Urban sewage, industrial sewage, including wastewater generated from various industries, have different compositions. Even in the same industry, due to differences in processes, the composition of the wastewater can vary, especially with fluctuations in the content of each component. The diversity of wastewater leads to complex changes in the corrosion environment and corrosion patterns of sewage treatment equipment.

Urban sewage is complex, causing severe corrosion to steel equipment such as grids, overflow plates, sludge scraping vehicles, submersible pumps, and railings above the secondary sedimentation tanks. Traditionally, epoxy coal tar coatings have been used for equipment protection, but corrosion often occurs severely within one year of use, even resulting in perforations.

The corrosion media in industrial wastewater treatment systems are especially complex, such as acids, alkalis, salts, and organic substances, which may appear in a mixed or alternating manner. The total amount and concentration of corrosion media are also unstable.

1. Corrosion Factors Facing Concrete

1.1 Concrete Corrosion Protection

The special difficulties of concrete corrosion protection.

The concrete surfaces of pools and tanks in wastewater treatment systems are significantly different from steel surfaces in terms of corrosion protection. Concrete structures have specific requirements for corrosion protection due to their construction characteristics. The surface of the concrete base is loose, rough, porous, and contains microcracks, which pose a major challenge to the adhesion and impermeability of protective coatings.

The moisture content in the concrete base creates certain difficulties for coating protection. The moisture content of the base comes from two sources: first, insufficient hydration of the cement and inadequate curing; second, capillary absorption caused by the high porosity and rough surface. Even if the concrete is fully cured, the absorption of moisture in humid climates may cause an excessive moisture content. Concrete structures are often in a damp state during use, and most of the pools and tanks are in semi-submerged, semi-enclosed environments, creating challenges not only during construction but also during maintenance. Concrete structures, unlike other equipment, cannot be easily repaired or replaced. Once reinforcement corrosion causes concrete cracking, it is difficult to take remedial measures. Many large pools and tanks are constructed in a modular format, where damage in one area can spread to the entire structure, causing serious risks.

1.2 Chemical Corrosion

Carbon dioxide in the atmosphere reacts with calcium hydroxide in cement to form an insoluble carbonate, which blocks the pores of the concrete. Carbon dioxide can also further react with calcium carbonate to form soluble calcium bicarbonate, causing the destruction of concrete.

Concrete structures are inherently alkaline, with a pore liquid pH > 12.5. Alkaline media generally do not cause corrosion to concrete structures, but acids can cause a certain degree of corrosion. Acidic water reacts with Ca(OH)2 to form soluble calcium salts, leading to the deterioration of concrete structures. Sulfuric acid, hydrochloric acid, and nitric acid can decompose calcium silicate and calcium aluminate in the concrete structure, causing surface degradation. When the water is of a sodium carbonate or calcium carbonate type, the calcium carbonate in the water is in an unsaturated state, which can continue to dissolve the carbonate in the concrete, weakening its strength. The higher the temperature of the acidic water, the more severe the corrosion. Long-term exposure to acidic water can react with Ca(OH)2 in concrete to form gypsum, and with calcium aluminate to form ettringite, which leads to the expansion and deterioration of the concrete structure.

1.3 Physical Action

Large reinforced concrete reservoirs are typically constructed outdoors and exposed to seasonal temperature fluctuations, making them susceptible to physical damage such as expansion, cracking, and penetration.

The movement of media can cause surface erosion due to the effect of flow speed, exposing new surfaces and accelerating the corrosion of concrete structures.

The porosity of concrete structures allows corrosive media to enter and be stored in the pores. When dried, crystallization or expansion occurs, increasing the internal stress of the concrete. Repeated cycles of drying and wetting can lead to the destruction of concrete structures.

In wastewater treatment pools made of concrete, freezing expansion can occur. When the water in the pores freezes, its volume increases by 9%, generating significant internal stress, which causes cracks and destruction in the concrete structure. This problem is more prominent in northern regions.

1.4 Microbial Corrosion

The corrosion of concrete structures is most typically manifested by the dissolution and dispersion of hydrates, leading to the expansion and deterioration of the concrete. A microbial film can often be observed on the pool walls of sewage treatment plants. This film attaches to the concrete surface and alters the local pH value and salt concentration, contributing to the corrosion of the concrete.

2. Corrosion Environment of Sewage Pipelines

2.1 Soil Corrosion

The external corrosion of buried pipelines is primarily due to soil corrosion. Soil is a complex mixture of solid, liquid, and gaseous phases, filled with air, water, and various salts, which give it electrolytic properties. Therefore, soil corrosion is the result of the interaction between underground pipelines and the salts or their aqueous solutions in the soil. Corrosion in coastal soil, with its high chloride ion content and sulfate-reducing bacteria, is more severe than that in inland soils. The external corrosion protection of pipelines in such environments requires enhanced protection levels, and a heavy-duty anti-corrosion coating combined with sacrificial anodes is recommended.

A heavy-duty anti-corrosion coating system for buried sewage pipelines can replace oil cloth wrapping, eliminating the need for fiberglass wrapping. It does not have pinhole defects and possesses excellent insulation properties, resistance to microbial erosion, good chemical resistance, strong adhesion, and low water absorption. This coating can be applied directly to steel surfaces with slight rust and is easy to construct with low surface treatment costs and a short construction period.

2.2 Sewage Corrosion

The sewage inside the pipeline contains acids, alkalis, salts, a wide pH range, organic matter, and microorganisms, with high flow velocities. The internal walls of steel pipes not only suffer from the severe corrosion of highly corrosive sewage, but the abrasive effects of high-flow sewage on the pipe wall are also significant. The recommended coating for the interior of pipelines is DreamNeng heavy-duty anti-corrosion graphene epoxy glass flake coating. It is suitable for steel surfaces, offering excellent adhesion, a smooth surface, resistance to salt spray, abrasion, and moisture, as well as excellent resistance to acids, alkalis, salts, oils, and microorganisms. It also has excellent resistance to cathodic delamination and works well with cathodic protection. This coating is widely used for heavy-duty corrosion protection in various steel structures and has a service life of over 15 years in sewage and seawater pipelines.

2.3 Environmental Definition

The corrosion environment for different systems is well defined in international standards like ISO 12944:

From ISO 12944's descriptions of various corrosion environments, the corrosion environment for sewage treatment plant pipelines is defined as follows: Sewage treatment water pipes: should be classified as Im 1 or Im 2, meaning the environment is water corrosion. In fact, the corrosion in sewage treatment plants is stronger than the corrosion levels in ISO 12944. The external walls of sewage treatment water pipes: should be classified as Im 3, indicating soil corrosion.

I. Reference Standards

Current national standards, relevant regulations, and mandatory standard provisions;

"Code for Construction and Acceptance of Building Anti-corrosion Engineering" (GB50212-2002)

"Standard for Corrosion and Rust Removal Levels of Steel Surface Before Coating" (GB8923-1988)

"Safety Regulations for Coating Operations and Safety Management Rules" (GB6514-1995)

"Quality Requirements for Anti-corrosion Coatings" (GB6514-1991)

"Safety Regulations for Coating Operations, Paint Process Safety, and Ventilation Purification" (DJ/T6931-1999)

"Code for Construction and Acceptance of Industrial Equipment and Pipeline Anti-corrosion Engineering" (HGJ229-91)

"Safety Regulations for Coating Operations and Safety of Surface Preparation Process" (GB7692-87)

"Noise Limits at Construction Sites" (GB12523-90)

ISO9001 Quality Management System Documents

ISO14001 Environmental Management System Documents

GB/T28001 Occupational Health and Safety Management System Documents

I. Design Basis

Environmental Conditions: The corrosion level of atmospheric environments on steel structures can be determined according to Table 1.

I. Industrial Wastewater Tank Interior Coating Solution

In wastewater treatment tanks, due to the presence of large amounts of corrosive media in industrial wastewater, such as waste acids, oxidizing chemicals, etc., the corrosion effect on concrete-based treatment tanks is significant. Untreated concrete tanks typically experience surface damage and a noticeable drop in strength within 2-3 months. Therefore, it is essential to carry out anti-corrosion treatment on concrete-based treatment tanks. The engineers at Mengneng Coatings introduce a coating anti-corrosion solution.

Table A: Concrete Interior Wall Coating System

II. Anti-corrosion Solution for Underground Pipeline Exterior Walls

The exterior walls of underground wastewater pipelines face corrosion hazards such as soil corrosion, electrochemical corrosion, microbial corrosion, stress corrosion, and physical wear. Moisture, salts, acidic and alkaline substances, and microorganisms in the soil react with the pipeline's exterior wall, causing metal corrosion, especially in saline and acidic soils. The electrolyte solution in the soil can also form micro-cells, leading to electrochemical corrosion, particularly at areas where different metals are in contact or where metal defects exist. Moreover, soil microorganisms (e.g., sulfate-reducing bacteria) can form biofilms on the pipeline's exterior wall, altering the local environment and accelerating the corrosion process. During installation and usage, pipelines may undergo mechanical stresses, which, in corrosive environments, can speed up the expansion of corrosion cracks. Hard particulate matter in the soil, when acted upon by water flow or geological movement, can also wear down the pipeline's exterior wall, further accelerating corrosion. To prevent these corrosion hazards, the engineers at Mengneng Coatings recommend using graphene glass flake anti-corrosion coatings.

Table B: Coating System for Underground Pipeline Exterior Walls

III. Corrosion Protection for Valves, Pumps, and Motor Equipment

In industrial wastewater treatment plants, valves and pumps, as key control equipment, face multiple corrosion issues. First, chemical corrosion is the most common, as the corrosive substances in wastewater such as acids, alkalis, and salts directly attack the metal parts of valves, leading to corrosion and damage. Next, electrochemical corrosion is particularly severe in wastewater containing electrolytes, where the potential differences between different metals accelerate the corrosion process. Additionally, microbial corrosion is also a serious problem, as microorganisms in wastewater form biofilms on the surfaces of valves and industrial pumps, altering the local environment and exacerbating corrosion. Finally, physical wear is another significant factor in valve corrosion, as high-speed flowing wastewater and solid particles within it can cause wear on the equipment surface, further accelerating the corrosion process. To prolong the equipment's service life, corrosion-resistant materials must be selected, and regular maintenance and inspections are required.

Table C: Paint Matching Design for Valves, Pumps, and Motors

I. Wastewater Treatment Plant Solutions

With the rapid acceleration of urbanization, wastewater treatment has become an important part of urban management and environmental protection. As one of the key facilities in wastewater treatment, the corrosion protection of wastewater pools is crucial for the stable operation of the treatment plant and environmental protection. The engineers at DreamCoatings will introduce the application of graphene low-surface treatment paint in the corrosion protection of wastewater pools, aiming to provide a reference for related professionals.

Table A: Design of Coating System for Concrete Wastewater Pool Interior Walls

I. Corrosion Factors in Farms

The wastewater from farms mainly comes from the cleaning water of the breeding areas. Keeping the breeding areas clean is one of the main measures to effectively prevent diseases in livestock and poultry. It also provides a good working and living environment for workers as well as animals, improving local air quality and reducing complaints from nearby residents. However, the wastewater after cleaning the breeding areas contains a large amount of organic matter, nitrogen compounds, heavy metal ions, and veterinary drug residues, so it cannot be discharged directly. It needs to be treated by a professional wastewater treatment plant using multiple treatment processes.

In addition to reducing the frequency of cleaning, laying bedding, and installing specialized manure removal equipment, farms can set up wastewater storage pits, tanks, or containers to centrally collect wastewater, thus reducing the frequency of wastewater transport and discharge, which in turn reduces related costs. The wastewater pits, tanks, and containers need to be equipped with sealing devices to prevent rainwater from mixing in and reduce secondary pollution.

The common methods for wastewater treatment in farms include: preliminary treatment using grids, filters, and septic tanks for physical filtration, which can effectively filter more than 50% of the solid materials in the wastewater; chemical treatment methods using coagulants such as ferric chloride and ferrous sulfate to precipitate suspended solids and colloidal substances in the wastewater; and finally, microbial treatment methods using activated sludge containing bacteria and microorganisms to degrade and oxidize farm wastewater, achieving discharge standards.

The corrosive nature of farm wastewater should not be overlooked during storage, transportation, and treatment. When animal feces ferment or dissolve in water, organic acids, hydrogen sulfide, ammonia, and other corrosive substances are formed. Not only are they highly corrosive by themselves, but they can also form electrolytes that accelerate electrochemical corrosion. The chemicals added during wastewater treatment are often in excess, and even when not in excess, the chloride ions and sulfite ions produced during hydrolysis have very strong corrosive properties.

II. Corrosion Protection of Wastewater Pits Coatings

To reduce the future maintenance costs of equipment and enhance its safety and reliability, Mengneng Coating's corrosion protection engineers point out that the equipment related to wastewater in farms must undergo proper corrosion protection. Even the steel structures in the farm (including beams, columns, and interior surfaces of the roof) should be properly protected. Mengneng's graphene glass flake anticorrosion coating is a suitable special product for this purpose. It can prevent organic corrosion, as well as resist alternating acids and bases, chloride ion corrosion, and ammonia corrosion. Additionally, the product is easy to apply, requiring only roller coating to achieve a thickness of 200-300μm (0.2-0.3mm). This product is widely used in more complex wastewater treatment projects and can solve the common corrosion problems in farms and wastewater treatment equipment.

A. Coating Design for Farm Wastewater Pits

III. Corrosion Protection for Steel Structures in Farm Buildings

The steel structures in farm buildings face various corrosion factors. Firstly, high humidity: Farms have relatively high humidity, and moisture easily condenses on the steel surfaces, accelerating corrosion. Secondly, animal excreta: Ammonia and acidic substances in animal waste can corrode steel structures. Thirdly, microbial corrosion: Microorganisms (such as bacteria and fungi) on the farm can form biofilms on steel surfaces, altering the local environment and intensifying corrosion. In addition, poor ventilation: Enclosed or poorly ventilated environments can lead to the accumulation of harmful gases, further accelerating corrosion of steel structures. Lastly, temperature changes: Day-night temperature fluctuations and seasonal variations cause thermal expansion and contraction in steel structures, generating stress that accelerates corrosion. To effectively prevent corrosion, common methods include using corrosion-resistant materials, applying high-performance coatings, maintaining good ventilation conditions, and performing regular inspections and maintenance.

B. Coating Design for Steel Structures in Farm Buildings

I. Corrosion Protection for Electroplating Industry Wastewater Pools

In recent years, with the promotion of energy conservation and emission reduction, the control of heavy metal pollution, and the implementation of new environmental protection laws, the expansion of the electroplating industry has slowed. Nevertheless, China is still considered a major player in the electroplating sector. It is expected that by 2023, the market size of China's electroplating industry will reach 182.76 billion yuan, with the automotive sector accounting for 20.86%, the electrical and electronics sector for 8.1%, the aerospace and defense sector for 16.5%, the jewelry application sector for 4.1%, and other applications accounting for 50.45%. This wide range of electroplating applications creates a large market for corrosion protection in electroplating wastewater pools, and it also raises higher demands on corrosion protection engineers in terms of material selection, combination, construction technology, service life, and cost-effectiveness. Advanced manufacturing will inevitably drive the development of advanced electroplating technologies, and advanced electroplating will certainly accelerate the development of corrosion protection technologies for wastewater pools. The author believes that corrosion protection for electroplating wastewater pools will become a key focus in the field of corrosion protection.

Electroplating is a surface treatment process that aims to deposit a metal coating on a substrate, altering the surface properties or dimensions of the base material. Electroplating can enhance the metal's corrosion resistance (the plating metal is usually corrosion-resistant), increase hardness, prevent wear, improve conductivity, lubricity, heat resistance, and surface aesthetics. During the process, alkaline, acidic, and some salt-based chemicals are used, which are harmful to the steel or concrete structures of electroplating wastewater pools. Therefore, corrosion protection for electroplating wastewater pools (whether steel or concrete) is crucial to ensure the normal operation of enterprises and improve cost-efficiency and social value.

II. Corrosive Components in Electroplating Wastewater Pools

Electroplating wastewater mainly comes from pre-treatment, cleaning of plated parts, floor washing, and the washing of hangers and anodes. The wastewater typically includes chromic wastewater (pH 2-4), cyanide wastewater (pH 9-11), chemical nickel plating wastewater (pH 4-9), electroplating nickel wastewater (pH 2-4), complex wastewater (pH 8-10), general organic wastewater (pH 6-10), general cleaning wastewater (pH 3-6), mixed floor wastewater (pH 2-10), comprehensive wastewater, and electroplating waste liquids (pH 3-6), as well as some oil contaminants. After treatment, the pH of the electroplating wastewater is generally between 6 and 9. Therefore, the electroplating wastewater pools initially contain both acids and alkalis, and after treatment, they may be either acidic or alkaline. The ideal pH is neutral (pH=7), but in practice, it is generally acidic. Hence, the selected corrosion protection materials for electroplating wastewater pools are mostly acid-resistant materials, though alkaline-resistant materials are also used. The optimal choice is materials that are resistant to both acids and alkalis and can also resist salts and oils.

Table A: Coating Design for Concrete Wastewater Pools

Table B: Coating Design for Steel Substrate Wastewater Pools

I. Surface Treatment of Sewage Tank Bodies

1.1 Concrete Surface

After safety inspection, use an electric angle grinder to grind the concrete surface and remove the loose layer. Chisel away cracked or loose concrete. Use a high-pressure water jet to clean the concrete surface of the pier columns, and let it dry naturally after washing.

Quality requirements: After washing, the concrete surface should be clean, firm, and free from looseness. Local convex concrete surfaces should be smoothed before proceeding to the next process.

1.2 Exposed Steel Structure Treatment

For exposed rebar and profiles, perform surface rust removal and apply two coats of epoxy anti-rust primer. The surface of the steel to be painted must be clean, dry, and free from grease, maintaining roughness and cleanliness until the first coat of paint is applied. All dust must be thoroughly removed, and according to ISO8502-3, the dust level must be less than Level 2.

Within 4 hours after surface treatment, before any yellowing occurs, the steel surface must be painted. If visible rusting, dampness, or contamination occurs on the steel surface, it must be re-cleaned to the specified level.

1.3 Surface Treatment During Repair

For large local depressions, holes, and cracks in the concrete, epoxy cement mortar can be used for local repair, followed by cement putty application. Alternatively, an epoxy sealing primer can be applied first, followed by the epoxy cement mortar for filling and smoothing.

II. Surface Treatment of Sewage Pipes

2.1 Oil Removal

Before sandblasting, remove grease or wetting agents left from the inspection film according to the SSPC-1 "Solvent Cleaning" standard.

2.2 Structural Treatment

Before secondary rust removal, inspect the welding and appearance of the structure to identify and address any structural defects:

— Free edges and corners should be treated with a grinder to achieve a radius of R≥2mm.

— Remove all slag, spatter, and weld beads.

— Pitted and porosity areas should be welded and smoothed.

— Surface peeling of steel should be ground off.

— Manual welds should be polished to reduce sharp surfaces.

2.3 Sandblasting Abrasive

The abrasive used for sandblasting should be dry, free of oil, and clean without impurities that could affect the performance of the coating. The conductivity of the abrasive must not exceed 250 μs/cm. River sand should not be used for sandblasting.

The abrasive should meet the required cleanliness and surface roughness standards, and no abrasive powder should remain on the cleaned surface.

The size of the abrasive must be able to achieve the required roughness for the coating system. The peak-to-valley height difference on the sandblasted surface must not exceed 0.1mm. Surface roughness levels should be evaluated according to ISO8503.

When sandblasting, care should be taken to avoid contamination from oil and/or water on the steel surface after sandblasting. Air compressors must be equipped with oil-water separators.

2.4 Sandblasting

The steel surface should be sandblasted to ISO 8501-1 Sa 2.5 level.

The roughness should meet the "medium requirements" of ISO8503-2, Ry5 30-85 microns.

After sandblasting, the prepared steel surface should be clean, dry, and free of grease, maintaining roughness and cleanliness until the first coat of paint is applied. All dust must be thoroughly removed, and according to ISO8502-3, the dust level should be less than Level 2.

Within 4 hours after surface treatment, before any yellowing occurs, the steel surface must be painted. If visible rusting, dampness, or contamination occurs, the surface must be re-cleaned to the required level.

2.5 Surface Treatment During Repair

For the closure welds after structural section installation or coating damage caused by transportation and handling, sandblasting or power tools should be used.

The sandblasting method and requirements can refer to secondary rust removal. Note: When repairing, attention should be paid to protecting the surrounding intact coating to avoid damage.

For power tool treatment, the surface should meet the ISO 8501-1 St 3 standard.

For the edges of the existing coating during repair, use a power sander with a disc to polish the damaged edges, forming a smooth ramp to ensure a smooth transition between the repair paint and the original coating, ensuring the overall good appearance and uniformity of the coating film.