Nansubuga, Irene, et al. “Enhancement of Biogas Potential of Primary Sludge by Co-Digestion With Cow Manure and Brewery Sludge”. International Journal of Agricultural and Biological Engineering, vol. 8, no. 4, Aug. 2015, pp. 86-94, https://www.ijabe.migration.pkpps06.publicknowledgeproject.org/index.php/ijabe/article/view/1616.