A²/O Process – Brownish Yellow Viscous Foam (In-depth Analysis)
July 7, 2026
I. Typical Phenomena
The foam is brownish yellow/tawny, highly viscous, hard to break and accumulates in large volumes on the aeration tank surface. Meanwhile, the activated sludge appears yellowish with fine flocs.
II. Root Causes (Sorted by A²/O Operating Conditions)
1. Excessive Organic Loading (Most Common Cause)
Sudden surge of influent COD and BOD triggers massive microbial proliferation and abundant young sludge; metabolic byproducts form viscous foam. Undegraded organic matter from the anaerobic/anoxic zones flows into the aerobic zone and worsens foaming.
2. Short Sludge Retention Time (SRT), Immature Sludge
Excessive sludge wasting leads to a system dominated by fresh zoogloea with strong viscosity, which readily generates thick foam. Unstable nitrifying bacteria further degrade sludge flocculation performance.
3. Locally Low Dissolved Oxygen (DO) in Aeration Tank
Insufficient overall aeration forces microbes to conduct anoxic metabolism and excrete massive extracellular viscous substances, thickening and darkening the foam.
4. Imbalanced Nitrogen and Phosphorus Nutrients
Deviation from the optimal C:N:P ratio of 100:5:1. Nitrogen or phosphorus deficiency disrupts microbial metabolism, inducing zoogloea to secrete large amounts of viscous substances and heavy foaming.
5. Abnormal Recirculation System
Improper internal and external recirculation ratios carry nitrate nitrogen from the anoxic zone into the anaerobic zone, inhibiting phosphorus release and organic matter degradation, which indirectly raises the organic load of the aerobic zone.
III. Targeted Countermeasures
Reduce influent loading: Cut down water intake volume and homogenize wastewater quality to prevent concentrated inflow of high-concentration wastewater.
Slash sludge wasting: Extend sludge retention time to mature sludge and restore flocculation capacity.
Appropriately raise DO level: Maintain aerobic zone DO at 2~3 mg/L to eliminate local anoxic zones.
Supplement nutrient salts: Test nitrogen and phosphorus concentrations and dose nitrogen/phosphorus fertilizers as required for deficiencies.
Optimize recirculation ratios: Adjust internal and external recirculation to minimize nitrate nitrogen crossover into the anaerobic zone.
Supplementary Distinction of Foam Types
Brownish yellow viscous foam: Caused by high organic loading / immature sludge (prevailing issue in A²/O systems)
Fine white foam: Derived from surfactants and detergents
Grey-black malodorous foam: Resulting from anaerobic decay and over-aged sludge
专业术语注释(便于海外工程使用)
A²/O Process:Anaerobic-Anoxic-Oxic Process 厌氧 - 缺氧 - 好氧工艺
