What is MBBR

I. What is MBBR?

The MBBR (Moving Bed Biofilm Reactor) process is based on the principles of biofilm technology. By adding a certain amount of suspended carriers to the reactor, the biomass and microbial diversity in the reactor are increased, thereby improving treatment efficiency. Since the density of the carriers is close to that of water, they become fully mixed with the water during aeration, creating a three-phase (gas-liquid-solid) environment for microbial growth. The collisions and shear forces between the carriers break air bubbles into finer particles, enhancing oxygen transfer efficiency.

Additionally, each carrier harbors different microbial communities—anaerobic or facultative bacteria inside and aerobic bacteria outside—effectively turning each carrier into a miniature reactor where nitrification and denitrification occur simultaneously, improving overall treatment performance.

II. Principles and Features of MBBR

1. Principles of MBBR

The MBBR process enhances reactor efficiency by adding suspended carriers to increase biomass and microbial diversity. The carriers, with a density similar to water, achieve complete mixing during aeration, creating a gas-liquid-solid environment for microbial growth. The turbulence and shear forces generated by the carriers improve oxygen utilization.

Each carrier functions as a microreactor, with anaerobic/facultative bacteria inside and aerobic bacteria outside, enabling simultaneous nitrification and denitrification.

MBBR combines the advantages of traditional fluidized beds and biological contact oxidation processes. It relies on aeration and hydraulic flow to keep carriers in a fluidized state, promoting both suspended activated sludge and attached biofilm growth. This maximizes reactor space utilization and leverages the strengths of both attached and suspended biomass. Unlike conventional fixed media, MBBR carriers continuously interact with wastewater, earning them the name "moving biofilms."

2. Advantages of MBBR

Compared to activated sludge and fixed-media biofilm systems, MBBR offers:

• High efficiency and operational flexibility (like activated sludge).

• Strong resistance to shock loads, long sludge age, and low excess sludge production (like traditional biofilm systems).

(1) Carrier Characteristics

• Made of polyethylene, polypropylene, polyurethane foam, etc.

• Near-neutral buoyancy (density ~1.0).

• Cylindrical or spherical shapes for easy biofilm formation, no clogging, and easy sloughing.

(2) Excellent Nitrogen Removal

• Carriers create aerobic, anoxic, and anaerobic zones, allowing nitrification and denitrification in a single reactor.

• Effective ammonia removal.

(3) High Organic Removal Efficiency

• High sludge concentration (5–10 times higher than conventional activated sludge, up to 30–40 g/L).

• Strong resistance to shock loads.

(4) Easy Maintenance

• No need for carrier support structures.

• Simple maintenance of aeration systems.

• Saves space and investment costs.

3. Disadvantages of MBBR

(1) Carrier Accumulation

• Carriers may accumulate in certain areas due to improper aeration or reactor design.

• Solutions: Optimize aeration layout and reactor structure.

• Recommended reactor length-to-depth ratio: ~0.5, with length ≤3 m for full fluidization.

(2) Effluent Screen Clogging

• Screens/grids are used to prevent carrier loss but may clog.

• Solutions: Use movable screens for manual cleaning or install air backflushing systems.

III. Evaluation Criteria for MBBR Carriers

1. Biofilm Adhesion Capacity

• Key indicator: Biofilm attachment = Protected surface area (design-dependent) × Biofilm density per unit area (carrier-dependent).

2. Carrier Performance

(1) Surface Properties

• Roughness: Rougher surfaces facilitate faster biofilm formation.

• Surface charge: Microorganisms are negatively charged; positively charged carriers promote growth.

• Hydrophilicity: Hydrophilic carriers favor microbial attachment.

(2) Hydraulic Properties

• Porosity: Higher porosity is better.

• Shape & size: Affects flow dynamics.

(3) Fluidization Performance

• Optimal density: 0.97–1.03 g/cm³ for easy fluidization.

3. Biofilm Maturity Assessment

• Visual inspection: Uniform biofilm distribution, darker color.

• Microscopic examination: Dense biofilm, diverse microbes (e.g., Vorticella, Epistylis), presence of rotifers/nematodes indicates maturity.

IV. Rapid Start-Up of MBBR

1. Carrier Filling Phase

• Add carriers gradually to avoid accumulation.

• Use intermittent aeration (reduce aeration at night).

• After 24–48 hrs, increase influent flow and check DO (maintain 1.5–2.0 mg/L).

• Full operation achievable in ~7 days.

2. Biofilm Cultivation

(1) Static Cultivation

• Seed sludge (10% of reactor volume) + nutrients (C:N:P = 100:5:1).

• Alternate aeration (1 hr) and static periods (2–4 hrs).

• After 4–5 days, continuous low-flow feeding begins.

(2) Dynamic Cultivation

• After ~6 days, switch to continuous flow (DO: 2–4 mg/L).

• Protozoa (e.g., amoebae, Vorticella) appear in 15–20 days.

• Mature biofilm (rotifers/nematodes) forms in ~20 days.

3. Biofilm Acclimation

• Adjust parameters (DO: 2–3 mg/L, aeration ≥5 hrs/day).

• Target biofilm thickness: 0.2–0.5 mm.

• Monitor until effluent BOD, COD, SS meet standards.

V. Common MBBR Engineering Issues

1. Biofilm Formation Time in Winter?

• Visible biofilm: 7 days.

• Effluent compliance: <30 days.

• Full maturity: >1 year (after seasonal adaptation).

2. Need for Bioaugmentation?

• Generally unnecessary (natural enrichment suffices).

• Specialized inoculants may help for industrial/refractory wastewater.

3. Requires Backwashing?

• No—biofilm sloughs off naturally due to aging/renewal.

4. Core of MBBR?

• Carriers + Fluidization.

• Optimal carrier shape: Flat cylinders (best balance of performance/durability).

5. Maximum Filling Ratio?

• Aerobic zone: ≤60%; Anoxic zone: ≤50%.