Carbon Monoxide Production during Bio-Waste Composting under Different Temperature and Aeration Regimes.

Despite the development of biorefinery processes, the possibility of coupling the "conventional" composting process with the production of biochemicals is not taken into account. However, net carbon monoxide (CO) production has been observed during bio-waste composting. So far, O2 concentration and temperature have been identified as the main variables influencing CO formation. This study aimed to investigate CO net production during bio-waste composting under controlled laboratory conditions by varying aeration rates and temperatures. A series of composting processes was carried out in conditions ranging from mesophilic to thermophilic (T = 35, 45, 55, and 65 °C) and an aeration rate of 2.7, 3.4, 4.8, and 7.8 L·h-1. Based on the findings of this study, suggestions for the improvement of CO production throughout the composting process have been developed for the first time. The highest concentrations of CO in each thermal variant was achieved with an O2 deficit (aeration rate 2.7 L·h-1); additionally, CO levels increased with temperature, reaching ~300 ppm at 65 °C. The production of CO in mesophilic and thermophilic conditions draws attention to biological CO formation by microorganisms capable of producing the CODH enzyme. Further research on CO production efficiency in these thermal ranges is necessary with the characterization of the microbial community and analysis of the ability of the identified bacteria to produce the CODH enzyme and convert CO from CO2.

The dynamics of macro- and microplastic quantity and size changes during the composting process.

The quantity and type of macro- and microplastics was investigated in rotting material during the composting process of two state-of-the-art composting plants in Austria. Microplastics >0.2 mm, were found already after the first turning event in both facilities. The generation of microplastics was more extensive in the plant that used shorter turning intervals during the first four weeks and generated approx. 21 particles per week and kg-1 DM. After 4 weeks of operation less microplastic particles were detected, which suggested that particles were fractionated to smaller sizes during processing. In addition, a total of nine composts from three different facilities that were operated in various settlement structures were compared. 7 to 232 macro- and microplastic particles per kg DM were found, whereas the highest plastic burden was observed in the composts made from biowaste that originated from the most densely populated area.

Influence assessment of a lab-scale ripening process on the quality of mechanically-biologically treated MSW for possible recovery.

In this study, the influence of an additional ripening process on the quality of mechanically-biologically treated MSW was evaluated in the prospective of recovering the end material, rather than landfilling. The biostabilised waste (BSW) coming from one of the MBT plants of Rome was therefore subjected to a ripening process in slightly aerated lab test cells. An in-depth investigation on the biological reactivity was performed by means of different types of tests (aerobic and anaerobic biological tests, as well as FT-IR spectroscopy method). A physical-chemical characterisation of waste samples progressively taken during the ripening phase was carried out, as well. In addition, the ripened BSW quality was assessed by comparing the characteristics of a compost sampled at the composting plant of Rome which treat source segregated organic wastes. Results showed that the additional ripening process allowed to obtain a better quality of the biostabilised waste, by achieving a much higher biological stability compared to BSW as-received and similar to that of the tested compost. An important finding was the lower heavy metals (Co, Cr, Cu, Ni, Pb and Zn) release in water phase at the end of the ripening compared to the as-received BSW, showing that metals were mainly bound to solid organic matter. As a result, the ripened waste, though not usable in agriculture as found for the compost sample, proved anyhow to be potentially suitable for land reclamation purposes, such as in landfills as cover material or mixed with degraded and contaminated soil for organic matter and nutrients supply and for metals recovery, respectively. In conclusion the study highlights the need to extend and optimise the biological treatment in the MBT facilities and opens the possibility to recover the output waste instead of landfilling.

Quantification of methane emissions from full-scale open windrow composting of biowaste using an inverse dispersion technique.

An inverse dispersion technique in conjunction with Open-Path Tunable-Diode-Laser-Spectroscopy (OP-TDLS) and meteorological measurements was applied to characterise methane (CH4) emissions from an Austrian open-windrow composting plant treating source-separated biowaste. Within the measurement campaigns from July to September 2012 different operating conditions (e.g. before, during and after turning and/or sieving events) were considered to reflect the plant-specific process efficiency. In addition, the tracer technique using acetylene (C2H2) was applied during the measurement campaigns as a comparison to the dispersion model. Plant-specific methane emissions varied between 1.7 and 14.3 gCH4/m(3)d (1.3-10.7 kg CH4/h) under real-life management assuming a rotting volume of 18,000 m(3). In addition, emission measurements indicated that the turning frequency of the open windrows appears to be a crucial factor controlling CH4 emissions when composting biowaste. The lowest CH4 emission was measured at a passive state of the windrows without any turning event ("standstill" and "sieving of matured compost"). Not surprisingly, higher CH4 emissions occurred during turning events, which can be mainly attributed to the instant release of trapped CH4. Besides the operation mode, the meteorological conditions (e.g. wind speed, atmospheric stability) may be further factors that likely affect the release of CH4 emissions at an open windrow system. However, the maximum daily CH4 emissions of 1m(3) rotting material of the composting plant are only 0.7-6.5% of the potential daily methane emissions released from 1m(3) of mechanically-biologically treated (MBT) waste being landfilled according to the required limit values given in the Austrian landfill ordinance.

Large scale study on measurement of respiration activity (AT(4)) by Sapromat and OxiTop.

In the run-up for amending the Austrian landfill ordinance, parameters were developed to assess the stability/reactivity of mechanically-biologically pretreated residual wastes. The Landfill Ordinance 2008 regulates limit values for Respiration Activity (="Atmungsaktivität") RA(4) (AT(4))<7mgO(2)*(g dry matter (DM))(-1), Gas Generation Sum GS(21)<20Nl*kgDM(-1) and alternatively Gas Evolution (="Gasbildung") GB(21)<20Nl*kgDM(-1). Methods for analysing these parameters were established by the Austrian Standards Institute (2004). As laboratory practice shows, these methods also are used for the assessment of other wastes (sewage sludge, commercial waste, material from abandoned sites, biowaste compost). For measurement of respiration activity in Austria mainly two methods are used: the Sapromat®-method and the OxiTop®-method. Whether respectively to what extent these two methods give same results, is discussed in this paper. Since 2009 at ABF-BOKU 169 respiration activity tests of samples taken from different stages of MBT - as well as biowaste composting processes, materials from landfills as well as abandoned sites and residues from anaerobic treatment plants were analysed parallel by Sapromat® and OxiTop®. The results manifest very strong correlation between the Sapromat® and OxiTop® method. The correlation coefficient is 0.993. As a very clear tendency OxiTop® gives lower amounts than Sapromat®. In average the lower values of OxiTop® are around 88%.

Modeling the final phase of landfill gas generation from long-term observations.

For waste management, methane emissions from landfills and their effect on climate change are of serious concern. Current models for biogas generation that focus on the economic use of the landfill gas are usually based on first order chemical reactions (exponential decay), underestimating the long-term emissions of landfills. The presented study concentrated on the curve fitting and the quantification of the gas generation during the final degradation phase under optimal anaerobic conditions. For this purpose the long-term gas generation (240-1,830 days) of different mechanically biologically treated (MBT) waste materials was measured. In this study the late gas generation was modeled by a log-normal distribution curve to gather the maximum gas generation potential. According to the log-normal model the observed gas sum curve leads to higher values than commonly used exponential decay models. The prediction of the final phase of landfill gas generation by a fitting model provides a basis for CO(2) balances in waste management and some information to which extent landfills serve as carbon sink.

How to enhance humification during composting of separately collected biowaste: impact of feedstock and processing.

Conventional parameters (loss on ignition, total organic carbon, total nitrogen, C/N-ratio, respiration activity (RA4), compost status (= 'Rottegrad'), NH4-N and NO3-N) are not correlated to humification. At best, they provide information on the biological stability (status of degradation) of composts. Humic substances which are a source of stable organic matter and nutrients are discussed as a parameter describing compost quality. Thus, in the present research project a photometric method evaluating humic acids was used to characterize the quality of 211 Austrian and foreign composts made from source-separated collected biowaste or sewage sludge. Furthermore, parameters influencing the formation of humic acids during the rotting process were investigated by implementing rotting experiments in the laboratory as well as in composting plants. The analysed composts showed humic acid contents between 2.5 and 47 %, calculated on a organic dry matter (oDM) basis. In addition to the duration of treatment the main influence on humification was the feedstock used. Stabilized sewage sludge, biowaste after intensive anaerobic pre-treatment or biowaste with low reactivity (RA4) or uniform composition (e.g. mainly grass) showed a low formation of humic acids. For optimum humification the feedstock needed to contain components that are well balanced from scarcely to easily degradable compounds. Processing also influenced humification. Open windrow systems and reactor systems allow the same quality to be produced when operated well, but optimizing mineralization (e.g. very intensive aeration) showed negative effects. The positive condition required for humification is an unhurried (not too intense) degradation with long-lasting biological activity in which microbes have enough time to use the metabolic products of degradation for humification.

Landfill modelling in LCA - a contribution based on empirical data.

Landfills at various stages of development, depending on their age and location, can be found throughout Europe. The type of facilities goes from uncontrolled dumpsites to highly engineered facilities with leachate and gas management. In addition, some landfills are designed to receive untreated waste, while others can receive incineration residues (MSWI) or residues after mechanical biological treatment (MBT). Dimension, type and duration of the emissions from landfills depend on the quality of the disposed waste, the technical design, and the location of the landfill. Environmental impacts are produced by the leachate (heavy metals, organic loading), emissions into the air (CH(4), hydrocarbons, halogenated hydrocarbons) and from the energy or fuel requirements for the operation of the landfill (SO(2) and NO(x) from the production of electricity from fossil fuels). To include landfilling in an life-cycle assessment (LCA) approach entails several methodological questions (multi-input process, site-specific influence, time dependency). Additionally, no experiences are available with regard to mid-term behaviour (decades) for the relatively new types of landfill (MBT landfill, landfill for residues from MSWI). The present paper focuses on two main issues concerning modelling of landfills in LCA: Firstly, it is an acknowledged fact that emissions from landfills may prevail for a very long time, often thousands of years or longer. The choice of time frame in the LCA of landfilling may therefore clearly affect the results. Secondly, the reliability of results obtained through a life-cycle assessment depends on the availability and quality of Life Cycle Inventory (LCI) data. Therefore the choice of the general approach, using multi-input inventory tool versus empirical results, may also influence the results. In this paper the different approaches concerning time horizon and LCI will be introduced and discussed. In the application of empirical results, the presence of data gaps may limit the inclusion of several impact categories and therefore affect the results obtained by the study. For this reason, every effort has been made to provide high-quality empirical LCI data for landfills in Central Europe.