MBBR media: biofilm carriers explained

A moving bed biofilm reactor (MBBR) fills part of an aeration tank with small plastic carriers. Bacteria grow as a thin biofilm on the protected surfaces of the carriers, and the carriers move freely with the air or mixer flow. Because the bacteria are fixed on the media, the plant holds far more biomass per cubic metre of tank than a suspended-growth plant, and it needs no sludge return. The result is a compact plant that copes with load variations and removes BOD, and with the right design, ammonia and nitrate. See MBBR vs SBR vs MBR for how it compares with other processes.

Protected surface

Biofilm grows inside the carrier, shielded from abrasion as the carriers collide. Only this protected surface counts as effective area for design. The outside surface is often colonised too, but is not counted.

Movement

In an aerobic tank, air from the diffusers keeps the carriers moving and supplies oxygen. In an anoxic tank, mixers keep the carriers in motion.

Self-regulating biofilm

The movement shears off excess biofilm, which keeps the layer thin and active. The sloughed biomass leaves the reactor with the water and is removed in the clarifier or settling stage.

Retention

Carriers are kept inside the reactor by sieves or perforated plates at the outlet, with openings smaller than the carrier.

Small cylindrical carriers with internal fins

The original and still common design: a cylinder of about 10 mm diameter and 7 mm length with internal fins, giving an effective surface area of about 500 square metres per cubic metre of carriers.

Larger carriers

Larger cylinders or wheel shapes of around 15 mm or more, with lower surface area per volume (around 350 square metres per cubic metre for one common design) but less tendency to clog and easier retention. Suited to high-strength or fibrous effluent.

Other shapes

Wheel, star, and sphere-based designs from different makers. Specific surface area for MBBR carriers generally ranges from about 350 to 1,200 square metres per cubic metre, and void ratio from about 60 to 90 percent.

Material

Most carriers are made of high-density polyethylene (HDPE) or polypropylene, with a density close to that of water so they stay in suspension without settling or floating to the top.

Media in suspended-growth plants (IFAS)

Carriers are also added to existing activated sludge tanks to add fixed biomass and increase capacity or nitrification without building new tanks.

Specific surface area

The protected surface per volume of carriers (m2/m3). Higher area means more biofilm per cubic metre of media, but very fine geometries clog more easily and are harder to keep moving.

Fill fraction

The share of the tank volume occupied by the carriers when at rest. Worked design examples commonly use about 40 percent, and manufacturers quote maximum fills of about 55 to 67 percent depending on carrier size. Higher fill needs good mixing so that all the carriers keep moving.

Surface in the reactor

Effective surface per cubic metre of reactor equals the carrier surface area times the fill fraction. For example, 500 m2/m3 at 67 percent fill gives about 335 m2 per cubic metre of reactor, which the carrier makers round to about 350.

1. LoadCalculate the daily load of the pollutant to remove: BOD, ammonia or nitrate, in grams per day.
2. Surface area loading rateChoose the design surface area loading rate (SALR, in g per m2 per day) for that pollutant and the required removal. It depends on temperature, dissolved oxygen, and effluent quality needed.
3. Surface neededDivide the load by the SALR to get the carrier surface area required.
4. Media volumeDivide that area by the carrier specific surface area to get the volume of carriers.
5. Tank volumeDivide the media volume by the fill fraction to get the reactor volume. Check against the hydraulic retention time and the mixing and aeration requirement.

Design SALR values vary with carrier, wastewater and temperature, so we take them from the carrier data and our own plant experience, not from a fixed figure.

Effective surface area

Ask for the protected surface area per cubic metre, not the total surface, and check which is quoted.

Shape and size

Match the carrier to the effluent: larger open carriers for fibrous or high-strength effluent, smaller ones for clean sewage. The outlet sieve must be sized to the carrier.

Material and density

HDPE or polypropylene with a density near water. Poor-quality or recycled plastic can be too light or too heavy and can degrade, so the carriers do not mix properly.

Dimension and quality

Check that carriers are consistent in size and shape and that the volume is true in cubic metres of bulk carriers, not a different measure.

Supply quantity

Order in cubic metres of bulk carriers. Ask how many pieces make up a cubic metre and how the volume was measured.

Mixing and aeration

The aeration or mixing system must keep all the carriers moving. Dead zones let the carriers pile up. See blowers for aeration.

Life and top-up

Media itself lasts for years when it is good quality. Plan for some loss and top-up at the first few years of operation.

Carriers piling up at the outlet sieve

Usually from low mixing, a blocked sieve or too high a fill. Check aeration pattern and clean the sieve.

Carrier loss

Carriers escaping through damaged sieves or carried over to the clarifier. Inspect sieves and the effluent channel.

Heavy, sticky biofilm and grease coating

Often from oil and grease reaching the tank. Improve pre-treatment (screening, oil removal) rather than the media.

Poor nitrification

Low dissolved oxygen, low temperature, low alkalinity or too short a retention time. Check these before blaming the media.

Foaming

Usually from the influent or from overloading rather than the media. See your O&M checklist: operator checklists.

We design MBBR plants and supply the media with a plant, or as a top-up or replacement volume for an existing reactor. Send the tank volume, current fill, flow and water analysis with the RFQ checklist. Make and carrier type are as per your specification and approved list, or we propose options in the quotation. See also other process media, STP and ETP.

Specific surface area range and design method: CED Engineering, MBBR Processes. Carrier dimensions, 500 m2/m3 area and fill rates: edie.net, technically speaking. Values vary by carrier maker and design, so confirm against the carrier datasheet for your project.