Mitigating inhibition of undissociated volatile fatty acids (VFAs) for enhanced sludge-rice bran composting with ferric nitrate amendment

- Fe(NO3)3 addition accelerated the VFAs degradation by 3 times for sludge composting.
- Fe(NO3)3 addition increased the final organics removal efficiency by 22%.
- Synergistic effect of denitrification and pH increase mitigated inhibition of VFAs.
- Fe(NO3)3 addition reduced the EC in compost material by 23% to 4.6 mS·cm−1.

This study investigated the effect of ferric nitrate on mitigating the inhibition of volatile fatty acids (VFAs) during the initial phase of sewage sludge composting amended with rice bran. During the 34-day lab-scale composting, the supplementation of ferric nitrate enhanced the degradation of VFAs by up to 3 times as compared to the control. The organic matters loss (OML) rate in the treatment reactor was almost doubled with supplementation of ferric nitrate as compared to the control reactor during the initial phase. Eventually the treatment reactor achieved a 39.0% OML by the end of composting, which was 22% higher than the control. Ferric nitration addition mitigated the inhibition of VFAs by stimulating denitrification which consumed protons and VFAs. Ferric nitrate addition also decreased the electrical conductivity by 23% in the final compost product, reducing the possibility of phytotoxicity issue upon soil application. In summary, the results demonstrated that ferric nitrate addition could be an effective strategy for enhanced sludge composting.