Stop Asking How Much Biomass To Retain. Ask Which Biomass, And Where.

Wastewater process intensification has long centered on a single instinct: hold more biomass in the same tank. Whether the technology is integrated fixed-film activated sludge (IFAS), a moving bed biofilm reactor (MBBR) or something else, the working assumption is that more retained biomass means more treatment capacity.

The problem is that biomass isn’t one thing. A biological nutrient removal (BNR) reactor contains fast-growing heterotrophs, slow-growing nitrifiers and phosphate-accumulating organisms (PAOs), each performing best at a different solids retention time (SRT). Conventional activated sludge runs at the SRT required by the slowest grower, forcing every other organism to share that operating point. The consequences are familiar: elevated MLSS, deteriorating settleability, filament outbreaks, clarifier stress and reduced flexibility.

Microbial Resource Partitioning (MRP) starts from a different question. Instead of asking how to retain more solids, it asks which organisms should be retained and where they should live.