Kunshan Jianbei WWTP MBBR Upgrade: 196,000 m³/day Case Study

Located in Kunshan, Jiangsu Province, the Jianbei North District Wastewater Treatment Plant stands as one of China's largest semi-underground municipal wastewater treatment facilities. With a total treatment capacity of 196,000 tons/day serving approximately 560,000 residents, the plant underwent a landmark MBBR (Moving Bed Biofilm Reactor) upgrade across four construction phases spanning 16 years. This case study examines the retrofit strategy, process configuration, and operational outcomes that elevated effluent quality from Class A to the stringent Class IV Surface Water Standard.

Plant Overview

The Kunshan Jianbei North District Wastewater Treatment Plant treats exclusively municipal wastewater and domestic sewage, making it a critical component of the region's environmental infrastructure. The facility occupies a total land area of approximately 115 km² within Kunshan's urban catchment. The core treatment process integrates pretreatment + modified AAO (Anaerobic-Anoxic-Oxic) Process with MBBR + high-efficiency sedimentation + V-type filtration + UV disinfection, representing a multi-barrier approach to advanced wastewater purification.

Treated effluent is discharged to a natural receiving water body (river), which imposes strict ecological requirements on nutrient removal - particularly for ammonia nitrogen and total phosphorus - to prevent eutrophication in the downstream aquatic environment.

Kunshan Jianbei North District WWTP - one of China's largest semi-underground municipal treatment facilities

Four-Phase Expansion History

The plant's development unfolded over four major construction phases, each expanding capacity while progressively incorporating advanced treatment technologies:

Phase I - Initial Commissioning (2005)

The first phase established a 50,000 tons/day baseline capacity. Construction was completed in January 2005, with commercial operation commencing in August of the same year. This phase laid the groundwork with conventional treatment units designed to meet the then-applicable Class A discharge standards.

Phase II - Expansion and Advanced Treatment with MBBR (2009)

Phase II added another 50,000 tons/day of capacity and, critically, introduced MBBR technology for enhanced biological nutrient removal. Commercial operation began in August 2009, marking the plant's first deployment of biofilm carrier-based treatment. This phase demonstrated early recognition that suspended-growth activated sludge alone would be insufficient to meet evolving regulatory expectations.

Phase III - Semi-Underground Expansion (2016)

Adding 48,000 tons/day, Phase III adopted a semi-underground construction design - an approach that minimizes visual impact, reduces odor emissions, and conserves surface land for community use. The phase passed formal acceptance on December 13, 2016.

Phase IV - Kunshan's First Semi-Underground WWTP (2019)

The fourth and final expansion added another 48,000 tons/day, bringing the plant to its current total of 196,000 tons/day. As Kunshan's first fully semi-underground WWTP section, it passed acceptance on May 23, 2019, setting a precedent for future urban wastewater infrastructure in the Yangtze River Delta region.

Plant layout diagram illustrating the multi-phase expansion and process unit arrangement

Upgrade & Retrofit: Class A to Class IV Surface Water Standard (2020–2021)

The most significant transformation commenced in September 2020 and reached completion in January 2021. The comprehensive upgrade and retrofit program elevated the plant's discharge standard from Class A of the Urban WWTP Standard to the far more demanding Quasi-Class IV Surface Water Standard. Achieving this required a systematic re-engineering of the biochemical treatment core - the AAO-MBBR reactors.

MBBR Biochemical Tank: Seven Key Improvements

The rectangular biochemical tank was the focal point of the retrofit. With an influent COD typically exceeding 100 mg/L, the upgraded system achieved effluent COD consistently below 20 mg/L - a removal efficiency exceeding 80%. Seven targeted improvements were implemented:

1. MBBR19 Biofilm Carrier Media

The plant selected MBBR19 carriers, manufactured from 100% virgin HDPE in black. The cylindrical carrier geometry features an internal cross-structure and protected surface area optimized for biofilm attachment while maintaining excellent fluidization characteristics. Virgin HDPE ensures chemical inertness, UV resistance, and a service life exceeding 10 years under continuous operation - essential for a facility of this scale where media replacement would involve significant downtime and cost.

MBBR19 carrier media - 100% virgin HDPE, engineered for high biofilm density and long service life

2. Inlet Flow Distribution Optimization

A critical hydraulic modification converted the inlet configuration from a wide-side pipe inlet to a long-side overflow weir inlet. This seemingly modest change produced substantial operational benefits: it optimized flow distribution across the tank cross-section, eliminated short-circuiting and dead zones, enhanced MBBR carrier fluidization uniformity, and significantly improved the tank's resistance to hydraulic and organic shock loads. Uniform flow distribution is a prerequisite for consistent biofilm performance - dead zones lead to carrier accumulation and reduced effective reactor volume.

Long-side overflow weir inlet modification - eliminating dead zones and improving carrier fluidization

3. Outlet Media Retention Screens

Dedicated MBBR retention screens were installed at the reactor outlet to prevent carrier media from escaping to downstream treatment units. Screen design is critical: excessive clogging increases head loss and maintenance frequency, while inadequate screening allows media loss that reduces effective biomass inventory and risks clogging downstream equipment. The stainless steel wedge-wire screens at Kunshan are sized to retain MBBR19 carriers while maintaining hydraulic throughput.

Stainless steel MBBR outlet retention screens - preventing carrier media loss

4. Carrier Fill Ratio

The retrofit employed a 10–15% fill ratio (carrier volume as a percentage of reactor effective volume), which is notably conservative for an MBBR retrofit. In greenfield MBBR designs, fill ratios of 30–55% are common, but retrofit projects must balance enhanced treatment capacity against existing tank geometry, aeration capacity, and mixing energy. The 10–15% fill ratio at Kunshan successfully delivered the required effluent improvement without exceeding the hydraulic and aeration limits of the existing infrastructure - a pragmatic, cost-effective approach to brownfield MBBR integration.

5. Combined Fine and Coarse Bubble Aeration

The aeration system employs a dual-mode configuration combining fine bubble disc diffusers and coarse bubble aeration hose:

Fine bubble aeration (disc diffusers) serves the dual purpose of MBBR carrier fluidization and dissolved oxygen supply for microbial metabolism. Fine bubbles provide higher oxygen transfer efficiency (OTE), typically 4.5–6.5% per meter of submergence, making them the energy-efficient backbone of biological treatment.

Coarse bubble aeration (aeration hose) supplements carrier fluidization in zones where fine bubble energy alone may be insufficient to maintain uniform media suspension. Coarse bubbles generate stronger turbulence and mixing patterns, preventing carrier stratification and ensuring consistent biofilm-wastewater contact throughout the reactor volume. This hybrid aeration strategy balances oxygen transfer efficiency against the mechanical requirements of carrier suspension - a critical design consideration for large-tank MBBR systems where uniform fluidization is more challenging than in compact reactors.

Hybrid fine and coarse bubble aeration system - balancing oxygen transfer with carrier fluidization

6. Foam Management Protocol

Light surface foam is recognized as a normal biochemical phenomenon resulting from microbial metabolism and extracellular polymeric substance (EPS) production in the biofilm. The plant operates with a tiered foam monitoring protocol: light, transient foam requires no manual intervention. Investigation and corrective action are triggered only when excessive foam accumulation, overflow, or abnormal odors are observed - potential indicators of influent toxic shock loads, sludge aging, or organic overload conditions that warrant operational adjustment.

7. Tank Wall Wear Protection

Long-term MBBR operation involves continuous carrier fluidization and collision with tank walls - a mechanical wear mechanism unique to MBBR systems. The Kunshan plant implements a routine inspection program covering anti-corrosion coatings, aeration equipment integrity, and retaining screen condition. Proactive repair and scheduled replacement of worn components prevent unplanned downtime and maintain structural integrity. This is a frequently overlooked aspect of MBBR operation that directly affects long-term asset life.

WWTP Configuration Types: A Comparative Perspective

The Kunshan plant's evolution from conventional activated sludge through multiple configurations provides valuable insight into wastewater treatment infrastructure planning. The semi-underground design adopted in Phases III and IV represents a growing trend in urban China, where land-use pressure and community odor concerns increasingly drive infrastructure design choices.

Comparative analysis of WWTP configuration types - above-ground, semi-underground, and fully underground

Key Takeaways for MBBR Retrofit Projects

The Kunshan Jianbei North District WWTP demonstrates that large-scale MBBR retrofit is both technically feasible and economically practical, even at capacities approaching 200,000 tons/day. The 16-year, four-phase development approach allowed the plant to incrementally incorporate advanced treatment technology while maintaining continuous operation - a model worth studying for municipalities facing similar upgrade challenges. The successful elevation from Class A to Class IV Surface Water standards, achieved with a conservative 10–15% MBBR fill ratio, underscores the effectiveness of well-engineered biofilm carrier integration within existing AAO infrastructure.

References & Data Sources

Site visit report, Kunshan Jianbei North District Wastewater Treatment Plant, June 2026. Plant operational data and design specifications verified during on-site inspection. Discharge standard comparison sourced from China's Urban WWTP Pollutant Discharge Standard (GB 18918-2002) and Surface Water Environmental Quality Standard (GB 3838-2002).