Printed circuit board wastewater treatment

2022-07-21 11:05
Waste water will be produced in the production process of PCB printed circuit board factory and circuit board industry.
1、 Overview
The company is a large-scale enterprise specializing in the production of multilayer circuit boards. In the process of PCB production, a certain amount of wastewater is produced in each process. According to the relevant national environmental protection regulations, it must be treated and discharged after reaching the standard. Entrusted by the company, it provides the design scheme of wastewater treatment engineering for the project.
The project adopts the design scheme of "pretreatment + chemical coagulation + biochemistry".
2、 Design conditions
2.1 wastewater quality and quantity
According to the environmental impact assessment and relevant data provided by the manufacturer and combined with our years of practical experience, the water quality and quantity are determined as follows:
1. During the design, according to the requirements of the manufacturer, our institute will construct the civil works at one time, and the equipment can be installed in batches according to the situation;
2. The oil removal waste liquid in the scheme is not divided. If it is alkaline oil removal, it can be directly connected to the film stripping and development waste liquid tank. If it is acidic oil removal liquid, it can be connected to the film stripping and development waste liquid tank.
3. Waste liquid refers to the replacement mother liquid, excluding rinsing water;
2.2 design effluent requirements
After treatment, the production wastewater meets the first-class discharge standard in the integrated wastewater discharge standard gb8978-2008:
3、 Wastewater treatment design process
3.1 classification and treatment of water
According to the classification of this scheme, wastewater can be divided into several categories:
A) Acidic high concentration waste liquid
Acid high concentration waste liquid comes from acid pickling and electroplating processes. This part of waste water contains a small amount of cu2+ ions, and other pollution components are low. Therefore, this part of waste liquid can be recycled first as a reagent addition, which can be used to adjust the pH value, such as acid regulation of acid precipitation tank, pH callback of secondary cyanide breaking tank, etc
B) Film removal developing waste liquid
High concentration ink waste liquid mainly refers to the high concentration organic waste liquid produced in the development and film removal processes. These waste liquids contain a large amount of photosensitive film, anti welding film residue, etc. their common feature is that the COD is very high, sometimes as high as tens of thousands, so they must be treated separately. In this scheme, the organic waste liquid is operated intermittently. After adjusting the pH value and separating out under acidic conditions to remove a large amount of COD and scum, the waste water still contains a large number of organic colloidal particles that are not caked and free or broken during fishing. If these organic particles are not removed, it will increase the difficulty of subsequent treatment. Traditional methods include
(1) . air flotation method
(2) Bag filter
(3) Plate and frame pressure filtration method
The air flotation method mainly uses pressurized air to contact with the solution, so as to improve the solubility of the air in the water, and then suddenly relieves the pressure through the releaser, so that most of the air dissolved in the water is re separated, forming countless tiny bubbles. The bubbles continue to adhere to the suspended solids in the rising filtration process, and rely on the rising force of the bubbles, constantly raise the suspended solids to the liquid level, and then scrape them out of the pool through the mud scraper, so that the suspended solids can be removed.
The advantages of air flotation are:
A. High degree of automation.
B. Capable of continuous operation and processing.
The disadvantages are:
A. The removal effect is general;
B. The dissolved gas tank is easy to be blocked;
The bag filter is simple to install and has high filtration quality, but the water production is limited, so it is only suitable for small flow wastewater treatment.
The plate and frame filter is a method of removing suspended solids by intercepting the liquid on the filter cloth of the plate and frame filter press through a high-pressure pump.
The advantages are: due to the large filtering area and high pressure, the water production is high and the filtering effect is good;
The disadvantage is that it is easy to block the filter cloth, resulting in failure to filter.
Based on the above process methods, our hospital plans to use an acid and alkali resistant filter press to filter the acid precipitation filtrate to further remove suspended organic particles and remove organic matter from the filter.
As the COD value of the wastewater after pretreatment is still very high, which is far from reaching the water quality standard of the effluent, this scheme adopts the quantitative injection of the wastewater into the organic rinsing water, and after coagulation and sedimentation, the "hydrolysis acidification + contact oxidation" method is used to further degrade the cod, so as to ensure the discharge of the effluent COD up to the standard.
C) Copper complex wastewater (total design volume: 400 m3 / D)
Copper complex wastewater mainly includes chemical copper wastewater and copper ammonia etching wastewater. Since the plant has no alkaline etching process, only the treatment of chemical copper complex wastewater is discussed.
The chemical copper waste water shows the following characteristics due to its high concentration of EDTA, HCHO, c10h8n2 (diazobenzene) and other components:
1. Because the wastewater contains EDTA, it can form a chelate with cu2+, which is difficult to be removed by traditional chemical coagulation method by breaking the bond and freeing cu2+.
2. Because the wastewater contains HCHO and c10h8n2 (diazobenzene), it shows high organic matter, which is specifically manifested in high CODcr.
If this part of wastewater directly enters the comprehensive wastewater treatment system without pretreatment, it will not only affect the standard discharge of total copper, but also the standard discharge of CODCr. According to the characteristics of chemical copper wastewater, our hospital adopts aluminum catalytic reduction method, and its basic principle is as follows:
Al°+40H-→H2AlO3+H2O+3l- E°+2.35
Cu+2+2l-→Cu° E°+0.34
Al°+40H-+Cu+2→H2AlO3+H2O+Cu°+l- E°+2.69
H2AlO3-+3H2O+2l-→ 【Al(OH)6】-3 + H2
At this time, note that OH - is necessary in the reaction formula, so the reaction can be carried out only when the pH is maintained at about 10. The waste water is quantitatively pumped into the aluminum replacement tank, and air agitation is used to accelerate the reaction effect. Because the effluent contains reduced copper powder and aluminum hydroxide suspended solids, the suspended solids are filtered out by the pump pumped bag filter, and the waste water is connected to the organic regulating tank.
This part of wastewater also contains high COD. If it is not treated, the effluent COD will be difficult to meet the standard. Therefore, it enters the aerobic section of the biochemical treatment system after breaking the complex and coagulating sedimentation, and then meets the discharge standard after removing COD.
D) Organic cleaning wastewater
This part of wastewater is mainly produced in the cleaning section of oil removal, development, film stripping and other processes, which is characterized by large amount of water and high COD concentration.
This part of wastewater will be treated by biochemical treatment system after pH adjustment, coagulation and sedimentation.
The wastewater treated by this system is all high concentration wastewater, of which the COD is high. Therefore, the activated sludge process is the core of the COD removal process in this scheme. After pH adjustment, the pretreated copper precipitation and copper ammonia wastewater enters the biochemical system to degrade cod.
The principle and characteristics of biochemical process are as follows:
1. Facultative treatment uses the first two stages of anaerobic treatment, hydrolysis and acidification. The hydrolysis stage is to hydrolyze complex macromolecular organics into small molecules of soluble organics by extracellular enzymes. The acidification stage is to transfer the soluble organic matter into organic acids, alcohols, aldehydes, CO2, etc. The facultative biochemical treatment section has a certain adaptability to the impact load of water volume and quality, and breaks the long-chain organic matter of surfactant in the circuit board wastewater, creating favorable conditions for the subsequent aerobic section.
2. At present, aerobic treatment system mainly includes contact oxidation method and activated sludge method. The advantage of the contact oxidation method is that the operation and management is relatively simple, but the disadvantage is that the filler support needs to be regularly stopped for maintenance, and the filler needs to be replaced for 2 ~ 3 years. Not only the maintenance is complex and the cost is high, but also the replacement needs to be stopped for more than 10 days, and the bacteria need to be re cultured after replacement, which is not as effective as the active sludge method. According to our practical engineering experience, the developing degumming solution is easy to coagulate. After long-term operation with contact oxidation method, the packing is seriously agglomerated, and the adhesive microorganisms have no shelter, resulting in a large reduction of microorganisms, and the biological treatment system is difficult to operate normally, which seriously affects the treatment effect; The advantage of activated sludge method is that it has good treatment effect and can control the operation state of sludge concentration control system without shutdown and maintenance. The disadvantage is that the operation is a little complicated, and there may be sludge bulking. This process adopts the dosing activated sludge method and adds bioflocculant in the coagulation area, which can effectively inhibit the problem of aerobic sludge bulking.
To sum up, after coagulation and sedimentation, the effluent enters the biochemical system for further treatment. There are two flow directions for the effluent, one is to discharge it together with the treated cleaning wastewater up to the standard, and the other is to discharge it after coagulation and sedimentation with the cleaning wastewater if the COD in the scheme cannot meet the standard.
E) Nickel containing cleaning wastewater
This part of wastewater mainly comes from the cleaning water after electroless nickel plating, chemical nickel and silver deposition. Because Ni is a kind of pollutant, according to environmental protection requirements, this part of wastewater must be treated separately, and then mixed into the comprehensive wastewater after reaching the standard, and discharged together after reaching the standard.
F) Cyanide containing wastewater
This part of wastewater mainly comes from the cleaning water behind the gold cylinder. Cyanide is a highly toxic substance. At present, the treatment methods of cyanide containing wastewater mainly include: electrolytic oxidation method, activated carbon adsorption method, ion exchange method, ozonation and ferrous sulfate method, etc. but at present, alkaline chlorination method is mostly used at home and abroad. Although some sodium hypochlorite is provided by electrolytic salt water, it is still different from electrolytic oxidation method.
A separate treatment system should be designed for cyanide containing wastewater, which should not be mixed with other wastewater, especially cu2+, ni2+, Fe plasma, which will cause great difficulty in treatment after the formation of complex. The cyanide breaking wastewater is discharged to the general cleaning wastewater treatment system for further treatment, and then discharged after removing heavy metals.

PCB printed circuit board factory
For cyanide containing wastewater, our hospital still adopts the alkaline chlorination method with stable technology and operation, and adopts secondary oxidation. The specific principle is as follows:
(1) Incomplete oxidation reaction
CN-ClO-H2O CNCl2OH-
According to relevant information, it is believed that the toxicity of cno- is only one thousandth of that of cn-. However, if the pH decreases after mixing with other wastewater, cno- will hydrolyze to produce ammonia, causing ammonia pollution and affecting the treatment of other metal ions. Therefore, complete oxidation treatment must be carried out.
(2) Complete oxidation reaction
The specific process parameters are:
(1) Primary oxidation: ph=10~11, hrt:30~60min
(2) Secondary oxidation: ph=9~9.5, hrt:30~60min
This part of wastewater must be treated separately, and then mixed into the comprehensive wastewater to meet the standard.
G) Comprehensive wastewater (general cleaning wastewater) (total design volume: 3000m3 / D)
The comprehensive wastewater mainly contains cu2+ and a certain amount of COD, and the pH is generally acidic. Most of the cu2+ in the wastewater exists in free state, so it is easy to remove it by adding NaOH to the wastewater. According to the solubility product principle, cu2+ forms suspended matter in the form of Cu (oh) 2.
According to relevant data, the COD content in the comprehensive wastewater is about 70mg/l. After the water quality and quantity are adjusted in the regulating tank, the comprehensive wastewater is lifted to the pH regulating tank by the lift pump. After the pH is adjusted, it enters the sedimentation tank for mud water separation after rapid mixing and slow mixing. The supernatant is discharged to the intermediate tank, and then discharged after reaching the standard through the sand filter tower.

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