Circular economy of urban carbon flows through innovative bio-waste recovery pathways.

Urban areas are both a problem area and a potential driver of the transformation to climate-friendly societies. In all urban sectors, the closing of carbon cycles, taking into account biogenic carbon from renewable raw materials, will play a central role in the decarbonization of urban processes to the greatest extent possible in the future.

The CirCles graduate program is investigating the potential of innovative recycling paths for urban biowaste. The inter- and transdisciplinary project aims to close urban carbon cycles by providing biowaste free of foreign matter, processing it in a targeted manner and recycling it sustainably. CirCles is divided into four subprojects, each of which is being worked on by one of the participating disciplines.

The project is carried out by the departments of Urban Water Engineering, Sustainable Marketing, Resource Management and Solid Waste Engineering as well as Grassland Science and Renewable Plant Resources of the University of Kassel and is carried out with the participation of Stadtreiniger Kassel, Abfallentsorgung Kreis Kassel, KASSELWASSER and Umwelt- und Gartenamt Kassel.

Project background

Biowaste includes food and kitchen waste, garden and park waste, waste from landscape conservation and waste that is similar in nature and composition to the aforementioned waste (cf. §3 of the Closed Substance Cycle Waste Management Act). In 2018, around 15 million tons of biowaste were treated in composting and fermentation plants throughout Germany, producing compost or biogas for energy recovery and digestate. In Kassel, about 32,000 tons of biowaste were generated in the same reference year (11,250 tons of biowaste, 11,000 tons of green waste and about 10,000 tons as biowaste fraction in residual waste).

Downstream recycling of biowaste depends decisively on its composition. Separate collection with good quality is a prerequisite for high-quality recycling. Foreign materials such as glass, metals and plastics can pose a problem for recycling and are the subject of legal regulations regarding both the treatment of the waste and the recycling products. In addition to these qualitative requirements for the recycling of biowaste, there is an urgent need for research and action in the area of waste prevention, since today an average household in Germany produces about 75 kg per capita of food waste each year, half of which could be avoided. Against this background, food waste from private households in Germany is to be reduced by half by 2030, thereby saving six million metric tons ofCO2 equivalents in greenhouse gas emissions.

Currently, activated carbons based on hard coal or coconut are mainly used for the production of activated carbons, which release up to 18 tCO2,eq per ton.

Alternatively, biowaste can also be used to produce biobased activated carbons by pyrolysis. However, due to the relatively high ash content of up to 20% of the dry mass, as well as high chlorine and sulfur contents, pyrolytic utilization of biowaste requires further processing in order to ensure both a high quality of the activated carbons (low ash content) and to prevent corrosion by chlorine and sulfur in the pyrolysis. The aim of further treatment of biowaste is therefore to reduce the mineral or ash content, as well as the chlorine and sulfur content, and thus to increase the carbon content. On the one hand, further processing achieves a significantly better quality of the biowaste for pyrolysis and, at the same time, a homogenization of the chemical-physical composition is achieved. This, in turn, is a prerequisite for the urban production of high-grade, climate-friendly activated carbon of constant quality against the background of closing urban material cycles. According to initial estimates, up to 2,300 t of biobased activated carbon can be produced per year from the 32,000 t of biowaste available in Kassel alone.

Organic trace substances from anthropogenic agents in industrial chemicals, household chemicals, pharmaceuticals or biocides occur ubiquitously in the environment in very low concentrations (ng/L to µg/L). Discharges via the municipal wastewater system are the dominant input pathway to water bodies for many of these substances. Therefore, in addition to source-oriented measures, advanced wastewater treatment is increasingly using separate elimination steps to reduce trace substance inputs from point sources such as wastewater treatment plants, primarily with downstream processes such as adsorption on activated carbon.

However, the targeted removal of organic trace substances from wastewater generates significant additional greenhouse gas emissions due to the necessary provision of auxiliary substances. In particular, the amount of activated carbon dosage required has a significant impact on theCO2 footprint of trace substance removal from wastewater. Against this background, only a sustainable substitution of the fossil activated carbons used can reduce the greenhouse potential of advanced wastewater treatment in the long term.

Market recovery options for biowaste have a substantial impact on the volumes and management of upstream material flows. Pyrolysis-based products have so far mainly been recorded as charcoal for private use and the catering industry. Other high-grade recycling processes, such as biobased activated carbon as required by Section 8 of the Closed Substance Cycle Waste Management Act (KrWG) and specified in the draft amendment to the Biowaste Ordinance, have not yet been recorded to any relevant extent despite the existing sustainability potential.

The acceptance of innovative recycling options that go beyond the provision of a transportable fuel with defined and controlled properties is characterized by the energy balance as well as the presumption of quality. A barrier to alternative utilization as biobased activated carbon can be contamination with pollutants that are present in the produced activated carbon either with the biomass, e.g., due to inadequate sorting and misdirection in material streams from household collection, or due to reactions in the carbonization process, especially in process adjustments resulting from heterogeneous inputs. Although the pollutants, especially heavy metals, are predominantly incorporated into the carbon structure of the pyrolysis coke, the pollutant content of the activated carbon is a relevant determinant of the acceptance of use. Only a stringent establishment and communicative enforcement of quality labels can help to increase user acceptance and product confidence through certified compliance with relevant maximum limits of pollutant contamination.

Project content

The explicit objective of subproject 1 is to measure relevant behavioral psychological determinants, such as self-efficacy and fatalism, for the targeted preparation of intervention among households to enable quality through biowaste that is as free of foreign matter as possible. In line with SDG 12.5, households should be motivated to carry out the collection and provision of solid biowaste in a way that enables efficient and xenobiotic-free recycling using pyrolytic processes. Beyond the technical determinants, such as the design of collection containers and collection cycles, the psychological factors of collection and sorting behavior are to be systematically recorded and quantified in terms of their influence. Based on this, it will be examined how knowledge about the utilization of activated carbon obtained from biowaste in wastewater treatment and possibly in urban landscaping can contribute to a substantial change in behavior.

Since activated carbon in urban gardening and landscaping offers the option ofCO2 sequestration in addition to organic soil improvement, increased hydrogen storage capacity, advantages in market utilization as a substitute for conventional phosphate fertilizer will also be examined.

Within the scope of subproject 2, the current status of biowaste management is first analyzed using the example of the Kassel region (city and surrounding area) on the basis of material and material flow analyses. The quantity, composition, and collection and recycling paths of biowaste will be recorded and presented in detail. Based on this, on the one hand theoretically mobilizable and on the other hand technically usable potentials for different recycling paths, especially in activated carbon products, are determined. To this end, in addition to existing waste analysis data, targeted surveys will be carried out for selected settlement and collection structures and possible measures for avoiding or better collecting biowaste (e.g. targeted education and public relations work, collection options at household level, incentives and punitive measures) will be evaluated. Critical pollutants and interfering materials are identified for the collected biowaste with respect to different recycling processes, and appropriate collection and treatment processes are designed to provide suitable material fractions for further recycling. Selected treatment and packaging steps are implemented on a pilot plant scale and evaluated with regard to the achievable material yields and qualities for further recycling, especially in activated carbon.

The developed management concepts and recycling chains for biowaste in the Kassel region will be balanced within the framework of scenario analyses using material and material flow models and backed up with inventory data (energy and material consumption, emissions). On this basis, life cycle assessments according to ISO 14040 will be prepared for different management models and recycling chains, and critical factors will be identified with regard to the ecological effects of biowaste management in urban areas. Subsequently, concepts will be designed that enable the ecologically optimal utilization of biowaste in high-quality products, e.g. activated carbon for wastewater treatment, and within the framework of regional value chains.

Subproject 3 involves the further processing of sorted biowaste into activated carbon. Based on comprehensive preliminary work, the first step will be to adapt and optimize the proven IFBB process for optimal treatment of the biowaste sorted in subproject 2.

The biomethane potential of the energy-rich liquid phase produced during hydrothermal conditioning and subsequent mechanical dewatering will be determined in fermentation tests in accordance with VDI4630 to determine the contribution made by the process to sustainable biogas and energy production in biogas plants or digestion towers.

The solid phase serves as feedstock for the production of plant and activated carbons in a continuously operated test reactor. In close coordination with subproject 4, the targeted development of adapted pyrolysis and activation profiles for the production of biogenic activated carbons by means of pyrolysis and steam activation is carried out by defined variation of relevant process parameters (e.g. pyrolysis and activation temperature, residence time, steam quantity for activation, separate pyrolysis with subsequent activation, pyrolysis with simultaneous activation). The aim here is to produce sustainable and high-quality activated carbons with consistent quality. The synthesis gas produced in the pyrolysis and activation process will be characterized and evaluated with respect to its potential for the production of green hydrogen

In addition to a comprehensive chemical-physical characterization of the products, the process chain will be balanced in terms of materials, energy and greenhouse gas emissions, and the results of subproject 2 will be made available for life cycle assessment. In addition, the suitability of biowaste for the production of vegetable carbon for potential environmental applications is evaluated.

Subproject 4 investigates the use of biobased activated carbons for the targeted removal of organic trace substances from wastewater matrices. The experimental part initially comprises batch and shaking tests on different biobased activated carbons from subproject 3 with the aim of characterizing the adsorption processes for selected trace substances in terms of kinetics and equilibrium state with simultaneous quantification of the organic background matrix. The laboratory experiments are performed in direct feedback with subproject 3 in the context of the selected process parameters of the pyrolysis and activation processes.

Another research aspect of subproject 4 concerns the simulation-supported description of the adsorption processes on biobased activated carbons in wastewater matrices for the substitution of fossil activated carbons. The results of the laboratory experiments are used for the calibration and parameterization of the mathematical model hypotheses. Extensive preliminary work on conventional activated carbons will be adapted and further developed for the new application and implemented in the simulation system used.

As a result, subproject 4 answers relevant questions about the real sustainability potential of the intended substitution of fossil activated carbons by activated carbons from biowaste for the targeted removal of organic trace substances from wastewater matrices.

Presentation of sustainability potentials

Each of the four subprojects refers to at least two Sustainable Development Goals (SDGs) of the United Nations (UN). The real sustainability potentials will be researched over a period of 3.5 years in spatially limited test areas in the city of Kassel.

Closing material cycles at the regional level by avoiding waste, handling biogenic materials efficiently, and collecting biowaste free of extraneous materials and recycling it in a high-quality manner contributes to sustainable consumption and production patterns (SDG 12, especially 12.3 and 12.5), reduces greenhouse gas emissions (SDG 13, especially 13.2), and strengthens regional value chains by using waste as secondary raw materials (SDG 11, especially 11.6). Further processing of biowaste using IFBB processes produces an energy-rich pressed juice that can be used for regional renewable energy production (SDG 7, in particular 7.2). Further processing of the produced press cake into high-quality activated carbon for the removal of trace substances from municipal wastewater directly promotes the development of sustainable industry and infrastructure (SDG 9, in particular 9.4) and thus also the establishment of sustainable production patterns (SDG 12, in particular 12.2, 12.4 and 12.5). Substituting activated carbons from fossil coals can reduce greenhouse gas emissions (SDG 13, especially 13.2) and promote the establishment of regional value chains (SDG 11, especially 11.6 and 11.a). Reducing inputs of trace substances contributes directly to the four SDG sub-goals "Improve water quality" (SDG 6.3), "Reduce illnesses caused by hazardous chemicals" (SDG 3.9), "Reduce marine pollution" (SDG 14.1), and "Manage chemicals in an environmentally sound manner" (SDG 12.4).

With the associated project partners