Search

Dewatering of pulp and paper mill biosludge is challenging, and it can make up half of overall wastewater treatment costs. By harvesting energy provided by nature, freezing-thawing can notably alter the physical structure of sludge flocs, thereby influencing dewaterability and anaerobic digestibility. Samples of biosludge from three pulp and paper mills (sulfite, kraft, and semi-chemical pulping) as well as biosludge digestate (i.e., biosludge after anaerobic digestion) were subject to freeze-thaw treatment, and they were subsequently tested in terms of dewaterability by using a gravity filtration-crown press unit, and anaerobic digestibility by means of biochemical methane potential assays. Gravity filtrate from dewatering of freeze-thaw treated biosludge was also tested for anaerobic digestibility. Freeze-thaw treatment improved the dewaterability of biosludge mill samples to a larger extent than dewatering polymer. Treatment at −10°C before dewatering increased the dry solid content of the dewatering cake from 13% to 21% (sulfite mill), from 7% to 26% (kraft mill), from 10% to 20% (digestate after 35 days of digestion), and from 17% to 23% (digestate after 60 days of digestion). Biosludge from the semi-chemical pulping mill was only dewaterable after freeze-thaw treatment, which enabled a final cake solid content of 45%. In contrast, the anaerobic digestibility of biosludge and digestate improved, if at all, only to a relatively small extent. A strong improvement in digestibility was only observed in the case of gravity filtrate from dewatering of freeze-thaw treated biosludge (sulfite mill), where the specific biogas yield increased from 111 to 310 mL/g chemical oxygen demand added. Visual inspection on untreated and freeze-thaw treated biosludge confirmed the assumption that the strong effect on dewaterability was caused by irreversible compaction and dehydration of sludge particles. Evidence for widespread rupture of bacterial cells was not confirmed, which may explain the comparably small effect on anaerobic digestibility. [ABSTRACT FROM AUTHOR]

Wastewater generated during pulp and paper production usually contains large amounts of organic matter that may be converted to biogas through anaerobic treatment. These effluents can also contain a variety of inhibitors such as resin acids and long-chain fatty acids (RFAs) among other compounds. A multivariate data analysis was conducted with performance and water quality data from an anaerobic reactor at a pulp and paper mill, which provides unprecedented evidence for the long-term inhibitory effects of RFAs during full-scale operation. Although RFAs in the reactor influent did not cause serious process disturbance, they significantly contributed to the variability in the percentage removal of soluble chemical oxygen demand (percent sCOD removal) (∼45% to ∼75%), as well as the variability in volatile fatty acid concentration in the effluent (2 to >20 meq/L). Furthermore, a linear correlation was found between RFA concentrations in the reactor sludge bed and the percent sCOD removal. An effective strategy to remove RFAs before anaerobic treatment is upfront removal of particles within the primary clarifier. Based on an exploratory model, a graphical method is introduced that allows for quick assessment of the extent to which resin acids are sorbed to particles, and associated potential for upfront settling. When the pH of high-solids wastewater is below 7, the majority of resin acids will be associated with particles and a considerable fraction of these can therefore be removed by means of settling or floatation. Finally, a novel liquid chromatography-mass spectrometry method that rapidly quantifies major RFA constituents was developed. In the full-scale reactor, dehydroabietic acid (DHA) represented the bulk (∼92%) of the resin acids within the granular sludge. DHA in sludge warrants special attention and should be monitored closely. [ABSTRACT FROM AUTHOR]