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Mechanical methods of sewage treatment are applied to extract insoluble mineral and organic impurities, the weighed particles in the size more than 5-10 mkm.
At first sewage are directed to the grating of purification structures to retain the large impurities, containing in them.
Sedimentation.
Rough mineral impurity and 90-95 % of floating oil products are retained in the sand traps, which are horizontal reservoirs, made from mass reinforced concrete. In the bottom of sand traps the clean-out pit is mounted, where the sediment is collected. To treat (clean) sewage containing more 100mg|l of no emulsifying hydrocarbons (oil, oil products), and also fine mineral impurities, it is possible to apply oil traps of the various designs. These are rectangular tanks in which separation of oil and water takes place due to a difference of their density.
Additional gravitational sewage treatment and averaging of their compound are carried out in the balancing reservoir or radial settling tanks, which are equipped by devices to collect oil products and to rake up sediment. The general mass of weighed particles (40-60 %) drops out in sediment for 1.5 hour. After cleaning in ponds of additional sedimentation the residual contents of oil products composites 70-100 mg / l, that is, it is decreased on 30-50 %.
-vertical settling tanks are applied at the discharge of sewage up to 10 000 m3/d.
- horizontal settling tanks are applied at the discharge of sewage up to 15 000 m3/d.
-radial settling tanks are applied at the discharge of sewage up to 20 000 m3/d.
Filtration. Emulsifying hydrocarbons and suspensions are removed by means of filtration on quartz sand, crush anthracite, quartz, a marble, a ceramic crumb, dolomite, polymer, etc.
Sandy filters are the steel vertical or horizontal tanks loaded by quartz sand by a layer 1м and operating under pressure 0,6МПа. Velocity of a filtration is 5-12 km/h. Efficiency of sewage treatment is 30-36 %. The residual contents of oil products is 45-55 mg /l. Low efficiency of sewage treatment is connected to plugging filtering checker by organic substances which can not be removed during washing.
Filters with floating media from polystyrene are single-layer and two-layer. Velocity of a filtration is 0.6-2 km/h. To regenerate granular filtering materials the intensive water-air washing is carried out.
To treat the sewage the filters «Polymer» with foam polyurethane media were developed. Residual contents of oil products is 10 mg /l, the velocity of filtration is 25-35 km/h.
Flotation.
Sewage treatment by flotation is to extract not dissolved impurities by means of finely dispersed air in sewage. Flotation installations are used to remove oil, oil products, and surface-active substances, solid particles, fiber materials from sewage.
Flotation is adhesion of particles of impurity and bubbles of air. Complex of bubbles-particles float on the surface. In this case particles are collected in the formed foam layer, which is removed from surface of water. There are some methods of flotation differing in the way of dispergation of air in the water.
Pressure head flotation is widely used. In this case the part of sewage is saturated by air under pressure. In order to intensify formation of the bubbles-particle complexes it is necessary to add in the water the different reagents increasing water-repellency treatment of particles surface. The degree of cleaning by flotation is 11 - 23 %.
The combination of pressure head flotation with coagulant treatment makes it possible to increase the sewage treatment degree from oil products on 90-95 % and mechanical suspensions is on 85-95 %.
When using pressure head flotation it is possible to use once-through or recirculation schemes. In case of once-through flotation the all sewage is directed to the carbonator, filled with a packer to intensify the process of dissolution air in water.
Flotation in a combination with coagulants and flocculants use (reagent flotation) is one of perspective method of sewage treatment and tertiary treatment. Thus, flotation makes it possible to treat sewage up to residual contents of pollution of 10-30 mg /l.
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