Carriers That Never Stop Moving: An MBBR Field Guide Built on Mill Effluent

The moving bed biofilm reactor (MBBR) was designed around a short list of refusals: no clogging, no backwashing, minimal head loss, and as much attached-growth surface as can be packed into a tank. Everything else follows from those four decisions.

The carrier does the work

Biofilm grows on small plastic carrier units that move freely with the water inside the reactor. In an aerobic reactor, aeration drives that movement; in an anoxic or anaerobic reactor, a mechanical stirrer does. A perforated screen at the outlet keeps carrier from escaping. Tanks are generally rectangular or cylindrical. A rectangular reactor is divided evenly into several compartments along its length, or left undivided; overall the flow through the reactor approximates plug flow, while within each compartment aeration fluidizes the contents and the flow is completely mixed. Side-mounted perforated aeration pipe circulates the carrier, whereas a cylindrical reactor uses microporous aeration heads at the bottom. Some reactors carry both aeration and stirring equipment so they can be switched conveniently to anoxic duty, and a cover is sometimes added to suppress air-lift and volatilisation.

The tank is filled with polyethylene or polypropylene suspended carrier whose specific gravity is close to water and whose specific surface area is large. Biofilm attachment surface inside the reactor can reach 500 m²/m³, and the actual specific surface area, the inner surface of the carrier, reaches 350 m²/m³. That is the entire trick: the biomass stays in the tank because it is stuck to something, not because a clarifier sends it back.

Where the idea came from

MBBR is a new type of biofilm reactor developed from fixed-bed reactors, fluidized-bed reactors and biofilters, and improved into a composite. It avoids the drawbacks of each ancestor: fixed beds need periodic backwashing, fluidized beds need the carrier fluidized, and submerged biofilters need media cleaning and aerator replacement. What it keeps from traditional biofilm processes is impact-load resistance, low sludge production and long sludge age. Because the sludge age is long, more nitrifying bacteria are retained and nitrogen removal improves over the activated-sludge process. The mechanism is unglamorous: wastewater passes, microorganisms multiply on the film and degrade the organic pollutants, and the water leaves cleaner.

Evidence from mill effluent

Broch and co-workers ran a pilot-scale MBBR on newsprint-mill wastewater. At a hydraulic retention time of 4-5 h, CODcr removal was 65%-75% and BOD5 removal 85%-95%; extending the hydraulic retention time moderately pushed those to 80% and 96%. Chandler and co-workers used plastic carrier in a two-stage MBBR pilot on papermill wastewater and, at a hydraulic retention time of 3 h, cut effluent BOD5 by 93% to an average concentration of 7.83 mg/L.

Earlier, in 1991, Rusten and co-workers treated neutral sulfite pulping wastewater with MBBR. At CODcr loadings up to 20-30 kg/(m³·d) and a carrier filling rate of 70%, total CODcr removal was 70% and BOD5 removal 96%. When CODcr loading was raised as far as 50 kg/(m³·d), removal held essentially constant at 60%-70%, which is the impact-load argument in one sentence.

Li Wenjun and colleagues at Sichuan University of Science & Engineering used a coagulation-MBBR sequence on the middle-stage wastewater of a mill running Neosinocalamus affinis with KP cooking and CEH three-stage bleaching, drawing on washing and screening wastewater, bleaching wastewater and remaining white water. Raw water came in at CODcr 1640 mg/L, SS 1330 mg/L, colour 187 times and pH 6.9, yellow-brown. Coagulation under optimal conditions brought CODcr to about 780 mg/L; MBBR biological treatment after that produced effluent colour of 23 times, CODcr 130 mg/L and SS <90 mg/L, for total CODcr and BOD5 removal of 92.1% and 93.3%.

What still needs fixing

Two variants have grown out of the known weaknesses. Running MBBR in SBR mode gives the Moving Bed Sequencing Batch Biofilm Reactor (MBSBBR), combining MBBR and SBR advantages; letting the carrier circulate with the flow gives the circulating moving-carrier biofilm reactor, combining MBBR with the internal-circulation reactor. Both are responses to the same complaint, that carrier movement inside the reactor is unbalanced and dead zones appear to varying degrees, and to the running argument about how to improve hydraulic flow characteristics and cut operating energy consumption.

Since its birth the process has attracted wide interest because it is simple to build, convenient to operate, efficient at organic-matter removal and strong on phosphorus and nitrogen removal, and because it suits the deep treatment of small- and medium-sized enterprise wastewater and organic wastewater generally. Overseas pilot and full-scale studies on domestic sewage and several industrial wastewaters have all produced good results. Carrier development continues, aiming at good adsorption performance, appropriate density, durability, corrosion resistance and low price. For plants whose existing process is struggling, the practical appeal is that MBBR capacity can often be obtained by upgrading what is already in the ground rather than by pouring new tanks.