## 1. Biofilm Process Is Not a Single Tank, but a Biomass Retention Mode
In the activated sludge process, microorganisms mainly exist in flocs suspended in water, and sludge return is adopted to sustain biomass inside the system. For the biofilm process, microorganisms attach to the surface of fillers, filter media or other carriers. Carriers serve as “habitats” instead of directly boosting treatment capacity. The usable specific surface area, mass transfer, shear force, biofilm clogging and sloughing determine the actual effective area available for microbial colonization.
Dissolved oxygen and substrate gradients form radially across the biofilm. The outer layer readily accesses oxygen and organic matter, while anoxic or even anaerobic conditions may develop in the inner layer. Accordingly, biofilms are favorable for retaining slow-growing microorganisms and enable the system to withstand loading fluctuations and variable water quality within a certain range. Nevertheless, thicker biofilms do not guarantee better performance. Excessively thick biofilms increase mass transfer resistance and may peel off in large pieces after aging.
**Key Conclusion**: The biofilm process is a broad category; contact oxidation, MBBR and BAF are reactor configurations implementing this principle.
## 2. Contact Oxidation: Fixed Fillers; Engineering Performance Hinges on Air Distribution and Hydraulics
Fixed or suspended fillers are installed inside aeration tanks for contact oxidation. Wastewater flows through the fillers, while aeration supplies oxygen and generates turbulence. This process accommodates both attached biomass and a certain amount of suspended biomass, featuring straightforward configuration and flexible retrofitting potential.
A commonly underestimated issue is the inherent uneven flow distribution within the filler zone. Poor air distribution creates dead zones; filamentous organisms, aged biofilm and influent suspended solids tend to tangle and cause clogging. Excessively dense fillers narrow flow channels and reduce maintenance space. The design priority of contact oxidation is not maximizing filler dosage, but matching the effective surface area, in-tank circulation, oxygen supply, scouring intensity and subsequent solid-liquid separation.
## 3. MBBR: Moving Carriers — Fluidization Does Not Equal Stable Treatment
In Moving Bed Biofilm Reactor (MBBR), specialized suspended carriers are added into reaction tanks. Aeration or mechanical agitation drives continuous carrier circulation, and screening sieves retain carriers inside the reactor. Compared with fixed fillers, MBBR largely avoids massive clogging of integral filler modules and maintains high attached biomass per unit tank volume. It is widely applied for tank retrofitting, enhanced nitrification and improved shock load resistance.
Core parameters of MBBR include carrier filling ratio, protected effective surface area, carrier fluidization status, sieve throughput and anti-clogging measures. Aeration needs to satisfy both oxygen demand and fluidization requirements. If carriers accumulate in corners, the nominal filling ratio becomes meaningless. Sieve clogging triggers water level rise, overflow and even structural damage. MBBR generally cannot produce clear effluent directly; sloughed biofilm and influent suspended solids still require removal via subsequent sedimentation, flotation or filtration units.
**Design Note**: The nominal specific surface area of carriers cannot replace the actual effective mass transfer area.
## 4. BAF: Functions as Both Bioreactor and Filter
Biological Aerated Filter (BAF) adopts granular filter media to form a fixed bed. Wastewater percolates through the filter bed, and aeration delivers oxygen inside the bed. Microorganisms attached to filter media degrade organic pollutants and achieve nitrification, while the filter bed intercepts suspended solids simultaneously. Distinct from contact oxidation and MBBR, BAF couples biological treatment and filtration within a compact, high-load filter bed.
Such compactness comes with obvious drawbacks. Trapped solids and accumulated biofilm continuously increase head loss, which necessitates periodic backwashing. Insufficient backwashing intensity causes bed compaction and short-circuit flow; excessive intensity may wash out excessive biomass. Backwash wastewater is not eliminated but diverted to the front-end process or dedicated treatment facilities, generating surges in flow rate and pollutant loading.
## 5. Standard Comparative Framework for the Four Processes
Both contact oxidation and BAF adopt fixed carriers. Contact oxidation relies on filler contact in open tanks, whereas BAF employs granular filter beds with inherent suspended solid interception capacity. MBBR uses freely mobile carriers retained by screening sieves. Contact oxidation and MBBR normally require subsequent solid-liquid separation units. BAF can intercept substantial suspended solids on its own, yet it requires investment in backwashing systems and continuous management of head loss.
MBBR offers advantages for existing tank retrofits and nitrification enhancement. Contact oxidation remains a practical option for small-scale projects with limited operational management and simple layout requirements. The compact advantages of BAF can be fully realized only when the site is constrained, influent pretreatment is adequate, and reliable backwashing is guaranteed. Final process selection depends on influent SS, biodegradability, water temperature, ammonia nitrogen loading, effluent standards, maintenance shutdown arrangements and coordination of sludge-water treatment systems.
**Operational Troubleshooting**: Poor fluidization of MBBR carriers should be investigated by checking air distribution, liquid level, filling ratio and water level difference across sieves.
## 6. Beyond Filler Parameters — Three Critical Mass Balances
1. **Oxygen Balance**: Covers carbonaceous oxidation, nitrification as well as energy consumption for fluidization or scouring;
2. **Solids Balance**: Tracks the fate of influent SS, proliferated sludge and sloughed biofilm;
3. **Hydraulic Balance**: Verifies peak flow, head loss, backwash water volume and impact induced by recirculation flow.
A complete process comparison cannot merely focus on nominal specific surface areas without addressing low-temperature nitrification performance, carrier fluidization, backwash wastewater handling and solid-liquid separation.
**Boundary Note**: The space saved by omitting secondary sedimentation tanks for BAF is often offset by the footprint of backwashing systems and backwash wastewater treatment facilities.
## Conclusion: The Choice Is Not Simply about Fillers, but the Entire Operational System
The strength of the biofilm process lies in stable biomass retention; its challenges stem from mass transfer limitations, biofilm sloughing, clogging and cleaning brought by attached microorganisms. No absolute superior or inferior option exists among contact oxidation, MBBR and BAF. Once the carrier status, gas-liquid flow pattern, solid routing and maintenance regime are systematically mapped out, the most suitable process will become evident.
