Two biological treatment systems with nearly identical configurations—the same bacterial strains, water quality, and operational parameters—after three months of biofilm cultivation, one operates stably with ammonia nitrogen removal exceeding 80%. The other? The biofilm repeatedly sloughs off, and the system crashes frequently. Where does the problem lie? I have seen too many people attribute such incidents to operational errors or bacterial strain quality. This is not the case. The answer often resides in a long-underestimated variable—**the carrier material**. In over several aquaculture water treatment projects I have participated in, this scenario has recurred, with the root cause repeatedly obscured. The carrier is not an "accessory" of a biological treatment system. It is the foundational condition that determines whether microorganisms can stably colonize. Errors in material selection are difficult to remediate through later operational adjustments. ## 1\. First Principles: The Physicochemical Basis of Microbial Attachment From first principles, microbial attachment to carrier surfaces is not a random process. It is a complex interfacial reaction driven by thermodynamic laws, electrostatic forces, and hydrodynamics. To truly understand this, the process must be deconstructed into two stages: **instantaneous physicochemical contact** and **persistent biological anchoring**. **Interfacial Thermodynamics and Surface Free Energy.** Whether microbial attachment can proceed spontaneously depends on the direction of change in the total free energy of the system. According to interfacial thermodynamic models, the free energy change $\Delta G_{adh}$ for microbial cells (b) attaching to a solid carrier (s) surface from an aqueous medium (l) can be expressed as: $\Delta G_{adh} = \gamma_{bs} - \gamma_{bl} - \gamma_{sl}$ where $\gamma_{bs}$, $\gamma_{bl}$, and $\gamma_{sl}$ represent the interfacial tensions between bacteria and carrier, bacteria and liquid, and carrier and liquid, respectively. When $\Delta G_{adh}$ is less than 0, attachment is spontaneous. Interfacial tension is composed of non-polar Lifshitz-van der Waals (LW) forces and polar Lewis acid-base (AB) forces. The presence of hydrogen bonds in aqueous environments often makes polar interactions dominant. The key conclusion? The smaller the difference in surface free energy (SFE) between the microbial cell and the carrier surface, the more negative the adhesion energy, and the easier it is for bacteria to spread and form initial attachment. This is not an empirical summary—it is a thermodynamic imperative. **DLVO Theory and Electrostatic Interactions.** Viewing microorganisms as colloidal particles—the classic DLVO theory does precisely this. The balance between van der Waals attraction and electrostatic double-layer repulsion determines whether bacteria can approach the carrier surface. The complication is that the vast majority of microorganisms carry a net negative charge under physiological conditions (derived from functional groups such as carboxyl, phosphate, and hydroxyl groups on the cell wall), while conventional plastic carriers like HDPE or polypropylene (PP) are also negatively charged. Like charges repel—this is a genuine engineering challenge. Overcoming this energy barrier typically involves three pathways: **ionic strength shielding**—cations in the water compress the electrical double layer, shortening the Debye length and weakening repulsion; **biological appendage penetration**—nanoscale structures such as flagella and pili directly traverse the electrostatic barrier to achieve physical contact; **potential modification**—introducing positively charged functional groups (e.g., amino groups) through surface modification to reverse the surface potential from negative to positive, enhancing electrostatic attraction to bacteria. Understanding these underlying mechanisms is a prerequisite for comprehending the entire biofilm cultivation process. From initial contact to stable biofilm formation, it is essentially a dynamic interplay between physicochemical driving forces and biological responses. ## 2\. Biofilm Formation: From 'Initial Contact' to 'Stable Colonization' Biofilm formation proceeds in four stages. Each stage imposes different requirements on the carrier material, and pitfalls vary accordingly. **Stage 1: Reversible Initial Attachment.** Microorganisms make physical contact with the surface through van der Waals forces and electrostatic attraction, at interaction distances less than 5 nm. Two surface parameters dictate everything at this moment: Zeta potential and contact angle (θ). When the carrier Zeta potential is below -30 mV, the initial attachment rate can decrease by 40–70%—this is not a trivial number. Hydrophobic surfaces (θ > 65°) exhibit initial attachment rates 2–4 times faster than hydrophilic surfaces.
