Wastewater treatment, due to different reasons for the formation of wastewater, the difficulty of wastewater treatment, etc., the required wastewater treatment technology is also different.
The quality of domestic sewage is usually relatively stable, and the general treatment methods include acidification, aerobic biological treatment, disinfection, etc. Industrial wastewater should be treated according to the specific water quality conditions for reasonable selection of the process flow. It is particularly important to note that for wastewater treatment using aerobic biological treatment, attention should be paid to the biodegradability of the wastewater, usually requiring COD/BOD50.3. If the requirements cannot be met, anaerobic biological hydrolysis acidification can be considered to improve the biodegradability of the wastewater, or physical or chemical methods of non-biological treatment can be considered.
2. Wastewater treatment level
This is the main basis for the selection of wastewater treatment process flow. The degree of wastewater treatment depends on the water quality characteristics of the wastewater, the destination of the treated water, and the self-purification capacity of the water body into which the wastewater flows. However, at present, the determination of the degree of wastewater treatment mainly follows the requirements of relevant national laws, regulations, and technical policies. Usually, environmental management departments control the discharge concentration of wastewater according to the "Comprehensive Discharge Standard for Wastewater" and related industry discharge standards. Some regions with higher economic development levels have also stipulated stricter local discharge standards. Therefore, no matter what kind of wastewater needs to be treated, or what kind of treatment process and treatment level is adopted, the treated water must meet the standards as the basis and premise. Prevent and control water pollution according to laws, regulations, and policies.
3. Construction and operation costs
When considering construction and operation costs, the premise should be that the treated water meets the water quality standards. Under this premise, the process flow with low engineering construction and operation costs should be valued. In addition, reducing the land area is also an important measure to reduce construction costs.
4. Difficulty of project construction
The difficulty of project construction is also one of the influencing factors in the selection of the process flow. For example, in places with high groundwater level and poor geological conditions, it is not suitable to use treatment structures with large depth and high construction difficulty.
5. Local natural and social conditions
Local natural conditions such as topography and climate also have a certain impact on the selection of wastewater treatment processes. For example, if the local climate is cold, a process that can operate normally in the cold season after taking appropriate technical measures and ensure that the water quality meets the standards should be adopted. Local social conditions such as raw materials, water resources, and power supply are also factors that should be considered when selecting the process.
In addition to water quality, wastewater volume is also an influencing factor. For wastewater with large changes in water volume and water quality, processes with strong shock load resistance should be considered first, or buffer equipment such as regulating tanks should be considered to minimize adverse effects.
7. Whether the treatment process will cause new contradictions
The wastewater treatment process should pay attention to whether it will cause secondary pollution problems. For example, pharmaceutical wastewater contains a large amount of organic matter (such as benzene, toluene, bromine, etc.), and organic waste gas will be emitted during the aeration process, affecting the surrounding atmospheric environment; the gasification wastewater from fertilizer plants is recycled after sedimentation and cooling treatment, and the cooling tower exhaust gas will contain cyanide, polluting the atmosphere; in the treatment of Rogor wastewater from pesticide plants, the alkaline method is used to degrade Rogor. If lime is used as an alkalizing agent, the sludge produced will cause secondary pollution; when treating wastewater from dyeing or dye factories, sludge disposal is a key issue to consider. In short, the selection of wastewater treatment processes should comprehensively consider various factors, and technical and economic comparisons of multiple schemes should be carried out before conclusions can be drawn.
Classification of common chemical methods for treating wastewater:
Adding electrolytes to colloidal turbid liquids to coagulate colloidal substances in the water and separate them from the water. Coagulants include aluminum sulfate, alum, polyaluminum chloride, ferrous sulfate, ferric chloride, etc. Mainly for oily wastewater, dyeing wastewater, gas station wastewater, wool washing wastewater, etc.
Acid-base neutralization, pH reaches neutral, using lime, limestone, dolomite, etc. to neutralize acidic wastewater, CO2 to neutralize alkaline wastewater, lime to neutralize sulfuric acid plant wastewater, dyeing wastewater, etc.
Adding oxidizing (or reducing) agents to oxidize (or reduce) substances in wastewater into harmless substances. Among them, oxidizing agents include air, bleaching powder, chlorine, ozone, etc., mainly for wastewater containing phenols, cyanides, sulfur chromium, mercury, dyeing, and hospital wastewater, etc.
Insert electrode plates into the wastewater. After energization, the charged ions in the wastewater become neutral atoms. Using power supplies and electrode plates, etc., for wastewater containing chromium and cyanide (electroplating), wool spinning wastewater.
Add a water-insoluble solvent to the wastewater to dissolve the solute in the wastewater into this solvent, and then separate the solvent using the difference in relative density between the solvent and water. Extraction agents include: butyl acetate, benzene, N-503, etc. Equipment includes pulsed sieve plate columns, centrifugal extractors, etc., for phenolic wastewater, etc.
6. Adsorption (including ion exchange)
Pass the wastewater through a solid adsorbent to adsorb the dissolved organic or inorganic substances in the wastewater onto the adsorbent, and the wastewater that passes through is treated. Among them, adsorbents include: activated carbon, coal slag, soil, etc. The adsorption tower is a regeneration device, dyeing, pigment wastewater, and can also adsorb phenols, mercury, chromium, cyanide, and remove color, odor, and taste, etc., used for deep treatment.
Modern wastewater treatment technology, according to the degree of treatment, can be divided into primary, secondary, and tertiary treatment:
Primary treatment mainly removes solid pollutants suspended in wastewater. Most physical treatment methods can only meet the requirements of primary treatment. After primary treatment, the BOD can generally remove about 30%, which does not meet the discharge standards. Primary treatment is a pretreatment for secondary treatment.
Secondary treatment primarily removes colloidal and dissolved organic pollutants (BOD, COD) from wastewater, achieving a removal rate exceeding 90% and ensuring that organic pollutants meet discharge standards. The treated wastewater is then either reused as reclaimed water or partially replenishes rivers and lakes.
Tertiary treatment further processes recalcitrant organic matter, nitrogen, phosphorus, and other soluble inorganic substances that can cause eutrophication. Main methods include biological nitrogen and phosphorus removal, coagulation sedimentation, sand filtration, activated carbon adsorption, ion exchange, and electrodialysis. Wastewater after tertiary treatment is mostly used to replenish rivers and lakes.
The entire process involves raw sewage passing through a coarse screen, then being lifted by a sewage pump, passing through a screen or sand filter, and then entering a sedimentation tank. After sand-water separation, the wastewater enters a primary sedimentation tank. This is the primary treatment (i.e., physical treatment). The effluent from the primary sedimentation tank enters biological treatment equipment, which includes activated sludge processes and biofilm processes. (Activated sludge processes use reactors such as aeration tanks and oxidation ditches, while biofilm processes include biofilters, rotating biological contactors, biological contact oxidation, and fluidized bed reactors). The effluent from the biological treatment equipment enters a secondary sedimentation tank. The effluent from the secondary sedimentation tank is disinfected and discharged or undergoes tertiary treatment. The end of primary treatment marks the beginning of secondary treatment. Tertiary treatment includes biological nitrogen and phosphorus removal, coagulation sedimentation, sand filtration, activated carbon adsorption, ion exchange, and electrodialysis. Some of the sludge from the secondary sedimentation tank is returned to the primary sedimentation tank or biological treatment equipment, while some enters a sludge thickening tank, then a sludge digestion tank. After dewatering and drying, the sludge is ultimately utilized.
