Rapid sludge treatment through phagotrophic algae cultivation using waste activated sludge pretreated with high-intensity alkali

The mixotrophic alga <i>Ochromonas danica</i> efficiently synthesizes lipids through phagotrophy and osmotrophy. This study explored its growth using alkali-pretreated waste activated sludge (WAS). Pretreatment at pH 12 for 24 h released 56.4% of WAS organics as microbial cells and dissolved substrates. During 96-hour cultivation, <i>O. danica</i> consumed 91.5% of liberated cells and 63.0% of dissolved organics, converting 28.6% of WAS organics into algal biomass containing 42.4% lipids (dry weight). This algal assimilation step proved pivotal in an integrated 30-day process (1-day alkali treatment, 4-day algal growth, 25-day anaerobic digestion), which achieved 61% total organic reduction - a 2.7-fold acceleration over anaerobic digestion with untreated sludge (23% in 30 days) and a 1.3-fold acceleration over anaerobic digestion with alkali treatment alone (48% in 30 days). The synergy between alkaline solubilization and algal phagotrophy thus simultaneously enhance WAS organics reduction while producing lipid-rich biomass, presenting a time-efficient strategy for sludge management.