Introduction – Moving-Bed Reactors

In the past 20 years, the moving-bed biofilm reactor (MBBR) has been established as a robust, flexible, and compact technology for wastewater treatment. The MBBR technology has demonstrated success with biochemical oxygen demand (BOD) removal, ammonia oxidation, and nitrogen removal applications, in a variety of different treatment configurations, designed to meet a wide range of effluent quality standards, including stringent nutrient limits.

Moving-bed biofilm reactors use specially designed plastic carrier elements for biofilm attachment, BIOFILL TYPES C and C-2 among others, held in suspension throughout the reactor by turbulent energy imparted by aeration, liquid recirculation, or mechanical mixing energy. In most applications, the reactor is filled between one-third and two-thirds full with carriers. Perforated plates or sieves located on the effluent-end of the reactor allow treated water to pass through to the next treatment step, but retain the media inside the reactor. Perhaps the most impressive aspect of an MBBR is in its versatility, allowing creative solutions by design engineers. The key differentiator for moving-bed technology when compared with other biofilm systems is that it combines many of the advantages of activated sludge with the advantages offered with biofilm systems, while, at the same time, trying to minimize the drawbacks of each.

• Like other submerged-bed biofilm processes, MBBRs help to promote a highly specialized active biofilm that is well-suited for the particular conditions in a reactor. This highly active specialized biomass results in high volumetric efficiencies and increased process stability, resulting in a more compact reactor.
• Unlike most other submerged-bed biofilm processes, the MBBR is a continuous flow-through process, eliminating the need for backwashing of the media to maintain performance; thus, headloss and operational complexity of the treatment step are minimized.
• Moving-bed reactors can offer much of the same flexibility and simplicity as activated sludge processes, allowing multiple reactors to be configured in a flow-through series arrangement to achieve multiple treatment objectives (i.e., BOD removal, nitrification, and pre- and post-nitrification). This occurs without the need for intermediate pumping.
• Unlike suspended-growth processes, biological performance in the MBBR does not depend on the solids separation step, because most of the active biomass is retained continually in the reactor. This solids concentration leaving the reactor with the treated flow is at least an order of magnitude lower in concentration. As a result, MBBRs are compatible with a variety of different separation techniques – not just conventional clarifiers.
• The versatility of MBBRs allows the technology to be considered in a variety of different potentional reactor geometries. For upgrades at existing plants, this makes MBBRs well-suited for retrofit installation to existing tanks.