MBBR

The need was compact biological treatment with greater capacity in the available tank volume.

MBBR emerged as a way to provide more biological treatment capacity within a limited reactor volume, particularly where conventional expansion would require additional tanks or site area. The requirement was relevant to compact new plants, smaller or decentralised installations and existing facilities facing higher loads or tighter outlet requirements.

Conventional activated sludge can meet these treatment duties, but increasing capacity may require additional aeration volume, greater oxygen-transfer capacity and sufficient secondary clarification capacity. Where nitrification is required, the biomass inventory and solids retention time must also be sufficient to retain the slower-growing nitrifying population. Where a site is already built out, extending biological tanks and clarifiers can be difficult or disruptive.

Earlier attached-growth systems retained biomass effectively, but their media arrangements could impose limitations on hydraulic distribution, access, mechanical reliability, backwashing or fluidisation. MBBR took a different route: biofilm was retained on mobile carrier elements that were kept in continuous movement within the mixed reactor.

The carrier inventory introduced a defined amount of protected biofilm surface within the reactor volume. This provided a way to increase retained attached biomass and biological capacity without relying only on additional tank volume, provided that loading, oxygen transfer, mixing and carrier movement could support the added duty.

In an existing activated sludge plant, upgrading may take several forms: a high-rate MBBR stage ahead of the activated sludge process, conversion of a suitable tank to stand-alone MBBR, or an IFAS arrangement in which attached and suspended biomass operate together. The available tank volume, hydraulics, oxygen transfer, carrier retention, existing sludge-return arrangement and downstream solids-separation capacity must support the selected configuration.

The defining development was protected biofilm surface on freely moving carriers.

MBBR was developed in Norway in the late 1980s through work led by Professor Hallvard Ødegaard and subsequently developed and commercialised through Kaldnes. Early full-scale application dates to 1989. The first K1 carrier was a small high-density polyethylene element with protected internal surfaces for biofilm growth.

Because the media was not fixed in place, the carriers could circulate throughout the reactor rather than forming a stationary bed. In an aerobic MBBR, aeration provides both oxygen transfer and the mixing energy needed to keep the carriers moving and distributed through the reactor. Biofilm develops mainly on protected carrier surfaces, while carrier contact and fluid shear help control excessive accumulation on exposed areas. Reliable carrier retention remains an essential reactor duty.

In a stand-alone MBBR, the carriers retain the attached biomass, so a return activated sludge loop is not used to maintain the biofilm inventory. Treatment capacity depends on the available carrier surface, biological loading and the conditions maintained within the reactor. Oxygen transfer, mixing, temperature, pH, alkalinity and inhibition must be considered according to the treatment duty.

An outlet retention screen keeps the carrier media inside the reactor. Sloughed biofilm and other suspended solids pass forward with the liquid and are removed in the downstream solids-separation stage.

A new reactor can be sized for the required attached-growth duty. An existing aeration tank can be considered for retrofit only after its usable volume, hydraulics, oxygen transfer, mixing, carrier retention and downstream solids-separation capacity have been checked.

Loading, mass transfer and detachment determine what the biofilm can deliver.

The biofilm on an MBBR carrier is not a uniform coating. Substrates and oxygen diffuse from the bulk liquid into the biofilm, creating concentration gradients through its depth. The active biofilm and achievable conversion rate therefore depend on loading, dissolved oxygen, temperature, carrier geometry, biofilm thickness and mass transfer.

Conditions within the biofilm depend on the wastewater load and the oxygen available in the reactor. Where readily biodegradable organic matter is still present, heterotrophic activity will account for much of the biological activity and oxygen demand. As the biodegradable carbon load is reduced, conditions may become more favourable for nitrification.

Biofilm thickness develops from the balance between growth, mass transfer, shear and detachment. Excess biomass is released into the bulk liquid and leaves the reactor as suspended solids. The quantity and characteristics of these solids affect the downstream separation duty.

Aeration must satisfy two duties in an aerobic MBBR: oxygen transfer and carrier movement. Insufficient air can limit biological conversion or lead to poor carrier distribution, while excessive aeration increases energy use and shear. The aeration system should therefore be designed for both biological demand and reliable carrier movement.

Surface loading is a central design check, but the load basis must be clear. Carbon duty may be expressed using BOD or an appropriate biodegradable COD basis. Where nitrification is required, ammonia loading and the corresponding design rate should be defined separately. Total COD alone does not define the biodegradable load available to the biofilm.

Conditions that may support MBBR selection.

MBBR can be applied as a stand-alone attached-growth biological stage or as part of a hybrid IFAS configuration. Selection should be based on the required biological duty, wastewater characteristics, loading conditions, available footprint and the ability of the overall process to support aeration, carrier retention and downstream solids separation.

IFAS should be distinguished clearly from stand-alone MBBR. In IFAS, attached growth on carriers operates alongside suspended activated sludge. The suspended-growth phase therefore retains RAS and waste-sludge control, while the carrier biofilm remains in the reactor. In a stand-alone MBBR, biomass is retained on the carriers and excess suspended solids are removed downstream.

Conditions that can reduce biological capacity or disrupt operation.

MBBR underperformance is often traceable to the influent basis, feed protection, carrier selection, oxygen transfer, mixing, carrier retention or downstream solids separation. These duties need to be resolved as part of the process route rather than left to equipment selection.

Suspended, fibrous, greasy or sticky solids at the reactor feed. Excess TSS, lint, hair, fibres, grease or sticky solids can coat carriers, lodge within carrier openings, restrict movement and blind the carrier-retention screen. Appropriate upstream screening and solids or grease removal should therefore be provided according to the wastewater characteristics.

Uncontrolled inhibitory or shock loads. Oxidising biocides, sanitisers, solvents, surfactants, metals, extreme pH, temperature shocks and sudden salinity changes can reduce biological activity. Source control, segregation, equalisation or upstream treatment should be considered where these conditions can occur.

An unresolved biodegradable-load and nutrient basis. Surface loading should be based on the biodegradable load relevant to the treatment duty, not total COD alone. The design basis should also define the nitrogen duty, nutrient balance, peak loads, temperature and any inhibition risk.

Unverified carrier capacity or underdesigned hydraulics and aeration. Carrier selection should be based on the declared surface area, design fill fraction and a reactor configuration that provides adequate mixing, oxygen transfer and carrier retention. Reactor hydraulics should be assessed as part of the same design basis.

What to look for in an MBBR proposal.

A well-developed proposal should make the design and performance basis clear without requiring disclosure of proprietary calculation methods. The review should provide enough information to show that the reactor, carrier media and surrounding process are configured for the wastewater, operating range and required outlet duty.

• Influent and biological duty. Flow and load range, biodegradable carbon basis, ammonia loading where nitrification is required, denitrification duty where applicable, temperature, pH, alkalinity where relevant, nutrient balance, peak and batch loads, inhibition assumptions and required effluent quality.
• Reactor and carrier basis. Effective reactor volume, carrier type, nominal protected surface area, design fill fraction, surface loading, volumetric loading and the basis for the selected conversion rates.
• Oxygen transfer, mixing and turndown. Process oxygen demand, blower flow and pressure, diffuser arrangement, oxygen-transfer basis, DO control, standby capacity, turndown and the mixing required to maintain carrier movement across the operating range.
• Hydraulics and carrier retention. Water levels, flow distribution, short-circuiting risk, outlet-screen geometry and approach velocity, cleaning access, and provisions to prevent carrier loss or accumulation.
• Upstream and downstream process boundary. Feed screening, equalisation, oil or fibre control, pH adjustment and other pretreatment where required, followed by downstream solids separation, polishing and sludge handling.
• Controls, commissioning and operating basis. Instruments, alarms, interlocks, duty and standby philosophy, start-up and biofilm development, performance-testing load range, operator tasks, cleaning requirements and critical spares.
• Guarantee boundary. Influent conditions, peak-load duration, temperature and inhibition assumptions, utility conditions, exclusions and required effluent performance.

Where MBBR fits in our design and proposal-review work.

For new systems, EnWater can develop the MBBR design basis, define the required reactor stages, establish carrier and reactor sizing, assess oxygen transfer and mixing, and define carrier retention and downstream solids separation. The design can then be carried through into equipment requirements and a comparable basis for supplier proposals.

For existing plants, EnWater can assess whether available tank volume, hydraulics, oxygen-transfer capacity, carrier retention and downstream solids separation can support an MBBR or IFAS upgrade.

Where supplier proposals are being considered, EnWater can review the technical basis, compare alternatives on a consistent basis and identify the requirements that need to be carried into detailed design and implementation.