The Integrated Fixed-Film Activated Sludge (IFAS) process originated from the contact oxidation method, which operated without sludge recirculation. This method relied mainly on microorganisms attached to biofilms for wastewater treatment and was once widely used. With the development of new types of media and the introduction of sludge recirculation systems, a combined process integrating biofilm on carriers and suspended activated sludge was formed. Its first application was in the upgrade of the Broomfield Wastewater Treatment Plant, and it has since been widely adopted across the United States, Canada, and Germany.
Immobilized activated sludge technology is a wastewater treatment method that enhances microbial stability and treatment efficiency by fixing microorganisms onto carrier materials. The following outlines its details:
1. Water Quality Features
COD (Chemical Oxygen Demand): 200–500 mg/L, mainly from organic matter.
BOD (Biochemical Oxygen Demand): 100–300 mg/L, reflecting the concentration of biodegradable organics.
Ammonia Nitrogen (NH₃-N): 20–50 mg/L, primarily from nitrogen-containing compounds in domestic sewage.
Total Phosphorus (TP): 5–10 mg/L, mainly from detergents and excreta.
Suspended Solids (SS): 100–300 mg/L, including particulate and colloidal substances.
2. Treatment Challenges
High organic load, making treatment more complex.
High concentrations of ammonia and phosphorus, which can cause eutrophication.
Elevated suspended solids, prone to clogging equipment.
1. Immobilized Activated Sludge
Principle: Microorganisms in activated sludge are immobilized on carrier materials to form biofilms or granular sludge, enhancing stability and treatment performance.
Methods: Embedding, adsorption, or cross-linking techniques are used to attach microbes to carriers.
2. Biodegradation
Organic Decomposition: Microbes metabolize organic matter into CO₂ and H₂O.
Ammonia Oxidation: Nitrifying bacteria oxidize ammonia into nitrite and nitrate.
Phosphorus Removal: Polyphosphate-accumulating organisms (PAOs) absorb and remove phosphorus through alternating anaerobic–aerobic processes.
3. Suspended Solids Removal
Removed via sedimentation, filtration, or adsorption.
1. Immobilization Stage
Carrier Selection: Porous materials (e.g., activated carbon, ceramsite) or gels (e.g., sodium alginate, polyvinyl alcohol) are selected.
Immobilization Method: Embedding, adsorption, or cross-linking techniques are applied.
2. Biodegradation Stage
Organic Removal: Under aerobic conditions, microorganisms degrade organics into CO₂ and H₂O.
Ammonia Oxidation: Nitrifying bacteria oxidize ammonia into nitrite and nitrate.
Phosphorus Removal: Polyphosphate-accumulating organisms absorb and remove phosphorus through anaerobic–aerobic alternation.
3. Suspended Solids Removal Stage
Sedimentation: Gravity settling removes suspended solids.
Filtration: Filtration equipment eliminates remaining solids.
Adsorption: Adsorbent materials capture residual particulates.
The IFAS process offers multiple advantages, including a small footprint, reduced sludge production, and high resistance to shock loads. It not only achieves efficient nitrogen and carbon removal but also resolves the sludge age conflict between biological nitrogen and phosphorus removal.
As such, IFAS is highly suitable for upgrading conventional activated sludge systems. In China, its application in both newly built and retrofitted wastewater treatment plants is on the rise. For instance, a wastewater treatment plant in Ningbo converted the end of its aeration tank into an aerobic tank and added 30% polyethylene fluidized-bed carriers. Once biofilm growth stabilized, ammonia removal efficiency increased from 67.6% to 86.7%, while sludge production decreased by nearly 30%.
Furthermore, Shreve and colleagues studied IFAS-based plants in eastern United States and found that, excluding 17 samples where trace organic contaminants (TrOCs) were not detected, the process achieved over 90% removal of TrOCs in the remaining samples. This indicates that IFAS is highly effective in eliminating trace organic pollutants from domestic sewage.
Given these advantages, IFAS is also being increasingly applied in the treatment of industrial wastewater.
