Pretreatment of eucalyptus kraft pulp mill waste activated sludge to enhance anaerobic digestion

Abstract

Pulp and paper (P&P) mills are currently one of the largest industrial water consumers and effluent generators worldwide. The biological effluent treatment processes currently in use generate a large volume of sludge (waste activated sludge – WAS), which requires great efforts and high costs to manage. An interesting option for sludge management is anaerobic digestion, a means of reducing sludge volume while generating renewable energy that can be used in the industrial processes. However, anaerobic technology needs to be improved, mainly due to the 30- to 40-day retention time required to digest pulp mill sludge in conventional anaerobic bioreactors. The thesis was structured in 3 chapters written as scientific papers. Chapter 1 presented a literature review about the potential of anaerobic digestion for sludge treatment in P&P mills and the application of pretreatment methods to enhance methane production, decrease the retention time and, ultimately, decrease the volume of the digesters. A case study examining the potential of integrating pretreatment and anaerobic sludge digestion in a Brazilian kraft pulp mill is also presented. The literature review resulted in 52 matches in which 16 articles were related to anaerobic digestion or co-digestion of P&P sludge without pretreatment and 20 articles had to do with anaerobic digestion of P&P sludge after pretreatment. A large discrepancy among the presented results made it difficult to assess the suitability of anaerobic digestion and the actual impacts of the sludge pretreatment on the viability of the system. Simulations were carried out using current data from a Brazilian kraft pulp mill and indicated the important role of sludge pretreatment in potential methane production. Chapter 2 aimed the application of thermal, thermal-alkaline and mechanical pretreatment methods in order to promote the solubilization of organic matter to enhance the anaerobic digestion. Chemical analyses showed an improvement in organic matter solubilization, with an increase greater than 1.2-fold for total COD, 7-fold for soluble COD and 4-fold for BOD 5 . In general, thermal-alkaline pretreatment showed the best results for all the evaluated parameters, with the advantage of requiring lower temperature and retention time when compared to thermal conditions. Chapter 3 aimed to evaluate the potential of methane production from the WAS of a bleached kraft pulp mill applying biochemical methane potential (BMP). After 30 days of testing, cumulative methane production increased from 90 mL CH 4 /gVS (raw sludge) to 181-194 mL CH 4 /gVS with thermally pretreated samples, and up to 203-204 mL CH 4 /gVS with samples which were pretreated using the thermo-alkaline technique. The pretreatment conditions of 175 °C for 15 min at pH 11 were sufficient to achieve both a high solubilization of WAS and a high methane production. The results showed that pretreatment could significantly enhance the anaerobic digestion of kraft pulp mill WAS, making this an attractive new option for sludge management in this sector. Keywords: Anaerobic digestion. Methane yield. Pretreatment. Pulp and paper mill. Secondary sludge.