As a seasoned salesperson in the water treatment industry, I'm excited to share insights into the Moving Bed Biofilm Reactor (MBBR) technology, a highly efficient wastewater treatment method known for its low sludge volume and simple operation. In this article, we'll delve into why the biofilm sometimes fails to form on the MBBR media, considering various aspects such as the system's working principle and factors influencing biofilm formation.
Principle of the MBBR Process
The MBBR media enables microorganisms to attach to the surface of the carrier and form a biofilm. When wastewater flows over the carrier surface, organic matter and dissolved oxygen in the water diffuse into the biofilm. The microorganisms within the biofilm metabolize and assimilate the organic matter in the presence of oxygen. The decomposition products then diffuse back into the water phase and air, effectively degrading the organic pollutants in the wastewater.
According to Characklis, Liu, and others, the formation of the microbial film typically goes through four stages: carrier surface modification, reversible attachment, irreversible attachment, and biofilm formation. This process can be divided into two main stages: microbial adsorption and sequestration growth.
Factors Influencing Biofilm Formation in MBBR
1. Carrier Surface Properties
The surface charge, roughness, particle size, and concentration of the MBBR carrier directly impact biofilm attachment and formation. Microorganisms usually have a negative charge on their surface under normal growth conditions. A rough carrier surface facilitates bacterial attachment and immobilization.
♦A larger surface area of the carrier increases the effective contact area between bacteria and the carrier compared to a smooth surface.
♦Rough parts of the carrier surface, such as holes and cracks, act as a shield to protect the adhered bacteria from hydraulic shear forces.
Smaller particle size carriers are more likely to generate biofilms due to their low mutual friction and large specific surface area. Carrier concentration is also crucial for biofilm formation. Wagner found that at very low carrier mass concentrations, even with a thick biofilm, a stable removal rate could not be achieved when treating refractory wastewater. However, at a carrier concentration of 20-30 g/L, the reactor could achieve a stable removal rate even with only 20% of the carriers having a thin biofilm.
2. Suspended Microbial Concentration
Generally, as the concentration of suspended microorganisms increases, the chance of contact between microorganisms and the carrier also increases. There is a critical concentration of suspended microorganisms during microbial attachment. Before this critical value, microbial transport and diffusion from the liquid phase to the carrier surface is the controlling step. Once this value is exceeded, microbial attachment and immobilization on the carrier surface are limited by the carrier's effective surface area and are no longer dependent on the concentration of suspended microorganisms.
3.Activity of Suspended Microorganisms
Microbial activity, described by the specific growth rate (μ), is crucial when studying the initial stages of biofilm formation. The amount and initial rate of attachment and fixation of nitrifying bacteria on the carrier surface are proportional to the activity of suspended nitrifying bacteria.
♦When the biological activity of suspended microorganisms is high, their ability to secrete extracellular polymers is also higher.
♦The energy level at which microorganisms are living is directly related to their growth rate.
♦The surface structure of microorganisms varies with their activity.
♦Factors such as microbial contact time with the carrier, hydraulic retention time (HRT), liquid phase pH, and hydrodynamic shear force also play a role.
Influencing Factors during the MBBR Biofilm Formation Process
1.Forces in the Biofilm Formation Process
These forces directly contribute to the interaction between microorganisms and the carrier surface, playing a crucial role in the entire biofilm formation process.
2.Effect of Carrier Surface Hydrophilicity
The surface of the GPUC carrier contains hydrophilic groups such as -OH and amide groups. Most microorganisms have good hydrophilicity, and the carrier surface and microorganism surface can form hydrogen bonding structures. The free energy of a hydrophilic carrier surface is lower than that of a hydrophobic one, making it easier for microorganisms in water to approach and adsorb onto the hydrophilic carrier surface for growth.
3.Effect of Temperature on Biofilm Formation
The suitable temperature range for aerobic microorganisms is 10~35℃. Water temperature significantly impacts the growth of nitrifying bacteria and the nitrification rate. The optimal growth temperature for most nitrifying bacteria is 25~30℃. When the temperature is below 25℃ or above 30℃, the growth of nitrifying bacteria slows down, and below 10℃, their growth and nitrification are significantly retarded.
Tests conducted at 10℃, 20℃, and 35℃ showed that at 10℃, the biofilm formation started slowly, with noticeable biofilm attachment after 7 days and maturation after 21 days, with a maximum attached biomass of 2.1 g/L. At 35℃, the biofilm started to form after 4 days and matured after about 19 days, with a maximum attached biofilm amount of 3.5 g/L. At 20℃, the biofilm started to form after 2 days and reached a maximum attached biofilm amount of 5.7 g/L after about 10 days. It is evident that temperature has a significant impact on biofilm formation, with faster initiation between 15-30℃.
Temperature is a key factor affecting biological activity and metabolic capacity, influencing the nitrification reaction process mainly through the growth pattern and biological activity of nitrifying bacteria. It affects the biochemical reaction rate and the oxygen transfer rate.
4.Effect of Carrier Specific Surface Area and Surface Roughness on Biofilm Adhesion Performance
A large specific surface area and roughness enhance the carrier's ability to capture microorganisms. Carriers with high surface roughness have a stronger ability to redistribute water flow, reducing the shear force on the biofilm and providing a favorable environment for the mixing and contact between microorganisms and substrate. The rough surface has a thicker laminar boundary layer than a smooth surface, offering a good static hydrodynamic environment and avoiding the adverse effects of water flow shear on the growth of attached microorganisms.