The basic method of wastewater treatment is to use various technical means to separate, remove and recycle the pollutants contained in sewage and wastewater, or convert them into harmless substances to purify the water.
There are many ways to treat sewage, which can generally be classified into four categories, namely biological treatment, physical treatment, chemical treatment and natural treatment.
1. Biological treatment
Through the metabolism of microorganisms, organic pollutants in the form of solutions, colloids and fine suspensions in wastewater are converted into stable and harmless substances. According to the different microorganisms, biological treatment can be divided into two types: aerobic biological treatment and anaerobic biological treatment.
The aerobic biological treatment method is widely used in the biological treatment of wastewater. According to the different process methods, the aerobic biological treatment method is divided into two types: activated sludge method and biofilm method. Activated sludge process itself is a treatment unit, it has a variety of operating modes. The treatment equipment of biofilm method includes biofilter, biological turntable, biological contact oxidation tank and biological fluidized bed, etc. The biological oxidation pond method is also called the natural biological treatment method. Anaerobic biological treatment, also known as biological reduction treatment, is mainly used to treat high-concentration organic wastewater and sludge.
2. Physical treatment
The methods of separating and recovering insoluble suspended pollutants (including oil film and oil droplets) in wastewater by physical action can be divided into gravity separation method, centrifugal separation method and sieve retention method. The treatment units that belong to the gravity separation method include sedimentation, floating (air flotation), etc., and the corresponding treatment equipment is grit chamber, sedimentation tank, grease trap, air flotation tank and its auxiliary devices, etc.; centrifugal separation itself is a kind of treatment unit, the processing devices used include centrifuge and hydrocyclone, etc.; the screen retention method has two processing units: grid screen retention and filtration. The former uses grids and screens, while the latter uses sand Filters and microporous filters, etc. The treatment method based on the principle of heat exchange is also a physical treatment method, and its treatment units include evaporation and crystallization.
3. Chemical treatment
A wastewater treatment method that separates and removes dissolved and colloidal pollutants in wastewater or converts them into harmless substances through chemical reactions and mass transfer. In the chemical treatment method, the processing units based on the chemical reaction of dosing are: coagulation, neutralization, redox, etc.; while the processing units based on mass transfer are: extraction, stripping, stripping , adsorption, ion exchange, electrodialysis and reverse osmosis, etc. The latter two processing units are collectively referred to as membrane separation technology. Among them, the treatment unit using mass transfer has both chemical action and related physical action, so it can also be separated from chemical treatment method and become another type of treatment method, called physical chemical method.
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Common sewage treatment process
1. Degreasing wastewater
The pollution indicators such as oil content, CODcr and BOD5 in the degreasing waste liquid are very high. Treatment methods include acid extraction, centrifugation or solvent extraction. The acid extraction method is widely used, adding H2SO4 to adjust the pH value to 3-4 for demulsification, steaming and stirring with salt, and standing at 45-60 t for 2-4 h, the oil gradually floats up to form a grease layer. The recovery of grease can reach 96%, and the removal of CODcr is more than 92%. Generally, the mass concentration of oil in the water inlet is 8-10g/L, and the mass concentration of oil in the water outlet is less than 0.1 g/L. The recovered oil is further processed and converted into mixed fatty acids which can be used to make soap.
2. Liming and hair removal wastewater
Liming and hair removal wastewater contains protein, lime, sodium sulfide, suspended solids, 28% of total CODcr, 92% of total S2-, and 75% of total SS. Treatment methods include acidification, chemical precipitation and oxidation.
The acidification method is often used in production. Under the condition of negative pressure, add H2SO4 to adjust the pH value to 4-4.5, generate H2S gas, absorb it with NaOH solution, and generate sulfurized alkali for reuse. The soluble protein precipitated in the wastewater is filtered, washed, and dried. become a product. The sulfide removal rate can reach more than 90%, and the CODcr and SS are reduced by 85% and 95% respectively. Its cost is low, the production operation is simple, easy to control, and the production cycle is shortened.
3. Chrome tanning wastewater
The main pollutant of chrome tanning wastewater is heavy metal Cr3+, the mass concentration is about 3-4g/L, and the pH value is weakly acidic. Treatment methods include alkali precipitation and direct recycling. 90% of domestic tanneries use alkali precipitation method, adding lime, sodium hydroxide, magnesium oxide, etc. to waste chromium liquid, reacting and dehydrating to obtain chromium-containing sludge, which can be reused in the tanning process after being dissolved in sulfuric acid.
During the reaction, the pH value is 8.2-8.5, and the precipitation is best at 40°C. The alkali precipitant is magnesium oxide, the chromium recovery rate is 99%, and the mass concentration of chromium in the effluent is less than 1 mg/L. However, this method is suitable for large-scale tanneries, and impurities such as soluble oil and protein in the recycled chrome mud will affect the tanning effect.
4. Comprehensive waste water
4.1. Pretreatment system: It mainly includes treatment facilities such as grille, regulating tank, sedimentation tank and air flotation tank. The concentration of organic matter and suspended solids in tannery wastewater is high. The pretreatment system is used to adjust water volume and water quality; remove SS and suspended solids; reduce part of the pollution load and create good conditions for subsequent biological treatment.
4.2. Biological treatment system: ρ(CODcr) of tannery wastewater is generally 3000-4000 mg/L, ρ(BOD5) is 1000-2000mg/L, which belongs to high-concentration organic wastewater, m(BOD5)/m(CODcr) value It is 0.3-0.6, which is suitable for biological treatment. At present, oxidation ditch, SBR and biological contact oxidation are more widely used in China, while jet aeration, batch biofilm reactor (SBBR), fluidized bed and upflow anaerobic sludge bed ( UASB).
Post time: Jan-17-2023
