Evaluation of pretreatment methods for increased biogas production

Summary The eutrophication of the Baltic Sea causes increased production of fast growing algae species which has direct and indirect negative impacts on the marine ecosystem causing depletion of habitats and anoxic sea bottoms. Furthermore the beach cast of algae and sea grass has increased which has negative effect on tourism industry and recreational values along the coast line. It has been suggested that anaerobic digestion of beach cast algae would be a way of mitigating the negative impacts of algae decomposing along the coast and producing a high value fuel, methane, simultaneously. However, the physical structure of algae and sea grass makes conventional anaerobic digestion difficult and pretreatment is needed to aid mechanical handling and microbiological digestion. This study investigated the effect of mechanical and enzymatic pretreatment on three different kinds of algae (Fucus vesiculosus, Furcellaria lumbricalis and filamentous red algae) and one sea grass (Zostera marina). The algae with the most promising results from the pretreatment were selected for batch methane potential tests where both pretreated and raw algae were tested to see the effect on biogas production. In addition an energetic analysis was made to compare the energy used for pretreatment to the methane produced. The results showed that the concentration of soluble COD could be greatly increased by mechanical or enzymatic treatment compared to that of raw algae. However during enzymatic treatment the algae was heated to 50 °C during 6.5h which for several algae had an almost equal effect on increasing the COD concentration even without the addition of enzyme. The batch methane potential tests showed that mechanical pretreatment increased the rate of production of methane for the two types of algae investigated. Enzymatic pretreatment increased the methane production for most samples, however to a much smaller extent than mechanical pretreatment. For algae with more rigid surface structure (Fucus vesiculosus) the final methane potential was also increased by 100% however for filamentous red algae the final methane potential was the same as for raw algae. The energetic analysis showed that the energy use for pretreatment (for mechanical pressing or for heating of sample during enzymatic pretreatment) lowered the net energy gain so much that the best option, out of an energy perspective, would be to digest raw algae.