(1) Good hydraulic characteristics

The hydraulic characteristics of fillers include specific surface area, porosity, and structural shape. The surface of the filler is the site where biofilm is formed and fixed, and a larger specific surface area is the primary condition for maintaining high concentration biomass in the reactor, which is the primary factor affecting the treatment effect of biofilm method. Generally speaking, a larger specific surface area is advantageous, but the larger the specific surface area, the more likely the reactor is to clog, and the greater the water flow resistance through the packing material. The larger the porosity of the filler, the higher its volume utilization rate, and the lower the water flow resistance, thereby reducing the possibility of reactor blockage and short flow. At the same time, the amount of filler used is reduced, and the infrastructure investment is lowered. However, the higher the porosity, the smaller the specific surface area and mechanical strength. The structural shape of the filler not only affects the surface area and porosity of the pen, but also affects the flow state between the fillers, thereby affecting the mass transfer between wastewater and biofilm and the renewal of biofilm. The shape of the packing also affects the hydraulic characteristics. When selecting the shape of the packing, several factors should be considered: it has a good protective effect on immobilized microorganisms, good mass transfer characteristics, and reduces the operating energy consumption of the reactor.

(2) Mechanical strength

In most biological processes, there are hydraulic shear forces of different intensities and friction and collision processes between fillers, especially in MBBR, where these effects are more pronounced. Therefore, as a biofilm filler, it must have mechanical strength corresponding to the biotechnology used. If the biofilm filler itself does not have a certain mechanical strength, it will inevitably cause varying degrees of damage during its operation. This will cause irregular changes in the biomass held in the biofilm reactor, leading to fluctuations in the effluent quality.

(3) Stability

Fillers must have good biological, chemical, and thermodynamic stability in order to prevent them from participating in biochemical reactions within the system. Biofilms produce various metabolic products during the metabolic process, some of which can corrode fillers. In practical applications, fillers should be inert and not participate in the biochemical reactions of biofilms. The filler itself is non biodegradable and exhibits significant inertness towards chemical reactions that occur in the environment, making it resistant to corrosion. At the same time, biofilm fillers should be inert to changes in the surrounding temperature, and do not react or deform during temperature changes.

(4) Adhesion of biofilm

The adhesion of biofilm on fillers depends on the physical and chemical properties of the filler surface.

Firstly, the porosity and roughness of the filler surface determine whether biofilm can be rapidly formed. The larger the roughness, the faster the biofilm formation. The pore size on the surface of the packing also affects the adhesion of the biofilm. Smaller pores have a strong retention effect on microorganisms. Only when more than 70% of the pore size distribution on the packing surface is within the range of 1-5 times the size of larger microorganisms in the reactor, can significant biological accumulation be achieved.

The second is the surface electrostatic effect and hydrophilicity of the filler. Microorganisms generally carry negative charges under the pH conditions of their living environment. If the carrier surface has a positive charge, it will facilitate the biological fixation process. According to the principle of low system free energy in physical chemistry, hydrophilic microorganisms are prone to adhere to the surface of hydrophilic fillers, while hydrophobic fillers facilitate the fixation of hydrophobic microorganisms on their surface. The hydrophilicity, hydrophobicity, and electronegativity of the filler surface can be achieved by modifying the filler surface or directly during the processing of filler raw materials.

(5) Moderate specific gravity

The specific gravity to water ratio of the filler should be close to 1. The MBBR process requires that the packing material with biofilm attached can move with the water flow under aeration conditions, so its specific gravity is required to be close to that of water.