MBBR Upgrade Of A Northwest China WWTP: Quasi-Class IV Effluent At 30,000 M³/d Without Shutdown Or Footprint Expansion (Case Study)

Winter performance dips. Aging equipment. Tight land. No room for shutdown. If any of these sound familiar, you are not alone. Across China, treatment plants built years ago are now facing stricter discharge standards, rising loads, and the reality of upgrading without interrupting operation. This article presents a real-world case from a plant in the Huangshui River basin of Qinghai Province that solved all of these constraints at once - using the Moving Bed Biofilm Reactor (MBBR) process combined with a denitrifying deep-bed filter and new UV disinfection. The result: effluent meeting quasi-Class IV surface water standards, achieved without expanding the plant footprint and without stopping existing operations during construction.

Background: Why Upgrade?

A Two-Phase Plant at High Altitude and Low Temperature

Located in the Huangshui River basin in Qinghai Province, the plant serves a region characterized by high altitude and low winter temperatures - precisely the conditions that punish conventional biological treatment. It was built in two phases with a total design capacity of 30,000 m³/d:

Phase I (5,000 m³/d) - built years ago with aging equipment, originally designed for Class 1B standard (GB 18918-2002).

Phase II (25,000 m³/d) - running at full capacity, designed for Class 1A standard.

The existing treatment train was: coarse screen → fine screen → aerated grit chamber → inverted AAO → fiber disc filter → UV disinfection. On paper this layout looked adequate, but five critical problems pushed the upgrade decision:

1. Equipment failure in Phase I. Aerators, submersible mixers, internal recirculation pumps, scraper mechanisms, and adjustable weirs were all damaged beyond repair - the phase was effectively idle.

2. Full load in Phase II. With Phase I offline, Phase II carried the entire load with no safety margin, leaving the plant vulnerable to any hydraulic or organic shock.

3. Winter non-compliance. At low water temperatures, effluent NH₃-N and CODCr occasionally exceeded Class 1A limits, forcing costly emergency measures.

4. Land constraints. There was no room for expansion beyond the existing boundary - any upgrade had to fit inside the current footprint.

5. No-shutdown requirement. The plant could not stop operation during construction, which ruled out any solution requiring prolonged tank outages.

Why MBBR Was Selected

After evaluating the available retrofit technologies, MBBR emerged as the clear winner. The biofilm carrier approach delivers a package of advantages that map directly onto the plant's constraints:

High volumetric loading - significantly higher biomass per unit volume than conventional activated sludge, enabling capacity gains inside existing tanks.

Enhanced nitrogen removal - the biofilm physically protects nitrifying bacteria at low temperatures, attacking the plant's most visible winter failure.

Resistance to shock loads - the biofilm buffers against hydraulic and organic fluctuations, restoring the safety margin Phase I's failure had eliminated.

Low sludge yield - less excess sludge reduces downstream handling costs, a growing concern as disposal standards tighten.

No sludge bulking - MBBR systems are inherently immune to sludge bulking, removing a chronic operational headache.

Retrofit-friendly - carriers can be added directly to existing tanks without structural changes, satisfying both the land and no-shutdown constraints.

The Design Challenge

Original Design Versus Actual Operating Reality

Table 1 shows the original design influent and effluent quality. The plant was specified to meet Class 1A limits, with NH₃-N at 5 mg/L and a relaxed 8 mg/L when water temperature drops to 12°C or below.

Note: values in parentheses for NH₃-N apply when water temperature is ≤12°C. Table 2 then shows the 2023 annual operating data. Under annual average conditions, the plant just barely met the existing standards - but the margins were razor-thin.

The winter data in Table 3 reveals the more concerning story. Effluent values consistently approached the upper limits, and during the coldest periods NH₃-N and CODCr actually exceeded the Class 1A standard. Compliance was only maintained through costly emergency measures such as chemical addition and reduced throughput - an unsustainable way to run a plant.

The gap between design intent and winter reality is the core technical justification for the upgrade. A process that could not protect nitrifying bacteria through cold spells would never hold tighter standards.

The Solution: Three-Pronged Approach

1. Restoring Phase I to Share the Load

The first priority was bringing Phase I back online so the plant could distribute load across both phases. Table 4 lists the complete equipment replacement program.

2. MBBR Retrofitting of the Biological Tanks

The core of the upgrade was converting the existing AAO basins into hybrid MBBR-activated sludge systems. Four design modifications were made:

Reconfiguration of flow zones. The original anoxic → anaerobic → anoxic adj. → anaerobic/aerobic adj. → aerobic sequence was simplified to anoxic → anaerobic/aerobic adj. → aerobic, increasing the anoxic zone volume that is critical for denitrification.

Installation of MBBR carriers. Suspended biofilm carriers (HDPE, ≥800 m²/m³) were added, with 2 mixers in Series 1 and 4 mixers in Series 2 to keep carriers fully fluidized.

Improved hydraulics. Partial walls were replaced with curved guide walls, creating a loop-channel aerobic zone for fully mixed plug-flow conditions - this eliminates dead zones and guarantees every carrier passes through the full treatment regime.

Carrier retention. Inlet and outlet screens were installed with perforated aeration to prevent clogging and carrier washout.

The tank volume redistribution is shown in Tables 5 and 6. The decisive change: the combined anoxic/anaerobic zone was converted entirely to anoxic volume, increasing anoxic HRT from 3.44 h to 4.61 h (Series 1) and from 4.01 h to 5.20 h (Series 2).

Because total tank volume is unchanged, the entire retrofit fit within the existing footprint - the land constraint was satisfied by design. The figures below illustrate the original layout, original process flow, upgraded process flow, and the biological tank layout before and after modification.

3. New Tertiary Treatment for Tight Standards

To meet the stricter effluent targets, two new tertiary units were added downstream of the biological stage:

Denitrifying deep-bed filter. With a design flow of 1,812.5 m³/h, a footprint of 34 m × 14 m in 4 cells (each 20 m × 3.5 m, 70 m²), and a filtration rate of 6.47 m/h (8.63 m/h during backwash), this unit uses quartz sand media (2–3 mm, depth 1.83 m) over a 0.5 m graded gravel support. It performs simultaneous solids removal and biological denitrification - the key step for pulling TN down to the quasi-Class IV limit.

UV disinfection channel. A new channel rated at 1,812.5 m³/h with 22 kW installed power replaces chemical disinfection for the final barrier, avoiding disinfection byproducts in the receiving water body.

Target Effluent Quality

Table 7 lists the upgraded design targets, which follow quasi-Class IV surface water standards (GB 3838-2002). Compared with the original Class 1A limits (CODCr 50, NH₃-N 5/8, TN 15 mg/L), these are significantly tighter - NH₃-N drops to 2.5 mg/L and TN to 10 mg/L.

MBBR Equipment Summary

Table 8 summarizes the MBBR equipment and materials added during the retrofit. The scale of the media addition - 2.21×10⁶ m² of effective biofilm surface area at ≥800 m²/m³ - illustrates how much biological capacity can be packed into existing tanks.

Key Lessons For Similar Projects

MBBR is a proven retrofit solution for cold-climate plants. The biofilm provides thermal protection for nitrifying bacteria at low temperatures - the exact failure mode this plant experienced every winter.

Capacity can be increased without expanding the footprint. Adding carriers to existing aerobic zones increases biomass by 50–100% without new construction.

Process reconfiguration matters. Simply adding carriers was not enough - reconfiguring flow zones and hydraulics was critical to achieving the targets. The anoxic volume increase was a deliberate design decision, not an afterthought.

Tertiary polishing is needed for tight standards. The deep-bed filter was essential for meeting the quasi-Class IV limits on TN and SS that the biological stage alone could not guarantee.

Phased construction without shutdown is achievable. MBBR components were installed while the plant remained in operation, proving that compliance upgrades do not have to mean service interruption.

Conclusion

The upgrade of this wastewater treatment plant in Northwest China demonstrates that the MBBR process, combined with appropriate tertiary treatment, can successfully address the common challenges of aging infrastructure, tightening discharge standards, cold-climate operation, and space constraints - all at once.

The combination of equipment replacement, MBBR retrofitting of the biological tanks, and the addition of denitrifying deep-bed filtration and UV disinfection produced a treatment train capable of meeting quasi-Class IV surface water standards - a significant improvement over the original design, achieved without expanding the plant footprint or interrupting operation. For plant owners and engineers evaluating their own upgrade options, this case provides a data-backed reference point for what MBBR technology can achieve under real operating conditions.

This article is based on the paper "Analysis of Upgrading Renovation of a Wastewater Treatment Plant in Northwest China Based on the MBBR Process" (Yang Jintao, 2026). If you are exploring MBBR solutions for your project, reach out to discuss the configuration best suited to your treatment requirements and site constraints.