Biofilm in MBBR Systems

Biofilm Development Stages

Biofilm formation on MBBR carriers follows a predictable sequence. During the first 1–3 days, pioneer bacteria attach to the carrier surface through reversible adsorption. This is followed by irreversible attachment (days 3–7) as cells produce extracellular polymeric substances (EPS) that anchor them to the substrate. Over weeks 2–4, the biofilm matures as microbial communities diversify and the EPS matrix develops a three-dimensional structure with channels for nutrient and oxygen transport.

Full functional maturation — particularly the establishment of stable nitrifying populations — requires 4–8 weeks depending on wastewater temperature, organic loading, and whether seed biofilm from an existing system is available. At maturity, the biofilm reaches a dynamic equilibrium where new growth at the base balances sloughing at the surface due to hydrodynamic shear.

The protected interior surfaces of MBBR carriers are critical for biofilm stability. Unlike external surfaces that are exposed to intense carrier-to-carrier collisions, the internal voids provide a sheltered environment where biofilm can develop undisturbed, resulting in higher biomass density and more diverse microbial communities.

Microbial Communities in MBBR Biofilm

MBBR biofilms host stratified microbial communities. The outer layers (closest to the bulk liquid) are dominated by fast-growing heterotrophic bacteria that consume dissolved organic carbon (BOD/COD). Beneath them, autotrophic nitrifiers — ammonia-oxidizing bacteria (AOB, primarily Nitrosomonas) and nitrite-oxidizing bacteria (NOB, primarily Nitrospira) — establish in zones where organic competition is lower.

In anoxic biofilms, denitrifying bacteria (Pseudomonas, Paracoccus, Thiobacillus) dominate, using nitrate as an electron acceptor. Under certain conditions, anaerobic ammonia oxidation (anammox) bacteria can also establish in the deeper, oxygen-depleted layers of aerobic biofilms, contributing to nitrogen removal.

Factors Affecting Biofilm Performance

• Organic Loading: High organic loading favors thick, heterotrophic biofilm that can outcompete nitrifiers. For nitrification, the SALR should be kept below 5 g BOD₅/m²·d to allow autotrophs to establish.
• Dissolved Oxygen: Oxygen diffusion into biofilm is limited to the outer 100–200 µm. Bulk liquid DO of ≥3 mg/L is required for effective nitrification in the inner biofilm layers.
• Temperature: Biofilm activity roughly doubles per 10°C increase (within the mesophilic range). Nitrification is particularly temperature-sensitive, with rates dropping sharply below 10°C.
• pH and Alkalinity: Nitrification consumes 7.14 mg alkalinity per mg NH₄-N oxidized. pH below 6.5 inhibits nitrifying bacteria. Alkalinity supplementation may be needed.
• Toxic Compounds: Heavy metals, chlorinated compounds, and high concentrations of certain industrial chemicals can inhibit or kill biofilm organisms. The protected biofilm structure provides some resilience, as toxicants must diffuse through the EPS matrix to reach active cells.