The efficient recycling of biowaste makes an enormous contribution to the bioeconomy and climate protection. Researchers in the Department of Waste Management and Emissions headed up by Prof. Dr.-Ing. Martin Kranert at the Institute for Sanitary Engineering, Water Quality and Solid Waste Management (ISWA) at the University of Stuttgart, are exploring the optimisation potential of biowaste recovery.
The German Closed Substance Cycle Waste Management Act (Kreislaufwirtschaftsgesetz) made separate collection of organic waste mandatory from 1st January 2015. Organic waste is collected in separate biowaste containers (green bins) and falls under the responsibility of public waste management authorities (administrative districts and cities). Recycling is a key component of the bioeconomy, as the recycling and further processing of raw materials not only saves resources, but also reduces greenhouse gas emissions.
Biowaste can be divided into different types of organic waste – organic food and kitchen waste that is collected in green bins, garden waste from private households, green cuttings from privately owned parks and gardens, shredded materials and green cuttings from municipal green space management activities.
Biowaste can be recycled in various ways, for example, in biomass power plants (energy utilisation) as well as in composting plants (material recycling, compost production). Biowaste can also be recycled in fermentation plants where the gas produced is used to generate energy and heat and the composted fermentation products are used as humus. “While green cuttings are usually transferred into composting plants after wood residues have been removed, organic food and kitchen waste is increasingly being used in fermentation plants with downstream composting plants,” says Kranert explaining that this development is the result of increasing fermentation plant capacities over the past few years.
Biogas from fermentation plants can be used in combined heat and power plants to produce electricity and heat. “This area has huge optimisation potential,” says Kranert explaining that biogas is currently mainly used for producing electricity. The heat produced could be used for heating greenhouses, schools and swimming pools. Moreover, micro gas grids could be installed to supply consumers further away from the plants and facilitate the pooling of several biogas plants. “Another interesting option is to further process the biogas and feed it into the gas grid. I am sure that excellent efficiency levels can be achieved and relatively high quantities of regenerative energy fed into the gas grid,” says Kranert. “This is already being done, but there is room for expansion.”
In addition to saving resources, the fermentation of biowaste has further advantages. Martin Kranert’s team is exploring flexibilisation measures that could be used to cover peak power requirements. Solar and wind energy are irregular sources of power, in contrast to biological waste that accumulates on a regular basis. Kranert explains that it would be economically advantageous to pursue measures that provide balancing energy through biogas plants instead of using them for generating baseload power, i.e. power stations that consistently generate the electrical energy required to cover minimum demand. “Plants like these could be controlled by applying targeted feeding management or by temporarily storing the biogas produced in a way that enables a demand-oriented energy production with a factor of 1.5 to 2,” says Kanert. An optimisation tool could then be used to develop flexible plans for biogas plants, enabling the demand-oriented production of biogas and also creating an economic advantage for the operators of biogas plants.
The different areas where biowaste can be used inevitably leads to competition between material flows. Herbaceous green cuttings and organic waste can be used in fermentation as well as composting plants. Competition in the use of biowaste also occurs when administrative districts operate their own waste management plants or when contractual quotas have to be met. “One example is mechanical-biological residual waste treatment plants. These plants also produce biogas; if green bins were no longer part of the material flow, the plants would become uneconomical,” explains Kranert. This is the reason why some – albeit only a handful according to Kranert - administrative districts have not yet established an area-wide biowaste collection system. The lack of biowaste treatment capacities is another reason why not all biowaste is collected separately from other types of waste. “The majority of administrative districts and cities such as Stuttgart that currently only collect part of the area’s biowaste have plans to expand their capacities.”
As the researchers have not yet collected all the information they need on biowaste types and quantity in Baden-Württemberg, they are now concentrating on the analysis of residual and waste biomass material flows in order to determine the potential of increasing the separate collection of biowaste quantities and efficiency of recycling biowaste. “We are also planning to determine the amount of biowaste that does not end up in green bins, compost, residual waste, shredding plants or that is disposed of illegally along forest edges,” says Lea Böhme, engineer and project coordinator. Böhme wants to identify biowaste collection optimisation potential along possible disposal routes. The researchers hope to be able to answer the following questions: how much biowaste is still thrown into residual waste bins, and how much and what kind of real biowaste is contained in the organic waste disposed of into green bins. The reason why the researchers are looking for answers to these questions is because the statistics usually only cover the quantities of collected biowaste, without stating how much paper or plastics are disposed of green bins.
Böhme is currently conducting analyses of waste in residual waste bins and green bins. "We take samples of residual waste and green bins at different seasons and in different areas. We hope that this will provide us with suitable data for carrying out spatial analyses and calculating the amount of household biowaste for Baden-Württemberg as a whole,” says Böhme. The researcher’s goal is to estimate the quantities of biowaste that still end up in residual waste bins. “We have found that between 17 and 35 percent of kitchen waste ends up in the residual waste bin,” says Böhme.
“Baden-Württemberg is already doing quite well,” says Kranert. However, as not all administrative districts have introduced area-wide collection and sorting of biowaste, they differ considerably in the biowaste quantities collected. In 2015, public waste management authorities collected 45 kg of biowaste per inhabitant per year in Baden-Württemberg. As far as the figures for Germany as a whole are concerned, the German Federal Statistics Office calculated that 57 kg/capita biowaste was collected in 2014. Baden-Württemberg has therefore set itself the goal of increasing the amount of biowaste collected by around one third, i.e. to 60 kg per inhabitant per year. Kranert and Böhme believe that in addition to establishing separate biowaste collection systems in all administrative districts, the waste management authorities could provide clearer and more uniform information on which type of waste needs to be disposed of in green bins and also increase collection frequency in the summer. “Many local authorities do not want meat to be thrown into green bins. But this shouldn't be a problem; in fact, this is where meat scraps belong,” conclude the experts.
Verpflichtende Umsetzung der Getrenntsammlung von Bioabfällen (Mandatory implementation of separate biowaste collection), TEXTE 84/2014
Environmental Research Plan of the Federal Ministry for the Environment, Nature Conservation, Building and Nuclear Safety
Federal Statistics Office press release no. 467 of 16th December 2015: 462 kg household waste per capita per year in 2014
Waste balance 2015 – German Ministry of the Environment
Rebellion gegen die Biotonne (Rebellion against Green Bins), Stuttgarter Zeitung, 1st June 2016