Air Nanobubbles improve heavy metal stability of sludge anaerobic digestion

https://www.sciencedirect.com/science/article/abs/pii/S2214714425016332

• Methane production was positively correlated with low valence/energy levels of HMI.

• SRF and CST were negatively correlated with low valence/energy levels of HMI.

• The ecological risk index of HMs was negatively correlated with methane production.

• The ecological risk index of HMs was positively correlated with SRF and CST.

• NBW decreased the ecological risk index of final digestate by 4.2 %–42.0 %.

Nanobubbled water (NBW) can improve the performance of sludge anaerobic digestion

(AD) to enhance digestate dewaterability. However, the effects of different doses of NBW

on the stability of heavy metals (HMs) remain unknown. This study found that NBW

increased the ultimate methane production of sludge AD by 11.7 %–16.0 %. Compared

with the corresponding control groups, the 40 % Air-NBW group (A40) presented the

best improvement in methane production and digestate dewaterability of the three

different dose groups (20 %, 40 %, and 60 % NBW: mixed substrates). NBW contributed

to decreasing the proportion of HMs in the unstable exchangeable/acid-soluble fraction

and promoting the conversion of heavy metal ions to stable low-valence states and low-energy levels.

These low-valence states and low-energy levels of HMs were positively correlated with digestate

dewaterability and methane production. NBW reduced the potential ecological risk index (RI)

of HMs by 4.2 %–42.0 %, with the A40 showing the greatest reduction in ecological risks.

The RI of HMs was negatively correlated with methane production and digestate dewaterability.

NBW can simultaneously improve methane production, digestate dewaterability, and

heavy metal stability in sludge AD, with 40 % NBW being the optimal dose.

This study provides a new sludge treatment strategy for controlling HMs pollution.

Among the three different dose groups, the 40 % NBW group (A40) exhibited

the best methanogenesis and dewaterability of the final digestate, with a

16.0 % increase in methane production and a 29.8 % and 13.4 % decrease in

SRF and CST, respectively, compared with the control. Additionally, NBW favored

decreasing the proportion of HMs with the unstable exchangeable/acid-soluble fraction

and promoted the conversion of HMI to stable low-valence states and low-energy levels in the digestate