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Sharing | Fundamentals of Water Treatment and Related Processes of Wastewater Treatment
Water treatment engineering primarily includes water supply treatment engineering and wastewater treatment engineering. Today, Jinzong Enterprise will share with you the fundamental knowledge of water treatment and the related processes of wastewater treatment.
Water supply treatment mainly refers to the process of removing various harmful impurities from raw water through clarification, filtration, disinfection, deodorization, taste removal, iron removal, softening, desalination, and other treatment steps, making it suitable for meeting the water quality standards of domestic and industrial use. Purified water production, seawater desalination, and drinking water preparation all fall under the scope of water supply treatment.
Wastewater treatment primarily involves purifying water quality through physical, chemical, physicochemical, and biological processes to meet the standards for discharge into a specific water body or reuse. Wastewater mainly includes domestic sewage, industrial wastewater, surface water, etc., covering a wide range of sources. In daily life, terms such as sewage, wastewater, and reclaimed water are often used interchangeably, all essentially falling under the concept of wastewater. In the following discussion, wastewater primarily refers to industrial wastewater, so the terms "wastewater" and "sewage" may be used interchangeably.
Wastewater treatment is categorized into primary, secondary, and tertiary treatment based on the degree of purification.
Primary Treatment: Mainly removes solid pollutants suspended in the wastewater. Most physical treatment methods can only meet the requirements of primary treatment. After primary treatment, approximately 30% of BOD is generally removed, which does not meet discharge standards. Primary treatment serves as pre-treatment for secondary treatment.
Secondary Treatment: Primarily removes colloidal and dissolved organic pollutants (BOD, COD substances) from wastewater, with a removal rate exceeding 90%. This process ensures that organic pollutants meet discharge standards and is considered the core process in water treatment, equivalent to the central stage in water supply treatment.
Tertiary Treatment: Further treats refractory organic matter, soluble inorganic substances such as nitrogen and phosphorus that can cause eutrophication, and other contaminants. Key methods include biological nitrogen and phosphorus removal, coagulation and sedimentation, sand filtration, activated carbon adsorption, ion exchange, and electrodialysis.
Given the broad meaning of wastewater, real-life wastewater treatment often involves a mixture of various types of wastewater. Even for the same type of wastewater, process selection may vary depending on the concentration of toxic and harmful substances. Therefore, in wastewater treatment, it is essential to apply treatment processes flexibly and skillfully choose to combine or treat different wastewater types separately.
There are many classification methods for wastewater treatment processes. We adopt the relatively common classification of physical, chemical, and biological methods. By selecting appropriate processes from these categories and combining them with specific membrane treatment technologies (such as electrodialysis, reverse osmosis, ultrafiltration, nanofiltration, and microfiltration), most wastewater treatment needs can be addressed.
Physical Methods: Employ physical or mechanical separation. Common physical methods include sedimentation, centrifugal separation, and flotation.
Chemical Methods: Involve adding chemical substances to react with toxic and harmful substances in the wastewater, achieving decomposition or separation. Common chemical methods include neutralization, oxidation, reduction, decomposition, coagulation, and chemical precipitation.
Physicochemical Methods: Utilize physicochemical separation principles to isolate specific substances in different states. Common physicochemical methods include stripping, air stripping, adsorption, extraction, ion exchange, electrolysis, electrodialysis, and reverse osmosis.
Biological Methods: Leverage microbial metabolism and degradation to convert organic matter in wastewater into inorganic substances. Common biological methods include aerobic treatment, anaerobic treatment, and enzymatic treatment.
Supplemental Explanation of Several Common Processes:
Coagulation: The process of aggregating colloidal particles and fine suspended solids in water through specific methods, such as dosing with chemical agents. Coagulation encompasses two stages: destabilization (coagulation in a narrow sense) and flocculation. Destabilization primarily refers to the process in which colloids lose their stability and form micro-aggregates, while flocculation mainly involves the aggregation of destabilized colloids or fine suspended solids into larger flocs. Chemicals capable of inducing such destabilization and flocculation are collectively referred to as coagulants.
Common coagulants include Polyaluminum Chloride (PAC), ferrous sulfate, ferric chloride, and Polyacrylamide (PAM), among others.
The effectiveness of coagulation is influenced by various conditions such as the composition of pollutants in the wastewater, pH level, water temperature, and flow dynamics. Different coagulants are selected based on specific circumstances. Generally, coagulation and sedimentation are more effective under alkaline or weakly alkaline pH conditions.
pH Neutralization: Also known as pH adjustment, this process involves dosing a certain amount of pH regulators into the wastewater to achieve a desired pH level. Common pH regulators include quicklime (calcium hydroxide), caustic soda flakes (sodium hydroxide), various strong acids, and citric acid, among others.
Oxidation-Reduction Method: This method transforms pollutants in wastewater into non-toxic and harmless substances through oxidation or reduction, thereby achieving treatment objectives. Common oxidation-reduction methods include chemical oxidation, Fenton oxidation, wet air oxidation, electrochemical methods, and photocatalytic oxidation. The most commonly used oxidants are air (oxygen), ozone, chlorine, and sodium hypochlorite, while the most common reductants include ferrous sulfate, iron filings, sodium bisulfite, and sodium borohydride.
1.Chemical Oxidation: A treatment technology that uses oxidants such as ozone, chlorine, potassium permanganate, chlorine dioxide, and hydrogen peroxide to oxidize pollutants in wastewater into carbon dioxide and water.
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Process Characteristics: Simple process, fast reaction rate, but high reagent cost. Chlorine-containing oxidants are primarily used for treating wastewater containing phenols, cyanides, and sulfides.
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Note: Ozone oxidation also has the effects of sterilization and increasing dissolved oxygen in water, thereby reducing COD and BOD in wastewater.
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2.Fenton Oxidation: A method that uses Fenton's reagent (a combination of ferrous ions and hydrogen peroxide) to treat wastewater. It is suitable for organic wastewater containing substances difficult to degrade by biological or general chemical oxidation methods, such as ethers, nitrophenols, chlorophenols, aromatic amines, and polycyclic aromatic hydrocarbons.
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Process Characteristics: Relatively complex process, but highly efficient, low-consumption, and produces no secondary pollution.
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Reaction Mechanism: H₂O₂, catalyzed by Fe²⁺, generates highly reactive hydroxyl radicals (·OH), which can degrade organic matter and reductive substances into inorganic substances like CO₂ and H₂O.
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Development Trends: Two main trends in Fenton technology are the photo-Fenton technology, which introduces UV light into the system, and the electro-Fenton technology, which combines the Fenton reaction with electrolysis. Electro-Fenton technology offers advantages such as in-situ generation of H₂O₂ and Fe²⁺, aeration to improve mixing in wastewater treatment, and auxiliary treatment through anodic oxidation and electro-adsorption.
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3.Wet Air Oxidation: A treatment process conducted under high temperature and pressure, using oxygen or oxygen from air to oxidize refractory organic matter in wastewater, breaking it down into smaller organic molecules and inorganic substances that are more amenable to biochemical treatment.
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Process Characteristics: Wide application range, highly efficient, rapid, low pollution, and allows for potential recovery and reuse of some materials. It is widely used in treating industrial wastewater such as cyanide-containing wastewater, coal gasification wastewater, sulfur-containing wastewater, phenol-containing wastewater, and papermaking black liquor.
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4.Electrochemical Method: A process that utilizes electrochemical reactions occurring at electrodes in wastewater to generate strong oxidants, gases, or flocculants, thereby removing pollutants. Based on the principle of action, it can be classified into electrolytic oxidation, electro-flotation, and electrocoagulation.
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Electrolytic Oxidation: Can be further divided into direct and indirect electrolysis. Direct electrolysis involves the direct oxidation or reduction of pollutants at the electrode surface for removal. Indirect electrolysis utilizes strong oxidants like hydroxyl radicals or hypochlorite ions generated on the electrode surface under the influence of the electric field to convert pollutants into more degradable or harmless substances, thereby purifying the wastewater.
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Electro-flotation: Utilizes gases (e.g., chlorine gas evolution if chloride ions are present in the wastewater) produced during electrolysis to cause volatile impurities and light suspended solids to float to the surface of the wastewater, achieving purification.
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Electrocoagulation: Involves the consumption of iron or aluminum anodes during the electrochemical wastewater treatment process, forming iron or aluminum salt flocculants in the wastewater to remove colloidal and suspended solids.
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5. Photocatalytic Oxidation: This method utilizes N-type semiconductors such as TiO₂ and ZnO as catalysts, combined with light to promote chemical reactions. It oxidizes organic pollutants in wastewater, converting them into non-toxic and harmless CO₂ and H₂O.
The mechanism involves the semiconductor, under light irradiation, causing water molecules to lose electrons and generate highly oxidizing ·OH (hydroxyl radicals). These radicals lead to the oxidation of various organic substances in the wastewater. The effectiveness of photocatalytic wastewater treatment depends on factors such as the catalyst's crystal form, crystallinity, specific surface area, light intensity, pH, and reactor design.
Aerobic Treatment: A method that utilizes aerobic microorganisms to degrade and metabolize pollutants in wastewater under oxygenated conditions. Common aerobic treatment methods include activated sludge processes, biofilm processes, biological contact oxidation, and biological fluidized bed reactors.
Anaerobic Treatment: A method that uses anaerobic microorganisms to degrade and metabolize pollutants in wastewater under oxygen-free conditions. Common anaerobic treatment methods include anaerobic ponds, anaerobic filters, anaerobic fluidized beds, anaerobic expanded beds, anaerobic rotating biological contactors, anaerobic tanks, and Upflow Anaerobic Sludge Blanket (UASB) reactors.
The operating conditions for anaerobic methods are generally more demanding than those for aerobic methods, but the overall cost is often lower. To achieve better treatment results, aerobic and anaerobic methods are frequently combined.
Bio-enzyme Treatment: This method involves organic matter forming free radicals through enzymatic reactions. These free radicals then undergo chemical polymerization to form high-molecular-weight compounds that precipitate out.
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