How to Treat Oily Wastewater?
Oily wastewater is any water that has become contaminated with oils, greases, and hydrocarbons. It is a major form of industrial wastewater that comes from a variety of sources including oil refineries, petrochemical plants, gas plants, automobile service stations, and more. Proper treatment of oily wastewater is crucial before discharging it into the environment in order to remove the oil and grease contaminants, which can be toxic to aquatic life. This blog provides an overview of the main sources of oily wastewater and the various methods and technologies used to treat it effectively.
Sources of Oily Wastewater
The petroleum industry is the largest source of oily wastewater. Activities like oil drilling, production, refining, and transportation produce significant volumes of oily wastewater. This includes produced water from oil wells which contains hydrocarbons, production chemicals, and heavy metals. Refineries also generate oily wastewater from processes like distillation, cracking, reforming, and coking.
Other sources include automobile service stations, engine repair shops, jetties and harbors, machine shops, food processing plants that use edible oils, soap and detergent plants, textile mills that use oils for fabric production, metal working facilities that use coolants and lubricants, airports due to fueling and aircraft washing activities, and more. The composition of oily wastewater varies widely based on the source.
Impacts of Discharging Untreated Oily Wastewater
Discharging untreated oily wastewater into the environment can have severe detrimental impacts:
1- The oil forms a layer on the surface of water bodies, preventing oxygen exchange. This leads to oxygen depletion in the water and can suffocate aquatic plants and animals.
2- Oil contamination makes water unfit for human consumption and industrial applications.
3- Oil contains toxic components like benzene, ethylbenzene, toluene, xylene, naphthalene and PAHs which can bioaccumulate up the food chain. This poses risks to human health.
4- It can disrupt biological treatment processes at downstream wastewater treatment plants.
5- The aesthetics of water bodies are reduced due to odor and fouling.
6- Oil prevents sunlight from reaching underwater vegetation. This affects photosynthesis and oxygen production.
7- Crude oils are very difficult to decompose naturally and can persist in the environment for years.
Goals of Oily Wastewater Treatment
The primary goals of any oily wastewater treatment system are:
1- Separate bulk free and dispersed oils and greases as much as possible
2- Remove soluble organic compounds that contribute to BOD, COD and TOC
3- Reduce total suspended solids (TSS) through clarification processes
4- Make the effluent safe for discharge into municipal sewers or surface waters
5- Produce an effluent that meets regulatory discharge standards and consent limits
6- Allow reuse of treated wastewater for appropriate applications
7- Generate minimal volumes of waste sludge
Treatment Technologies and Processes
A variety of methods exist for treating oily wastewater effectively to meet regulatory discharge limits. These can be used alone or in combination as part of an overall treatment system.
1. Gravity Separation:
Gravity separation methods use the density difference between the oil and wastewater to allow separation. Common techniques include:
- API Oil-Water Separators: These consist of simple containers where the oily water is held for sufficient residence time to allow gravity separation of free oil. Oil rises and accumulates at the top while the clearer water is discharged from the bottom.
- Parallel Plate Interceptors: These have parallel inclined plates which improve separation efficiency by increasing the surface area. Oil droplets coalesce and flow upwards while water moves downwards.
- Tilted Plate Interceptors: These have tilted parallel plates to facilitate sludge removal. Efficiency is higher than parallel plate interceptors.
2. Dissolved Air Flotation (DAF):
In DAF systems, air bubbles are injected into the oily wastewater under pressure. Upon release of pressure, the bubbles rise and carry the oil droplets with them. The oil accumulates as froth which is skimmed off. Chemical coagulants can be added to agglomerate fine oil particles for enhanced removal. DAF requires proper sludge handling.
3. Skimmers:
Skimmers are used to remove the accumulated oil layer from separators and DAF units. Different types exist - weir, disc, drum, belt, tube skimmers that use various mechanisms to collect the floating oil for downstream recovery.
4. Centrifugation:
Hydrocyclones and centrifuges use centrifugal forces to separate oil and solids from water. The spinning motion causes the higher density phase to move outwards where it can be collected. No chemicals are required but energy consumption is high.
5. Media Filtration:
This technique uses a bed of filter media like sand, gravel or anthracite to trap oil droplets and solids. Media filtration is often used as a final polishing step. Backwashing is required to remove accumulated oil and solids from the media.
6. Membrane Filtration:
Membrane systems like ultrafiltration and microfiltration provide a physical barrier for separating oil and particles. Ultrafiltration membranes have pore sizes ranging from 0.01 to 0.1 microns while microfiltration membranes range from 0.1 to 10 microns. High pressures are required to overcome osmotic pressure. Oil and solids accumulate on the membrane surface and need cleaning.
7. Chemical Coagulation/Flocculation:
Chemical coagulants like alum, ferric chloride or synthetic polymers are added to destabilize the oil emulsion. The oil particles agglomerate into flocs which can be separated by flotation or sedimentation. This is often used with DAF systems to improve efficiency. Proper pH control is critical.
8. Electrochemical Processes:
Electrocoagulation uses sacrificial anode materials like aluminum or iron to introduce coagulating metal cations into the oily wastewater when current is applied. The cations destabilize emulsions and neutralize charges on oil droplets. Electroflotation employs electrolysis to produce hydrogen and oxygen bubbles which float the oil to the surface. Electrochemical methods avoid chemical addition but require electrical energy.
9. Biological Treatment:
Biodegradation of oil by microorganisms like bacteria, fungi and yeasts can assist with wastewater treatment. Conventional activated sludge systems can adapt over time if properly acclimatized. Other biological processes like trickling filters, rotating biological contactors, sequential batch reactors and membrane bioreactors can also degrade organic oil contaminants. Nutrient addition and suitable environmental conditions are needed.
10. Tertiary Polishing:
Various tertiary treatment methods like activated carbon adsorption, chemical oxidation, ozonation and UV irradiation can be used as final polishing steps to remove residual organics and solubilized oil. These help meet stringent discharge limits. Multi-stage treatment trains incorporating both physicochemical and biological methods are commonly designed for optimal oily wastewater treatment.
Summary
Oily wastewater generated from various industries must be properly treated before discharge into the environment. A combination of gravity separation, chemical, biological and physical methods is commonly utilized to remove bulk oil, emulsified and dispersed oil, suspended solids and soluble organics from the wastewater. The specific treatment techniques and sequences are designed based on the wastewater characteristics and the desired discharge quality. Well-designed systems can successfully treat oily wastewater to meet regulatory consent limits in an efficient and cost-effective manner.
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