Whereas the AS technology relies on free-flowing bacteria flocs, the MBBR technology relies on fixed-film technology where bacteria are attached to inert biological carrier profiles (bio-elements, biomedia, biological carriers) that are retained within the reactor by screens. This eliminates the need for a return activated sludge system and reduces the requirements for subsequent filtration steps.

Commonly, the MBBR technology is favoured over AS plants when space is limited and/or costly, especially for Municipal wastewater treatment, since the MBBR components often present a higher initial investement cost per reactor volume. However, this should be balanced with the fact that the reactor volumes required for BOD removal and biological nutrient removal (BNR) are usually 40-60% of what is needed with AS technology. For pure nitrification purposes, MBBR technology requires approximately 25-40% of the volume required by AS technology.

Thus, the MBBR technology offers a powerful alternative to AS systems for high-strength industrial wastewater treatment and for pre- or post-treatment upgrades in existing WWTP's that require upgrading to meet new ammonia discharge limits.