Stuart Thomas Wagland

Test methods to aid in the evaluation of the diversion of biodegradable municipal waste (BMW) from landfill.

A wide range of waste characterization methods are available, each developed for a specific purpose such as determining compost stability, or for landfill acceptance criteria. Here test methods have been evaluated for the purpose of assessing waste treatment process performance and monitoring the diversion of biodegradable municipal waste (BMW) from landfill. The suitability factors include the timescale of the method, applicability to a wide range of materials and ability to indicate the long-term biodegradability of organic waste samples. The anaerobic test methods, whilst producing reliable results, take at least several weeks to complete, therefore, not allowing for regular routine analysis often required for diversion assessments. Short-term tests are required which can correlate with, and, therefore, estimate, values obtained from long-term anaerobic methods. Aerobic test methods were found to offer a significantly improved timescale compared with anaerobic test methods; however, they have limitations due to not measuring the full extent of sample biodegradability. No single test method was found to be completely sufficient for routine biodegradability analysis suitable for monitoring the BMW diversion from landfill. Potential areas for further research include spectrographic FT-IR or enzyme-based approaches such as the ECD or EHT methods.

Comparison of coal/solid recovered fuel (SRF) with coal/refuse derived fuel (RDF) in a fluidised bed reactor

An experimental study was undertaken to compare the differences between municipal solid waste (MSW) derived solid recovered fuel (SRF) (complying with CEN standards) and refuse derived fuel (RDF). Both fuels were co-combusted with coal in a 50 kW fluidized bed combustor and the metal emissions were compared. Synthetic SRF was prepared in the laboratory by grinding major constituents of MSW such as paper, plastic, textile and wood. RDF was obtained from a local mechanical treatment plant. Heavy metal emissions in flue gas and ash samples from the (coal+10% SRF) fuel mixture were found to be within the acceptable range and were generally lower than that obtained for coal+10% RDF fuel mixture. The relative distribution of heavy metals in ash components and the flue gas stream shows the presence of a large fraction (up to 98%) of most of the metals in the ash (except Hg and As). Thermo-gravimetric (TG) analysis of SRF constituents was performed to understand the behaviour of fuel mixtures in the absence and presence of air. The results obtained from the experimental study will enhance the confidence of fuel users towards using MSW-derived SRF as an alternative fuel.

Solid Recovered Fuel: Influence of Waste Stream Composition and Processing on Chlorine Content and Fuel Quality

Solid recovered fuel (SRF) produced by mechanical-biological treatment (MBT) of municipal waste can replace fossil fuels, being a CO(2)-neutral, affordable, and alternative energy source. SRF application is limited by low confidence in quality. We present results for key SRF properties centered on the issue of chlorine content. A detailed investigation involved sampling, statistical analysis, reconstruction of composition, and modeling of SRF properties. The total chlorine median for a typical plant during summer operation was 0.69% w/w(d), with lower/upper 95% confidence intervals of 0.60% w/w(d) and 0.74% w/w(d) (class 3 of CEN Cl indicator). The average total chlorine can be simulated, using a reconciled SRF composition before shredding to <40 mm. The relative plastics vs paper mass ratios in particular result in an SRF with a 95% upper confidence limit for ash content marginally below the 20% w/w(d) deemed suitable for certain power plants; and a lower 95% confidence limit of net calorific value (NCV) at 14.5 MJ kg(ar)(-1). The data provide, for the first time, a high level of confidence on the effects of SRF composition on its chlorine content, illustrating interrelationships with other fuel properties. The findings presented here allow rational debate on achievable vs desirable MBT-derived SRF quality, informing the development of realistic SRF quality specifications, through modeling exercises, needed for effective thermal recovery.

Solid Recovered Fuel: Materials Flow Analysis and Fuel Property Development during the Mechanical Processing of Biodried Waste

Material flows and their contributions to fuel properties are balanced for the mechanical section of a mechanical-biological treatment (MBT) plant producing solid recovered fuel (SRF) for the UK market. Insights for this and similar plants were secured through a program of sampling, manual sorting, statistics, analytical property determination, and material flow analysis (MFA) with error propagation and data reconciliation. Approximately three-quarters of the net calorific value (Q(net,p,ar)) present in the combustible fraction of the biodried flow is incorporated into the SRF (73.2 ± 8.6%), with the important contributors being plastic film (30.7 MJ kg(ar)(-1)), other packaging plastic (26.1 MJ kg(ar)(-1)), and paper/card (13.0 MJ kg(ar)(-1)). Nearly 80% w/w of the chlorine load in the biodried flow is incorporated into SRF (78.9 ± 26.2%), determined by the operation of the trommel and air classifier. Through the use of a novel mass balancing procedure, SRF quality is understood, thus improving on the understanding of quality assurance in SRF. Quantification of flows, transfer coefficients, and fuel properties allows recommendations to be made for process optimization and the production of a reliable and therefore marketable SRF product.

Rare earth elements and critical metal content of extracted landfilled material and potential recovery opportunities

Rare earth elements (REEs), Platinum group metals (PGMs) and other critical metals currently attract significant interest due to the high risks of supply shortage and substantial impact on the economy. Their uses in many applications have made them present in municipal solid waste (MSW) and in commercial and industrial waste (C&I), since several industrial processes produce by-products with high content of these metals. With over 4000 landfills in the UK alone, the aim of this study was to assess the existence of these critical metals within landfills. Samples collected from four closed landfills in UK were subjected to a two-step acid digestion to extract 27 metals of interest. Concentrations across the four landfill sites were 58±6mgkg(-1) for REEs comprising 44±8mgkg(-1) for light REEs, 11±2mgkg(-1) for heavy REEs and 3±1mgkg(-1) for Scandium (Sc) and 3±1.0mgkg(-1) of PGMs. Compared to the typical concentration in ores, these concentrations are too low to achieve a commercially viable extraction. However, content of other highly valuable metals (Al and Cu) was found in concentrations equating to a combined value across the four landfills of around

Development of an image-based analysis method to determine the physical composition of a mixed waste material.

400 million, which increases the economic viability of landfill mining. Presence of critical metals will mainly depend on the type of waste that was buried but the recovery of these metals through landfill mining is possible and is economically feasible only if additional materials (plastics, paper, metallic items and other) are also recovered for reprocessing.

Energy recovery from human faeces via gasification: A thermodynamic equilibrium modelling approach.

An experimental study was undertaken to assess the suitability of an image-based approach for determining the physical composition of mixed wastes. Waste samples were investigated at six different waste sorting facilities each visited twice during the study. These materials were physically sorted to examine the composition of these materials; during each of these surveys the waste was also processed to obtain digital images covering an area of 30 m(2), representing approximately 250-500 kg (3× mechanical bucket loads) of mixed waste. The images were processed using ERDAS Imagine software to assess the area covered by each component within the waste material. The composition determined from the image analysis was compared with results from the physical hand sorting. The image analysis results indicated a strong correlation with the physical results (mean r=0.91), however it was evident that components such as film plastics and paper were being over-estimated by the image analysis approach. This manuscript provides initial results, demonstrating the potential of an image-based method, and discusses further research requirements and future applications of this technique.

An experimental investigation of the combustion performance of human faeces

Highlights • On dry basis, typical human faeces contain 83 wt.% organic fraction and 17 wt.% ash.• The LHV of dry human faeces ranged from 19 to 22 MJ/kg, values similar to wood biomass.• Syngas from dry human faeces had LHV of 15–17 MJ/kg at equivalence ratio of ∼0.31.• Energy is best recovered from moist human faeces at equivalence ratio above 0.6.• Recoverable exergy potential from moist human faeces can be up to 15 MJ/kg.

Investigation of the application of an enzyme-based biodegradability test method to a municipal solid waste biodrying process

Highlights • Dry human faeces have a Higher Heating Value (HHV) of 24 MJ/kg.• Faeces combustion was investigated using a bench-scale downdraft combustor test rig.• Combustion temperature of 431–558 °C was achieved at air flow rate of 10–18 L/min.• Fuel burn rate of 1.5–2.3 g/min was achieved at air flow rate of 10–18 L/min.• Combustion temperature of up to 600 ± 10 °C can handle 60 wt.% moisture in faeces.

Compositional and physicochemical changes in waste materials and biogas production across 7 landfill sites in UK

This paper presents a study to evaluate the recently developed enzymatic hydrolysis test (EHT) through its repeated application to a waste treatment process. A single waste treatment facility, involving a biodrying process, has been monitored using three different methods to assess the biodegradable content of the organic waste fractions. These test methods were the anaerobic BMc, aerobic DR4 and the EHT, which is a method based on the enzymatic hydrolysis of the cellulosic content of waste materials. The input municipal solid waste (MSW) and the output solid recovered fuel (SRF) and organic fines streams were sampled over a period of nine months from a single mechanical biological treatment (MBT) facility. The EHT was applied to each stream following grinding to <10 mm and <2 mm, in order to investigate the effect of particle size on the release of dissolved organic carbon (DOC) from enzyme hydrolysis. The output organic fines were found to more biodegradable than the MSW input and SRF output samples in each of the test methods, significantly (p<0.05) for the EHT and DR4 methods, on the basis of DOC released and oxygen consumed, respectively. The variation between sample replicates for the EHT was higher where sample sizes of <2 mm were analysed compared to sizes of <10 mm, and the DOC release at each phase of the EHT was observed to be higher when using particle sizes of <2 mm. Despite this, additional sample grinding from the <10 mm to a smaller particle size of <2 mm is not sufficiently beneficial to the analysis of organic waste fractions in the EHT method. Finally, it was concluded that as similar trends were observed for each test method, this trial confirms that EHT has the potential to be deployed as a practical operational biodegradability monitoring tool.