Enhancing Volatile Fatty Acid Accumulation in Seaweed-Arrested Anaerobic Digestion via a Two-Tier Framework of Engineering Diagnostics and Microbial Surveillance

This study presents a two-tier framework for brown seaweed–arrested anaerobic digestion (SW-AAD) by coupling engineering diagnostics (tier-1) with microbial surveillance (tier-2) to transform conventional digesters into volatile fatty acid (VFAs) biorefineries. In 70-day lab-scale batch tests (volume: 2 L), engineering diagnostics revealed that sequential additions of brown seaweed elevated salinity to 3.3% ash content, stabilized pH at 5.8–6.1, collapsed the CH₄/CO₂ ratio from 2.98 to <0.04, suppressed biogas by 96%, and boosted carbon-conversion efficiency from 10% to 52%. More than half of the influent carbon was redirected from methane to a liquid-phase VFAs pool, dominated by butyrate, hexanoate, and acetate, peaking at 14.5 g L⁻¹. Microbial surveillance using 16S rRNA sequencing presented a 10-fold decline in methanogens, alongside an increase in salt- and acid-tolerant acidogenic microbial families, including Lachnospiraceae, Ruminococcaceae, and Clostridiaceae, as well as seaweed-derived microbial families, including Psychromonadaceae, and Marinomonadaceae. Temporal synchrony and asynchrony-resolved analysis confirmed a strong correlation between two tiers, revealing an operational window featured with two patterns: a rapid engineering parameter response (where changes in engineering parameters were followed by a decline in methanogens within 0–3 days) and a slower microbial restructuring (where the surge in VFA concentration lagged the enrichment of seaweed-derived acidogens by 1–5 days). Based on this operation window, feedback control strategies were simulated for SW-AAD, indicating that cumulative VFAs (cVFAs) yield could be bolstered from 11.38 to 16.13 g COD L⁻¹ (+41.8%). A techno-economic assessment (TEA) revealed that this VFA-targeted SW-AAD can reduce capital expenditure by ∼20% compared to conventional AD. This study underscores the promise of asynchrony-resolved analysis of SW-AAD systems for robust and economically viable VFAs production from organic waste.