Ten thematic analyses on MBBR technology

The basic design principle of MBBR is to enable continuous operation without blockage, eliminating the need for backwashing, minimizing head loss, and providing a large specific surface area. This can be achieved by growing biofilm on smaller carrier units, which move freely with the water flow in the reactor. In aerobic reactors, aeration drives the movement of the carriers; in anoxic/anaerobic reactors, mechanical stirring moves the carriers. To prevent the loss of packing material in the reactor, a porous filter screen can be installed at the reactor outlet. MBBRs generally have a rectangular or cylindrical structure. Rectangular reactors are evenly divided into several sections along the length of the pool using baffles, or they may be undivided. Overall, the water flow in the reactor is plug flow, while in each section, due to aeration fluidization, the water flow is completely mixed. The pool is filled with polyethylene or polypropylene suspended packing material with a specific gravity close to water and a large specific surface area. The biofilm attachment surface area in the reactor can reach 500 m²/m³, and the actual specific surface area (inner surface area of the packing material) reaches 350 m²/m³. Perforated aeration pipes aerate on one side, causing the packing material to circulate in the pool. The cylindrical reactor has microporous aeration heads at the bottom. In addition, some reactors are equipped with not only aeration devices at the bottom of the pool but also stirring devices. These stirring devices allow the reactor to be conveniently and flexibly applied under anoxic conditions. Sometimes, to prevent the stripping and volatilization caused by aeration, a cover can be added to the top of the reactor.

I. During winter low-temperature commissioning, how long does it take for the packing material to become coated with biofilm?

It can achieve good standards within a month. Biofilm formation is actually a process. We look at biofilm formation from two perspectives: first, when biofilm is visibly apparent on the packing material, which takes about seven days; second, the time to reach standards, which is about one month in winter; third, the time for complete biofilm maturation, which is longer. From a professional standpoint, complete biofilm maturation requires at least one winter-summer cycle for the colonies to stabilize. In summary, while from an academic perspective, stability is achieved after a winter-summer cycle, from an effectiveness perspective, the effluent meets standards within 30 days, and from a visual perspective, it takes about seven days.

II. MBBR treatment process: Is it necessary to add additional microbial agents?

Strictly speaking, MBBR does not require the addition of microbial agents. It achieves natural enrichment of nitrifying bacteria or denitrifying bacteria through reasonable parameter optimization. The biofilm conditions are conducive to the attachment of relevant bacteria, such as anaerobic ammonia oxidation bacteria, which attach under specific conditions. However, under special water quality conditions, such as some difficult-to-degrade water quality or water sources with a single origin, some specific microbial agents have special effects, so microbial agents can be added for initial inoculation, but subsequent addition is unnecessary. In summary, it is not needed for domestic sewage, but it can be a research topic for specific wastewater conditions.

III. Some experts in the industry say that the packing material is prone to clogging and the flow pattern is difficult to control. What are your thoughts?

This shouldn't be called a problem, but rather a challenge. It is because of these challenges that different technical teams and manufacturers are selected to overcome them. Overcoming these challenges leads to world-leading technology; otherwise, the technology will gradually be eliminated from the market.

IV. MBBR: Does it require denitrification backwashing?

The biggest advantage of MBBR compared to traditional biofilms is that it does not require backwashing because the biofilm sheds automatically. Our research shows that when the biofilm activity is strong, the secretion of extracellular polymeric substances is high, and its viscosity is strong. When it ages, the extracellular secretion decreases, its viscosity weakens, and it automatically sheds during fluidization, with new biofilm growing subsequently. Therefore, it does not require washing.

V. What is the core and magic of MBBR?

The core of MBBR is twofold: the packing material and fluidization. The packing material serves as a carrier. There is no unified research yet on the extent of its performance impact, but its shape does affect fluidization. Therefore, the most commonly used packing material domestically and internationally is the flat cylindrical type. Research on packing materials continues, and various types can be tested to determine which performs best. However, from an engineering perspective, performance, biofilm formation speed, final stable effect, lifespan, wear resistance, etc., should be evaluated. Therefore, currently, the core of suspended carriers lies in fluidization.

VI. MBBR system filling rate issues?

The currently verified maximum filling rate is 67%; the maximum achievable aerobic zone in engineering is 60%, and the anoxic zone is 50%.

VII. Must the MBBR packing material be modified?

I believe that the packing material does not need to be modified; existing packing materials are sufficient. Numerous engineering practices have proven that the packing material can still achieve good results. In my opinion, packing material modification is still in the research stage and has not yet reached the engineering stage.

VIII. Water temperature at 3 degrees Celsius, can MBBR still operate?

In Xinjiang, China, practical cases have shown stable operation at water temperatures of 7-8 degrees Celsius. Water temperatures of 3 degrees Celsius have not been encountered in China, but according to information, the Nordheim wastewater treatment plant in Norway (serving the Winter Olympics) uses snowmelt water with a temperature of 3 degrees Celsius, and it has been shown to achieve stable standards.

IX. Regarding the first application in China of a large-scale MBBR wastewater treatment plant, do its aeration and push-flow devices coexist?

China's first Class A standard water plant, and the first to use a large-scale IFAS/MBBR system, is a circulating flow pool type, similar to the traditional bottom aeration oxidation ditch model, with both push-flow devices and aeration, but aeration cannot be present near the push-flow devices during installation.

X. Is the packing material prone to sludge bulking?

The main cause of sludge bulking is filamentous bacteria. Reports from abroad show that fillers help mitigate sludge bulking because they can break up the 'long and stringy' filamentous bacteria in the sludge system. If the sludge flocs are normal and their size is much smaller than the filler, the filler will not break up the sludge. Therefore, based on foreign research, MBBR is beneficial for improving sludge settleability. Our engineering practice has not revealed any significant sludge bulking characteristics in systems using MBBR.