Advances in wastewater treatment in paint production.

Abstract: The sources and hazards of wastewater produced by paint production are reviewed, and its special physical and chemical properties are expounded. Aiming at the unique properties of paint production wastewater, the paper summarizes the commonly used coating wastewater treatment methods at home and abroad from the aspects of general methods and composite methods, points out the advantages and disadvantages of each method, and provides reference and basis for the treatment of wastewater in future paint production.
key words: paint wastewater, water treatment technology, composite method
0 introduction
the source of paint wastewater is mainly divided into two parts: first, equipment cleaning wastewater, and second, process wastewater

. Due to the small scale of production in the coatings industry, the variety, the use of intermittent discharge, water quality and water fluctuations are relatively large, wastewater contains a variety of toxic, difficult to biodegradable polymer and organic compounds. Such wastewater belongs to a high concentration of organic wastewater, its color, turbidity, chemical oxygen demand (COD) as high as 105 mg/L or more, in the treatment of such wastewater, often encounter COD continued high, can not completely break the milk, turbidity is difficult to reduce, affecting the subsequent biochemical treatment effect, resulting in wastewater discharge is not up to standard and other issues. Paint wastewater discharged without treatment will remain in the environmental medium for a long time, causing serious pollution to the environmental system, so paint wastewater must be treated to meet the standard before discharge.
At present, the main treatment methods of wastewater produced in domestic and foreign coatings are still coagulation precipitation, adsorption, biology and a small number of advanced oxidation methods, etc., but in practice, most of the above treatment methods are used composite processes to improve treatment efficiency.
1 General method
1.1 Coagulation precipitation method
The basic principle of coagulation method is to put coagulants in wastewater, because coagulants are electrolytes, forming gels in wastewater, and the collosome substances in wastewater occur in electricity and, forming velvet deposition. Coagulation precipitation can not only remove small particle size suspended particles in wastewater, but also remove chroma, oil, microorganisms, nitrogen and phosphorus and other nutrients, heavy metals and some small molecules of organic matter.
in the process of coagulation and precipitation treatment of wastewater, there are many factors that affect the coagulation effect. Among them are the effects of water samples: for different water samples, the treatment effect of the same coagulant may vary greatly due to the different components in the wastewater. The principle of coagulation mainly depends on three kinds of functions: compression double-layer action, adsorption of electricity and co-action and net sweeping effect. The condensation and flocculation phenomena caused by these three effects are generally called coagulation. Coagulation precipitation method is applicable to B/C small, poor biochemicity or suspension of more wastewater, such wastewater is not suitable for direct treatment by biochemic method, so before biochemic treatment should be used coagulation precipitation method for pretreatment.
, Peng Yufan and others selected 3 kinds of coagulants (iron salt, aluminum salt, calcium salt), ICI Segu paint company's 25 paint wastewater was carried out pollutant degradation test. The test results show that the chemical coagulant can effectively precipitate a large number of suspended solids in the wastewater of the coating, the COD removal rate can reach 90%, which creates extremely favorable conditions for subsequent biochemical methods, and by comparison, it is found that the treatment effect of iron salt is the best and the cost is low, which can be used as the ideal coagulant for wastewater treatment in industrial production.the
coagulation precipitation method needs to add a large number of coagulants (such as lime, polyacrylamide) and flocculants, the operation is slightly more complex, but the general water quality fluctuations of paint wastewater, in order to achieve good treatment results, this method is usually used as a pretreatment method of other deep treatment processes, and other methods combined.
1.2 adsorption method
adsorption method is the use of porous solid adsorption agent to absorb one or more components of the water sample on the surface of the adsorbent, and then through appropriate solvent, heating or blowing methods to decompose the prediction group adsorption, so as to separate the pollutants in the wastewater from the wastewater.
adsorbent is generally a porous solid with good physiculation stability, of which activated carbon is the most widely used, long-standing and relatively low-cost adsorbent. After pretreating the raw liquid of paint wastewater, Liu Shicai and so on, the activated carbon with composite ratio is used for adsorption treatment, so that the coating wastewater treatment meets the standard of industrial wastewater discharge and the cost is relatively low. In recent years, there have also been reports of other new adsorbents being used in coating wastewater treatment, with Fagbenro using natural oblique zeolite (CPL) for water treatment, resulting in the safe discharge of coating wastewater, removing about 90% of SS (suspension) and 79% of COD.
1.3 Biofilm Method
Biofilm Method is a kind of wastewater aerobic biological treatment technology, is a fixed membrane method, mainly to remove dissolved and colloidal organic pollutants in wastewater. Treatment techniques include biofiltration tanks (common biofiltration tanks, high-load biofiltration tanks, tower biofiltration pools), bio-turntables, bio-contact oxidation-ready and bio-fluidized beds.
the design and operation of wastewater treatment plants using membrane bioreaacters. The test results show that the membrane bioreactive device has a good removal effect on COD and SS in the water after coagulation filtration pre-treatment, the average removal rate of COD is 91%, the average removal rate of water COD< is 91%, the average removal rate of SS is more than 85%, the water SS<14 mg/L, the water quality meets the requirements of Shanghai's Comprehensive Sewage Discharge Standard (DB31/1997).
But the disadvantage of this method is that in the biological membrane operation process, membrane cleaning is an indispensable part of membrane bioreacter operation, only the membrane assembly for proper cleaning, generally with 1% NaOH plus 2.5% NaOCl solution soaked membrane assembly (6 h), and then with 1% sulfuric acid solution immersion (4 h) to make the system stable operation, stable membrane flow and good water quality. Membrane cleaning process operation trouble, need to invest a lot of human and material resources.
1.4 Advanced Oxidation Method
According to the definition of Bolton, etc., the advanced oxidation process is an oxidation technique that degrades organic matter based on the active production of strong oxides (strong oxidizing freelances). It is mainly based on the production of hydrogen and oxygen freelance fundamentals (HO.), and other oxidizing substances such as sulfur-oxygen free fundamentals and chlorine-containing free fundamentals. Advanced Oxidation Technologies (AOPs) have the advantage of being "environmentally friendly" compared to other chemical and biological treatment technologies because they do not simply transfer pollutants from one phase to another (e.g. chemical subsidion and adsorption processes), but rather break down large molecular organic pollutants into smaller, more easily disposed of or non-toxic substances, thus not producing large amounts of toxic waste.
advanced oxidation technology based on hydroxyl free radicals (HO.) was produced earlier, and the technology has been perfected. The more common advanced HO.based oxidation processes today include the Fenton process: the use of Fe (II) to biocompose H2O2 to produce highly active HO. In addition, based on the Fenton-like process, technologies such as fenton-like process, light-Fenton process, electro-Fenton process are derived to further improve oxidation efficiency. The main advantage of the Fenton process is its low cost and ability to magnetically separate the remaining iron.
U.Kurt and others treat paint wastewater in a mixer, using rust particles as catalysts, and adding a certain amount of hydrogen peroxide, because rust particles are inefficient, so the reaction time in the mixer is about 70 h. This test also confirmed that within 70 h, when the rust cast addition is 10 g, the removal rate of COD is 80%, and rust as a reaction catalytic substance at the same time will be used as waste in the roll-up process. This test shows that rust can be used as a treatment agent for paint wastewater, but the efficiency is very low.
advanced oxidation technologies such as photo catalysis and catalytic ozoneization, fenton catalytic oxidation techniques have obvious advantages: simple operation, low cost and no special equipment required. However, due to the special properties of solid-phase catalysts containing metal ions, most polyphase Fenton catalysts are based on a single active redox point to achieve their catalytic reaction, so these catalysts almost all have low activity under neutral conditions, low processing efficiency and low utilization rate of hydrogen peroxide (H2O2) and other urgent problems.
2 Composite Method
2.1 Coagulation - Electrochemical Method
Larissa F. da Silva s14) uses Coagulation (Aluminum Sulfate) - Electrochemical Method (BDD electrode) coupling treatment coatings to produce wastewater (mainly made of water-based acrylic resin). The wastewater treated by two methods (combination method and separation method) is compared, and the treatment efficiency is evaluated according to typical parameters. There are two factors that need to be assessed, one is the feasibility of treating wastewater using these two methods separately and jointly, and the other is the possibility of determining the discharge of wastewater into the water body. The results show that the optimal dose of aluminum sulfate using chemical coagulation is only 12 mL/L, however, the water quality of the water discharged by coagulation alone cannot meet the discharge requirements and requires the joint use of electrochemical methods (i-10 mA/cm2, t-90 min) in order to discharge the treated wastewater into the water body.
2.2 Coagulation precipitation - Fenton reagent catalytic oxidation - activated carbon adsorption process
Chen Shaowei, etc. , with "coagulation precipitation - Fenton reagent catalytic oxidation - activated carbon adsorption" composite process for the production of wastewater in a paint factory workshop in Shanghai. The test first used aluminum sulfate as a coagulant to remove collosites and large molecule organic pollutants from wastewater, the amount of water added was 400 mg/L, and the water COD after coagulation and precipitation could be reduced from 5,900 mg/L to 1,500 mg/L from infested water; FeSO4 cast plus 1,540 mg/L, pH is 6.0, reaction time is more than 4 h, reaction temperature is room temperature, the removal rate of wastewater COD is about 90%;
2.3 Coagulation gas float - hydrolytic acidification - bio-contact oxidation
HuJin, etc.) for water-based paint wastewater test research, wastewater main water quality indicators are: COD concentration of 6,000 to 12,000 mg/L, BOD5 concentration 1 600 to 3 400 mg/L, SS concentration 600 to 2 400 mg/L, NH3-N concentration 18 to 35 mg/L, if discharged directly, will cause great harm to the environment. The waste water is treated by the combination process of coagulation float-hydrolytic acidification-bio-contact oxidation, and the reliability and stability of the operation of the system are studied through the combination of small test research and engineering commissioning and operation research, so that the discharge water meets the Sewage Comprehensive Discharge Standard (GB 8978-1996) Level 2 discharge standard. Through the screening of coagulants, PAC (polymerized aluminum chloride) and PAM (polyacrylamide) were determined to be the optimal coagulants, with the optimum dosing of 125 mg/L and 7 mg/L respectively. Through the analysis of the changes of various indicators with HRT (hydrodynity dwell time), it is found that the removal rate of COD and SS increases with the extension of HRT until HRT is stable at 8 h, at which time the removal rate of COD and SS is 34.3% and 51.4%, respectively, and the hydrodynium dwell time of the hydrolytic acidifier treatment coating wastewater in the test is determined to be 8 h. After hydrolytic acidification, the B/C value of wastewater was increased from 0.33 to 0.41.
the above"coagulation float - hydrolytic acidification - bio-contact oxidation - activated carbon combination process" joint operation, wastewater water quality index is: COD concentration 92 mg/L, BOD5 concentration 25.4 mg/L, SS concentration 14.3 mg/L, NH3-N concentration 8.44 mg/L, water discharge index is stable and meets the "Sewage Comprehensive Discharge Standard" (GB 8978-1996) Level 2 emission standards.
2.4 Materialization - Two-segment bio-contact oxidation method
zo red shadow, etc.) using materialization-two-segment bio-contact oxidation series process to treat paint wastewater, the results show that: when the design of water quality CODcr ≤4 000 mg/L, CODcr, BOD5, SS average of 3,600 mg/L, 1 275 mg/L At 1,457 mg/L, after coagulation precipitation, gas floating, bio-contact oxidation, and two-stage bio-contact oxidation treatment, the average CODcr removal rate of each process was 47.5%, 43.8%, 77.5%, 72.5%, the total removal rate of CODcr was 98.2%, and the water quality of the secondary sink reached the level 1 emission standard. The results of the plant determination show that after the second sink water is treated by activated carbon adsorption and industrial cycle cooling water processor, the water quality meets the design specification requirements of industrial cycle cooling water, namely: pH 6.5 to 9, turbidity level I, total alkalinity ≤ 7 mmol/L, total E. coli cluster ≤ 1,000 /L. The water quality meets the emission standard requirements of level 1 of the Water Pollution Emission Limit (DB 44/26-2001).
3 Conclusion
Coatings industry in China is now large-scale application, in the production process produced by the composition of wastewater changeable, poor biochemistry, the traditional general treatment method for the treatment of such wastewater is not ideal, can not meet the emission requirements set by the state. In order to meet the requirements of national standards, generally need two or more traditional methods of compound use, how to series a variety of processes, so that each process can achieve the best conditions, is a long time before the focus of research.
, people around this idea to develop and study a lot of treatment of paint wastewater methods, and achieved good results. Other popular technologies, such as the use of photo-catalytic technology, the active persulphate to produce strong oxidizing free-form and other advanced oxidation technology to treat coating industrial wastewater, the emergence of such technology for the treatment of paint wastewater provides a new way of thinking; Development and research economy is good, stability is high, removal rate is strong and not.