Key Considerations for Contact Oxidation Process Operation
Introduction
The contact oxidation process is a biofilm-based wastewater treatment method where microorganisms grow attached to carriers (e.g., elastic or fibrous fillers) in an aerated tank. It efficiently removes organic matter (BOD/COD) and ammonia (NH₄⁺-N) through microbial degradation.
Key Advantages:
- High Efficiency: Effective for both organic and nitrogen removal.
- Compact Design: Smaller footprint vs. activated sludge systems.
- Resilience: Tolerates load fluctuations better.
- Low Sludge Yield: Reduced sludge disposal costs.
Key Operational Considerations & Solutions
The contact oxidation process requires careful management to maintain efficiency. Common challenges include:
1. Selection of Biofilm Carriers
Biofilm carriers serve as the medium for microbial attachment and directly affect the biomass, spatial distribution, metabolic activity, and even water/air distribution in the contact oxidation tank. In addition to general requirements such as long service life and moderate cost, factors like wastewater characteristics and concentration must also be considered.
- High-concentration wastewater
- Microbial growth is rapid, leading to thick biofilms.
- Recommended carriers: Elastic carriers that facilitate biofilm sloughing.
- Low-concentration wastewater
- Microbial growth is slow, resulting in thin biofilms.
- Recommended carriers: Soft fiber or composite carriers with high specific surface area to enhance biofilm adhesion.
- Nitrification zone (for biological nitrogen removal)
- Nitrifying bacteria are strict aerobes and grow only on the biofilm surface.
- Recommended carriers: Suspended or elastic 3D carriers with uniform spatial distribution and high specific surface area.
- For suspended carriers:
- Relative density should be slightly higher than water when biofilm is attached.
- This allows the carriers to mix thoroughly with wastewater under aeration, improving mass transfer and promoting the shedding of excessive biofilm.
2. Preventing Excessive Biofilm Growth and Balling
In systems with fixed or suspended carriers, a loosely hung carrier should be installed in each tank section for periodic inspection. If the biofilm becomes excessively thick (black, foul-smelling) and effluent quality deteriorates, "biofilm stripping" measures should be taken:
- High-flow/high-aeration flushing: Instantly dislodges thick biofilms.
- "Anoxic starving" method: Temporarily stop aeration to induce anaerobic fermentation, weakening biofilm adhesion before flushing.
For industrial wastewater with sticky pollutants (e.g., sugars in beverage wastewater, oligomers in acrylic fiber wastewater, PVA in textile wastewater):
- Carrier selection: High-porosity floating or elastic 3D carriers to minimize balling.
- Remediation: High-intensity air/water flushing; replace carriers if severely balled.
3. Timely Removal of Excess Sludge
Sludge in contact oxidation tanks primarily comes from:
- Shed aged biofilms.
- Incompletely removed suspended solids from pretreatment.
Impacts of sludge accumulation:
- Large sludge flocs increase effluent COD due to endogenous decay.
- Clogging of fine-bubble aerators.
Mitigation measures:
- Regularly inspect sludge deposition and suspended solids concentration.
- Sludge removal methods:
- Pump out black sludge.
- Loosen sludge via intensified aeration before discharge.
- Temporarily install auxiliary air lances to resuspend sludge in dead zones.