Annimari Lehtomäki

Biogas production from boreal herbaceous grasses--specific methane yield and methane yield per hectare.

The objective of this study was to determine the specific methane yields of four grass species (cocksfoot, tall fescue, reed canary grass and timothy) cultivated under boreal conditions as well as how harvesting time and year of cultivation affects the specific methane yields per ha. The specific methane yields of all grasses and all harvests varied from 253 to 394 Nl CH4/kg volatile solids (VS) added. The average specific methane yield of the 1st harvest of all grasses was higher than the 2nd harvests. In this study the methane and energy yields from different harvest years were ranged from 1200 to 3600 Nm(3) CH4/ha/a, corresponding from 12 to 36 MWh(CH4)/ha/a. The methane yield per hectare of the 1st harvest was always higher than that of the 2nd harvest per hectare because of the higher dry matter yield and specific methane yield. High biomass yield per hectare, good digestibility and regrowth ability after harvesting are important factors when choosing grass species for biogas production. If 30% of fallow and the second harvest of grassland were cultivated grasses and harvested for biogas production in Finland, the energy produced could be 4.9 TWh(CH4).

Storing energy crops for methane production: effects of solids content and biological additive.

The effect of storage on chemical characteristics and CH4 yield (taking into account loss of VS during storage) of a mixture of grasses and ryegrass, ensiled as such (low solids content) and after drying (medium and high solids) with and without biological additive, were studied in field and laboratory trials. Up to 87% and 98% of CH4 yield was preserved with low solids grass (initial TS 15.6%) and high solids ryegrass (initial TS 30.4%), respectively, after storage for 6months, while under suboptimal conditions at most 37% and 52% of CH4 yield were lost. Loss in CH4 yield was mainly due to VS loss, presumably caused by secondary fermentation as also suggested by increasing pH during storage. Biological additive did not assist in preserving the CH4 yield.

Development of microbial populations in the anaerobic hydrolysis of grass silage for methane production

Six batch leach bed (LB) reactors, installed in parallel and connected to a common upflow anaerobic sludge blanket reactor, were fed with grass silage and operated at 35 (+/-1) degrees C. The development and distribution of microorganisms, which firmly and loosely attached to solid materials, and presented in the leachate in the LB reactors, were investigated by 16S rRNA gene-based terminal restriction fragment length polymorphism and clone library analyses. The phylotypes and their relative abundance changed in the respective bacterial community throughout the 49-day run and showed differences between the communities. Large numbers of phylotypes were detected from day 10 onwards. On day 17, the majority of phylotypes in the bacterial community firmly attached to solid residues affiliated to the classes Clostridia and Bacteroidetes. There were high numbers of the phylotypes in the leachate bacterial community. They were closely related to members of classes Clostridia, Bacteroidetes, Betaproteobacteria, Alphaproteobacteria, Gammaproteobacteria, and OP10. The Clostridium-like species clearly dominated the bacterial community. Archaea were only found in the solid residues on day 17 and in the leachate on days 10 and 17. The majority of the Archaea fell within the hydrogenotrophic genus Methanobacterium. The organism assigned to the aceticlastic genus Methanosarcina was only present in the solid residues.

Impact of crop species on bacterial community structure during anaerobic co-digestion of crops and cow manure.

The bacterial communities in three continuously stirred tank reactors co-digesting cow manure with grass silage, oat straw, and sugar beet tops, respectively, were investigated by 16S rRNA gene-based fingerprints and clone libraries. The analyses revealed both clearly distinct and similar phylotypes in the bacterial communities between the reactors. The major groups represented in the three reactors were Clostridia, unclassified Bacteria, and Bacteroidetes. Phylotypes affiliated with Bacilli or Deltaproteobacteria were unique to the sugar beet and straw reactor, respectively. Unclassified Bacteria dominated in sugar beet reactor while in the straw and grass reactor Clostridia was the dominant group. An increase in organic loading rate from 2 to 3kg volatile solids m(-3) d(-1) resulted in larger changes in the bacterial community in the straw compared to grass reactor. The study shed more light on the evolution of bacterial community during anaerobic co-digestion of different crops and manure to methane.