Case Study · MBBR Technology

How Two-Stage Anaerobic-Aerobic MBBR Treats Rural Sewage at 8°C - And Still Meets Class A Standards

A field study from Changzhou, Jiangsu Province, China demonstrates that with the right carrier media, recirculation strategy, and process design, MBBR systems can achieve over 80% removal of COD, ammonia nitrogen, TN, and TP in winter conditions - where conventional processes routinely fail.

Key results at 8–10°C water temperature · HRT 8h

83.8%

COD removal

173.7 → 28.1 mg/L

84.9%

NH₃-N removal

28.6 → 4.3 mg/L

80.1%

TN removal

49.5 → 9.9 mg/L

85.7%

TP removal

2.73 → 0.39 mg/L

The challenge

Why Winter Kills Conventional Rural Sewage Treatment

China's rural areas discharged 33.71 billion tons of domestic sewage in 2022. Many decentralized treatment facilities - designed and tested under normal temperature conditions - fail to meet discharge standards during winter, directly polluting lakes, farmland, and local water supplies.

The root cause is biological: nitrifying bacteria perform optimally at 25–30°C. Below 15°C, their metabolic capacity drops sharply. Below 5°C, activity nearly stops. The result is cascading failure - COD removal slows, nitrification stalls, denitrification loses its carbon source, and phosphorus removal collapses along with PAO (polyphosphate-accumulating organism) activity.

This study, conducted from September 2022 to April 2023 in Luoyang Town, Wujin District, Changzhou City, tested whether a specifically engineered two-stage anaerobic-aerobic MBBR process could overcome these limitations at water temperatures of 8–10°C.

Influent water quality

COD	100–200 mg/L
NH₃-N	15–40 mg/L
TN	20–50 mg/L
TP	3–5 mg/L
pH	6–8
Water temp.	7–10°C

Process design

Three Modifications That Made the Difference

Air Stripping Recirculation

Nitrified liquid returned from the sedimentation tank to equalization tank A1 at a recirculation ratio of 200–400%. This replenishes the carbon source for denitrification - the biggest bottleneck in cold-weather nitrogen removal.

MBBR Biofilm Carriers

Pre-cultured and acclimated MBBR biological carriers added to all reactor stages. The carriers increase contact specific surface area and biomass, hosting low-temperature-adapted nitrifying bacteria that maintain activity at 8°C where suspended sludge cannot.

Aeration-Enhanced Phosphorus Removal

Excess sludge removed by aeration stirring and air stripping. This enhances phosphorus release and uptake cycling by PAOs and prevents sludge age from becoming excessive - a critical factor in maintaining PAO viability at low temperatures.

System operating parameters

Stage	HRT	DO target	Function
Anaerobic A1	3 h	< 0.5 mg/L	Phosphorus release, denitrification with returned nitrified liquid
Aerobic O1	2 h	2.0–4.0 mg/L	COD degradation, nitrification, phosphorus uptake
Anaerobic A2	2 h	< 0.5 mg/L	Further denitrification, secondary phosphorus release
Aerobic O2	1 h	2.0–4.0 mg/L	Final nitrification, phosphorus uptake, effluent polishing
Total HRT	8 h	-	Recirculation ratio: 200–400%

MBBR experimental device two-stage anaerobic aerobic reactor

Treatment results

Stage-by-Stage Removal Performance at 8–10°C

COD Removal - 83.80% total

Even at low temperatures, COD removal exceeded 80% - demonstrating that the two-stage process maintains effective organic degradation through both anaerobic transformation (PAOs, denitrifying bacteria) and aerobic microbial degradation. The MBBR carriers provide sufficient biomass to compensate for the reduced per-cell metabolic rate at 8°C.

Stage	COD (mg/L)	Removal
Influent	173.68	-
After A1	101.74	41.42%
After O1	63.80	63.84%
After A2	54.87	68.41%
Effluent	28.14	83.80%

NH₃-N Removal - 84.93% total

Ammonia nitrogen removal was concentrated in the aerobic stages (O1, O2), where nitrifying bacteria on the MBBR carrier surfaces maintained high nitrification capacity despite the 8–10°C water temperature. This is the key advantage of biofilm carriers over suspended sludge in cold environments: the carrier biofilm community can be pre-acclimated to low temperatures, hosting cold-adapted nitrifier strains that are absent in conventional activated sludge systems.

Stage	NH₃-N (mg/L)	Removal
Influent	28.61	-
After A1	18.47	35.43%
After O1	10.91	61.86%
After A2	10.51	63.27%
Effluent	4.31	84.93%

TN Removal - 80.06% total

Air stripping recirculation at 200–400% ratio was the decisive factor. By returning nitrified liquid to A1, sufficient carbon was available for denitrification even when influent COD/N ratios were low - the most common cause of TN non-compliance in cold-weather rural treatment.

Influent TN	49.54 mg/L
After A1+O1+A2	16.39 mg/L
Effluent TN	9.88 mg/L (−80.06%)

TP Removal - 85.71% total

TP actually increased to 6.82 mg/L after A1 - a sign that PAOs were actively releasing phosphorus in the anaerobic zone, confirming biological phosphorus uptake was functioning. Subsequent aerobic stages and excess sludge removal via air stripping reduced TP to 0.39 mg/L.

Influent TP	2.73 mg/L
After A1 (P release)	6.82 mg/L ↑
Effluent TP	0.39 mg/L (−85.71%)

MBBR biofilm microbial community microscopy low temperature

Microbial community analysis

High Microbial Diversity Maintained at Low Temperature

High-throughput sequencing of biofilm communities from the anaerobic and aerobic zones revealed that microbial diversity remained high even at 8–10°C - a direct result of the MBBR carrier providing protected micro-environments where diverse communities could establish and resist washout.

Anoxic Zone (A)

OTU count	1,014
Shannon index	4.24 (high diversity)
Simpson index	0.05
Dominant phylum	Proteobacteria 83%
Key genus	Acinetobacter 13.2%

Aerobic Zone (O)

OTU count	973
Shannon index	4.45 (high diversity)
Simpson index	0.05
Dominant phylum	Proteobacteria 77%
Key genus	Lysobacter 20.3%

Key functional microorganisms identified

Genus	Key function in the system
Lysobacter	Secretes phosphatase to degrade phosphorus; dominant in aerobic zone at 20.3%
Acinetobacter	Decomposes organic matter into organic acids as substrate for nitrifiers/denitrifiers; uses nitrate as electron acceptor
Flavobacterium	Converts organics to acids/fatty acids in anaerobic zone; products fed to aerobic stage for further degradation
Acidovorax	High-capacity degradation of complex organics (proteins, sugars, fats); reduces COD effectively
Pseudoxanthomonas	Nitrification: oxidises ammonia nitrogen to nitrite and nitrate
Stenotrophomonas	COD degradation + antibiotic production inhibiting harmful bacteria, protecting beneficial microbial activity
Brevundimonas	Facultative anaerobe with both nitrification and denitrification functions; degrades macromolecular organics

Conclusions

What This Means for Rural MBBR Plant Design

1	MBBR carriers are essential for cold-weather compliance The carrier biofilm hosts pre-adapted low-temperature nitrifiers that suspended sludge cannot maintain. At 8°C, carrier-based systems achieve what conventional activated sludge cannot.
2	High recirculation ratio (200–400%) compensates for carbon shortage In cold weather, low-C/N influent is the primary reason denitrification fails. Returning nitrified liquid at high ratio replenishes the carbon source and keeps denitrification active throughout winter.
3	8h HRT is sufficient when all three modifications are applied together The combined approach - MBBR carriers + air stripping recirculation + aeration-enhanced sludge removal - achieves Class A effluent (GB 18918-2002) and Jiangsu DB32/T 3462-2018 compliance at 8°C without extending hydraulic retention time beyond what is practical for rural decentralized systems.

Specifying MBBR carriers for a rural or decentralized treatment project?

Juntai supplies pre-cultured MBBR bio carriers suitable for low-temperature environments, including small-scale packaged units and rural sewage treatment applications. Factory-direct supply, no minimum order.

→ Contact our team View MBBR carrier range

How Two-Stage Anaerobic-Aerobic MBBR Solves Winter Rural Sewage Treatment Failure At 8–10°C