Methane fermentation of aquatic biomass

Abstract

Four aquatic biomass species were anaerobically fermented to methane as part of an evaluation of these biomass as potential energy resources. Two freshwater weeds (Duckweed ( Lemna sp.) and Hydrilla verticillata) and two marine algae ( Gracilaria ceae and Ulva lactuca) were evaluated. Volatile solids, ash content, calorific values, and elemental analyses are reported for these biomass. All four were fermented at mesophilic (37°C) conditions in 50 1 CSTR units using a rich nutrient feed of essentially equal parts by weight sewage sludge and aquatic biomass in an approximately 5% solids concentration slurry and with a 26-day retention time. In addition, the two freshwater weeds were evaluated in a similar manner at thermophilic (60°C) conditions. Bioconversion efficiency was based on measured energy out, as methane, and measured energy in, as calorific values of the biomass. It was found that 25 to 34% of the energy value in the freshwater weeds was recovered at mesophilic conditions, a low value perhaps due to the fact that steady-state conditions were not reached in the fermenters. For the marine species, 27 to 45% of the energy value was recovered under the same conditions. Conversion of the freshwater weeds at thermophilic conditions, however, was from 32 to 46%. It was found that by assuming all total volatile solids in the seaweed had an oxidation state equivalent to cellulose the same bioconversion efficiency was obtained as measured directly, appearing to indicate a high fraction of biodegradable polysaccharides. Freshwater weeds, however, demonstrated a much lower conversion based on calorimetric values than with the assumption that all volatile material was cellulosic in nature. This may indicate that bioconversion of a cellulosic fraction occurred, but that residual higher energy components in the biomass such as lignin were nonbiodegradable under these conditions. Results of the bioconversion of alkaline pretreated (saturated lime) Duckweed were approximately equal to those with the untreated biomass. An inhibition investigation to explain lower than anticipated bioconversion was conducted on the two marine species based on their high degree of sulfonation of polysaccharides. This hypothesis proved to be invalid, and slow acclimatization of innoculating microorganisms was given as a possible explanation of observed results. Further, in situ or batch fermentation were carried out at mesophilic conditions using minimal inoculum. Both freshwater aquatic biomass were evaluated in 2 1 units, while Hydrilla was also evaluated in a 50 1 unit. It was found that 80% of the methane was evolved in the first two months of operation. Moreover, bioconversion performance in these simple mesophilic in situ units were equal to that in the CSTR units at thermophilic conditions, namely 34–46% conversion. Obtaining baseline biomass conversion results in simple in situ units appears most practical. Pretreatment alternatives and novel processing techniques which improve on conventional CSTR technology appear to be required to improve bioconversion energy efficiencies.