Abstract: In view of the fact that automobile painting wastewater contains pollutants such as resin, surfactant, heavy metal ions, oil and pigment. In particular, electrophoresis wastewater and painting wastewater have complex components, high concentration and poor biodegradability. In this paper, water quality treatment, coagulation and sedimentation, coagulation and air flotation, sand filtration and other technologies are used to treat painting wastewater, and good results have been achieved: the removal rate of CODCr is over 80%. The actual operation shows that the process is reasonable and feasible in technology and economy.
Treatment technology of typical coating wastewater from automobile manufacturing
Abstract: This paper aims at the pollution of coating wastewater from automobile manufacturing, including resins, surfactants, heavy metal ions, oils, paints, dyes, etc., especially ELPO wastewater and coating wastewater, which have complex components and high concentration. The coating wastewater was treated by separation pretreatment, coagulation sedimentation, air flotation and sand filtration, and good results were obtained: the removal rate of CODCr could reach above 80%. The operation of the device proves that it is technically and economically feasible under this condition.
Keywords: coating wastewater; Quality treatment; Coagulation and precipitation; Coagulation air flotation; Sand filtration; fenton reagent
Keywords: coating wastewater; Separation pretreatment; Coagulation and precipitation; Air flotation; Sand filtration; fenton reagent
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The painting of automobile and its parts is one of the links that produce the most wastewater in the automobile manufacturing process. Paint wastewater contains pollutants such as resin, surfactant, heavy metal ions, oil, PO43-, paint, pigment, organic solvent and so on, and its CODCr value is high. If it is not handled properly, it will cause serious pollution to the environment. For this kind of wastewater, the traditional method is to directly treat mixed wastewater by coagulation, but the treatment effect is not ideal, and the effluent quality is unstable, so it is difficult to meet the discharge standards. In particular, the spray paint wastewater contains a large number of water-soluble organic solvents, and the direct treatment effect by coagulation is very poor. After field investigation, a lot of analysis, investigation and small-scale test in an automobile factory in Shanghai, according to the characteristics of painting wastewater, we adopted the secondary treatment method of water quality pretreatment and subsequent treatment, and chose Fenton oxidation-coagulation precipitation and air flotation physicochemical process for treatment, which reached the discharge standard and the CODCr removal rate reached over 80%.
1 waste water source and main pollutants
1. 1 Sources of coating wastewater and harmful substances
Coating wastewater mainly comes from pre-degreasing, degreasing, surface conditioning, phosphating, passivation and other body pretreatment processes; Cathodic electrophoresis process and intermediate coating and topcoat spraying process.
The main toxic and harmful substances contained in waste water are as follows:
Pretreatment before painting: nitrite, phosphate, emulsified oil, surfactant, Ni2+, Zn2+.
Primers: low solvent cathodic electrophoretic paint film, lead-free cathodic electrophoretic paint film, pigment, powder, epoxy resin, butanol, ethylene glycol monobutyl ether, isopropanol, dimethyl ethanolamine, polybutadiene resin, dimethyl ethanol, paint, etc.
Intermediate coat and topcoat: organic solvents such as xylene and banana oil, paint film, pigment and powder.
1.2 quality and quantity of wastewater
The designed water treatment capacity of this project is 60m3/h.
The waste water discharged from paint workshop is divided into intermittent waste tank liquid and continuous flushing water.
The intermittent discharge of wastewater mainly comes from the waste liquid from the reverse tank of the pretreatment tank and the waste liquid discharged from the painting section. The wastewater concentration is high, and the primary discharge is large. See table 1 for water quality.
Table 1 quality of wastewater discharged intermittently
dirty
Dye
thing
source
come
water
defective
CODCr
Mg/l
oil
Mg/l
PO43-
Mg/l
zinc ion
Mg/l
Nickel ion
Mg/l
Cadmium ion
Mg/l
carbon black
Mg/l
PH other
Pre-degreasing tank 2500~ degreasing tank waste liquid, and post-spraying immersion tank waste liquid.
4000
300~
950
250~400 9.5~ 1 1
Waste liquid from ground regulating tank 15~30 8.5~ 10.5
400 ~ 600100 ~150 20 ~ 306 waste bath liquid from phosphating bath, post-spraying bath and soaking bath.
Waste bath solution of passivation bath, post-spray bath and impregnation bath 50- 100- 1-34-5.
Electrophoresis waste bath solution 3000~
20000
8 1 7~9
3000 5~6 middle coat and top coat painting room sink paint residue.
The continuous discharge of wastewater mainly comes from the post-spraying of pretreatment process and the overflow wastewater of impregnation tank. Compared with the intermittent discharge of wastewater, its concentration is low and its total discharge is large. See Table 2 for its water quality.
Table 2 Water quality of continuously discharged wastewater
source
come
water
defective
dirty
Dye
thing
CODCr
Mg/l
oil
Mg/l
PO43-
Mg/l
zinc ion
Mg/l
Nickel ion
Mg/l
Cadmium ion
Mg/l
carbon black
Mg/l
pH value
Degreased washing wastewater 300 25 10~20 7~8
20~30 12 8 6 of rinsing wastewater after phosphating
Rinsing wastewater after passivation10 ~150.1.5 ~ 6
Deionized water spray tank spray wastewater 3900 1~3 4
Circulating deionized cleaning wastewater 400 6
Rinse the overflow wastewater after swimming 100~ 1000 8 7~9.
2. Design of coating wastewater treatment process
One of the keys of automobile painting wastewater treatment process is reasonable turbidity separation. For some wastewater that is difficult to be treated or affects the subsequent treatment, intermittent pretreatment should be carried out according to its nature and discharge law, and then centralized and continuous treatment with other wastewater can not only achieve better and stable treatment effect, but also be reasonable and feasible in economy.
2. 1 coating wastewater treatment process
See figure 1 for the process flow of coating wastewater treatment.
Figure 1 Treatment Process of Painting Wastewater Treatment Station in an Automobile Factory
2.2 Intermittent pretreatment
2.2. 1 degreasing waste liquid
Acidification pretreatment is carried out on the degreasing waste liquid, and inorganic acid is added to the degreasing waste liquid to adjust the pH value to 2-3, so that the higher fatty acid soap in the emulsifier can separate out fatty acids insoluble in water and soluble in oil, thus demulsifying the degreasing waste liquid and separating out oil.
In addition, after adding acid, the anionic surfactant in degreasing waste liquid is easy to decompose in acidic solution, losing its stability and the original lipophilic and hydrophilic balance, thus achieving the purpose of demulsification. After pretreatment, CODCr decreased from 2500 ~ 4000 mg/L to 1500 ~ 2400 mg/L, and the removal rate was about 40%. The oil content decreased from 300 ~ 950 mg/L to 50 ~ 70 mg/L, and the removal rate reached 90% ~ 95%.
2.2.2 Electrophoresis waste liquid
Cathode electrophoresis wastewater contains a large number of high molecular organic compounds, the highest CODCr can reach 20000mg/L, and also contains a large number of electrophoresis residues, which exist in the form of fine suspended matter or negatively charged colloid in water. Cationic polyacrylamide (PAM) and polyaluminum chloride (PAC) were added as coagulants in the treatment process, and the pollutants in wastewater were quickly removed by the adsorption bridging effect of flocculants. The pH value of electrophoresis waste liquid should be between 1 1 ~ 12, and the precipitation effect is good. The CODCr of the effluent after the reaction is about 2000 mg/L.
Spray painting wastewater
Fenton reagent (H2O2+FeSO4 _ 4) was used to pretreat painting wastewater, so that the organic matter in it was oxidized and decomposed, and the removal rate of CODCr was about 30%. Then add polyaluminum chloride and polyacrylamide for coagulation and precipitation. After these two steps, the total removal rate of CODCr can reach 60% ~ 80%, from 3000 ~ 20000 mg/L to 1200. The effluent is discharged into the mixed wastewater regulating tank.
Fenton reagent has strong oxidation ability. When the pH value is low (controlled at about 3), H2O2 is decomposed by Fe2+ to generate hydroxyl radical (OH), which leads to more other radicals, thus triggering a series of chain reactions [1]. Through the reaction between OH with strong oxidizing ability and organic matter, the refractory organic matter in wastewater is partially oxidized, and the C-C bond of organic matter in wastewater is broken, and finally it is decomposed into H2O, CO2, etc. , reducing CODCr. Or coupling or oxidation occurs, changing the density and structure of its electron cloud, forming intermediate products with small molecular weight, thus changing its solubility and coagulation sedimentation. At the same time, Fe(OH)3 generated by Fe2+ oxidation exists in colloidal form at a certain acidity, which has coagulation and adsorption properties, and can also remove some suspended solids and impurities in water. The effluent is further removed by subsequent coagulation and sedimentation to achieve the purpose of purification [2]
2.3 Continuous treatment
All kinds of pretreated wastewater are discharged into the balance regulating tank and mixed with other wastewater to enter the continuous treatment process. The CODCr of mixed wastewater is about 700 ~ 900 mg/L, and the continuous treatment is divided into two stages: coagulation sedimentation and coagulation air flotation.
In coating wastewater, oil, polymer resin (epoxy resin), pigment (carbon black), powder and phosphate are stably dispersed in aqueous solution in the form of colloid under the action of surfactants, solvents and various additives. Chemical agents can be added to destroy the stable system formed by colloidal fine suspended particles in water, so that they can aggregate into flocs with obvious sedimentation performance, and then form precipitation or scum to be removed [3].
After adding a certain amount of inorganic flocculant into wastewater, the potential of emulsified oil or polymer resin can be neutralized, the electric double layer can be compressed, and the collision of colloidal particles can promote agglutination, complete the destabilization process and form fine flocs. In this way, solid particles generated by metal ions and phosphate ions in coating wastewater under alkaline conditions can form precipitates [4]. Therefore, coagulation treatment can effectively remove oil, polymer resin, pigment and powder from automobile painting wastewater [5].
Among heavy metal ions and phosphates, the optimum pH values of Ni(OH)2 precipitate generated by Ni2+ and Ca3 (PO4) 2 precipitate generated by PO43- are above 10. However, the optimum pH range for Zn2+ to generate hydroxide precipitation is 8.5 ~ 9.5. If the pH value is too high, ZnO22-will be formed and dissolved. Therefore, the two-stage coagulation reaction should be carried out separately to remove Ni2+, PO43- and Zn2+ respectively. At the same time, the inclined plate sedimentation tank and air flotation tank are used for solid-liquid separation after coagulation reaction, so that heavy metal compound precipitation with high specific gravity can be removed by inclined plate sedimentation tank and organic matter with low specific gravity can also be removed by air flotation tank.
2.3. 1 coagulation and sedimentation
The first stage is coagulation and sedimentation, and the pH value is adjusted to 10 ~ 10.5.
The reaction tank is a plug-flow reaction tank, which is divided into three compartments. Alkali is added to the first tank to adjust the pH to 10 ~ 10.5, CaCl2 _ 2 and FeSO4 _ 4 are added to the second tank, and coagulant PAM is added to the third tank. After the reaction, it enters the inclined plate sedimentation tank for solid-liquid separation. The residence time of the three compartments is 65438 05 minutes, 65438 05 minutes and 7.5 minutes respectively. The surface load of inclined plate sedimentation tank is designed as 2m3/m2 h, and the removal rate of CODCr in first-order reaction is 50% ~ 60%. Fig. 2 is a schematic diagram of a primary reaction tank.
Fig. 2 schematic diagram of primary reaction tank
Coagulation and air flotation
The reaction tank of secondary reaction also adopts plug-flow reaction tank, which is divided into three grids. Adding acid to the first cell to adjust the pH to 8.5 ~ 9, adding PAC to the second cell, and adding PAM to the third cell, and after the reaction, entering an air flotation cell for solid-liquid separation. The three-cell residence time of the secondary reaction tank is 65438 00 minutes, 65438 00 minutes and 5 minutes respectively. The dissolved air water in the air flotation tank is designed to be 30% of the treated water. The removal rate of CODCr in the second reaction is 20% ~ 25%, and Zn2+ and some surfactants are also removed by air flotation.
2.4 Advanced treatment
The advanced treatment adopts sand filtration and activated carbon filtration. From the operation situation, the effluent after sand filtration can meet the discharge standard (CODCr≤300mg/L). The filtration speed of the sand filtration device is controlled at10 ~12m3/(m2 h). Backwashing water is provided by the water in the monitoring water tank after being pressurized, and the backwashing intensity is controlled at16 ~18l/(m2 s).
The effluent after sand filtration can meet the discharge requirements, so activated carbon filtration is only an emergency safeguard measure, which is rarely used in general.
2.5 Sludge treatment
The quality of sludge treatment directly affects the operation of sewage treatment station. Because of the high oil content of sludge, the direct pressure filtration effect is poor. Adding Ca(OH)2 into the sludge concentration tank and adjusting the pH value to about 10 can achieve better filter pressing effect. The plate-and-frame filter press can reduce the water content of sludge from 99% to 75% ~ 80%.
2.6 Analysis of removal rate of continuous treatment
The removal rate of continuous treatment process is shown in Table 3.
Table 3 Continuous processing efficiency
Removal rate of CODCr at outlet
50% ~ 60% of the outlet of inclined plate sedimentation tank
20% ~ 25% of the outlet of the air flotation tank
Sand filter outlet 15%
3 Analysis of therapeutic effect
The project has been running since 2002, and the treatment effect is stable. Table 4 is the data summary of the monitoring analysis report of Shanghai Environmental Monitoring Center Station in 2004. The monitoring time is 3 days, and samples are taken 12 times a day (once every 1 hour, including the inlet and outlet of wastewater treatment device).
Table 4 Monitoring Data of Total Discharge Outlet of Wastewater Treatment Facilities
class monitor
project
The import of wastewater treatment equipment * the export of wastewater treatment equipment meets the comprehensive wastewater discharge standard of Shanghai (DB 31199–1997).
Minimum concentration (mg/L) Maximum concentration (mg/L) Average concentration (mg/L) Minimum concentration (mg/L) Maximum concentration (mg/L) Average concentration (mg/L)
PH 6.94 8.96 8.32 7.57 8.85 7.88 6 ~ 9
CODCr 434 759 625 73132115.6 300.
SS 93 351204 21145 29 350 Class III standard.
BOD 536145 87 48316.9150 three-level standard.
Oil 2.611.55.10.10.90.610 secondary standard.
Secondary standard of Zn2+**-0.02 1.6 0.09 4.0
Mn2+**-0.05 0.26 0. 16 5.0 secondary standard
Ni2+* *-Nd 0.180.091.0 Emission Standard for Class I Pollutants
Benzene second-class standard neodymium neodymium neodymium neodymium neodymium neodymium 0.2
Toluene Secondary Standard Nd Nd Nd Nd Nd Nd Nd Nd Nd Nd 0.2
Xylene secondary standard neodymium neodymium neodymium neodymium neodymium 0.6
* The entrance of wastewater treatment unit refers to the entrance of continuous treatment unit.
* * Zn2+, Mn2+ and Ni2+ were not analyzed in this monitoring, and the daily analysis data listed in the table are the wastewater treatment station of this factory.
It can be seen from the above table that the treated wastewater is evaluated according to Shanghai Comprehensive Wastewater Discharge Standard (DB 31199-1997), in which CODCr, BOD5 and SS are evaluated according to the third-level standard (wastewater is discharged into Anting Water Purification Plant after treatment), and the rest are evaluated according to the second-level standard and the maximum allowable pollutant of Class I..
At present, the treatment device runs stably and the effluent can meet the standard.
4. Technical and economic analysis
Engineering cost and operating cost are issues that people must consider and care about when choosing treatment methods. Compared with the general centralized physical and chemical treatment, this project saves chemicals, reduces the sludge output, reduces the operation cost to a certain extent, and more importantly, ensures the effluent quality to meet the standard stably. The technical and economic indicators of this project are shown in Table 5.
Table 5 Technical and economic indicators of this treatment project
Total investment/unit volume sewage investment of 10,000 yuan/first year operation cost of 10,000 yuan/unit volume sewage treatment fee of 10,000 yuan/yuan /m3.
800 1. 1 1 30 1.67
* The annual working day is 250 days, and the daily water treatment capacity is 720 m3.
5. Conclusion
1. This project adopts different processes such as water quality treatment, coagulation and sedimentation, coagulation and air flotation and sand filtration to treat automobile painting wastewater, which is reasonable and feasible in technology and economy. The actual operation results show that the removal rate of heavy metals, SS and oil is over 90%, and the removal rate of CODCr is over 80%.
2. The quantity and quality of automobile painting wastewater change greatly, so special attention should be paid to the balance of quantity and quality of wastewater and the pretreatment of water quality. Engineering practice has proved that the intermittent pretreatment of degreasing wastewater, electrophoresis wastewater and painting wastewater is not only beneficial to the improvement of subsequent treatment efficiency, but also of great significance to the stable operation of the whole system and the stable discharge of water.
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Liu Shaogen, Huang Xianhuai Physicochemical-Biochemical Treatment of Automobile Production Wastewater [J] Water Supply and Drainage, 200 1, 27 (12): 53 ~ 56
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