The metabolic potential of plastics as biotechnological carbon sources - Review and targets for the future.

The plastic crisis requires drastic measures, especially for the plastics' end-of-life. Mixed plastic fractions are currently difficult to recycle, but microbial metabolism might open new pathways. With new technologies for degradation of plastics to oligo- and monomers, these carbon sources can be used in biotechnology for the upcycling of plastic waste to valuable products, such as bioplastics and biosurfactants. We briefly summarize well-known monomer degradation pathways and computed their theoretical yields for industrially interesting products. With this information in hand, we calculated replacement scenarios of existing fossil-based synthesis routes for the same products. Thereby, we highlight fossil-based products for which plastic monomers might be attractive alternative carbon sources. Notably, not the highest yield of product on substrate of the biochemical route, but rather the (in-)efficiency of the petrochemical routes (i.e., carbon, energy use) determines the potential of biochemical plastic upcycling. Our results might serve as a guide for future metabolic engineering efforts towards a sustainable plastic economy.

Chemical recycling: A critical assessment of potential process approaches.

Increased media coverage of plastic pollution in the environment and import bans on plastic waste in several countries have resulted in plastic waste becoming one of the most discussed waste streams in recent years. In the European Union (EU), only about one-third of the post-consumer plastic waste is recycled; the rest goes to energy recovery and landfilling in equal parts. In connection to the necessary increase in efforts to achieve the ambitious EU recycling targets, chemical recycling is currently receiving more and more attention. The assumption is that chemical recycling processes could open up new waste streams for recycling and generate valuable raw materials for the chemical industry. Although there exists no legal definition for chemical recycling, there is more or less agreement that it covers the conversion of plastic polymers into their monomers or chemical building blocks. Techniques such as gasification, pyrolysis and liquefaction as well as solvolysis can be used for chemical recycling. So far, only few large-scale plants for chemical recycling exist worldwide. This article presents the different processes by means of examples from (formerly) running installations and their suitability for plastics recycling is assessed. However, to date, only few chemical recycling plants are in continuous operation, and further scientific evidence for the ecological and economic benefits is still necessary for final evaluation.

Thermal treatment of carbon-fibre-reinforced polymers (Part 2: Energy recovery and feedstock recycling).

The use of carbon fibre (CF)-reinforced plastics has grown significantly in recent years, and new areas of application have been and are being developed. As a result, the amount of non-recyclable waste containing CF is also rising. There are currently no treatment methods for this type of waste. Within this project different approaches for the treatment of waste containing CF were investigated. Main subject of the research project were large-scale investigations on treatment possibilities and limits of waste containing CF in high temperature processes, with focus on the investigation of process-specific residues and possible fibre emission. The results showed that the two conventional thermal waste treatment concepts with grate and rotary kiln firing systems are not suitable for a complete oxidation of CFs due to the insufficient process conditions (temperature and dwell time). The CFs were mainly discharged via the bottom ash/slag. Due to the partial decomposition during thermal treatment, World Health Organization (WHO) fibres occurred in low concentrations. The tests run in the cement kiln plant have shown the necessity of comminution for waste containing CF. With respect to the short testing times and moderate quantities of inserted CF, a final evaluation of the suitability of this disposal path was not possible. The use of specially processed waste containing CF (carbon-fibre-reinforced plastic (CFRP) pellets) as a carbon substitute in calcium carbide production led to high carbon conversion rates. In the unburned furnace dust, which is marketed as a by-product of the process, CFs in relevant quantities could be detected.

Thermal treatment of sewage sludge in Germany: A review.

Sewage sludge from municipal wastewater treatment plants in Germany is currently disposed of via thermal treatment and agricultural utilization or used for landscaping. Increasing focus on hygiene, soil protection and most recently on phosphorus recovery combined with the associated legal changes leads to an increased relevance of thermal sewage sludge treatment processes. This article reviews existing technologies for thermal treatment of sewage sludge with a view to the situation in Germany. Thermal sewage sludge treatment can be divided into different processes: drying reduces high water contents of mechanically dewatered sewage sludge and often precedes subsequent treatment processes. Today, most of the sewage sludge in Germany is incinerated, about half in mono-incineration, mostly in stationary fluidized beds, and the other half in co-incineration, in particular in coal-fired power plants, cement kilns or, to a lesser extent, waste incineration plants. Some alternative thermal processes, mainly pyrolysis and gasification, but also metallurgical approaches, are tested in bench or pilot scale. Recent amendments to the German Sewage Sludge Ordinance will restrict the disposal route of co-incineration in future. Consequently, a significant increase in mono-incineration capacity is expected. These processes should enable the combination of environmentally friendly disposal and phosphorus recovery.

Status of waste-to-energy in Germany, Part I - Waste treatment facilities.

This study gives a detailed overview over the German waste-to-energy sector in 2015. The aim is to quantify the available treatment capacities and the energetic potential of waste in Germany. The work is based on an extensive data collection and evaluation, both from literature sources as well as from a survey among operators of waste treatment plants. The present Part I, gives an overview of all treatment facilities in Germany that convert waste into energy. It was found that in total, almost 320 PJ of end energy are produced in German waste treatment plants: 225 PJ a-1 of heat; and 90 PJ a-1 of electricity. This is a share of about 3.7% of the German end energy consumption.

Thermal treatment of carbon fibre reinforced polymers (Part 1: Recycling).

The increasing use of carbon fibre reinforced polymers requires suitable disposing and recycling options, the latter being especially attractive due to the high production cost of the material. Reclaiming the fibres from their polymer matrix however is not without challenges. Pyrolysis leads to a decay of the polymer matrix but may also leave solid carbon residues on the fibre. These residues prevent fibre sizing and thereby reuse in new materials. In state of the art, these residues are removed via thermal treatment in oxygen containing atmospheres. This however may damage the fibre's tensile strength. Within the scope of this work, carbon dioxide and water vapour were used to remove the carbon residues. This aims to eliminate or at least minimize fibre damage. Improved quality of reclaimed fibres can make fibre reuse more desirable by enabling the production of high-quality recycling products. Still, even under ideal recycling conditions the fibres will shorten with every new life-cycle due to production-based blending. Fibre disposal pathways will therefore always also be necessary. The problems of thermal fibre disintegration are summarized in the second part of this article (Part 2: Energy recovery).

[Particulate emissions from residential wood combustion : Evaluation under real-life operating conditions and toxicological relevance]. / Partikuläre Emissionen aus Einzelraumfeuerungen für Holzbrennstoffe : Bewertung unter realen Betriebsbedingungen und toxikologische Relevanz.

Due to their high emission of particulate matter, wood fired furnaces have become a focal point of public discussion in Germany. Log-fired single room heaters can be identified as a main contributor to this matter. The particulate matter emitted by outdated as well as modern furnaces directly affects the pollution inside residential areas. This is demonstrated by a test campaign of a fibrous filter system developed by the Technology of Fuels Unit at RWTH Aachen University. The filter system captures the emitted particles and retains them inside a highly porous media. Particles from different households and combustion systems were collected over half a heating season. Afterwards, the chemical composition of the accumulated particulate matter was analysed. Based on the particle composition, the furnace operation can be evaluated and consequently improved. This method can be seen as an integral evaluation of the emitted particles of small scale furnaces under real-life operating conditions.

Technical, economical, and climate-related aspects of biochar production technologies: a literature review.

For the development of commercial biochar projects, reliable data on biochar production technologies is needed. For this purpose, peer-reviewed scientific articles on carbonization technologies (pyrolysis, gasification, hydrothermal carbonization, and flash carbonization) have been analyzed. Valuable information is provided by papers on pyrolysis processes, less information is available on gasification processes, and few papers about hydrothermal and flash carbonization technologies were identified. A wide range of data on the costs of char production (between 51 US$ per tonne pyrolysis biochar from yard waste and 386 US$ per tonne retort charcoal) and on the GHG balance of biochar systems (between -1054 kg CO(2)e and +123 kg CO(2)e per t dry biomass feedstock) have been published. More data from pilot projects are needed to improve the evaluation of biochar production technologies. Additional research on the influence of biochar application on surface albedo, atmospheric soot concentration, and yield responses is necessary to assess the entire climate impact of biochar systems. Above all, further field trials on the ability of different technologies to produce chars for agricultural soils and carbon sequestration are essential for future technology evaluation.

A review of the fate of engineered nanomaterials in municipal solid waste streams.

Significant knowledge and data gaps associated with the fate of product-embedded engineered nanomaterials (ENMs) in waste management processes exist that limit our current ability to develop appropriate end-of-life management strategies. This review paper was developed as part of the activities of the IWWG ENMs in Waste Task Group. The specific objectives of this review paper are to assess the current knowledge associated with the fate of ENMs in commonly used waste management processes, including key processes and mechanisms associated with ENM fate and transport in each waste management process, and to use that information to identify the data gaps and research needs in this area. Literature associated with the fate of ENMs in wastes was reviewed and summarized. Overall, results from this literature review indicate a need for continued research in this area. No work has been conducted to quantify ENMs present in discarded materials and an understanding of ENM release from consumer products under conditions representative of those found in relevant waste management process is needed. Results also indicate that significant knowledge gaps associated with ENM behaviour exist for each waste management process investigated. There is a need for additional research investigating the fate of different types of ENMs at larger concentration ranges with different surface chemistries. Understanding how changes in treatment process operation may influence ENM fate is also needed. A series of specific research questions associated with the fate of ENMs during the management of ENM-containing wastes have been identified and used to direct future research in this area.

Post-combustion syntheses of PCDD/F and PBDD/F from halogen-rich fuel is suppressed by a pebble heater technology.

GOAL, SCOPE AND BACKGROUND: Changes in German and European legislation shifted processing of polymer-rich shredding residues (SR) from landfill to thermal treatment. However, when waste of electric and electronic equipment (WEEE) is the source of SR, thermal treatment is complicated by halogens as well as the presence of polybrominated dioxins and furans (PBDD/F) and brominated flame retardants (BFR). Hence, WEEE requires high temperature incineration with sufficient residence times. Post-combustion synthesis of polyhalogenated dioxins and furans (PXDD/F) is dominant in the temperature range between 250-450 degrees C. Thus, a very rapid gas cooling from 450 degrees to 250 degrees C is important for proper raw gas treatment. The pebble heater technology developed by ATZ Entwicklungszentrum (Sulzbach-Rosenberg, Germany) might serve as an alternative to the state-of-the-art quench cooling. It is based on the application of a pebble bed of natural bulk material, which the exhaust gases flows through radially. It provides an excellent heat transfer and a temperature gradient in the range of 1,500-2,000 K/m. The paper presents data of a pilot application of the pebble heater technology for the treatment of raw gas derived from the incineration of polymeric materials from WEEE. METHODS: A liquid fuel was chosen in order to minimise technical modifications of the plant. It was analysed for halogens by x-ray fluorescence, for brominated flame retardants by HPLC-UV/MS and for PXDD/F by GC-HRMS. Combustion gases were rapidly cooled down to temperatures below 200 degrees C and emissions of PBDD/F and PCDD/ F were estimated without further off-gas treatment. PBDD/F emissions were computed as PCDD/F toxicity equivalents applying two different calculation approaches. RESULTS AND DISCUSSION: PCDD/F emissions accounted for 0.04 ng I-TEQ/Nm3 and are in compliance with European emission limits. Calculated PBDD/F toxicity equivalents exceeded the emission limit of 0.1 ng I-TEQ/Nm3 by factors of 75 and 208 depending on the calculation approach. A mass balance of PBDD/F and PCDD/F congeners revealed an efficient elimination of more than 95% in most cases. Lower reduction rates (76% for 2,3,7,8-TeBDF and 82% for 1,2,3,7,8-PeBDF) were attributed to incomplete combustion. An intended recovery of halogens by one-stage scrubbing downstream of the pebble heater was ineffective, recovering 28% of the applied chlorine and 9% of the bromine, only. CONCLUSIONS: Our pilot incineration test indicates that the pebble heater technology can effectively suppress a post-combustion synthesis of PCDD/F and PBDD/F, resulting in low PCDD/F emission levels without further off-gas treatment. The presented data state that WEEE is sensible to incomplete combustion, which will lead to increased PBDD/F emissions without increasing PCDD/F emission limits. This finding is especially relevant for small and low-technical incineration appliances, which have been reported to treat WEEE in developing countries and are considered to serve as a significant source of PXDD/F these days. RECOMMENDATIONS AND PERSPECTIVES: Monitoring of PCDD/F emissions only might considerably underestimate the total emission of dioxins and dioxin-like compounds. It is therefore an ineffective means for assessing resulting health risks, at least for those waste treatment plants which are considered to handle the increasing amounts of PBDD/ F-containing polymers from WEEE in future. Consequently, it is recommended to initiate a screening programme for PXDD/F emissions in large scale incineration facilities which are capable of treating WEEE shredder residues.

Energetic utilisation of refuse derived fuels from landfill mining.

The residence of municipal solid waste within a landfill body results in a significant change of material properties. Experiences with the energetic utilisation of the burnable fractions from formerly landfilled waste are hardly documented, the influence of refuse derived fuels (RDF) from such materials on the performance of modern waste-to-energy plants is not sufficiently described in scientific literature. Therefore this study focuses on the energetic utilisation of refuse derived fuel from landfilled waste, processed in a mechanical waste treatment facility, and the impact of the material on the operation of the incineration plant. Additionally, the possibility of direct combustion of non-pre-treated excavated landfill material has been evaluated in the same facility. First, sampling and analysis of the fuel has been carried out. Based on this, a large-scale combustion experiment was planned and conducted in an industrial waste-to-energy plant. Steam mass flow rate, concentration of harmful substances in the raw gas, as well as total emissions of the facility have been monitored in detail. Furthermore, the influence of the landfilled material on the additive consumption has been determined. The combustion residues (bottom ash) were also sampled and analysed. Based on the evaluation of operating data and analysis of both fuel and residue, suitable thermal treatment approaches for the refuse-derived fuel and the non-pre-treated excavated material have been assessed.