Wastewater is liquid waste (water) generated in the production process of factories and factories as well as in the daily activities of people.
Every day, the amount of wastewater discharged into the environment is extremely large, so it requires appropriate treatment measures before being discharged into the sources of lakes, rivers and streams.
THEORETICAL BASIS OF WASTE WATER TREATMENT
Wastewater in general contains many different pollutants, which require different appropriate treatment methods. In general, wastewater treatment methods are divided into the following categories:
− Methods of scientific treatment;
− Chemical and physicochemical treatment methods;
− Biological treatment methods.
Methods of processing
− Aerobic biological treatment with microorganisms that grow in the form of adhesion such as activated sludge process, trickling filter tank, high-load filter tank, biological disc, nitrate reaction tank with fixed membrane.
– Wastewater often contains insoluble substances in suspension. To separate these substances from wastewater. Often mechanical methods are used such as filtering through a screen or screen, settling under the action of gravity or centrifugal force and filtering. Depending on the size, physico-chemical properties, concentration of suspended matter, wastewater flow and level of need to be cleaned, an appropriate treatment technology should be selected.
a. Garbage barrier
– Wastewater leading to the treatment system must first pass through the garbage screen. Here, large-sized components (garbage) such as rags, garbage, canned goods, trash, plastic bags… are kept. This avoids clogging of pumps, pipes or channels.
– Depending on the size of the gap, the screen is classified into coarse, medium and fine. Coarse screens have a distance between bars from 60-100mm and fine screens have a distance between bars from 10-25mm. According to the shape, it can be divided into garbage screens and garbage screens. Garbage screens can also be fixed or mobile. Garbage screens are made of metal, placed at the entrance to the channel, inclined at an angle of 45 – 600 if manual cleaning or at an angle of 75 – 850 if cleaned by machine. The cross-section of the bars can be round, square or mixed.
b. Sand listening
– Sand settling tank is designed to separate insoluble inorganic impurities from 0.2mm to 2mm in size from wastewater to ensure pump safety from sand and gravel erosion, pipeline blockage and avoid affect the biological works behind. Sand settling tanks can be classified into two types: horizontal settling tanks and vertical settling tanks.
– In addition to increasing the efficiency of sand deposition, air blown sand settling tanks are also widely used. The flow velocity in the horizontal clarifier should not exceed 0.3 m/s. This velocity allows sand, gravel and other inorganic particles to settle to the bottom, while most other organic particles do not settle and are treated in subsequent works.
– The settling tank is responsible for settling the suspended sediment particles available in the wastewater (phase 1 sedimentation tank) or the residue generated from the flocculation process or biological treatment process (phase 2 settling tank). According to the flow, settling tanks are classified into: horizontal settling tanks and vertical settling tanks.
– In the horizontal settling tank, the water flows horizontally through the tank at a speed not greater than 0.01 m/s and the retention time is 1.5 – 2.5 h. Horizontal settling tanks are often used when the wastewater flow is greater than 15000 m3/day.
– For vertical settling tanks, the top of the waste moves vertically from the bottom up to the overflow wall with a velocity of 0.5 – 0.6 m/s and the retention time in the tank ranges from 45 – 120 minutes. The settling efficiency of vertical clarifiers is usually 10-20% lower than that of horizontal clarifiers.
Flotation Tank – The flotation method is often used to separate impurities (in solid or liquid form) dispersed insoluble, poorly self-settling from the liquid phase. In some cases, the process is also used to separate solutes such as surfactants. In wastewater treatment, the flotation process is often used to remove suspended substances and thicken biological sludge. The basic advantage of this method is that it is possible to completely eliminate small, light, slow-settling particles in a short time.
– The flotation process is carried out by injecting small air bubbles into the liquid phase. These air bubbles will adhere to the residue particles. When the density of the aggregate of bubbles and sediment is less than the density of water, the residue will follow the foam to the surface.
– The efficiency of the flotation process depends on the number and size of air bubbles and the solids content. The optimal size of air bubbles is in the range of 15 – 30 µm (normally 50 – 120 µm). When the solid particle content is high, the probability of collision and adhesion between particles will increase, so the gas consumption will be reduced. In the flotation process, the stabilization of the bubble size is important.