October 25, 2006 | General

Fifth ORBIT Conference Probes Anaerobic Digestion (Germany)

BioCycle October 2006, Vol. 47, No. 10, p. 53
Biogas projects are described at the Weimar, Germany sessions that brought many researchers together to compare latest advances.

RECOVERY of organic matter and its importance for energy, nutrient and humus supply was a focal point for ORBIT 2006, which took place at the University of Weimar in Germany, September 13-15, 2006. The theme was “Biological Waste Management – From Local to Global.” A number of the sessions were aimed at new developments in anaerobic digestion.
Explained Prof. Werner Bidlingmaier, who served as Orbit’s president this year, “traditional subjects of composting and anaerobic digestion were presented together with various matters dealing with technology and product quality. The new aspect addressed the complex issues of sustainable energy recovery from biomass and the need for a strategy to preserve humus content world-wide, especially in the endangered zones with devastation tendencies.”
In addition to Dr. Bidlingmaier, others directly involved in the Conference organization included Marco de Bertoldi of the University of Udine; Luis F. Diaz of Cal Recovery, Inc.; Josef Barth of the European Composting Network; and Eckhard Kraft, Scientific Secretary. The program offered best practice oriented sessions by 230 presenters from 38 countries, including presentations by R. Cossu of the University of Padua, Martin Kranert of Stuttgart University, Harold Keener of The Ohio State University, Heribert Insam of Innsbruck University; and Ed Stentiford of the University of Leeds.
Innovative Technologies For Hotels
Describing a technology called Modulaare, which combines anaerobic treatment of organic wastes and membrane filtration for wastewater treatment in one decentralized system, Martin Kranert of Stuttgart University provided details of Germany and its tourist industry. The objective of Modulaare is to test the practical operation of the unit as well as its power production. The modular system combines an anaerobic digestion facility for quantities of organic residuals (kitchen, garden) and a membrane facility to purify the hotel’s wastewater. Output will be purified process water, fertilizer and biogas.
In the context of this research and demonstration project, a membrane facility will be installed in a Turkish hotel complex. Its effectiveness and the different areas of application for the cleaned process water will be determined and examined. The resulting excess sludge will go directly into the fermentation system. The analysis program of the biogas module covers both the technical adjustment to the input material and the optimization and simplification of the handling. Cosubstrates, processing, digester, fermentation residues and biogas will be examined. The wastewater resulting from the fermentation process will be supplied to the membrane facility. A further important aspect is the generation of an energy concept for the optimal use of the biogas and the combination with further regenerative sources of energy.
Digestion Of MSW
In The Batch Process
In a paper on MSW digestion in a thermophilic sequential batch process, R. Adhikari and his colleagues at the Asian Institute of Technology in Thailand presented data on how digestion is classified into two processes – batch and continuous process. The sequential batch anaerobic composting (SEBAC) process uses a combination of high solid fermentation and leachate recycling between new and stabilized reactors to provide moisture, nutrients and inocula for rapid start-up. Researchers have proved that biogas production is relatively low under mesophilic (37°C) than thermophilic (55°C) conditions. In addition, the digestion period can be considerably shortened under higher operating temperature. State-of-the-art research on sequential batch anaerobic digestion needs further investigation, especially by employing several cycles of sequencing operation with increased leachate cross-recirculation rate for an attempt to further optimize the process performance.
A pilot-scale sequential batch anaerobic digestion process in three consecutive cycles under thermophilic condition was investigated. Adhikari and colleagues evaluated the effect of increasing leachate cross-circulation rates at sequential cycles with cross-recirculation rate of 0.34, 0.46, and 0.58 m3 leachate/m3 of waste per day for Cycle I, II, and III respectively. This process showed increasing specific methane yield of 184, 217, and 239 L CH4/kg VS which correspond to process efficiency of 63 percent, 74 percent and 82 percent for Cycle I, II, and III respectively. The start-up period decreased from seven days in Cycle I to five days in Cycle III. Higher recirculation rates enhance biogas production but also shorten digestion period. Therefore, thermophilic sequential batch anaerobic digestion, along with several cycles and an increased recirculation rate, enhanced the organic solid waste which generates high biogas yield and high process efficiency.
Biogasification For Kitchen Waste Management In Japan
A small proportion of kitchen waste is composted in Japan – only 0.1 percent – but municipal governments have been considering biogasificaton since it can generate energy. In three facilities that can treat 16 to 55 tons/day, engineers at Hokkaido University investigated characteristics of biogas, odor and liquid residues. Some scenarios were developed on how a major city might decide to adopt a biogasification facility.
At each site (located in Hokkaido), waste is separated into kitchen waste, combustible waste, noncombustible waste and resources (used paper, cans, glass, etc.) and placed in plastic bags for paid collection. Plastic bags for kitchen waste are collected twice a week and transported directly to biogasification facilities.
To investigate characteristics of biogas emissions, composition, odor concentration and liquid effluent were measured. At each facility, heat recovered from power generators was used for heating the methane fermentation tank, road heating in winter, and other applications.
In their concluding analysis, the main components of biogas were methane and carbon dioxide, with methane concentrations at 59 to 63 percent. Biogas amounts were 110 to 160 m3/ton of kitchen waste. Concentrations of hydrogen sulfide and nitrous oxide in biogas were about 1,000 ppm and less than 5 ppm respectively. Largest source of odor was the solubilization tank and odor index, measured by the triangular odor bag method. Measured values of BOD, COD, SS, total nitrogen and coliform bacteria count in the liquid effluent were high. The researchers proposed biogasification of kitchen waste in a major Japanese city based on results of the surveys.
Improving Efficiency Of Anaerobic Treatment Processes
The Department of Waste Management at Braunschweig Technical University in Germany is conducting a variety of waste recycling research. A main topic is pre and posttreatment related to anaerobic digestion, with vacuum-heat-extraction (VHE) demonstrating a good approach to residual and biowaste. Primary focus of digestion studies has been on the fermentation process.
Write T. Bahr, R. Liebeneiner and K. Fricke of Braunschweig’s Leichtweib Institute: “To enhance the efficiency, some changes need to be introduced in pretreating fermentable materials. Improvements are required in the fields of: Concentration of substances which ferment in an anaerobic way by abscission of nonfermentable matter; and Increase in bioavailability of organic matter.”
Pretreatment by conditioning considerably affects the degradation rate and consequently methane production and energy demand of biogas plants, explain the researchers. That’s why pretreatment has a key position in optimizing the whole system. Anaerobic degradation rates for digestion are on average 40 to 50 percent in relation to fermentable organic material. The VHE technology applies to both aspects (energy production and energy demand), leading to significant improvements in anaerobic processes. The ongoing work at the University of Braunschweig will determine the functional capabilities of VHE in terms of pretreatment technology for residual and biowaste as well as agricultural residues. Its purpose is to compile a complete treatment design with both ecological and economical balance sheet analyses.
Fermentative Production Of Biohydrogen From Organic Residuals
At the Hamburg University of Technology in Germany, R. Stegmann and colleagues are studying thermophilic production of biohydrogen from renewable biomass. Various substrates (raw and waste materials) and processing factors like temperature, pH, etc. are being investigated to increase hydrogen production.
For the investigation of the fermentative production of biohydrogen, laboratory tests were conducted at thermophilic process temperature of 60°C in batch operation. For inoculation of the process, heat pretreated digested sewage sludge was used. Previous investigations showed that the heat shocking of the mixed culture represents an effective method for enrichment of hydrogen producing bacteria. The model substrate glucose and seven more different carbon sources were tested. Glucose was investigated in two test systems – Sensomat System (500 ml) and a Completely Stirred Tank Reactor (CSTR) – with an optional pH control (30-1). Raw agricultural products and waste materials were only tested in the Sensomat System.
The highest specific hydrogen production was achieved for the monosaccharide glucose (10 g/l) in both test systems. In the CSTR at a controlled pH of 5.5, the accumulated values of the produced biogas resulted in the highest production of 280 Nl H2/kg VSS and 171 NI CO2/kg VSS ever achieved by the authors. That conversion of hydrogen from the substrate to the gaseous phase is equal to a yield of 56 percent (assuming a maximum hydrogen yield of four mol H2/mol substrate) or 112 percent (assuming the more realistic maximum yield of 2 mol H2/mol). Thus, based on the first results from the CST operation, it can be assumed that both the scale up and pH control are promising measures for increasing the fermentative production of biohydrogen. In the Sensomat System, a high hydrogen production of maximal 221 Nml H2/g VSS (yield of 45 percent and 90 percent resp.) was achieved for glucose.
The values of hydrogen produced from corn and potato starch (both 20 g/l) are 211 Nml H2/g VSS and 123 Nml H2/g VSS respectively. Those hydrogen production values correspond to a conversion yield of 75 percent for corn starch and 45 percent for potato starch, assuming the more realistic maximum yield of 2 mol H2/mol. Agricultural products and waste materials are very promising substrates for the production of biohydrogen.
For a list of the anaerobic digestion research papers presented at the Orbit Conference, e-mail secretary@orbit-online.net. Or visit www.orbit-online.net.

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