March 23, 2007 | General

High Quality Composts From Anaerobic Digestion Residues

BioCycle March 2007, Vol. 48, No. 3, p. 55
Supporting the trend to increased biogas production, cocomposting of residuals was found to be an appropriate alternative of closed loop management in Austrian research.
Katharina Meissl and Ena Smidt

IN AUSTRIA, approximately 480,000 tons of waste per year are generated and collected using a biowaste bin system. Composting is a commonly used method in the processing of biogenic residuals. Another approach, especially for leftovers, is anaerobic digestion with biogas production. Over the last decade, much attention has been given to improving biogas yields, with less attention to the solid residues in complex cellular structures, e.g. lipids and lignocellulosic substances. Due to the high remaining content of organic matter and reactivity potential, further treatment is required.
In our research at the BOKU – University of Natural Resources and Applied Life Sciences in Vienna, Austria, we focused on advanced aerobic treatment of residues from anaerobic digestion of leftovers and kitchen waste originating from an industrial waste treatment plant by means of cocomposting in order to reveal potential synergistic effects.
In the cocomposting process, residues from anaerobic digestion are composted with biogenic waste with intent to “crack” anaerobically stabilized and resistant organic matter under aerobic conditions. To illustrate the usable potential of residues from anaerobic digestion, the latter were aerated during an additional aerobic liquid phase. The advanced aerobic treatment of anaerobic residues was performed during the liquid phase (dilution 1:1 with deionized water) in shaken Erlenmeyer flasks (volume of 250 ml) at 30°C.
Cocomposting of residues from anaerobic digestion were carried out in lab-scale experiments in small glass vessels in a climatic chamber to show synergistic effects. Furthermore, two reactors of 7 L volume were operated for temperature control. The substrates used are shown in Table 1.
Process control was performed using modern analytical methods such as Fourier Transform Infrared (FT-IR) spectroscopy and thermogravimetry. Determination of extractable humic acids was carried out to assess the composting process as humic acids represent an appropriate quality parameter in addition to standard parameters established by the Austrian Compost Ordinance (2001). It is well known that humic substances offer several environmental benefits, such as carbon sequestration and positive effects on soil and plants: suppression of plant diseases, slow release of nitrogen and nutrients, positive effects on moisture, temperature and structure of soils. Therefore the improvement of humic acid contents by means of composting is a target to be aimed for, leading to the production of high quality marketable composts.
For chemical analyses, infrared spectroscopy and thermogravimetry, the material was air-dried, ground in an agate mill and screened through 0.63 mm. For biological tests, fresh material (< 11.2 mm) was used.
Humic substances were extracted using a 0.1 molar solution of sodium pyrophosphate (pH 10.5) and separated according to their solubility in acidic or alkaline solutions according to Gerzabek et al. (1993).
Infrared spectroscopic investigations were carried out in the mid-infrared area (wavenumber 4000-400 cm-1) using the KBr technique. Table 2 compiles relevant infrared bands for compost assessment.
Thermal analyses were performed using an STA 409 CD Skimmer (Netzsch GmbH) that enables the recording of thermograms and DSC-curves simultaneously. All samples were combusted under oxidizing conditions within a temperature range from 30 to 950°C.
The biological tests respiration activity (RA4d) of the Input (In) was 165 mg O2 g-1 DM and gas forming potential (GFP21d) was 97 NL kg-1 DM. Due to the limited capacity of the plant, the retention time of anaerobically digested leftovers and kitchen waste was only two weeks. After two weeks of anaerobic digestion, the respiration activity (RA4d) was 16.9 mg O2 g-1 DM and gas forming potential (GFP21d) was 30 NL kg-1 DM. Both parameters exceeded the limits established for stabilized MBT-waste by Austrian Standards (Austrian landfill directive 2004) that requires a respiration activity (RA4d) of < 7 mg O2 g-1 DM and a gas forming potential (GFP21d) of < 20 NL Gas kg-1 DM.
After two weeks of anaerobic digestion, the output material (Out) was aerated to “crack” the anaerobic stabilized organic matter in order to advance the stabilization process by aeration. Figure 1 represents the infrared spectra of the input material for anaerobic digestion (In), the output material after two weeks (Out), and samples that underwent the subsequent aerobic treatment (aerated Out 8 and 14 weeks) in the liquid phase. The spectrum of the input material for anaerobic digestion (In) characterizes a nondegraded material. Indicator bands are the aliphatic methylene bands at 2920 and 2850 cm-1, the bands of aldehydes, ketones, carboxylic acids and esters at 1720 cm-1, the amide II bands at 1580 and 1540 cm-1 as well as the amine band at 1320 cm-1and the band of 1240 cm-1 assigned to the C-N vibration of amines and amides and the C-O vibration of carboxylic acids. After two weeks of anaerobic treatment all indicator bands decrease or disappear and the input material seems to be stabilized. However, during aerobic treatment, an increase (8 weeks) and incidental decrease (14 weeks) of the indicator band at 1240 cm-1 is detectable. A temporary increase of this band might be due to degradation of proteins including the biomass from anaerobic digestion, indicating the possibility of decomposing anaerobically stabilized organic matter under aerobic conditions.
The thermograms show the mass loss during combustion at a constant heating rate. Figure 2 illustrates the changing thermal behavior (thermograms) depending on anaerobic and aerobic treatment of the samples (In), (Out), (aerated Out 1 and 14 weeks). Generally the thermograms of biogenic materials are characterized by three to four characteristic steps of weight losses. The first step at ~80-105°C indicates the loss of residual water. Two peaks are assigned to the loss of organic matter (at around 300°C and 470°C). If carbonates are present, their decay takes place at around 690°C. Stabilization is reflected by a curve shift towards higher temperatures.
In Figure 2, stabilization during anaerobic digestion (In to Out) is illustrated by the curve shift towards higher temperatures. During the aeration of the anaerobic residues (Out) the curve shifts back towards a lower temperature range, indicating the degradation of complex structures and biomolecules as previously shown by infrared spectroscopy (Figure 1). After 14 weeks, the curve shifts back again towards higher temperatures and reaches approximately the temperature range and stability of the initial output material from anaerobic digestion.
FT-IR spectroscopy and thermogravimetry both provide more detailed information on the progressing processes.
A fundamental parameter of compost quality is the content of humic acids. Figure 3 shows the curve of humic acid contents of the variants investigated. The highest content of humic acids was found in the variant containing the reference and output material from anaerobic digestion (R+Out). The mixture containing input material for anaerobic digestion (R+In) did not reach the reference value for humic acid formation. As illustrated in Figure 3, cocomposting experiments in the glass vessels and in the reactors (7L) shown as dotted line are very similar to each other. Numerous organic compounds are involved during the humification process; to this regard, the composition of input material for anaerobic digestion, especially leftovers, did not provide the necessary building blocks for humification. Former investigations by Smidt et al. (2004) have shown that humification is the outcome of a well-balanced mixture of organic compounds, microbial activity and moderate aeration. However, anaerobic digestion seems to be capable of providing suitable organic compounds for advanced aerobic treatment. It has to be emphasized that the residues from anaerobic digestion in the presented process were relatively reactive due to the short retention time in the reactor (see RA4d and GFP21d). This fact could be the reason that enough biodegradable components are still available promoting the microbial activity.
Figure 4 shows the content of humic acids (black) and the remaining organic matter (gray) at the beginning (a) and the end (b), (b7L) of the process. Humification and mineralization are both identifiable. In the rotting processes of the input material for anaerobic digestion (In) and the reference mixed with input material (R+In), mineralization got the highest values, while humification did not reach the level of the reference contents. The decrease of organic matter in the process (R-Out) observed yielded the lowest values, while humification was the best compared to the other variants.
It is well known that high quality composts can increase and enrich the terrestrial carbon pool. The decrease of organic matter in soils causes momentous problems in some European areas (COM (2002) 179, Montanarella, 2002) and worldwide. Therefore, it is aimed to produce high quality composts rich in stable organic matter to remediate agricultural soils and to prevent soil organic matter depletion.
After a six month cocomposting process of residuals from anaerobic digestion and biogenic waste, the final products of the composting plant near Salzburg mentioned above reach humic acid contents of about 30-34 percent ODM. Humic acid contents in a range from 5 percent ODM to 45 percent ODM were found in foreign and Austrian composts.
It can be concluded that cocomposting of residues from anaerobic digestion leads to high quality composts. Further investigations will be performed using various residues from anaerobic treatment such as sewage sludge and residues from anaerobic digestion of different biogenic input materials. A crucial point is the retention time in the digestion reactor depending on plant capacities. The influence of remaining reactivity and available substrate components on humification will be in the focus of interest. Modern analytical methods such as infrared spectroscopy and thermal analysis should support this purpose.
With respect to the increasing trend of biogas production, cocomposting of residues from anaerobic digestion is an appropriate alternative of closed loop management.
More details concerning cocomposting of residues from anaerobic digestion can be found in our upcoming paper, “High Quality Composts By Means Of Cocomposting Of Residues From Anaerobic Digestion” (Meissl and Smidt in the Spring 2007 issue of Compost Science & Utilization).
The authors are with the Institute of Waste Management, BOKU – University of Natural Resources and Applied Life Sciences, Vienna, Austria. K. Meissl can be e-mailed at:
Gerzabek, M.H., Danneberg, O. & Kandeler, E. 1993. Bestimmung des Humifizierungsgrades. In Bodenbiologische Arbeitsmethoden. Schinner , Öhlinger, Kandeler, Margesin (Eds), pp. 107-109.
Smidt E., Binner E. & Lechner P. 2004. Humic acid formation in composts – the role of microbial activity. Proceedings of the European Symposium on Environmental Biotechnology ESEB 2004, W. Verstraete (Ed), pp. 143-146.
Smidt, E., Schwanninger, M. 2005. Characterization of waste materials using FTIR Spectroscopy: Process Monitoring and quality assessment, Spectroscopic Letters, 38: pp. 247-253.

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