The basic design idea of MBBR is continuous operation without clogging, no backwashing, small head loss, and a large specific surface area. This is achieved by growing the biofilm on small carrier units that move freely with the water flow within the reactor. In aerobic reactors, aeration drives the carrier movement; in anoxic/anaerobic reactors, mechanical stirring moves the carrier. To prevent loss of the carrier from the reactor, a perforated screen can be installed at the reactor outlet (Figure 1). MBBR is generally rectangular or cylindrical in structure. A rectangular reactor is evenly divided into several compartments along its length, or left undivided. Overall, the water flow is in plug-flow within the reactor, while in each compartment, due to aeration fluidization, the flow is completely mixed. The tank is filled with polyethylene or polypropylene suspended carrier with a specific gravity close to water and a large specific surface area; the 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. A perforated aeration pipe aerates from one side, causing the carrier to circulate within the tank. A cylindrical reactor has microporous aeration heads at the bottom. In addition, some reactors are equipped not only with aeration devices at the bottom but also with stirring devices, which allow the reactor to be conveniently and flexibly used under anoxic conditions. Sometimes, to prevent the air-lift and volatilization effects brought by aeration, a cover can be added above the reactor.

Figure 1. Moving Bed Biofilm Reactor

Broch et al. used a pilot-scale MBBR to treat newsprint-mill wastewater; at a hydraulic retention time of 4–5 h, the CODcr and BOD5 removal rates were 65%–75% and 85%–95% respectively; with a moderate extension of the hydraulic retention time, the CODcr and BOD5 removal rates could be increased to 80% and 96% respectively.

MBBR is a new type of biofilm reactor developed from fixed-bed reactors, fluidized-bed reactors and biofilters as an improved new-type composite biofilm reactor. It overcomes the shortcomings of fixed-bed reactors (which require periodic backwashing), fluidized-bed reactors (which require carrier fluidization), and submerged biofilters (which require cleaning of the media and replacement of aerators), while retaining the characteristics of traditional biofilm processes such as impact-load resistance, low sludge production and long sludge age. Compared with the activated-sludge process, because of the longer sludge age it can retain more nitrifying bacteria and achieve better nitrogen removal. Its main principle is that as wastewater continuously flows through the reactor carrier, a biofilm forms on the carrier; microorganisms multiply and grow on the biofilm while degrading the organic pollutants in the wastewater, thereby purifying it [1].

Research and Development Prospects of MBBR for Pulping and Papermaking Wastewater

Since the birth of the MBBR reactor, because this process concentrates the advantages of biofilters, fixed beds and fluidized beds, it has attracted extensive interest from experts and scholars worldwide, especially because it is simple to build, convenient to operate, highly efficient in organic-matter removal, and strong in phosphorus and nitrogen removal — and is particularly suitable for the deep treatment of small- and medium-sized enterprise wastewater and organic wastewater.

In 1991, Rusten et al. used MBBR to treat neutral sulfite pulping wastewater; under conditions of CODcr up to 20–30 kg/(m³·d) and a carrier filling rate of 70%, the total CODcr removal rate was 70% and the BOD5 removal rate was 96%; when CODcr gradually increased to up to 50 kg/(m³·d), the removal rate remained essentially constant at 60%–70%.

Abroad, pilot and full-scale studies using MBBR to treat domestic sewage and certain industrial wastewaters have all achieved good results. Tests show that the moving-bed biofilm wastewater-treatment process is suitable for small- and medium-sized domestic sewage and industrial organic wastewater. Among these, integrated or buried wastewater-treatment devices have very good prospects for promotion and application in China. However, the moving-bed biofilm reactor still has some problems, for example, the movement state of the carrier within the reactor is unbalanced, and dead zones exist to varying degrees within the tank. How to improve the hydraulic flow characteristics inside the reactor and reduce operating energy consumption is a question worthy of in-depth discussion in the development of moving-bed biofilm reactors. Operating MBBR in the SBR mode is an improvement of MBBR technology; this reactor is called the Moving Bed Sequencing Batch Biofilm Reactor (MBSBBR), which combines the advantages of both MBBR and SBR. Another improvement is to let the carrier circulate and move with the water flow within the reactor; this reactor is called the circulating moving-carrier biofilm reactor, which combines the advantages of both MBBR and the internal-circulation reactor.

Chandler et al. used plastic carrier in a two-stage MBBR to treat papermill wastewater in a pilot test; the results showed that at a hydraulic retention time of 3 h, effluent BOD5 could be reduced by 93%, with an average concentration of 7.83 mg/L.

Li Wenjun et al. of Sichuan University of Science & Engineering used a coagulation–MBBR method to experiment on the middle-stage wastewater of a certain mill (including washing/screening wastewater, bleaching wastewater and remaining white water). The mill uses Neosinocalamus affinis as raw material with KP cooking and CEH three-stage bleaching. Its main water-quality indicators were: CODcr 1640 mg/L, SS 1330 mg/L, color 187 times, pH 6.9, yellow-brown color. After coagulation treatment under optimal conditions the effluent CODcr was about 780 mg/L; after further MBBR biological treatment the effluent color was 23 times, CODcr 130 mg/L, SS <90 mg/L, and the total CODcr and BOD5 removal rates were 92.1% and 93.3% respectively.

According to the literature, experts are developing carrier with good adsorption performance, appropriate density, durability, corrosion resistance and low price to improve the treatment effect and promote its perfection and development. Although domestic research and application are still in their early stages, foreign test results have proved that MBBR can not only treat papermaking wastewater efficiently and stably, but the MBBR process can be relatively easily obtained by upgrading existing treatment processes. Its characteristic of suiting small- and medium-sized enterprises is especially suitable for the current situation of China's papermaking enterprises being small in scale, and is worthy of our attention and reference [2].