Industrial wastewater treatment engineering

Wastewater Treatment

Wastewater treatment (wastewater treatment methods) is the use of physical, chemical, and biological methods to treat wastewater, purify it, reduce pollution, and even achieve wastewater recycling and reuse, making full use of water resources.

Separation and recovery of insoluble suspended pollutants (including oil films and oil droplets) in wastewater through physical actionwastewater treatment methods, which can be divided into gravity separation, centrifugal separation, and screening and interception methods. Treatment methods based on the principle of heat exchange also belong to physical treatment methods.

Throughchemical reactionsand mass transfer to separate and remove pollutants in dissolved orcolloidalstate in wastewater, or to convert them into harmless substances. In chemical treatment methods, the treatment units based on chemical reactions by adding reagents are: coagulation, neutralization, oxidation-reduction, etc.; while the treatment units based on mass transfer include: extraction, steam stripping, stripping, adsorption, ion exchange, and electrodialysis and reverse osmosis, etc. The latter two treatment units are also collectively calledmembrane separation technology. Among them, the treatment units that use mass transfer have both chemical and related physical effects, so they can also be separated from chemical treatment methods and become another type of treatment method, called physicochemical methods.

   

 

 

    Through themetabolic actionof microorganisms, theorganic pollutants, converting them into stable, harmless substances. According to the type of microorganisms used, biological treatment can be divided into aerobic biological treatment andanaerobic biological treatmenttwo types.Wastewater biological treatmentis widely used, and traditionally, aerobic biological treatment is divided intoactivated sludge processandbiofilm processtwo categories. The activated sludge process itself is a treatment unit with various operating modes. Treatment equipment belonging to the biofilm process includes trickling filters, rotating biological contactors,Biological contact oxidation tankand biological fluidized bed, etc.Biological oxidation pond methodalso known as natural biological treatment method.Anaerobic biological treatment methodalso known as biological reduction treatment method, mainly used for treating high-concentration organic wastewater and sludge. The main treatment equipment used isDigestion tank.

 

 

biological contact oxidationwastewater treatment, which usesbiological contact oxidationprocess to fill the biological reaction tank with filler. Thewastewateris submerged in the entire filler and flows through the filler at a certain flow rate. A biofilm is formed on the filler, and the wastewater comes into extensive contact with the biofilm. Under the metabolic action of microorganisms on the biofilm, the organic pollutants in the wastewater are removed, and the wastewater is purified. Finally, the treated wastewater is discharged into the biological contact oxidation treatment system and mixed withdomestic sewagefor treatment, and then discharged after chlorination disinfection to meet standards. Biological contact oxidation is a biofilm process between the activated sludge method and the biological filter. Its characteristic is that it is equipped withfiller, the tank bottomaerationto oxygenate the wastewater and keep the wastewater in the tank in a flowing state, to ensure sufficient contact between the wastewater and the filler submerged in it, and to avoid the defect of uneven contact between wastewater and filler in the biological contact oxidation tank. Thisaeration deviceis called blast aeration.

According to the degree of treatment, wastewater treatment (mainly urban domestic sewage and someindustrial wastewater) can generally be divided into three levels.

The task of primary treatment is to remove suspended solid pollutants from wastewater. For this purpose, physical treatment methods are often used. Generally, after primary treatment, the removal rate of suspended solids is 70% to 80%, while the biochemical oxygen demand (BOD) removal rate is only about 25% to 40%. The degree of wastewater purification is not high.

 

    The task of secondary treatment is to significantly remove organic pollutants in wastewater. Taking BOD as an example, generally after secondary treatment, the BOD in wastewater can be removed by 80% to 90%. For example, after urban sewage treatment, the BOD content in the water can be less than 30 mg/L. Most treatment units of aerobic biological treatment methods can meet this requirement.

 

 

The task of tertiary treatment is to further remove pollutants that were not removed by secondary treatment, including organic matter, phosphorus, nitrogen, and soluble inorganic matter that microorganisms cannot degrade. Tertiary treatment is synonymous with advanced treatment, but the two are not entirely identical.Tertiary treatmentis one or more additional treatment units added after secondary treatment to remove specific pollutants from wastewater, such as phosphorus and nitrogen. Advanced treatment, on the other hand, often aims at wastewater recovery and reuse, with treatment units or systems added after secondary treatment. Tertiary treatment is costly and complex to manage, but it can make full use of water resources. A few countries have built some tertiary wastewater treatment plants.

 

Pre-treatment units. Pre-treatment processes for municipal wastewater can include: coarse screening (bar screens), fine screening, comminution, flow measurement, pumping, grit removal, pre-aeration, flotation, flocculation, and chemical treatment. Domestic sewage treatment generally does not use flotation, flocculation, or chemical treatment. The adoption of these methods sometimes depends on the industrial wastewater present in municipal wastewater. Flotation is used to remove fine suspended solids, oils, and greases, either in a separate unit or in a pre-aeration tank with grease removal and sometimes grit removal. If the petroleum industry and meat processing plants have adequate pre-treatment, municipal treatment plants may not require a flotation unit. High-strength municipal wastewater can benefit from flocculation, with or without chemical addition, to improve primary treatment efficiency and prevent overloading of secondary treatment processes. Chlorination of raw wastewater is sometimes employed to control odors and improve settling characteristics. Variations in the arrangement of pre-treatment units depend on the characteristics of the raw wastewater, subsequent treatment processes, and the pre-treatment units employed. Some general principles are frequently applicable to unit arrangements. Bar screens are used to protect pumps and prevent solids from accumulating in sludge hoppers or grit chambers. Small treatment plants typically place a grit chamber before constant-speed lift pumps. In large treatment plants or where variable-speed pumps are used, the grit chamber may be located after the pumps. In most independent domestic sewage plants, sludge hoppers are located after the lift pumps, but if a large sludge load is anticipated, the sludge hopper should be placed before the pumps. Primary treatment units Primary treatment is sedimentation. However, what is commonly referred to as primary treatment includes pre-treatment processes. All large municipal treatment plants employ raw sewage sedimentation, which must be located before conventional biological filters. Raw wastewater without sedimentation can be treated using the complete mix activated sludge process; however, due to sludge disposal and operating costs, such processes are only used in small towns.

 

 

  Secondary treatment units. Biological secondary treatment uses the activated sludge process, trickling filters, or stabilization ponds. In new wastewater treatment plant designs, high-rate trickling filters have widely replaced low-rate trickling filters, and completely mixed activated sludge is replacing conventional activated sludge. Stabilization ponds are generally limited to small towns.

 

 

In large treatment plants, high-rate biological filtration and completely mixed activated sludge are the two most common methods currently used for secondary treatment. The advantage of biological filters is their ease of operation and their ability to accept shock loads and overloads without causing complete failure. Completely mixed activated sludge can withstand shock loads but will fail under prolonged overload. For example, when the BOD load of a biological filter increases from the design load of 45 lb/1000 ft³/day (R=1) to 90 lb/1000 ft³/day (R=2), the efficiency drops from 77% to 70% (see Figure 11-32). An activated sludge unit subjected to the same degree of overload will fail due to sludge bulking and loss of activated sludge solids in the effluent. The BOD removal efficiency drops from 90% to less than 50%. The advantage of the completely mixed activated sludge process is its high BOD removal efficiency, its ability to treat high-strength wastewater, and its adaptability for future conversion to advanced treatment. For secondary treatment of strong wastewater with 300 mg/L settleable BOD, activated sludge secondary treatment can remove at least 90% of the BOD, resulting in an effluent BOD of 30 mg/L or less. A single-stage high-rate biological filter can only remove 77% or less of the BOD, resulting in an effluent BOD of approximately 70 mg/L. To achieve BOD removal efficiency comparable to the activated sludge process, two stages of filtration are required.

Sludge disposal. Primary sedimentation and secondary biological flocculation concentrate the organic matter in wastewater into sludge, the volume of which is much smaller than the volume of wastewater treated. However, the disposal of accumulated waste sludge is a major economic factor in wastewater treatment. The initial investment in sludge processing equipment is about one-third of the investment in the treatment plant. The process of pumping, storing, and concentrating waste sludge from sedimentation tanks. Settled solids clarified from the effluent of biological filters, or excess activated sludge, are often returned to the head of the treatment plant for removal with primary sludge. Raw sludge can be stored at the bottom of primary sedimentation tanks awaiting treatment or pumped to a holding tank for storage. Pumped sludge can be concentrated in a thickening tank, which is usually a gravity unit placed before sludge processing. Excess activated sludge is mixed with the withdrawn primary sludge. In system design, a holding tank is usually used in conjunction with a sludge thickening tank. Excess activated sludge can be thickened separately before processing, or mixed sludge thickening can be used.

Various methods for processing and disposing of raw sludge. Common sludge processing methods include anaerobic digestion and vacuum filtration, often using centrifugation and wet combustion. Conventional disposal methods include landfilling, incineration, manufacturing soil conditioners, and ocean dumping. In coastal cities, ocean dumping is often the most economical, while landfilling is the customary method when land is available. Although incineration is more expensive, it is often the only feasible disposal method within urban areas.

 

 

A municipal treatment plant must carefully consider all possible sludge disposal processes. The best chosen method should be the most economical and appropriate for environmental conditions. Factors such as the transportation of processed sludge through residential areas, future use of landfill sites, groundwater pollution, air pollution, other potential public health hazards, and scenic issues must be considered.

Phosphorus and nitrogen removal. In recent years, much research has been conducted on feasible phosphorus removal methods in wastewater treatment plants. Research has also been done on developing methods for nitrogen removal and complete water recovery. Several pilot plants and small-scale production treatment plants for phosphorus removal are in operation, but experience data as a precedent for designing large-scale equipment is still limited. Although water quality standards specify limits for phosphorus and nitrogen, large-scale application of nutrient removal methods is still in the future.

Town size. For small towns, the primary considerations in selecting wastewater treatment processes are operation management, control, and sludge disposal. Methods that do not require sludge disposal (stabilization ponds) or only occasional sludge withdrawal (extended aeration) are superior for small villages and subdivisions. Larger municipalities often adopt systems that require more control and maintenance, such as contact stabilization and oxidation ditch treatment plants. Types of conventional wastewater treatment plants built by towns of different sizes. Many existing treatment systems are no longer in common use, such as septic tanks and other types chosen based on unique local conditions.

 

 

The series of treatment units in a biological filter treatment plant includes: a sludge hopper with an independent grit removal trough, a primary sedimentation tank, a biological filter, a final sedimentation tank with gravity return to the original wastewater wet well circulation pipeline, a single-stage digestion tank for treated sludge, and drying beds. The following wastewaters are returned to the original wastewater wet well: grit removal trough effluent, final sedimentation tank return sludge, drying bed leachate, and digestion tank supernatant. The treatment plant can be equipped with a bypass pipe at the influent manhole or after the biological filter.

The activated sludge plant treatment unit series includes: mechanically cleaned bar screens and a grinder to return shredded solids to the original wastewater, constant and variable speed lift pumps with standby gas engine units, a Parshall flume, a clarification-type aeration sedimentation tank with an independent grit remover, a primary sedimentation tank, a completely mixed activated sludge secondary treatment tank with a gravity return pipeline for excess sludge to the wet well, a vacuum filter for raw sludge from the sludge holding tank, and a landfill for sludge cake disposal. The filtrate from the vacuum filter is returned to the wet well. Due to the buried depth of the wastewater pipes, the original wastewater cannot bypass by gravity before the wet well. Two lift pumps are equipped with standby gas engine units that can operate during power outages. Bypass pipelines are provided after the lift pump station and after the primary sedimentation tank.

Adopt reasonablewater treatment processes, combined with deep water treatment, the treated water can reach the water reuse standards of GB5084-1992, CECS61-94, etc., and can be recycled for a long time, saving a large amount of water resources.

Commonly used agents

(1) Flocculants: Sometimes also called coagulants, they can be used as a means to enhance solid-liquid separation and are used in process stages such as primary sedimentation tanks, secondary sedimentation tanks, flotation tanks, and tertiary or advanced treatment.

(2) Coagulant aids: Assist flocculants in their function and enhance coagulation effects.

(3) Conditioners: Also known as dewatering agents, they are used for conditioning excess sludge before dewatering. Their types include some of the above-mentioned flocculants and coagulant aids.

(4) Demulsifiers: Sometimes also called destabilizers, they are mainly used for pre-treatment of oily wastewater containing emulsified oil before air flotation. Their types include some of the above-mentioned flocculants and coagulant aids.

(5) Antifoaming agent: mainly used to eliminate the large amount of foam generated during aeration or stirring.

(6) pH adjusting agent: used to adjust the pH of acidic and alkaline wastewater to neutral.

(7) Oxidizing and reducing agent: used for the treatment of industrial wastewater containing oxidizing or reducing substances.

(8) Disinfectant: used for disinfection treatment after wastewater treatment before discharge or reuse.

[Technical Overview]

Micro-electrolysis technology is an ideal process for treating high-concentration organic wastewater. This process is used for the treatment of high-salt, difficult-to-degrade, and high-color wastewater. It can not only significantly reduce COD and color, but also greatly improve the biodegradability of wastewater. This technology treats wastewater by utilizing the "primary battery" effect generated by the micro-electrolysis packing filled in the micro-electrolysis equipment without applying electricity. When water flows through, countless "potential differences of 1.2V" are formed within the equipment.Primary battery". The "primary battery" uses wastewater aselectrolyte, through dischargeto form current for electrolytic oxidation and reduction treatment of wastewater, so as to achieve the purpose of degrading organic pollutants. The newly generated [?OH], [H], [O], Fe2+, Fe3+, etc. produced during the treatment process can undergo oxidation-reduction reactions with many components in the wastewater, such as destroying the chromogenic groups of colored substances in colored wastewater orAuxochrome, even chain scission, to achieve degradation and decolorization; the generated Fe2+ is further oxidized to Fe3+, and their hydrates have strong adsorption-flocculation activity, especially after adding alkali to adjust the pH value to generateferrous hydroxideandferric hydroxide colloidflocculants, whose flocculation ability is far superior to general agentshydrolysisof ferric hydroxide colloid obtained can flocculate dispersed fine particles, metal particles, and macromolecular organic matter in the water body. Its working principle is based on the combined action ofelectrochemistry, oxidation-reduction, physical, andflocculation and sedimentation. This process has the advantages of a wide range of applications, good treatment effects, low cost, short treatment time, convenient operation and maintenance, and low power consumption. It can be widely used in the pre-treatment and deep treatment of industrial wastewater.

 

 

[Technical Features]

(1) Fast reaction rate, generally only half an hour to several hours for industrial wastewater;

(2) Wide range of organic pollutants treated, such as difficult-to-degrade organic substances containing azo, double bonds, nitro, and halogen groups, which have good degradation effects;

⑶ Simple process flow, long service life, low investment cost, convenient operation and maintenance, low running cost, stable treatment effect. Only a small amount of micro-electrolysis filler is consumed during the treatment process. The filler only needs to be added periodically and does not need to be replaced; it can be directly added when needed.

⑷ After micro-electrolysis treatment, the wastewater will form native ferrous or ferric ions in the water, which have a better coagulation effect than ordinary coagulants. No additional iron salts or other coagulants are needed. The COD removal rate is high, and it does not cause secondary pollution to the water.

⑸ It has good coagulation effect, high removal rate of color and COD. At the same time, it can greatly improve the biodegradability of wastewater.

⑹ This method can achieve the effect of chemical precipitation to remove phosphorus and can also remove heavy metals through reduction.

⑺ For high-concentration organic wastewater treatment projects that have been built but do not meet standards, this technology can be used as a pre-treatment for the wastewater from the existing project to ensure that the wastewater is discharged stably after treatment. It can also be used to separately divert and treat the wastewater with higher concentration from the production wastewater.Micro-electrolysisTreatment.

⑻ Each unit of this technology can be used as an independent treatment method, or as a pre-treatment process for biological treatment, which is beneficial for sludge settling and biofilm formation.

[Applicable Wastewater Types]

⑴. Dye, chemical, pharmaceutical wastewater; coking, petroleum wastewater; ------The BOD/COD values of the above wastewater are significantly increased after treatment.

⑵. Printing and dyeing wastewater; leather wastewater; papermaking wastewater, wood processing wastewater;

------Good application in decolorization, and effective removal of COD and ammonia nitrogen.

⑶. Electroplating wastewater; printing wastewater; mining wastewater; other wastewater containing heavy metals;

------Heavy metals can be removed from the above wastewater.

⑷. Organophosphorus agricultural wastewater; organochlorine agricultural wastewater;

------Greatly improves the biodegradability of the above wastewater, and can remove phosphorus and sulfides.

New filler

[Technical Overview]

It is produced by a multi-metal alloy composite catalyst and high-temperature microporous activation technology, belonging to a new type of direct-addition non-caking micro-electrolysis filler. Applied to wastewater, it can efficiently remove COD, reduce color, and improve biodegradability. The treatment effect is stable and lasting, and it can avoid phenomena such as filler passivation and caking during operation. This filler ismicro-electrolysisreaction to continuously act as an important guarantee, bringing new vitality to the current treatment of chemical wastewater.

[Iron-Carbon Primary Battery Reaction]

Anode: Fe - 2e →Fe2+ E（Fe / Fe2+）=0.44V

Cathode: 2H﹢ + 2e →H2 E(H﹢/ H2）=0.00V

When oxygen is present, the cathode reaction is as follows:

O2 + 4H﹢ + 4e → 2H2O E (O2）=1.23V

O2 + 2H2O + 4e → 4OH﹣ E（O2/OH﹣）=0.41V

    The main source of zinc in electroplating and metal processing wastewater is drag-out from electroplating or pickling. Pollutants are transferred to the rinse water through the metal rinsing process. The pickling process involves immersing metal (zinc or copper) in strong acid to remove surface oxides, followed by immersion in a brightener containing chromic acid for brightening. This wastewater contains a large amount of hydrochloric acid, heavy metal ions such as zinc and copper, and organic brighteners. It is highly toxic, and some also contain highly toxic substances that are carcinogenic, teratogenic, and mutagenic, posing a great hazard to humans. Therefore, electroplating wastewater must be carefully recycled and treated to eliminate or reduce its pollution to the environment. Electroplating mixed wastewater treatment equipment consists of a regulating tank, dosing tank, reduction tank, neutralization reaction tank, pH adjustment tank, flocculation tank, inclined tube sedimentation tank, chamber filter press, clean water tank, air flotation reaction, activated carbon filter, etc. Electroplating wastewater treatment adopts the iron filings internal electrolysis treatment process. This technology mainly uses activated industrial iron filings to purify wastewater. When wastewater contacts the filler, electrochemical reactions, chemical reactions, and physical effects occur, including catalysis, oxidation, reduction, displacement, co-precipitation, flocculation, adsorption, and other comprehensive effects, to remove various metal ions in the wastewater and purify the wastewater.

 

 

 

    Wastewater containing heavy metals mainly comes from enterprises such as mining, smelting, electrolysis, electroplating, pesticides, pharmaceuticals, paints, and pigments. If heavy metal wastewater is not treated, it will seriously pollute the environment. In wastewater treatment,heavy metalsvary in type, content, and form depending on the production enterprise. In addition to heavy metals, it is very important in wastewater treatment. Since heavy metals cannot be decomposed or destroyed, they can only be transferred to their location and their physical and chemical forms changed to achieve the goal of removing heavy metals. For example, during wastewater treatment, afterChemical precipitationAfter treatment, heavy metals in wastewater are converted from dissolved ionic forms into insoluble compounds and precipitate out, transferring from water to sludge. After ion exchange treatment, heavy metal ions in wastewater are transferred toion exchange resinand then transferred from the ion exchange resin to the regeneration waste liquid after regeneration. Therefore, the principle of heavy metal removal from wastewater is:

Principle 1 of heavy metal removal: The most fundamental is to reform production processes and avoid or minimize the use of highly toxic heavy metals.

Principle 2 of heavy metal removal: Adopt reasonable process flow, scientific management and operation to reduce the amount of heavy metals used and lost with wastewater, and minimize the amount of wastewater discharged. Heavy metal wastewater should be treated at the point of generation, not mixed with other wastewater to avoid complicating treatment. It should not be discharged directly into the city without heavy metal removal treatmentsewerto avoid expanding heavy metal pollution.

Methods for removing heavy metals from wastewater can generally be divided into two categories:

Method 1 for heavy metal removal: Convert dissolved heavy metals in wastewater into insoluble formsmetal compoundsor elements, removed from wastewater by precipitation and flotation. Applicable methods include neutralization precipitation, sulfide precipitation, flotation separation, electrolytic precipitation (or flotation), membrane electrolysis, and other wastewater treatment methods.

Method two for heavy metal removal: Concentrates and separates heavy metals in wastewater without changing their chemical form. Applicable methods includereverse osmosis, electrodialysis, evaporation, and ion exchange. These wastewater treatment methods should be used individually or in combination based on wastewater quality, quantity, and other factors.