Secondary Treatment Technology Comparison

Conventional activated sludge keeps biomass suspended and relies on a separate secondary clarifier. MBBR adds plastic biofilm carriers so biomass grows attached, increasing capacity without larger tanks. MBR replaces the clarifier with membrane filtration, allowing very high MLSS (8,000–15,000 mg/L) and excellent effluent quality in a small footprint. SBR performs fill, react, settle and decant in a single tank on a timed cycle. Each suits different constraints of footprint, effluent target and existing infrastructure.

MBR is the most compact because it operates at very high biomass concentration and needs no secondary clarifier. MBBR is also compact relative to conventional activated sludge because attached biofilm increases treatment capacity per unit volume. Conventional activated sludge with a separate clarifier typically needs the largest footprint.

The decision follows the binding constraint: if the limit is footprint, MBR or MBBR retrofit is favoured; if it is biological capacity in existing tanks, MBBR carriers can be added; if very high effluent quality or reuse is required, MBR is strong; if operation must be simple and flows are variable, SBR fits. Start from measured plant data — flows, loads, effluent targets and available volume — rather than from a preferred technology.

Yes. A common, cost-effective upgrade is to convert existing aeration tanks by adding MBBR carriers (IFAS) to raise biological capacity, or to retrofit membranes into part of the tank volume for MBR. Reusing existing civil structures avoids new construction and is often the fastest route to added capacity or nitrification.