In addition, 4.4 million metric tons of green waste including garden and park waste and materials from landscape conservation are collected at decentralised collection points. Added to this varied range of waste types are leftovers from large canteens, hotels and hospitals, out-of-date food from supermarkets and various remnants from food production, such as slaughterhouse waste, with an estimated volume of three to four million metric tons.

The potential of organic domestic waste and green waste is the farthest from being exhausted. These fractions still largely go into composting plants. The German Federal Association for Compost Quality records around 980 such plants, but only 100 fermentation plants for biogenic and green waste. In 2009, these biogas plants processed a mere 17 percent of the collected organic waste, with the rest not used for energy production but simply composted.

Combination of fermentation and composting

The Renewable Energy Sources Act is aimed at increasing this rate. Fermentation is not in competition with composting, but rather is a first step in the process. More and more frequently, biogas plants are being integrated into existing composting plants, which should have a minimum capacity of 10,000 metric tons per year. The composting plants no longer compost the organic waste, but the fermentation residue, which does not lead to a tangible change in the quality of the resulting compost. The biogas could be used for the generation of electricity or heat, or as a fuel, replacing fossil energy sources. According to a study carried out by the IFEU Institute, the CO₂ savings of biogas fermentation as opposed to mere composting can be up to 160 kilogrammes per metric ton of organic waste.

In addition to wet fermentation, dry fermentation technologies are increasingly used, which in turn distinguish between continuous flow and batching processes. In the latter, a wheeled loader charges pits or garages with the substrate, which is then sprayed with process water (percolate). The biogas is then generated in the pits and in an additional percolate fermenter respectively. Horizontal plug-flow digesters are the most widespread for continuous flow processes. Here, waste material is pulverised, diluted to a paste and pressed through the pit using an axial shaft. Percolation plants can handle substrate that has not been pre-treated. In technical terms, they are more simply constructed, meaning that operation requires scant automation, but that they tend to produce lower quantities of gas over comparable residence times.

Irrespective of the process used, the average gas yield per metric ton of organic waste is 100 standard cubic metres. And this is where another advantage comes into play: 70 metric tons of organic waste supply the same amount of biogas as one hectare of maize silage. Organic waste fermentation thus avoids creating competition for land with food and animal feed production. However, this is dependent on the separate collection of organic waste. If this waste must be treated together with residual waste, the only option is incineration and subsequent ash disposal, which removes biomass – and thus important nutrients – from the biosphere, preventing material cycling.