Optimization of free nitrous acid pre-treatment conditions for enhancing short-chain fatty acid recovery from sludge: role of nitrite fate in fermentation pathways

Abstract

Free nitrous acid (FNA) pretreatment has emerged as an effective strategy for enhancing short-chain fatty acid (SCFA) production from waste-activated sludge (WAS). However, a comprehensive understanding of how individual pretreatment parameters influence fermentation performance remains limited. This study systematically optimized FNA dose, pre-treatment temperature, pH, and duration to enhance SCFA yield. It also introduces a different mechanistic perspective highlighting the fate of NO₂⁻–N as a key determinant in modulating fermentation pathways. FNA doses of 3.7–7.3 mg L⁻¹, temperatures between 5 and 20 °C, and pH levels of 5.5–6.0 minimized NO₂⁻–N decomposition during pre-treatment and increased its exposure to anaerobic bacteria during fermentation, which was favorable for enhancing SCFA production by suppressing methanogens. The findings underscore that residual NO₂⁻–N concentration, which is governed by the extent of NO₂⁻–N decomposition during the pre-treatment phase, critically influences fermentation performance. The study also found that shorter pre-treatment durations (12–24 h) and lower temperatures (5 °C) produced SCFA yields comparable to those from 48 h treatments at 20 °C. These conditions reduce the need for heating and allow smaller tank sizes, making the process more energy- and cost-efficient, especially in colder climates. Under optimal conditions (FNA dose: 2.4 mg L⁻¹, pH: 6.0, temperature: 5 °C, duration: 24 h), the FNA reactor yielded 141 mg COD g⁻¹ VSS of SCFA, 4.4 times higher than the control. Microbial community analysis revealed that FNA-induced fermentation suppressed SCFA-consuming bacteria while enriching SCFA-producing bacteria. This study provides critical insights for optimizing FNA pre-treatment and scaling up sludge fermentation across diverse environmental conditions.