The rupture of algal cell walls and hydrolysis of particulate algal bipolymers are the rate limiting steps in the AD process. Unicellular algae often have rigid cell walls composed of cellulose, hemicellulose, and chitin. In general, the amount of algal organic matter converted to biogas during AD varies from 20 to 60%. This level is unacceptably low, and effective methods to hydrolyze the recalcitrant fraction of algal biomass is an essential prerequisite for making methane generation commercially viable. Our group compares two alternative pretreatment methods: thermochemical and enzymatic.
Thermochemical Pretreatment of Algal Biomass to Enhance AD Process Two algal species known to have strong cells walls, Nannochloropsis species (sp). and Chlorella sp., were tested. Chemical pretreatment. Various amounts of 50% NaOH were added to 300 ml of algal solution to achieve final concentrations between 0-20 g NaOH/L (see figure below).
Thermal treatment. Samples were autoclaved at 121°C for 30 min. Thermochemical treatment. NaOH addition was followed by autoclaving as described above.
The influence of pretreatment on methane production was measured using biomethane potential (BMP) tests (see figure on the right). Thermochemical pretreatment was found to be superior to thermal and thermochemical methods alone for algal biomass hydrolysis. Both biogas and methane production increased by 30-40% compared to untreated samples. In addition, the optimal chemical levels for thermochemical pretreatment were determined such as doses of NaOH in the range of 4-5 g/L.
Enzymatic Pretreatment of Algal Biomass to Enhance AD Process Enzymatic hydrolysis of algal cell walls and biopolymers is a promising alternative to energy-consuming catalytic hydrolysis at high temperature. Enzymatic pretreatment can be used for algal cell disruption before the lipids extraction and to enhance AD efficiency. Enzymes are highly specific so it is important to identify which enzymes are able to hydrolyze target algal species efficiently. Therefore, our study includes the screening of commercially available enzymes. To date, we have characterized 14 carbohydrases, including cellulases, hemicellulases, and amylases, according to standard enzyme activity assays.