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| Characteristics, process and process of electroplating wastewater treatment |
| Time：2018-09-11 Source： Views：1944 |
1 pollution characteristics of waste water in electroplating industry
The water quality of electroplating wastewater is very complicated. The wastewater contains heavy metal ions such as chromium, zinc, copper, nickel, cadmium, and other toxic impurities such as acid, alkali and cyanide. The wastewater of the industry has the following characteristics:
(1) the composition is complex, and pollutants can be classified into two categories: inorganic pollutants and organic pollutants.
(2) the variation of water quality is large, and there are various kinds of pollutants in production wastewater, and the coefficient of variation of CODcr is large.
(3) Wastewater is highly toxic and contains a large number of heavy metal ions. If it is discharged directly without treatment, it will cause great pollution to the surrounding water bodies.
2 Determination of process plan
The production sewage of a company mainly comes from acid and alkali treatment before plating and rinsing after plating. In addition, a certain amount of waste acid is discharged regularly.
(1) pretreatment of production wastewater
Removal of Cr6+
At present, chromium containing electroplating wastewater is mainly treated by oxidation-reduction precipitation process.
Oxidation-reduction method refers to the use of strong oxidants or strong reductants, the toxic substances in wastewater oxidation or reduction to non-toxic or low toxic substances. Hexavalent chromium in electroplating wastewater mainly exists in the form of CrO42 -, in the form of Cr2O72 -, in the acidic condition, the reduction reaction takes place under the action of ferrous ions, the reduction reaction is faster. The reduced chromium exists in the form of Cr (OH) 3 precipitation under alkaline conditions, and the sludge obtained is a mixture of trivalent chromium and iron hydroxide precipitation. Ferrous sulfate is used to reduce hexavalent chromium. Considering the incomplete reduction reaction, the amount of ferrous sulfate in actual operation is 2.5-3 times that of theoretical calculation, so the amount of sludge is large.
The specific process is as follows:
ferrous sulfate
It
Electroplating wastewater, reduction reaction, PH neutralization, flocculation sedimentation, reaching the standard discharge
The basic principles are:
Cr2O72-+6Fe2++14H+=2Cr3++6Fe3++7H2O
Cr3++3OH-=Cr (OH) 3
It can be seen from the above process that the reduction of hexavalent chromium by ferrous sulfate is carried out under acidic conditions, and the sludge production is large, which makes the sludge disposal more difficult.
Removal of other metal ions
Cr6+ in electroplating wastewater exceeds the national discharge standards, and contains a large number of Zn2+, Cu2+, Ni2+, Fe2+ and other metal ions. Therefore, the method of aeration oxidation under alkaline conditions can not only make the pH value reach the discharge standards, but also effectively remove heavy metal ions in wastewater. Its principle is as follows:
2HCl+Ca (OH) 2=CaCl2+2H2O
Zn2++2OH-=Zn (OH) 2
Ni2++2OH-=Ni (OH) 2
Cu2++2OH-=Cu (OH) 2
Fe2++e=Fe3+
Fe3++3OH-=Fe (OH) 3
First, PH is adjusted to over alkali. Since zinc ions precipitate at PH=6.4, then completely precipitate at PH=9.3 (2.0 mg/l) and begin to dissolve at PH=10.5, it is divided into two stages. The PH of the first stage reaction tank must be controlled in the range of 9.5-10.
In the first-order reaction, Fe3+ ion can completely precipitate when PH=4.1, Cu2+ ion can form basic salt precipitation when PH=5.0, PH=7.2 can completely precipitate, Cr3+ ion can completely precipitate when PH=4.9, and can completely precipitate when PH=6.8, and begin to dissolve when PH=12. Since Ni2+ ions begin to precipitate at PH=7.7 and can not be completely precipitated until PH=10.5 (1.0 mg/l), Ni2+ and Fe2+ can not be completely precipitated in the primary reaction, so secondary reaction is needed. In the secondary aeration oxidation reaction, PH must be controlled in the range of 10.5-11.
(2) biochemical treatment of production wastewater
After two-stage precipitation treatment, the PH value and heavy metal ion index in the wastewater have been qualified, but the CODcr in the wastewater exceeded the standard due to the presence of additives and other organic substances. The CODcr value in the wastewater was about 200 mg/l after two-stage precipitation according to the determination. The national standard is 100mg/l, so the wastewater is treated by aerobic biochemical method after two-stage precipitation pretreatment, and the wastewater reaches the national standard.
Electroplating additives are mainly divided into leveling agents, stress eliminating agents, surfactants, brighteners, auxiliary brighteners, etc., mainly aldehydes, coumarins, saccharin and decomposition products, most of which are biochemical substances. Aerobic biological treatment process is divided into activated sludge process and biofilm process. Activated sludge process has SBR and its modified method and AB method. Biofilm process has contact oxidation method and biofilter. Among them, SBR and its modified and biological contact oxidation are the most popular biochemical treatment processes in the world.
1. SBR and its improved type
SBR is the abbreviation of sequential batch activated sludge process, which is a kind of activated sludge treatment technology operated by intermittent aeration, also known as sequential batch activated sludge process.
SBR sewage treatment technology and its improved type are different from traditional sewage treatment technologies. It uses time division instead of space division, unsteady biochemical reaction instead of steady biochemical reaction, static ideal precipitation instead of Dynamic Precipitation and so on. It realizes the combination of orderly and intermittent operation in operation. SBR process is a kind of activated sludge treatment process in a single reactor, and makes the unit operation and time form of sewage treatment continuous treatment method.
A partition wall is arranged in the SBR reaction pool, and the reaction pool is divided into the pre-reaction zone and the main reaction zone. The bottom of the wall has holes connected. Each cycle of water intake, reaction, precipitation, decanting and idle five processes are carried out in the same pool cycle, SBR process compared with other treatment processes, SBR process makes the sewage treatment structure greatly simplified.
A. Organic pollutants are oxidized and decomposed by microorganisms during the aeration period because the aeration system supplies oxygen to the reaction tank. At the same time, NH3-N is converted to NO3-N by nitrifying bacteria.
B. Stop aeration during sedimentation and separate mud and water. At the same time, microorganisms use the residual dissolved oxygen in the water to oxidize and decompose. The reaction tank gradually becomes aerobic.
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