Performance of Biofilm-Based Secondary Treatment Systems in Treating High-Ammonia Municipal Wastewater

Performance of Biofilm-Based Secondary Treatment Systems in Treating High-Ammonia Municipal Wastewater

Keywords
Biofilm Reactor; High-ammonia wastewater; Secondary treatment; Nitrification; Municipal wastewater; Biofilm stability; Nitrogen removal

1. Introduction

Municipal wastewater with elevated ammonia concentrations presents a major challenge for secondary biological treatment systems. Excess ammonia not only increases oxygen demand during treatment but also threatens receiving water bodies through eutrophication and aquatic toxicity. Conventional activated sludge systems often experience unstable nitrification performance when ammonia loading fluctuates significantly or when environmental conditions become unfavorable.

Biofilm-based secondary treatment systems have gained increasing attention because attached-growth microorganisms exhibit stronger resistance to shock loading and environmental changes compared to suspended-growth systems. Biofilms provide a stable environment for nitrifying bacteria, allowing higher biomass retention and longer microbial survival. This study investigates the effectiveness of biofilm-based reactors in treating municipal wastewater containing high ammonia concentrations and evaluates the operational parameters influencing nitrogen removal efficiency and process stability.

2. Materials and Methods

Pilot-scale biofilm reactors were operated using synthetic carrier media with high specific surface area to support microbial attachment. Municipal wastewater influent containing ammonia concentrations ranging from 40 to 80 mg/L was continuously introduced into the system. Hydraulic retention times (HRT) were adjusted between 6 and 12 hours to evaluate treatment performance under different flow conditions.

Aeration was maintained to provide dissolved oxygen concentrations between 2 and 4 mg/L, supporting nitrification activity within the biofilm layer. Water quality parameters monitored included ammonia nitrogen (NH₄⁺-N), nitrite, nitrate, COD, BOD₅, dissolved oxygen, and suspended solids. Biofilm thickness and microbial composition were analyzed periodically using microscopic observation and microbial sequencing techniques to evaluate bacterial adaptation and community dynamics during long-term operation.

3. Results

The biofilm-based treatment system demonstrated strong performance under high ammonia loading conditions. Ammonia removal efficiency remained above 90% during stable operation, while total nitrogen removal reached approximately 75–85% depending on hydraulic loading and aeration intensity. COD and BOD removal efficiencies exceeded 88%, indicating effective simultaneous removal of organic matter and nitrogen compounds.

Biofilm development was observed rapidly during the initial operational phase, with nitrifying bacteria gradually dominating the microbial community. The attached-growth structure improved resistance to hydraulic shock loads and maintained stable nitrification performance even during short-term fluctuations in influent quality.

Operational optimization revealed that moderate hydraulic retention times combined with stable dissolved oxygen concentrations provided the best overall treatment performance. Excessively high aeration increased operational energy demand without significantly improving nitrogen removal efficiency. The study also observed that thicker biofilms could create internal anoxic zones, supporting simultaneous nitrification and denitrification within the same reactor environment.

4. Discussion

The results confirm that biofilm-based secondary treatment systems are highly suitable for municipal wastewater with elevated ammonia concentrations. The attached-growth mechanism allows microorganisms to remain protected within the biofilm matrix, improving resilience against environmental stress and influent variability.

One of the most important findings was the system’s ability to support simultaneous nitrification and denitrification through oxygen gradients inside the biofilm layer. This reduces the need for separate anoxic treatment zones and simplifies reactor design. In addition, the biofilm structure enhances biomass retention, enabling stable nitrification even under relatively short hydraulic retention times.

The study also emphasizes the importance of balancing aeration intensity and biofilm thickness. While adequate oxygen is necessary for nitrification, excessive aeration may disrupt biofilm stability and increase operational costs. Proper carrier filling ratio and mixing conditions are therefore critical for maintaining long-term treatment efficiency.

5. Conclusion

Biofilm-based secondary Wastewater Treatment systems demonstrate excellent performance in treating municipal wastewater with high ammonia concentrations. The technology provides stable ammonia and nitrogen removal, strong resistance to hydraulic and organic shock loading, and effective simultaneous organic matter degradation.

The study concludes that biofilm reactors offer a reliable and energy-efficient alternative to conventional activated sludge systems, particularly for municipalities experiencing ammonia loading fluctuations or stricter nitrogen discharge regulations. With optimized aeration and operational control, biofilm-based systems can maintain high treatment efficiency while supporting long-term process stability.