Proceedings

Composting: From Effectivity to Energy Efficiency

Bauerschlag, N., Kaufield, S., Pretz, Th., RWTH Aachen University, Germany

(free)

Abstract:

Over the past decade 3.8 million tonnes of biowaste have been collected annually in curb side collection systems in Germany. Most of these are treated in composting plants with an artificial aeration system which consumes up to 60 % of the overall energy. Mostly set up in the 1990s, such composting plants have since depreciated in value. Since few investigations have analyzed the energy efficiency of these plants, the aim of the research project is to improve the energy efficiency by implementing novel processing techniques. Here two screw extruders were incorporated into an existing windrow composting plant used to press water and biowaste before composting. Using this new processing technique the input material is conditioned to require less artificial aeration. In addition the organically enriched press water could be used for digestion. This novel processing technique may significantly reduce energy consumption of composting plants

Keywords: biowaste, composting, energy efficiency, digestion, biogas

Introduction

In Germany the treatment of biowaste has reached high quality standards. Whether it is accomplished by composting or digestion, biowaste is treated to fulfil sustainable recycling.

The separate collection system for biowaste has been introduced in about 80 per cent of German households. (Henssen 2009) Although there are slight differences in the nationwide access concerning the curb side collection system and the amount per capita of biowaste, the annual amount has stagnated at about 3.8 million tonnes over the past three years. (Statistisches Bundesamt 2010)

Biowaste is a very complex type of waste, not only because of the monthly changes relating to amount, stucture and quality, but also in concerns of its treatment. Biowastes which are seperatly collected in late spring to early autumn are richer in stucture and lower in moisture. These can be easily treated in composting plants. Yet, biowaste which is collected from late fall to early spring is characterised by more moist and low stuctured material. Here, a digestion plant would allegorise the adequate treatment. However, a composting plant also has to cope with this task making this kind of waste treatment a very sensitive one throughout the year.

Despite the fact that biowaste is better for one of the mentioned treatments, composting plants represent the more technically mature treatment which is sufficiently time-tested since the early nineties of the last century. Thus, in Germany there are more composting plants then digestion plants.

Nevertheless, the number of newly built composting plants has dropped over the last years while the number of annually built digestion plants has increased. One of the main reasons for this ongoing can be found in the particular energy balance of each treatment. The composting process consumes a large amount of energy. In comparison to this, the treatment of digestion produces methane-enriched biogas which can be used in a combined heat and power plant (CHP) upgrading its overall energy balance.

Due to the above facts and because over half of all composting plants in Germany have been in operation for more than ten years and have therefore depreciated in value, further investments can be made for novel techniques. By this the treatment of composting can experience a further upgrade of its prevailing effectivity and, on top of that, could lead to an efficiency concerning the overall energy consumption.

Within the current joint research project “Energieeffizinete Bioabfallverwertung (EnBV)” (FKZ 0327846 A&B), which is funded by the Federal Ministry of Economics and Technology of Germany, the implementation of two screw extruders into an operating windrow composting plant is evaluated and examined under the aspect of energetic efficiency. The main goal is to lower the amount of energy which is needed to compost one metric ton of biowaste in the overall composting process. Before the actual composting process, the biowaste is irrigated and compressed straight afterwards increasing the bulk density and decreasing the amount of degradable organic matter. Firstly, this leads to a higher throughput and secondly the press water can be optionally used in a digestion process.

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