Key Takeaways & Executive Findings
- •• • Rapid start-up: AGS system reached 75% of design capacity (20,000 m3/d) by day 10 and 90% by day 26, with a maximum hydraulic load of 25,001 m3/d, demonstrating operational resilience and reduced commissioning time. • • Enhanced granulation: Sludge average particle size increased to 2.6 times the inoculum by day 44 and 3.2 times by day 110, with granules (>200 μm) comprising 17.3% and 33.4% of the sludge, respectively, indicating robust granule formation. • • High pollutant removal: Average removal efficiencies of 83.2% for COD, 97.0% for NH4+-N, 75.9% for TN, and 94.4% for SS were achieved over 4 months, even under low BOD5/TN ratios (<4), highlighting effective nitrogen removal. • • Cost and footprint reduction: Compared to conventional A/O process, AGS reduced electricity consumption by 77.3%, chemical usage by 25.4%, sludge production by 30.4%, and footprint by 65.6%, offering significant economic and land-saving benefits.
Abstract
This study presents a full-scale engineering practice of retrofitting an idle upflow anaerobic sludge blanket (UASB) reactor into an aerobic granular sludge (AGS) system for treating low-strength municipal wastewater. The design capacity was 20,000 m3/d (maximum 24,000 m3/d), achieving separate treatment of industrial and domestic wastewater to reduce operational costs. Systematic analysis covered hydraulic capacity enhancement, effluent quality, pollutant removal efficiencies, sludge granulation progress, and operational costs. Results showed rapid start-up: the system reached 75% of design capacity by day 10 and 90% by day 26. During a 4-month operation, average removal efficiencies for COD, NH4+-N, TN, and SS were 83.2%, 97.0%, 75.9%, and 94.4%, respectively, even under low influent BOD5/TN ratios (typically below 4). Granulation progressed quickly: by day 44, average particle size was 2.6 times that of the inoculum and over 4 times that of flocs, with granules (>200 μm) accounting for 17.3%; by day 110, these values increased to 3.2 times and 5 times, with granule proportion reaching 33.4%. Compared to the previous year (June–August), the AGS process reduced electricity consumption, chemical consumption, and sludge production by 77.3%, 25.4%, and 30.4%, respectively, while saving 65.6% of footprint. This ten-thousand-ton case provides a practical basis for AGS technology application in China.
1. Introduction
Municipal wastewater treatment in China faces increasing pressure from stricter discharge standards and carbon neutrality goals. Conventional activated sludge processes are energy-intensive and require large footprints. Aerobic granular sludge (AGS) technology offers a compact, low-energy alternative, but its application has been limited to high-strength wastewaters (COD 500–800 mg/L) in international cases. Chinese municipal wastewater typically has low organic strength (COD 100–300 mg/L), making direct technology transfer challenging.
This study addresses the bottleneck by retrofitting an idle UASB reactor into an AGS system for low-strength domestic wastewater at a 20,000–24,000 m3/d scale. The approach leverages existing infrastructure to reduce capital costs and demonstrates that AGS can achieve rapid start-up, efficient nutrient removal, and significant operational savings under local water quality conditions, providing a viable pathway for domestic AGS adoption.
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WANG Xianglian, MA Lin, JIA Chunfang, CHEN Kai, LIN Sijie, XU Wei, WU Weijun, LYU Jianhao, YANG Caoling, HU Qing (2026). Ten-Thousand-Ton Scale Engineering Practice of Retrofitting a UASB Reactor into an Aerobic Granular Sludge Process. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202510041
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Frequently Asked Questions
What are the critical operational parameters for achieving rapid granulation in low-strength wastewater, and how does the system maintain stability under low BOD5/TN conditions?
The system achieved rapid granulation by day 44, with average particle size 2.6 times the inoculum, and maintained high TN removal (75.9%) even at BOD5/TN below 4. This was attributed to the selection of slow-growing bacteria and the intermittent operation mode, which favored granule formation. The presence of Nitrospira at 9.9% relative abundance and diverse denitrifiers contributed to robust nitrogen removal.
How does the AGS process compare economically with the conventional A/O process in terms of energy, chemical, and sludge disposal costs?
Compared to the previous A/O process, AGS reduced electricity consumption by 77.3%, chemical usage by 25.4%, and sludge production by 30.4%. These reductions translate to substantial operational cost savings, making AGS economically attractive despite potential higher initial investment for retrofitting.
What are the scalability bottlenecks when retrofitting existing UASB reactors to AGS, and how were they addressed in this project?
Key bottlenecks include ensuring adequate mixing and aeration for granulation, managing influent variability, and achieving stable biomass retention. In this project, the UASB reactor was divided into three parallel AGS tanks with a total effective volume of 10,301.88 m3, and the system was operated in a sequencing batch mode with a cycle time of 12.4 hours. This design allowed for controlled feast-famine conditions, promoting granulation and process stability.
What is the maximum hydraulic capacity of the retrofitted AGS system, and how does it handle peak flow conditions?
The design capacity is 20,000 m3/d with a maximum of 24,000 m3/d. During operation, the system achieved a maximum treatment capacity of 25,001 m3/d, indicating a 25% safety margin over the design maximum, which provides operational flexibility during peak flow events.
How does the microbial community structure contribute to the observed nitrogen removal efficiency, and what are the implications for process optimization?
High-throughput sequencing revealed that Nitrospira (9.9% relative abundance) and multiple denitrifying genera coexisted, forming a synergistic community for nitrification and denitrification. This microbial diversity enhances process resilience and allows for efficient nitrogen removal even under low carbon-to-nitrogen ratios, suggesting that maintaining such community structure is key to stable performance.
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