Pulp and paper mills have waste streams that can be used for the farming of algae. The mills wastewater treatment provides heat to the algae ponds and the flue gas provides carbon dioxide. A current project is aiming at producing oil, for biodiesel or lubricating oil, from mill-farmed algae. Ten pools have been constructed and seeded with different strains of algae. After harvest and separation of oil, there is a leftover algae pulp. The fate of this algae remnant is an important part in achieving economic and energy viability for the whole process.
The purpose of this work was to study the potential to produce biogas from algae grown in pulp and paper mill wastewaters, for different algae strains and pond conditions, with differing degrees of oil separation before the anaerobic digestion stage. The methane potential of the various algae pulps was tested by anaerobic digestion batch assays under thermophilic conditions for 23 days.
Pre-treatment of substrate is sometimes done in order to reduce the necessary retention time of biogas production. One batch of algae was subjected to either thermal pre-treatment or NaOH or comminution, to assess the effect on these methods on methane production.
Ammonia concentration and pH in the digestate was analyzed to monitor the anaerobic digestion process, and the algae pulp’s content of fat, protein and carbohydrates were analyzed to characterize the substrate. In addition, the dewaterability of algae and of algae digestate was examined, as the dewaterability affects transport costs and possible use of the digestate.
During the first year of operation, the algae in the range pond produced 186 ml CH4/g VS. The pre-treatments did not significantly affect the methane potential. The second year of operation saw more adapted strains of algae which are expected to produce somewhat different results [data from these experiments can be presented at the conference].
The research is of importance for the development of the biogas market as new substrates are needed to reach the volumes that are necessary to achieve economic viability for biogas plants. By characterizing the algae, it should be easier to see which role they are suitable to play in a co-digestion mix. Furthermore, the production of bio-oil and biogas together benefit the bioenergy system as a whole.
algae, anaerobic digestion, biogas, methane potential, pre-treatment, pulp mill, wastewater
Green Gas Research Outlook Sweden (GGROS 2015) Örnsköldsvik 23-25 mars 2015