Natural Freezing-Thawing and Its Impact on Dewaterability and Anaerobic Digestibility of Biosludge

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.