Enhancement of methane production from the anaerobic digestion of chemical pulp and paper mill effluents

This doctoral dissertation was completed under the Erasmus Mundus Joint Doctorate program Environomical Pathways for Sustainable Energy Services (SELECT+) for the degree of Doctor of Science (Technology) from Aalto University and the degree of Doctor from the Universitat Politècnica de Catalunya.Sustainability of resources such as energy, water and waste have become important drivers in our current economy. For large industries that are water and energy intensive like pulp and paper (P&P) mills, this is a specifically relevant issue. The large and heterogenous volumes of effluents in P&P mills make it a difficult task to properly treat before discharge. Anaerobic digestion is an efficient wastewater technology that cleans the effluent, reduces wasted sludge and simultaneously produces methane that can be further used as energy. The research work described in this thesis aims to enhance the methane production from the anaerobic digestion of different chemical P&P mill effluents. Through effluent characterization, variability of process parameters, promoting agents and the assessment of biochemical methane potential (BMP) tests, the benefits of three potential methods to enhance methane yields of wastewaters were evaluated. In addition, the conversion of chemical pulp fibers directly to methane was explored. The attained results described the different improvements that can be made to enhance methane production. Easily degradable effluents such as hydrolyzed filtrates and evaporator condensates contain high concentrations of sugars and acetic acid respectively, which could encourage an oversaturation of acids during acidogenesis. An inoculum to substrate ratio (ISR) of 2 proved to be the optimal in order to add the required buffering capacity to neutralize the pH and produce significantly high methane yields of up to 333 mLCH4/gVS. The high productivity of these effluents can be then considered for co-digestion with harder to treat effluents such as lignin-rich streams. Lignin clearly hinders methane production indicated by the negative linear correlation found between lignin content and methane yield. The co-digestion of lignin-rich effluent with evaporator condensates from neutral sulfite semi-chemical (NSSC) pulping proved to enhance the overall methane productivity of the mill´s wastewater treatment. Furthermore, the assessment of hydrotalcites (HT) addition indicated a contribution towards an increase in methane yield, faster production rates and a greater lignin removal. The poor performance of calcined HT suggests that the advantages of HT addition came from the layered sheet structure. Finally, besides establishing the methane potential in various P&P effluents, the examination of methane productivity of different pulp fibers and its products proved to be a promising new energy alternative to explore. Brown, oxygen delignified and bleached pulp gathered biodegradabilities of up to 90% and methane yields as high as 380 mLCH4/gVS. With the current changing bio-economy this last approach paves the way in exploring alternative and novel uses for chemical pulp mill products