source:LASEN Environmental Protection release date:2025-02-26 Number of reads:4994
1. Project Background
A refinery generates a large volume of oily wastewater during its production processes. Direct discharge of this wastewater would cause severe pollution to surrounding water bodies. The refinery discharges approximately 2,0XX tons of wastewater per day, which contains high levels of petroleum hydrocarbons, sulfides, volatile phenols, chemical oxygen demand (COD), suspended solids, and other pollutants.
2.Problem Statement
The large volume of wastewater increases the difficulty and cost of treatment.
The wastewater contains a complex mixture of pollutants, including ammonia nitrogen, oils and greases, heavy metals, sulfides, volatile phenols, and recalcitrant cyclic organic compounds, making it difficult for conventional treatment processes to effectively remove all contaminants.
Significant fluctuations in both wastewater quality and flow rate pose challenges to the stable operation of treatment processes.
Petroleum hydrocarbons in oily wastewater can adsorb onto the surface of activated sludge flocs or biofilms, impairing the activity of aerobic microorganisms and adversely affecting biological treatment processes.
3.Client Requirements
The treated wastewater must meet the Grade I criteria of the "Integrated Wastewater Discharge Standard," with significant reductions in pollutant concentrations, including chemical oxygen demand (COD), biochemical oxygen demand (BOD), and petroleum hydrocarbons.
Wherever possible, partially treated wastewater should be reclaimed and reused to achieve resource recycling and reduce production costs.
The treatment process must be stable and resilient to shock loads to effectively handle fluctuations in both wastewater quality and flow rate.
4.Treatment Challenges
The pollutants are complex in composition and difficult to degrade, making it impossible for a single treatment process to meet the required effluent standards. A combination of multiple treatment processes is therefore necessary, which increases the complexity of both process design and operational management.
The dispersed oil and emulsified oil in oily wastewater form stable systems that do not readily separate by flotation, necessitating specialized treatment methods such as dissolved air flotation and the addition of coagulants, which increase both treatment costs and operational complexity.
Significant fluctuations in both wastewater quality and flow rate can make it difficult to maintain stable operation of treatment equipment and process parameters, necessitating real-time monitoring and adjustments, which places high demands on the technical expertise and management capabilities of operating personnel.
5.Treatment Strategy
A combined treatment process is adopted: "oil separator + dissolved air flotation (DAF) tank + coagulation and sedimentation + biological treatment + advanced treatment."
Pretreatment: Large debris and suspended solids are removed from the wastewater using equipment such as bar screens and equalization tanks, while simultaneously balancing the flow rate and quality of the wastewater to create stable conditions for subsequent treatment processes.
Oil Removal：
Oil removal is achieved using an oil separator, which also functions as a primary sedimentation tank to remove coarse, settleable solids, thereby reducing the dosage of coagulants required in subsequent treatment stages.
A dissolved air flotation (DAF) tank is employed to further remove petroleum oils and hydrophobic fine suspended solids from the wastewater. A dissolved air tank is installed upstream of the DAF unit, where compressed air—supplied by an air compressor—is pressurized and thoroughly mixed with water and coagulants inside the tank to generate fine microbubbles. The coagulant dissociates in the wastewater into charged ions that neutralize the opposite charges on fine particles and colloidal substances, thereby destabilizing emulsified oils. The microbubbles then attach to the dispersed oil droplets and suspended solids, forming flocs that rise together to the water surface, where they are skimmed off and removed.
Coagulation and sedimentation: Coagulants are added to aggregate suspended solids and colloidal particles in the wastewater into larger flocs, which then settle out, further removing impurities from the wastewater.
Biological treatment: Biochemical processes such as the activated sludge process and biofilm technology are employed to degrade organic matter, nitrogen, phosphorus, and other pollutants in the wastewater. Among these, the A/O (Anoxic/Oxic) process offers advantages including a simple process flow, fewer treatment structures, lower capital costs, no requirement for external carbon sources, and reduced operating expenses. It effectively removes organic pollutants and partially eliminates nitrogen from the wastewater.
Advanced treatment: Advanced treatment technologies such as sand filtration, carbon filtration, and reverse osmosis are employed to further purify the water and remove residual contaminants, ensuring that the effluent meets required discharge standards.
Disinfection and discharge: The treated wastewater undergoes disinfection to eliminate bacteria and viruses, ensuring water safety before discharge. A portion of the treated water can be reclaimed and reused for non-process applications within the refinery, thereby achieving resource recycling and reuse.
6.Treatment Performance
After treatment, the concentrations of pollutants such as oil, COD (Chemical Oxygen Demand), and BOD (Biochemical Oxygen Demand) in the wastewater were significantly reduced. The effluent quality meets the Grade A criteria of the "Integrated Wastewater Discharge Standard," effectively minimizing pollution to surrounding water bodies.
By reusing a portion of the treated wastewater, the refinery has achieved resource recycling, reduced production costs, and improved water use efficiency.
The treatment process operates stably and demonstrates strong shock resistance, effectively coping with fluctuations in both wastewater quality and flow, thereby ensuring the reliable operation of the wastewater treatment system.
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