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Definition of oily wastewater
Oily wastewater refers to wastewater containing lipids and various oils. The characteristics are high COD and BOD, with a certain odor and color, flammable, easy to oxidize and decompose, generally lighter than water, and difficult to dissolve in water. Oily wastewater is a type of industrial wastewater with serious hazards, and its pollution mainly manifests in the following aspects:
Deteriorating water quality and endangering aquatic resources;
Endangering human health;
Affects crop production;
Impact on natural landscapes;
Affects clean natural water sources.
The form of oil in water
Suspended oil: particle size ≥ 100 μ m, can float quickly after standing, floating on the water surface as a continuous phase oil film;
Dispersed oil: with a particle size of 10-100 μ m, suspended and dispersed in a water tank. After sufficient standing time or external force, it can condense into larger oil droplets and float to the water surface, or further reduce in size and convert into emulsified oil;
Emulsified oil: with a particle size of 0.1-10 μ m (extremely fine oil droplets), it is stably dispersed in water in the form of water in oil due to the influence of surfactants at the oil-water interface. It is difficult to achieve oil-water separation solely by static method.
In general oily wastewater, not all three types of oil mentioned above may be present, but they are present in representative industries such as electroplating wastewater. The concentration of oil and fat is generally between 300-500mg/L, with emulsified oil accounting for the largest proportion.
There are several main methods for treating oily wastewater, summarized as follows:
1. Sedimentation separation method
The sedimentation separation method is a primary treatment that utilizes the density difference between oil and water phases and the immiscibility between oil and water for separation. Sedimentation separation is carried out in the oil separation tank, and common types include laminar flow, parallel plate, corrugated plate, etc.
2. Filtering method
By utilizing the interception, inertial collision, screening, surface adhesion, coalescence and other mechanisms of granular media filter bed, oil content in water can be removed, which is generally used for secondary treatment or deep treatment. Common granular media filter materials include quartz sand, anthracite, glass fiber, polymer materials, etc. The filtration method equipment is simple, easy to operate, and has low investment costs.
3. Membrane separation method
The membrane separation method for treating oily wastewater uses porous membranes as the separation medium to intercept oil and surfactants in the oily wastewater, allowing water molecules to pass through and achieve oil-water separation. The key to membrane separation technology is the selection of membranes and components. According to pore size, it can be divided into microfiltration, ultrafiltration, reverse osmosis, etc. The most suitable for oily wastewater with high discharge requirements and low treatment capacity.
4. Condensation method
Agglomeration method is to add a certain proportion of flocculant to wastewater, generating lipophilic flocs in the wastewater, allowing micro water and oil droplets to adsorb on them, and then using sedimentation or air flotation methods to remove the oil. Commonly used inorganic coagulants include aluminum sulfate, ferrous sulfate, ferric chloride, polyaluminum chloride, and polyaluminum chloride, as well as organic coagulants such as polyacrylamide and acrylamide. The pH range of different coagulants varies. To enhance the flocculation effect, two coagulants are often used in combination. Common polyaluminum chloride powders are yellow and white, while polyacrylamide is classified as anionic, cationic, and nonionic. This method requires a large amount of medication and slag discharge, making it suitable for treating large amounts of wastewater with low oil content, such as dairy oil or other fine suspended solids.
