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Analytical methods of examination avoid time-consuming regulations
| | Samples collected from all 16 mechanical-biological waste treatment plants (MBWTs) in Austria were analysed during a 33-month research project. The project, which is supported by the fund for the promotion of scientific research, began in January 2007 and will be concluded in September this year.
By Ena Smidt, Katharina Böhm, Johannes Tintner and Erwin Binner from the Institute of Waste Management of the University of Natural Resources and Applied Life Sciences (Boku) in Vienna
The aim of the project is to employ modern analytical methods such as infrared spectroscopy and thermal analysis to forecast time-consuming, expensive and error-prone parameters. In accordance with Austrian landfill regulations, since January 2004 residual waste may no longer be deposited at an untreated state. Apart from incineration, mechanical-biological treatment is another method of stabilising the material in terms of gaseous emissions.
The criteria for depositing waste at mass landfill sites are determined by threshold values. The compliance with respiration activity (AT4), gas generation (GS21) and calorific value must be proven prior to depositing waste. Earlier examinations showed that the reactivity parameters AT4 and GS21 are reflected in the infrared spectrum and the calorific values are reflected in the thermal behaviour of the waste. In this project, forecasting models are being developed that are based on a large number of samples, infrared spectroscopic and thermoanalytical analyses as well as statistical methods of evaluation.
Photo: Institut für Abfallwirtschaft, Boku
The criteria for depositing waste at mass landfill sites are determined by threshold values.
The models are to enable the practical application of these fast and environmentally friendly methods, which supply further information about the material in addition to the determining of parameters. Due to the great number of samples, it was possible to gain a good overview of the situation of the plants by means of the additional determination of conventional parameters.
Sample taking and analysis
The forecasting models require a great number of samples, which represent a broad range of varying reactivities and calorific values. The samples were taken at various times throughout the year in order to record seasonal fluctuations in the composition of the input material. Thus it was later possible to derive the regularity of the process management. An approximate total of 300 samples were analysed.
The samples made it possible to measure the gaseous composition of the material throughout the rotting process. Figure 1 shows an example of the gaseous composition, which was measured at two different points of one plant in various stages of the rotting process. Most plants provided a similar picture. Apart from the infrared spectroscopic and thermoanalytical analyses, respiration activity (AT4), gas formation potential (GS21) and calorific value formed the basis for creating a model. In order to complete the picture, the parameters ignition loss, total organic carbon (TOC), total nitrogen (TN), ammonia and nitrate nitrogen, low-molecular-weight carbonic acids (from acetic acid to valeric acid), conductivity and pH value of each of the rotting phases were determined. In doing so, ongoing processes were not analysed but various batches that were available when taking the samples. Nevertheless, plant-specific flows of each of the parameters can still be illustrated.
However, it also showed that the deterioration caused by fluctuations in process management was not always regular. Measured according to the parameters pH value, total nitrogen (TN in per cent of dry mass, DM) and conductivity (Cd in mS cm-1), the respiration activity of the samples was analysed prior to and after eight weeks of constant treatment. It showed that the respiration activity in most of the materials had greatly reduced after eight weeks and that some MBWT samples had already reached the threshold value required by the Austrian landfill regulations of 7 mg O2 g-1 DM.
In the course of the project the validity of the conventional parameters is also to be analysed. Total parameters such as total organic carbon (TOC) and total nitrogen (TN) only provide a rough picture of the decomposition process. The dynamics of the processes, particularly the initial intensive decomposition phase, are well reflected in the amounts of ammonia-nitrogen and low-molecular-weight carbonic acids. As expected, the biological tests (respiration activity and gas formation) supply the greatest amount of information on the behaviour of the material and the process kinetics.
Preparing residue-free samples
The diverse structures and varying consistence of the products and substances which make up the residual waste require differing steps in their mechanical treatment. It is quite challenging to produce a residue-free sample of MBWT material of the corresponding exactitude for weighing in the milligram range (for infrared spectroscopy and thermoanalysis) and the required reproducibility.
The undertaking only succeeded after several weeks spent finding the best possible way of breaking down the samples. Due to the high proportion of fibrous content (cellulose) to be found in fresh waste, multi-step processing using a cutting mill prior to grinding in a centrifugal mill proved to be necessary in order to produce particles of a uniform size. The latter is absolutely essential for good reproducibility.
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