The objective of this study was to enrich the nutrient content of compost and to investigate the passivation and solubilization of plant micronutrients (Fe, Al, Cu, Ni, Zn, Na, Mn), macroelements (P, K, Mg, Ca), and heavy metals (Cr, Cd, Pb) during sewage sludge composting with nutrient-rich biomass ash additives. T0: 0%, T1: 3.5%, T2: 7.0%, and T3: 14.0% dry weight (DW), weight/weight (w/w) biomass ash was added to the sewage sludge + sawdust mixture (volume, 1:1) to obtain the final NPK content and monitored over a 45-day period. Sawdust was used as auxiliary material. The sequential extraction method was used to determine the elemental species. Cr, Cd and Pb showed higher affinity to the residual fraction and occluded in the oxide fraction, which decreased the bioavailability factor (BF) (<1% BF–Cr, 21% BF–Cd and 9% BF–Pb) compared to the control treatment (46% BF–Cr, 47% BF–Cd and 80% BF–Pb). As the amount of biomass ash increased (T1-T3), the percentages of residual Cr (Res-Cr) (10–65%), exchangeable Cd (Exc-Cd) and organically bound Cd (Org-Cd) (14% and 21%), and oxides-Pb (Oxi-Pb) (20–61%) increased. In all composts, Fe, Al, and Cu were associated with organically bound and oxides-entrapped fractions. More than 50% of total Mn and Mg were concentrated mainly in exchangeable fractions, suggesting high mobility and bioavailability (42% BF–Mn and 98% BF–Mg). Ni, Zn, and Na tended to be present in oxide-bound, organically bound, and residual fractions, while K and P were associated with exchangeable and organically bound fractions. The overall results suggest that composting sewage sludge with biomass ash may be the best strategy and technique to overcome soil application bottlenecks because it passivates heavy metals and improves the bioavailability of plant nutrients. Display omitted •Biomass ash effect on the speciation of micro-macro elements was investigated.•Biomass ash effect on the passivation of heavy metals was researched.•Biomass ash addition reduced the bioavailability of all elements except Mn and Mg.•Mn and Mg were concentrated in exchangeable fractions (41% and 48%, respectively).•Biomass ash has a high effect on reducing the bioavailability of heavy metals (<1% BF–Cr, 21% BF–Cd, 9% BF–Pb).