Hydrodynamic effects on bulk-suspension microbial abundance and a preliminary conceptual safe operating window for bioelectrochemical systems

Hydrodynamic conditions can influence mass transfer, suspended-cell distributions, and electrode-associated biofilms in bioelectrochemical systems. This study evaluated mechanical agitation in a batch single-chamber microbial fuel cell (MFC) and developed a preliminary conceptual Safe Hydrodynamic Operating Window (SHOW) framework. The surviving archive contains one bulk-suspension microbial count for each of six agitation conditions (0, 50, 100, 150, 200, and 250 rpm) on days 0, 5, 10, 15, and 20, yielding 30 longitudinal observations. Counts were obtained by direct microscopic enumeration after a 1-g bulk-reactor sample was diluted in demineralized water to 10⁻⁵; five counting-chamber areas were read and the mean was converted to cells mL⁻¹. The mean across the five sampled days decreased descriptively from 6.644 × 10⁶ cells mL⁻¹ at 0 rpm to 3.32 × 10⁵ cells mL⁻¹ at 250 rpm, a 95.0% difference. Because these values are repeated time points and the archive does not document independent reactor replication, no independent-groups inferential test or statistically validated rpm threshold is claimed. The 50–150 rpm interval is retained only as a hypothesis-generating candidate region for future validation. The measured fraction was bulk reactor suspension, not scraped or eluted electrode biomass; therefore, direct biofilm retention and detachment were not measured. Validation of SHOW requires independent reactors together with biofilm, hydrodynamic, electrochemical, treatment-performance, energy, and reliability measurements.