Thomas Pretz

Central sorting and recovery of MSW recyclable materials: A review of technological state-of-the-art, cases, practice and implications for materials recycling

Todays waste regulation in the EU comprises stringent material recovery targets and calls for comprehensive programs in order to achieve them. A similar movement is seen in the US where more and more states and communities commit to high diversion rates from landfills. The present paper reviews scientific literature, case studies and results from pilot projects, on the topic of central sorting of recyclable materials commonly found in waste from households. The study contributes, inter alia, with background understanding on the development of materials recovery, both in a historical and geographical perspective. Physical processing and sorting technology has reached a high level of maturity, and many quality issues linked to cross-contamination by commingling have been successfully addressed to date. New sorting plants tend to benefit from economies of scale, and innovations in automation and process control, which are targeted at curtailing process inefficiencies shown by operational practice. Technology developed for the sorting of commingled recyclables from separate collection is also being successfully used to upgrade residual MSW processing plants. The strongest motivation for central sorting of residual MSW is found for areas where source separation and separate collection is difficult, such as urban agglomerations, and can in such areas contribute to increasing recycling rates, either complementary to- or as a substitute for source separation of certain materials, such as plastics and metals.

Intelligent solid waste processing using optical sensor based sorting technology

Solid wastes are always collected as mixtures of different materials. They gets crushed, classified and sorted in solid waste treatment plants. Among these processes the sorting is the determining step for recycling and reuse. Traditional sorting technologies like magnetic sorting and eddy current sorting are only able to process some special kinds of ingredients of waste mixture roughly, such as the separation of ferrous and non-ferrous metals. Since there exist corresponding force fields between waste particles and separators. Some other properties of the solid particles such as the colours, shapes and texture features could also be considered as sorting criterions but there is no sufficient force field between these properties and separators. In this paper, an indirect sorting process by using optical sensor and mechanical separating system was developed and introduced. By using this system the particle sizes and positions, colours and shapes of each waste particle are able to be determined and used as sorting criterion. The mechanical sorting device consists of a compressed air nozzle which is controlled by computer, the target particles which were recognized by sensor were blown out of the main waste stream. Feature recognition by using optical sensor yield good results. This research provides a new approach for multi-feature recognition of sensor based sorting technology.

Separate collection of plastic waste, better than technical sorting from municipal solid waste?

The politically preferred solution to fulfil legal recycling demands is often implementing separate collection systems. However, experience shows their limitations, particularly in urban centres with a high population density. In response to the European Union landfill directive, mechanical biological waste treatment plants have been installed all over Europe. This technology makes it possible to retrieve plastic waste from municipal solid waste. Operators of mechanical biological waste treatment plants, both in Germany and the Netherlands, have started to change their mechanical separation processes to additionally produce plastic pre-concentrates. Results from mechanical biological waste treatment and separate collection of post-consumer packaging waste will be presented and compared. They prove that both the yield and the quality of plastic waste provided as feedstock for the production of secondary plastic raw material are largely comparable. An economic assessment shows which conditions for a technical sorting plant are economically attractive in comparison to separate collection systems. It is, however, unlikely that plastic recycling will ever reach cost neutrality.

Fine grain separation for the production of biomass fuel from mixed municipal solid waste

The main goal of the project MARSS (Material Advanced Sustainable Systems) is to build a demonstration plant in order to recover a renewable biomass fuel suitable for the use in biomass power plants out of mixed municipal solid waste (MMSW). The demonstration plant was constructed in Mertesdorf (Germany), working alongside an existing mechanical-biological treatment plant, where the MMSW is biological dried under aerobe conditions in rotting boxes. The focus of the presented sorting campaign was set on the processing of fine grain particles minor than 11.5mm which have the highest mass content and biogenic energy potential of the utilized grain size fractions. The objective was to produce a biomass fuel with a high calorific value and a low content of fossil (plastic, synthetic) materials while maximizing the mass recovery. Therefore, the biogenic components of the dried MMSW are separated from inert and fossil components through various classification and sifting processes. In three experimental process setups of different processing depths, the grain size fraction 4-11.5mm was sifted by the use of air sifters and air tables.

Text recognition for information retrieval in images of printed circuit boards

In order to achieve an efficient and environment-friendly recycling of printed circuit boards (PCBs), a comprehensive analysis of their material composition is essential. Besides sophisticated chemical and physical methods for a direct material analysis, an indirect method based on information retrieval provides a less costly and more efficient alternative. During the process of information retrieval, PCBs and their components need to be recognized based on their appearance and the corresponding text information. Their material composition is then available through a pre-established database. Therefore, a practical text recognition is necessary for a successful data analysis prior to PCB recycling. Our paper is focusing on two key aspects of text recognition: binarization and final recognition of text objects using optical character recognition (OCR) engines. For binarization of text contents, a novel local thresholding method using an adaptive window size along with background estimation is presented. Several state-of-the-art algorithms and the proposed method were evaluated for comparing their binarization performance on text objects in PCB images. With respect to a data set containing manually created references, our novel method provides superior results. Furthermore, in contrast to previous work on text recognition, an additional evaluation of available open source OCR engines was conducted to asses technical limitations of OCR applications. We show that the quality of text recognition can be significantly improved if the binarization approach accounts for these technical limitations of OCR software. The presented method and results are expected to provide improved OCR performance also in other applications.

Accurate laser triangulation using a perpendicular camera setup to assess the height profile of PCBs

Recycling of printed circuit boards (PCBs) is still subject to current research. For identifying valuable materials in electronical components on the PCBs the height profile of PCBs is a important feature. Laser triangulation with a perpendicular laser is a classical approach to generate height profiles. However, in such a setup it is not possible to acquire different modalities such as panorama images and the height profile simultaneously. The solution is to use a setup in which the laser is slanted and the camera is perpendicular to the PCBs. A new triangulation approach employing raycasting and intersecting a laser plane and a ray of sight is needed to retain accuracy of the height profile generation. Evaluation results show that the accuracy of height measurement with an average error of 0.077 mm is still sufficient for detecting small surface mounted devices on PCBs with a minimal height of about 0.2 mm.

Chapter 6 – Metal Waste

Publisher Summary This chapter describes the fundamentals of metal recycling. It considers the process of mechanical processing of scrap. These processes are all that is required to produce metals that can be used without any difficulties and without further treatment in metallurgical plants. These metallurgical processes include the machinery that is actually a part of the downstream equipment involved in smelting and refining and thus these processes complete the recycling loop for metals. Mechanical treatment of different waste streams containing metals involves the use of well-established methods, which, as a rule, is profitable because of the high market value of the recovered metals and also because metals do not change their properties with use and hence can be recycled an unlimited number of times. The main aim of the processing methods is to achieve high recovery values as well as the best possible grades of the final metallic products. Newly developed systems such as sensor-based sorters are increasingly being implemented to improve separation efficiencies. Finally, re-smelting of the reclaimed products constitutes the closing of the complete recycling loop for metals.

A methodical approach for the assessment of waste sorting plants.

A techno-economical evaluation of the processing result of waste sorting plants should at least provide a realistic assessment of the recovery yields of valuable materials and of the qualities of the obtained products. This practical data is generated by weighing all the output products and sampling these products. Due to the technological complexity of sorting plants, for example, lightweight packaging waste treatments plants and the high expenditures concerning time and costs of sampling with subsequent manual sorting for quality determination, usually only final products undergo such an investigation. Thereby, the transferability of the results depends decisively on the boundary conditions (extent, throughput of the plant, process parameterization). Given that the process is too complex, not all relevant information of the process steps can be determined by sampling. By model calculations and/or adjustment of reasonable assumptions, information concerning weak points in the process can be identified, which can be used for further plant optimization. For the example of the recovery of beverage cartons from co-collected and mechanically recovered mixtures of lightweight packaging waste, a methodical approach for the assessment of processing results will be presented.

Commingled Waste Collection as Chance for Technical Separation: Alternative Collection Systems

The most relevant parameter for the profitability of a deposit is its raw materials concentration. With the view on secondary raw materials from municipal solid waste (MSW), the concentration depends on the population density and the specific waste generation rate. To recover a secondary raw material from MSW, collection is the first step and at the same time, the bottleneck, as typically the efficiency of the separate collection of recyclables decreases with increasing population density. Also, the effort of collecting many different recyclables as a single fraction, with each of these fractions making up a small specific amount per household, often only leads to collection costs being too high to be compensated by revenues from recycling or waste fees. As a compromise between losing recyclables due to high degrees of contamination when collected in mixed household waste, and exploding collection costs for too many single fractions, recyclables are often collected as a commingled fraction of selected materials, which can technically be efficiently separated, and then be directed to recycling plants. Local waste management structures, such as contractual periods and distribution of responsibilities, lead to specific collection and treatment systems with individual efficiencies, which is demonstrated by different examples, as implemented in Europe.

Rotation estimation for printed circuit board recycling

Electronic devices are nowadays an integral part of everyday life. The number of discarded electronic items has grown significantly over the last years. Due to the amount of precious materials used in the manufacturing of these devices recycling of electronic devices is becoming more and more important. Currently, the processes to regain some of these precious materials such as gold, copper, scarce elements etc. are not able to differentiate electronic waste according to its material composition. To enhance these processes, as much information as possible needs to be retrieved per electronic waste item. In particular, information used for the classification of the processed printed circuit boards (PCBs) is important as PCBs are extensively used in electronic devices. One key aspect of this classification process is the estimation of the orientation of the PCBs in this process (e.g. on a conveyor belt). In this paper typical properties of PCBs with respect to orientation estimation are introduced and three different orientation estimation algorithms are evaluated and discussed. These approaches comprise the Hough transform, structure tensors and orientation estimation via Fourier transform. Finally, all presented algorithms are evaluated based on images of PCBs with known orientation. The results indicate, that the Hough transform yields the best results for completely visible PCBs while structure tensors performed best on partially visible PCBs.