Abstract
Lignocellulosic residues, such as straw, are currently considered as candidates for biogas production. Therefore, straw fermentations were performed to quantitatively estimate methane yields and cell counts, as well as to qualitatively determine the microbiome. Six fully automated, continuously stirred biogas reactors were used: three mesophilic (41 °C) and three thermophilic (58 °C). They were fed every 8 h with milled wheat straw suspension in a defined, buffered salt solution, called ‘synthetic manure’. Total reflection X-ray fluorescence spectrometry analyses showed nickel and tungsten deficiency in the straw suspension. Supplementation of nickel and subsequently tungsten, or with an increasing combined dosage of both elements, resulted in a final concentration of approximately 0.1 mg/L active, dissolved tungsten ions, which caused an increase of the specific methane production, up to 63% under mesophilic and 31% under thermophilic conditions. That is the same optimal range for pure cultures of methanogens or bacteria found in literature. A simultaneous decrease of volatile fatty acids occurred. The Ni/W effect occurred with all three organic loading rates, being 4.5, 7.5, and 9.0 g volatile solids per litre and day, with a concomitant hydraulic retention time of 18, 10, or 8 days, respectively. A maximum specific methane production of 0.254 m3 CH4, under standard temperature and pressure per kg volatile solids (almost 90% degradation), was obtained. After the final supplementation of tungsten, the cell counts of methanogens increased by 300%, while the total microbial cell counts increased by only 3–62%. The mesophilic methanogenic microflora was shifted from the acetotrophic Methanosaeta to the hydrogenotrophic Methanoculleus (85%) by tungsten, whereas the H2-CO2-converter, Methanothermobacter, always dominated in the thermophilic fermenters.
Highlights
Lignocellulosic residues, such as wheat straw or other straw varieties, are currently considered strong candidates to substitute energy crops for low-cost, sustainable biogas and energy production, without competing for food production or land-use change [1].About 30 million metric tons of cereal straw with 0.8 kg straw/kg grain are produced annually in Germany [2], which represents 14% of the total amount of agricultural residues.Whereas in China, the total output of rice, wheat, and corn straw reached 764 million tons [3]
Gradual purging of the trace elements in the inoculum sludge, technical modifications, and process stabilization occurred in the start-up phase of about 140 days and an hydraulic retention times (HRT)
The results showed that, for a long-term fermentation with an automated, intermittently-fed continuously stirred tank reactor system (CSTR) system, the adapted microbiology worked quite optimally, without the need for any effluent recirculation or an extra thermal/chemical pre-treatment of wheat straw
Summary
Lignocellulosic residues, such as wheat straw or other straw varieties, are currently considered strong candidates to substitute energy crops for low-cost, sustainable biogas and energy production, without competing for food production or land-use change [1]. Whereas in China, the total output of rice, wheat, and corn straw reached 764 million tons [3]. These agricultural residues are mostly unused or even burnt on the field. When such residues are converted into biogas via anaerobic digestion, the digestate may be returned to the fields as valuable inorganic and organic fertilizer to enhance crop production.
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