Treatment of wastewater using coagulation-flotation-secondary biological treatment-mechanical filtration

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Treatment of wastewater by coagulation-flotation-secondary biological treatment-mechanical filtration. Authors: Chen Huogui, Zeng Youqun, Shenzhen Best Environmental Protection Engineering Company. Abstract: In factories in the aluminum alloy surface treatment industry, there are generally processes such as oil removal, acid and alkali washing, oxidation, and rinsing with clean water; in some cases, there are also processes such as dyeing and sealing； Using degreasing powders, various acids and bases, as well as various industrial detergents, red and black dyes, etc ； Some aluminum parts require multiple cleaning processes; as a result, such factories generate large amounts of wastewater, and the water contains various pollutants such as pH, CODcr, petroleum compounds, phosphates, Zn2+, Cr6+, etc. Some of this wastewater also has a high color intensity. Therefore, although the pollution concentration of this type of production wastewater is not high, its large volume and the variety of pollutants present result in relatively severe pollution of water bodies. Therefore, larger surface treatment plants for aluminum alloy parts, while considering the treatment of production wastewater to meet regulatory standards, often seek to reuse this wastewater for economic reasons, typically for purposes such as toilet flushing and irrigation of green spaces. Practice has shown that treating such industrial wastewater using the coagulation-flotation-secondary biological treatment-mechanical filtration process can ensure that the effluent meets environmental protection standards as well as relevant requirements for reuse. The production wastewater generated during the surface treatment of aluminum components at an electronics company in Shenzhen is treated using the aforementioned process. The designed treatment capacity is 1,000 m3/day. The initial pH, CODCr, petroleum content, phosphate levels, and Zn2+ concentrations in the wastewater are respectively 3–6, 200–350 mg/L, 12–30 mg/L, 50–75 mg/L, and 20–30 mg/L. After treatment, the removal rates for CODCr, petroleum content, phosphates, and Zn2+ reach 90%, 93%, 99.5%, and 96% respectively, ensuring that the effluent meets the local first-class standards for sewage discharge. Keywords: coagulation flotation, secondary biological treatment, mechanical filtration, wastewater from aluminum parts surface treatment plants 1. Introduction Different aluminum alloy surface treatment plants, in addition to following the same basic processes of acidic degreasing/alkaline etching/phosphoric acid oxidation, also employ value-added processes such as dyeing and sealing; the surfaces are typically dyed black, red, etc. Cleaning is carried out once or multiple times between these processes, with some plants even using recycled cleaning solutions ； As a result, the discharged process wastewater varies greatly; the main pollutants include acids and bases, COD, phosphates, petroleum compounds, fluorides, LAS, color intensity, etc., and it may also contain the heavy metal chromium. Therefore, this type of wastewater is characterized by large volumes and a variety of pollutants, making it a type of industrial wastewater that is difficult to degrade. The treatment of such wastewater generally relies on physicochemical methods, with biological treatment may also be used in addition. This paper briefly describes the physicochemical and biochemical treatment of such wastewater, as well as the wastewater reuse process, demonstrating that appropriate biochemical treatment can also yield good results. A certain electronics company in Shenzhen specializes in the production of coils, drive frames, aluminum brackets, and hardware components. Its manufacturing process consists of two main steps: degreasing/acid-alkali cleaning, and rinsing with clean water. The production wastewater from this factory is primarily generated during the surface treatment of aluminum parts. Oil-removing powders, hydrofluoric acid, sulfuric acid, nitric acid, phosphoric acid, and industrial detergents are mainly used in the production process. The wastewater discharged from production processes contains a large amount of foam, high concentrations of phosphates, significant amounts of suspended and colloidal organic matter, as well as some components of floating oil. It is characterized by high phosphate concentrations and poor biodegradability. Considering the characteristics of such industrial wastewater, the treatment process is designed to first employ coagulation and air flotation to remove most of the organic matter, phosphates, and petroleum compounds through coagulation; followed by secondary biological treatment, using highly adapted specialized microorganisms to eliminate the remaining organic matter. Finally, secondary fiber ball filtration is used to remove fine suspended particles from the water, thereby meeting the requirements for recycled water. This process features a high treatment capacity, impact resistance, and stable effluent quality, achieving excellent results in practical applications. 2. Volume, quality of wastewater and discharge standards 2.1 Volume of wastewater: Designed wastewater volume: 1000 m3/day, with operation 24 hours a day. Drainage pattern: Stable discharge 24 hours a day, 3 shifts per day. 2.2 Wastewater quality Based on the results of multiple on-site sampling tests, the water quality is shown in Table 1. Table 1: Quality of raw production wastewater – Parameters include pH value, suspended solids, LAS, total zinc. Wastewater quality: 3–6, 80–100, 25–30, 20–30. Parameters for CODcr, phosphates, and petroleum products: Wastewater quality: 200–350, 50–75, 12–30. 2.3 Wastewater discharge standards: The wastewater must meet the first-level standards of the Guangdong Provincial Standard “Emission Limits for Water Pollutants” (DB44/26-2001) after treatment; the specific criteria are shown in Table 2. Table 2: Wastewater discharge standards
Parameter: pH value; Suspended solids; LAS; Total zinc
Wastewater quality requirements: 6.0–9.0; ≤60; ≤5.0; ≤2.0
Parameter: CODcr; Phosphates; Petroleum substances
Wastewater quality requirements: ≤90; ≤0.5; ≤5.0; 2.4
Requirements for reclaimed water quality: The quality of reclaimed water must meet the requirements for toilet flushing and landscape irrigation. Therefore, the water quality of recycled water shall comply with the standards specified in the \"Quality Standards for Miscellaneous Domestic Water\" CJ25.1-89 for use in flushing toilets and for urban greening projects; excerpts of the relevant water quality parameters are shown in Table 3. Table 3: Excerpts from the water quality standards for domestic miscellaneous water use. Parameters: pH value, suspended solids, LAS. Waste water quality standards: pH 6.5–9.0; suspended solids ≤10; LAS ≤1.0. Other parameters: CODcr, turbidity, color. Waste water quality standards: CODcr ≤50; turbidity ≤20 degrees; color ≤30 degrees. 3. Waste water treatment process and its characteristics 3.1 Waste water treatment process The waste water treatment process is shown in Figure 1. Figure 1 Process Flow 3.2 Process Characteristics This wastewater treatment and reuse project consists of a pretreatment system, a coagulation and air flotation treatment system, a biological treatment system, a mechanical filtration system, and a sludge treatment system. The process characteristics are as follows: (1) The pretreatment system includes a grid well and a regulating tank, which are used to remove debris, buffer water volume, and homogenize water quality; an air agitation system is also installed. (2) The coagulation flotation treatment system includes a coagulation reaction zone, a sludge-water separation zone, a clear water zone, etc., and its principle is the chemical coagulation flotation method. First, the pH value of the wastewater is adjusted, followed by the addition of coagulants and flocculants. Air flotation is then used to remove insoluble organic substances and phosphates from the wastewater in the form of scum, while also eliminating the majority of petroleum compounds. This step is highly targeted: since aluminum parts must be degreased with detergent before surface oxidation, the \"floc\" formed as a result of the coagulation reaction of petroleum-based pollutants is not always easy to settle; therefore, air flotation is a good method for removing it. (3) The anaerobic biological treatment system only undergoes hydrolysis and acidification processes. Semi-soft fillers are installed in the tank; the biofilm on these fillers breaks down some complex and difficult-to-degrade macromolecular organic substances into simpler, more easily degradable organic compounds. Suspended and colloidal organic substances are converted into soluble forms. This process also reduces the concentration of suspended solids in the wastewater, thereby increasing significantly the proportion of soluble organic substances and biodegradable organic substances in it, improving the biodegradability of the wastewater and creating favorable conditions for subsequent aerobic biological treatment. (4) The aerobic biochemical treatment system adopts a two-stage aerobic process. The first stage uses a biological contact oxidation tank, and the second stage also uses a biological contact oxidation tank. In aerobic environments, aerobic microorganisms oxidize organic substances in water in an environment with a continuous supply of oxygen; they have a high capacity to break down acids, alcohols, and sugars. The energy generated from this decomposition is used for their reproduction and growth. The organic substances that serve as energy are ultimately converted into CO2, NH3, and H2O, while another portion of these organic substances is used by the microorganisms to synthesize new cells. This method has advantages such as high volume load, high biological activity, low sludge production, good and stable effluent quality, low energy consumption, and no sludge bulking problem; as a result, it can steadily ensure that the effluent quality parameters remain within the range suitable for reuse. (5) Mechanical filtration: Since the suspended solids level in the recycled water is relatively low, being below 10 mg/L, a two-stage fiber filtration method is required to ensure that the suspended solids level remains below this threshold. 3.3 Main process design parameters: (1) The effective volume of the integrated regulating tank is 170 m3, with a retention time of 4.0 hours. According to the factory, the emission per hour varies little from day to day, indicating that the volume of wastewater discharged is relatively stable. Therefore, a comprehensive adjustment time of 4.0 hours is sufficient to meet the requirements for balancing water volume and quality, as well as to satisfy the operational needs of subsequent processes (mainly the backwashing time of filters). (2) Coagulation reaction zone: effective volume of 28 m3, reaction time of 40 min. The coagulation reaction time is 15 to 20 minutes, while the flocculation reaction time is 20 to 30 minutes. (3) Coagulation air flotation tank: It uses a conventional pressure dissolved air flotation system with fully automatic control. The upward flow velocity in the air flotation contact chamber is 10 mm/s, with a nominal residence time of 2 minutes ； The flow velocity in the air flotation separation chamber is 1.5 mm/s, the surface load is 3.23 m3/(m2·h), and the nominal residence time is 1 hour ； (4) PH callback tank: It acidifies the clear water discharged from the alkaline coagulation flotation process for subsequent biochemical treatment; the nominal residence time is 10 minutes, and the effective volume is 6.95 m3. (5) Hydrolysis-acidification tank: nominal retention time of 2.0–3.0 hours, effective volume of 125 m3; water distribution is achieved through pipes, and composite fillers are installed inside; sludge load Ns = 0.68 kg CODcr/(kg MLVSS·day). (6) Biological contact oxidation tank: This tank is designed in two stages, with a nominal retention time of 3.0–4.0 hours, an effective volume of 166.67 m3, and a packing load of 1.2 kg COD/m3·day. A micro-porous aeration system is used. (7) Coagulation sedimentation tank: A inclined-tube sedimentation tank is used, with a coagulation reaction zone installed in the front section. The reaction time in the coagulation zone is 15–20 minutes. The surface load in the sedimentation zone is 0.4~0.6 m3/m2·h. (8) High-speed fiber filter: The filter layer height is 1.3 m, and the filtration rate is 25–32 m/h. Wastewater treatment efficiency: The project commenced construction in June 2002, was completed and put into trial operation in October 2002, and passed the environmental protection inspection in March 2003. The entire facility has operated efficiently and stably since it was put into use. The treatment effects are shown in Table 3. The data in Table 3 show that after treatment, all parameters of the treated wastewater meet the required standards. Table 4: Wastewater treatment performance table Parameter CODCr (mg/L) SS (mg/L) Petroleum compounds (mg/L) Phosphates (mg/L) Regulator tank 338.5 087.6 027.9 068.1 Outlet of air flotation tank 231.8 741.6 3.37 0.71 Outlet of anaerobic tank 63.7 622.3 02.41 0.52 Outlet of aerobic tank 34.1 216.1 02.30 0.035 Outlet of filter 32.1 85.2 01.85 0.034 As can be seen from Table 4, after being treated using the “coagulation-air flotation + hydrolysis-acidification + contact oxidation” method, the overall removal rates for CODCr, suspended solids, petroleum compounds, and phosphates are 90.5%, 94.1%, 93.4%, and 99.9%, respectively. Reflections and Discussions (1) Since the petroleum-based pollutants in production wastewater originate from the protective oil film on the surface of aluminum parts, emulsification reactions occur easily, resulting in their coagulation into \"floc\". However, this type of \"alum flower\" contains a certain amount of oil, making it sticky and prone to clumping together to float on the surface. Based on this characteristic, the coagulation flotation method achieves good sludge-water separation results. It can be seen that the air flotation process not only effectively removes petroleum-based pollutants from this wastewater, but it also pre-oxygenates the wastewater; thus, it improves the biodegradability of the wastewater in two ways, laying a good foundation for subsequent biochemical treatment processes. (2) Since aluminum parts must be repeatedly cleaned with industrial detergents during the production process, the wastewater generated is not only in large quantities but also rich in industrial detergent components. When aerated and stirred, this wastewater produces a large amount of foam; therefore, air stirring is not suitable in the coagulation zone and the pH adjustment zone. An antifoaming agent should be used before the aerobic tank to alter the surface activity of the detergents, otherwise the aerobic tank will fail to operate properly due to the excessive foam produced by aeration. (3) Due to the industrial detergents and small amounts of dyes used in the production process, which are all high-molecular compounds that are difficult to biodegrade, when designing further biochemical treatment processes, it is necessary first to use hydrolysis and acidification steps to convert complex macromolecules and insoluble organic substances into smaller molecules and soluble organic substances. Only then can contact oxidation be used to oxidize and decompose these smaller molecules and soluble organic substances, thereby achieving better biochemical treatment results. (4) A mechanical stirrer is used in the hydrolysis-acidification tank to enhance the contact between the wastewater and sludge, eliminate gradients within the tank, prevent stratification, and improve efficiency. (5) For microbial growth, carbon, nitrogen, phosphorus, and other trace elements must be absorbed in certain proportions. However, such production wastewater contains insufficient nutrients; therefore, substances such as nitrogen and phosphorus are added to the wastewater on a regular basis to provide the growth elements required for microbial metabolism. (6) The aerobic treatment section employs the contact oxidation method. Contact oxidation is a biological treatment technique that is widely used at present and boasts high efficiency. Due to the large specific surface area of the filler in the tank, the oxygenation conditions within it are good, resulting in a higher biomass per unit volume in the oxidation tank compared to the activated sludge process and biological filters ； The aeration device in the tank is located beneath the packing, ensuring adequate oxygen supply and favorable mass transfer conditions. This results in high biological activity within the tank and a rapid renewal of the biofilm. The concentration load that can be handled is several times higher than that of other biological treatment methods, thereby allowing for reduced space requirements and lower energy consumption. The contact oxidation tank is divided into two compartments; each compartment can have a different sludge load, allowing the microorganisms in each compartment to adapt to the respective load conditions (size, properties). This facilitates the specialized cultivation and acclimatization of the microorganisms, thereby improving treatment efficiency. (7) A secondary high-efficiency fiber filter is installed after the water exits the coagulation and sedimentation tank, to ensure that the suspended solids level meets the reuse requirements of below 10 mg/L. It should be noted that efficient fiber filtration is different from fiber ball filtration, and the efficiency of fiber ball filtration is far inferior to that of fiber filtration. Special attention should be paid when selecting filter media, as it needs to be determined based on the water quality requirements. (8) During the production process, due to poor workshop management by the factory, production consumables such as rubber hoses, rubber strips, rubber particles, and rubber gloves end up being discharged into the wastewater treatment tank along with the wastewater, which causes the wastewater lift pumps to become clogged and damaged frequently. Furthermore, since the factory operates 24 hours a day with continuous production and drainage, the process design must take into account the need to have one backup pump for each pumping unit in order to ensure the smooth operation of production. Otherwise, it will lead to unimaginable consequences. Conclusion (1): The coagulation-flotation-secondary biochemical-mechanical filtration method for treating the wastewater generated by aluminum parts surface treatment plants and its reuse offers advantages such as high treatment efficiency, simple operation, resistance to shock loads, and easy daily maintenance. The water quality remains stable and meets the requirements for reuse, thereby achieving excellent environmental and economic benefits. (2) The overall removal rates of CODCr, suspended solids, petroleum compounds, and phosphates by this process are 90.5%, 94.1%, 93.4%, and 99.9%, respectively. About the author: Chen Huogui, male, born in September 1973. He graduated from Guangdong University of Technology in 1997, obtaining a Bachelor’s degree in Engineering. Since graduating, I have been mainly engaged in the design, construction, commissioning of wastewater treatment projects, as well as related research work.