Valorization of organic wastes to high-value biochemicals: Performance, economic and environmental impact assessment of free nitrous acid, heat, and sonication pretreatment strategies

This study evaluates the effects of free nitrous acid (FNA), heat, and sonication pretreatments on waste-activated sludge (WAS) and mixed substrates for enhanced recovery of volatile fatty acids (VFA) and phosphorus. It highlights the superior performance of combining pretreatment with co-fermentation over fermenting pretreated WAS alone. Among the three pretreatments applied to WAS, heat and sonication favored methanogenic activity, resulting in 1.4–1.9 times lower VFA yields than FNA pretreatment. FNA pretreatment promoted acidogenic conditions due to residual NO 2 − -N, producing 135 mg COD/g VS (p < 0.05), significantly higher than heat, sonication, or untreated sludge. These acidogenic conditions also enhanced apatite phosphorus (AP) migration by 40–45 %, doubling PO 4 3- -P release relative to other methods. In mixed substrates, all pretreatment methods improved VFA yields by 15–25 %, with FNA achieving the highest yield of 443.6 mg COD/g VS, 1.25 times greater than untreated mixed substrates. Furthermore, pretreatment enhanced VFA production kinetics (k 1 ) by 1.4–1.5 times, offering the potential to reduce fermentation reactor volume and associated capital costs. Economic analysis demonstrated that FNA pretreatment yielded a net revenue three times higher than untreated sludge and 12 % higher than untreated mixed substrates. In contrast, heat and sonication resulted in minimal or negative economic returns, and struvite recovery was unfeasible due to the limited availability of soluble PO 4 3- -P. Life cycle assessment revealed that sonication pretreatment resulted in the highest environmental impact among the three strategies. • FNA pretreatment of WAS favored acidogenic activity with higher AP migration. • Heat and sonication treatment enhanced methanogenic activity and AP accumulation. • FNA pretreatment of the mixed substrate enhanced the VFA production by 1.25 times. • Pretreatment accelerated the VFA production kinetics by 1.4–1.5 times. • FNA pretreatment is cost-effective for VFA recovery from WAS and mixed substrate.