Metabolic energy distribution drives concurrent selective microbial growth and polyhydroxyalkanoate (PHA) storage in municipal activated sludge.

Municipal wastewater treatment plants have the potential to deliver benefits beyond providing clean water by resource supply supporting circular economy goals. Waste activated sludge (WAS) can be directly used for producing PHA-rich biomass with controlled polymer quality. This study investigated the metabolic conditions enabling concurrent PHA storage and selective microbial growth of the PHA producing bacteria in municipal WAS for enhanced direct accumulation productivity. A proposed metabolic framework links NADH production and ATP distribution to PHA synthesis and active biomass growth. Altered NADH and ATP flux ratios during PHA synthesis were systematically tested in accumulation experiments by varying the acetate to butyrate ratio in the feed supplied to a municipal WAS. Increasing butyrate fraction results in metabolism with a relative overflow of ATP supply, beyond the requirements of PHA synthesis. This overflow correlated to an enhanced selective growth of the PHA accumulating community, boosting progressively the accumulation outcomes from 0.48 gPHA/gVSS for acetate to 0.61 gPHA/gVSS for butyrate feeding while doubling the volumetric productivity. The metabolic framework suggests that more reduced substrates for PHA production, like butyrate, can generate excess NADH. If this NADH is converted to ATP and a surplus exists, then biomass growth will ensue with continued PHA storage in the newly formed biomass. These findings advance the understanding of principles underlying enhanced PHA accumulation in WAS, motivating further developments to valorise municipal WAS as a resource for PHA production supply chains.