<p>This study demonstrates a circular strategy by converting spent nickel-loaded humic acid-coated magnetite (Ni-HA@Fe₃O₄) adsorbent into a value-added supplement for anaerobic digestion (AD) of Tofu wastewater (TW). The spent Ni-HA@Fe₃O₄ composites were generated through nickel removal from synthetic nickel wastewater using synthesized HA@Fe₃O₄, yielding materials (R-0 to R-8) with nickel content declining from 68.48 to 49.68&#xa0;mg·g⁻¹ over eight regeneration cycles. In batch AD of tofu wastewater (TW), the spent adsorbent (R-0) substantially enhanced biomethane production. Kinetic analysis using the modified Gompertz model showed a 2.54-fold increase in biomethane production potential (<i>G</i>_m) relative to the control, and outperformed nickel-only and HA@Fe₃O₄-only groups by factors of 1.41 and 1.49, respectively. Correspondingly, the maximum daily biomethane production rate <i>R</i>_mwas elevated by 2.73-, 1.41-, and 1.44-fold, while the lag phase (<i>λ</i>) was reduced by 2.73-, 1.59-, and 1.84-fold compared to the same groups. Although repeated regeneration gradually diminished efficacy, all experimental groups (R-0 to R-8) still improved the biomethane production potential compared to the control group. These results confirm that spent Ni-HA@Fe₃O₄ can be effectively utilized as an additive for AD, offering a sustainable route that integrates wastewater treatment, solid waste management, and enhanced bioenergy recovery.</p>