The Effect of Anaerobic Treatment of Pulp Mill Effluents on Reactor Performance and Granular Sludge

Pulp mill wastewaters contain high concentrations of organic compounds that can be partially converted into methane. Granulation of sludge is the key to successful treatment of pulp mill effluents in high rate anaerobic reactors. This research focused on the anaerobic treatment of a high strength pulp mill alkaline effluent from sulphite pulping (AE) of softwood chips, which was characterized by rich COD content and relatively high concentrations of resin acids and long-chain fatty acids (RFAs) known to be compounds inhibitory to methanogens. Two continuous reactor experiments were conducted to study the treatment of this softwood AE. In the first experiment, different concentrations of AE were treated for one month in four reactors. In the second experiment, increasing loadings of AE were added to the test reactor over a nine-month period. The negative impact of the addition of AE on reactor performance and granulation was confirmed, shown as poorer organic removals, lower biogas production, and smaller and weaker granules in the AE sludge. Larger amounts of RFAs (>50mg RFAs / g TSS sludge) were found to associate with the sludge solids receiving a high AE loading, with palmitic acid being the most dominant RFA. RFA loadings were found to be significantly negatively correlated to the biogas production. Therefore, RFAs were proposed to play an important role in the negative impact of AE. Microbial community analysis using pyrotag sequencing of amplified 16S rRNA genes from various samples collected from the two studies revealed that the communities of the sludge treating AE were very different than those of the AE-free sludge. In terms of the organisms affected by the addition of AE, Oscillospira was significantly positively correlated to AE loadings. The sludge treating AE also contained significantly lower percentages of methanogens. Furthermore, no clear sign of sludge acclimating to AE with enhanced biogas production was found. It is recommended that AE should be pretreated to reduce its toxic effect and to achieve greater COD removals toward higher biogas production.Ph.D