Purification of oily wastewater

Oily wastewater is formed as a result of production processes and at each of these stages wastewater is formed with its own specific composition and sometimes the presence of specific components (which, in particular, are determined by the chemical composition of the initial crude oil).

Let's list the most common types of oily wastewater:

  1. Water after desalination of oil, including from electric desalination plants (ELOS).
  2. Condensates of atmospheric and vacuum distillation.
  3. "Acidic" condensates of catalytic cracking and hydrocracking.
  4. Wastewater from an oil refinery after pyrolysis.
  5. Sulphurous-alkaline effluents of a high concentration oil refinery resulting from the processing of sulphurous oil and the purification of petroleum products with alkalis.

Oily wastewater, in addition to oil and petroleum products themselves, contains a number of specific components, for example, phenols, naphthenic acids, sulfides and hydrogen sulfide, mercaptans, organic acids and aldehydes, ammonium ions. Oily wastewater from enterprises after desalination is characterized by high salinity, in particular, chloride content up to 15 g/l and, accordingly, has high corrosive activity.

Oily wastewater is formed after washing barrels, on closed production sites, on draining and filling platforms. Highly polluted are oily wastewater generated during the cleaning and washing of tanks of a tanker, which are produced in the case of receiving a higher-quality product than previously transported, as well as during periodic cleaning of tanks from sediments (rust, other solid residues).

Sludge oil-containing wastewater is formed from food storage tanks. Surface oily wastewater is the effluent of enterprises formed during the melting of snow and precipitation. The volumes of surface oily wastewater are calculated using appropriate techniques.

Here we have indicated the main types of oily wastewater. However, a large amount of oily wastewater can be generated, for example, not directly related to oil refining. The composition of such oily wastewater is determined mainly by commercial petroleum products used at this enterprise and their physico-chemical properties, the main pollutants may be fuels for various purposes (gasoline, ligroins, gas oils, kerosene, diesel fuel, etc.), various lubricants, fuel oils, etc.

Features of the composition of oily wastewater

Oily wastewater has its own specifics regarding its composition and the behavior of petroleum products in the aquatic environment, which ultimately determines the methods of purification of oily wastewater. The main features determining the behavior of petroleum products in water are their lower density compared to the density of water (gasoline 0.70—0.76, diesel fuel 0.8—0.9, jet fuel 0.8—0.85, fuel oil 0.94—1.0 g/cm3) and low solubility. The latter for light fractions of oil (gasoline) in water does not exceed 20-30 mg / l, for kerosene 70-90 mg / l, and for heavy fractions it is practically zero. Petroleum products, once in the water, are mostly in a coarse (droplet) state and, due to their lower density, are easily released onto the surface of the water, forming a floating film or layer. Another, smaller part of petroleum products may end up in a finely dispersed state, forming an oil-in-water emulsion. It is generally believed that a true emulsion is formed with colloidal droplet sizes of petroleum products (approximately 0.1 microns). But in the effluents of an oil refinery, persistent emulsions are also observed with large droplet sizes. The stability of the emulsion is due to surface tension, kinetic stability of particles, their low concentration, suspended particles present in oily wastewater can act as stabilizers of emulsions.

Technology of purification of oily wastewater

The performance of the oily wastewater treatment plant is not limited. A promising and modern scheme for the effective treatment of oily wastewater, compiled taking into account the latest advances in technology and technology.

Advantages of the oil-containing wastewater treatment plant of JSC Engineering Center Barents Region:

  • Compactness and small footprint of the oily wastewater treatment plant with the highest quality of oily wastewater treatment ;
  • Easy installation (the oil-containing wastewater treatment plant is supplied in large factory-ready units);
  • The small collection area of oily sludge during the treatment of oily wastewater, with the possibility of supplying highly contaminated oily wastewater and the rectangular geometry of the main part of the height of the oily wastewater treatment plant, allow the installation to be placed outdoors;
  • The possibility of a closed design of an oil-containing wastewater treatment plant with rotating covers and a nozzle for gas extraction during the treatment of oil-containing wastewater;
  • The ability of an oil-containing wastewater treatment plant to operate in a nitrogen environment;
  • There is no need to erect a separate building of explosive and fire-hazardous categories with complex engineering systems for the installation of oily wastewater treatment, which allows significant savings on the implementation of the oily wastewater treatment project as a whole;
  • The maximum possible use of existing communications, networks, and facilities for the installation of oily wastewater treatment;
  • Introduction of new oil-containing wastewater treatment plants and reconstruction of old oil-containing wastewater treatment plants without stopping the process of oil-containing wastewater treatment, reducing the productivity of oil-containing wastewater treatment, or temporarily deteriorating the existing level of oil-containing wastewater treatment for the period of preparation, installation and start-up;
  • The maximum possible use of the hydraulic profile of existing oily wastewater treatment plants, i.e., if possible, the exclusion of new additional pumping for a new oily wastewater treatment plant;
  • Reduction of energy costs through the use of modern innovative highly efficient equipment for the treatment of oily wastewater, controls and regulation;
  • Reduction of capital costs for the treatment of oily wastewater, abandonment of the construction of new buildings;
  • Reduction of capital costs through the use of thoughtful technological solutions and the use of innovative, patented equipment for the treatment of oily wastewater;
  • The possibility of adapting the oil-containing wastewater treatment plant to the size of existing channels, reservoirs and structures;

To achieve the specified requirements for the quality of purified water, it is proposed to use an oil-containing wastewater treatment plant based on the pressure flotation method.

 

Flotation treatment of oily wastewater

The pressure flotation method for the treatment of oily wastewater is one of the most versatile, compact and short-term methods of water conditioning and sediment compaction. Pressure flotation provides a high degree of purification from suspended solids of different nature, BOD, petroleum products, surfactants, fats and other undesirable impurities, a high concentration of flotation sludge during the purification of oily wastewater. The installation of flotation treatment of oily wastewater based on the pressure flotation method is in many cases the foundation for the creation of closed water use systems. The core of the purification of oily wastewater based on the pressure flotation method is the saturation of part of the clarified water with air at a pressure of 4-6 atm. and its mixing with purified water in a flotation plant. Subsequent decompression, directly in the body of the flotation device, leads to the formation of micro-bubbles of air (20-50 microns in size), which adhere to the particles of contamination and float to the surface of the oily wastewater treatment plant, forming a flotation sludge. The flotation sludge from the oil-containing wastewater treatment plant is collected from the entire surface into the center of the flotation device by a special collector. The dissolution of gas in water during the purification of oily wastewater occurs in air saturation plants. During the purification of oily wastewater, part of the clarified water on the flotation device is taken away by a recirculation pump and supplied to the water saturation units with air at a pressure of ~ 5.5 atm. The special design of the water saturation unit with air makes it possible to achieve effective dissolution of gas in water in an oil-containing wastewater treatment plant. After the water saturation units with air, the gas-saturated (saturated) water passes through a pressure reducing valve, which causes a pressure loss from 5.5 atm. to the pressure in the supply pipeline of the oily wastewater treatment plant. At the same time, a huge number of microbubbles are released from the saturated water to purify oily wastewater. An air-water mixture from a saturator is supplied to the treated water to purify oily wastewater. This water enters the central part of the oily wastewater treatment plant. From the central distribution column, the mixture is distributed throughout the entire volume of the oily wastewater treatment plant. In the process of purification of oily wastewater, intensive separation of pollutants occurs. Pollutants entrained by air bubbles rise to the surface of the water and form a stable layer of flotation sludge after purification of oily wastewater. A carriage rotates along the rail of the cylindrical part of the oily wastewater treatment plant, on which a spiral collector is installed. Rotating, the spiral collector scoops up the flotation sludge and dumps it into the central pipe to remove oily wastewater from the treatment plant. After cleaning the oily wastewater, the flotation sludge exits the flotation device by gravity. Water without contamination is collected in the annular channel of the oily wastewater treatment plant, fixed on the inner surface in the middle part of the cylindrical body.

After cleaning the oily wastewater, the water exits the flotation device by gravity through an adjustable overflow mounted on the outer wall of the flotation device. The settled impurities are removed from the oil-containing wastewater treatment plant under the action of hydrostatic pressure using clamping valves in the lower part of the flotation device. The installation has a very reliable and easily configurable flotation sludge removal system during the treatment of oily wastewater. The water level in the bath of the oily wastewater treatment plant is regulated. The flotation device has a spiral collection of flotation sludge to remove the maximum amount of sludge during the treatment of oily wastewater. The speed of the collector of the oily wastewater treatment plant is regulated through the frequency converter of the motor depending on the amount of flotation sludge. This makes it possible to maintain a high concentration of the flotation sludge removed from the oil-containing wastewater treatment plant.