Characteristics, processflow of electroplating wastewater treatment
Electroplating pickling sludge is mainly the sludge produced in the production process of electroplating and stainless steel processing industry. It itself contains a large amount of heavy metals nickel and chromium. It is a kind of heavy metal pollutant. If the electroplating sludge is not treated with hazardous waste, the heavy metals cannot be decomposed in rivers, lakes and soil, which will only increase the concentration of heavy metals in water and the content of metals in soil. Fish, shrimp, plants and other organisms are in this environment, These heavy metal elements will be enriched and finally reach the human body through the biological chain. Heavy metals will cause malformations and other hazards in the human body, which will seriously affect human health. Therefore, it is very important to treat the hazardous waste of electroplating sludge. In short, if it is not treated with hazardous waste, it will cause irreparable pollution to the environment.
1 pollution characteristics of electroplating wastewater
The wastewater quality of electroplating industry is complex. The wastewater contains heavy metal ions such as chromium, zinc, copper, nickel and cadmium, as well as impurities with great toxicity such as acid, alkali and cyanide. Wastewater from this industry has the following characteristics:
(1) Pollutants with complex composition can be divided into inorganic pollutants and organic pollutants.
(2) The water quality changes greatly, the types of pollutants in each production wastewater are diverse, and the CODcr change coefficient is large.
(3) The 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 body
2. Determination of process scheme
The production sewage of a company mainly comes from the acid and alkali treatment of plated parts before plating and rinsing after plating. In addition, a certain amount of waste acid will be discharged regularly.
(1) Pretreatment of production wastewater
① Removal of Cr6 +
At present, chromium containing electroplating wastewater is mainly treated by oxidation-reduction precipitation process
Redox method refers to the use of strong oxidant or strong reductant to oxidize or reduce toxic substances in wastewater into non-toxic or low toxic substances. In electroplating wastewater, hexavalent chromium mainly exists in the form of CrO42 - and cr2o72 - under acidic conditions. The reduction reaction occurs under the action of ferrous ions, and the reduction reaction is fast. The reduced chromium exists in the form of Cr (OH) 3 precipitation under alkaline conditions. The obtained sludge is the mixed precipitation of trivalent chromium and iron hydroxide. Considering that the reduction reaction is not complete, the amount of ferrous sulfate in actual operation is 2.5-3 times of the theoretical calculation, so the amount of sludge is large.
The specific process is as follows:
ferrous sulfate
↓
Electroplating wastewater → reduction reaction → pH neutralization → flocculation sedimentation → discharge up to standard
Its basic principle is:
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 production of sludge is large, which also increases the difficulty of sludge disposal.
② . removal of other metal ions
In addition to exceeding the national discharge standard, Cr6 + in electroplating wastewater also contains a large number of metal ions such as Zn2 +, Cu2 +, Ni2 +, Fe2 +, etc. Therefore, the method of aeration oxidation under alkaline conditions can not only make the pH value meet the discharge standard, but also effectively remove heavy metal ions in wastewater. The 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, adjust the pH to peralkaline. Since zinc ions start to precipitate at pH = 6.4, completely precipitate (2.0mg / L) at pH = 9.3, and dissolve at pH = 10.5, there are two-stage reactions, and the pH of the primary reaction tank must be controlled within the range of 9.5-10.
In the first-order reaction, Fe3 + ions can completely precipitate at pH = 4.1, Cu2 + ions can form basic salt precipitation at pH = 5.0, pH = 7.2 can completely precipitate, Cr3 + ions can start to precipitate at pH = 4.9, pH = 6.8 and dissolve at pH = 12. Since Ni2 + ions begin to precipitate at pH = 7.7 and can not completely precipitate (1.0mg / L) until pH = 10.5, Ni2 + and Fe2 + cannot completely precipitate in the primary reaction, so a secondary reaction is required. In the secondary aeration oxidation reaction, the pH must be controlled within 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 exceeds the standard due to the presence of additives and other organic substances (part of the CODCr in the wastewater is produced by ferrous iron and part by organic substances). According to the measurement, the CODcr value after two-stage precipitation is about 200mg / L, while the national standard is 100mg / L. therefore, after two-stage precipitation pretreatment, Aerobic biochemical treatment is adopted to make it meet the national standard.
Electroplating additives are mainly divided into leveling agent, stress relief agent, surfactant, brightener, auxiliary brightener, 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 includes SBR and its improved type, AB process, etc; Biofilm process includes contact oxidation process, biofilter, etc. Among them, SBR and its improved and biological contact oxidation are the popular processes of sewage biochemical treatment in the world.
① , SBR and its improved
SBR process is the abbreviation of sequential intermittent activated sludge process. It is an activated sludge water treatment technology operating according to intermittent aeration, also known as sequential instruction activated sludge process.
SBR Sewage Treatment Technology and its improved type are different from traditional sewage treatment technology. It adopts time division operation instead of space division operation, unsteady biochemical reaction instead of steady-state biochemical reaction, static ideal precipitation instead of dynamic precipitation, etc. It realizes the combination of orderly and intermittent operation in operation. SBR process is a method to carry out activated sludge treatment process in a single reaction tank and make the unit operation and time form of sewage treatment continuous.
There is a partition wall in the SBR reaction tank, which divides the reaction tank into pre reaction area and main reaction area, and the bottom of the wall is connected with holes. The five processes of water inflow, reaction, sedimentation, decanting and idling in each cycle are carried out repeatedly in the same tank. Compared with other treatment processes, SBR process greatly simplifies the sewage treatment structure.
a. During the aeration period, as the aeration system supplies oxygen to the reaction tank, organic pollutants are oxidized and decomposed by microorganisms, and NH3-N is transformed into NO3-N by nitrifying bacteria.
b. During the sedimentation period, stop aeration and separate mud and water. At the same time, microorganisms use the remaining dissolved oxygen in the water for oxidative decomposition. The reaction tank gradually changes from aerobic state to anoxic state and begins denitrification.
c. After the precipitation in decanting period, decant the water to discharge the supernatant, and the water level in the tank gradually decreases. At this time, the reaction tank gradually transitions to anaerobic state and continues denitrification.
d. During the idle period, the water level in the pool rises from the lowest water level to the highest water level.
Compared with the traditional activated sludge process, SBR process and its improved type have the following characteristics:
a. The process flow is simple, eliminating sedimentation, secondary sedimentation tank, sludge reflux and sludge reflux equipment.
b. The floor area is saved, 20% - 30% less than that of ordinary aeration method.
c. Low operation cost, high degree of automation control and convenient management. The absorption rate of oxygen is high and the operation cost is saved by 25%.
d. High treatment efficiency, reliable operation stability, strong load impact resistance and good effluent quality.
e. The effect of nitrogen and phosphorus removal is good.
f. Sludge sedimentation is good.
② Biological contact oxidation
Biological contact oxidation is a biofilm process between activated sludge process and biofilter process. There are fillers in the contact oxidation tank. Some microorganisms grow on the surface of the filler in the form of biofilm, and some grow in flocculent suspension and in water. At this stage, biological contact oxidation method is to set filler in the tank, immerse all the filler with oxygenated sewage and flow through the filler at a certain speed. The filler is covered with biofilm, and there is also a certain amount of activated sludge in the sewage. The sewage is in contact with biofilm and activated sludge, and the sewage is purified under the action of microorganisms. It can be said that biological contact oxidation is a biological treatment method between activated sludge method and biofilm method, which has the dual efficiency of activated sludge and biofilm.
The biological contact oxidation process has the following characteristics:
a. The volume of biological contact oxidation method is high. The equipment with the same size and volume has short treatment time, high treatment capacity, saving floor area and saving energy compared with ordinary aeration method.
b. Low operation cost, high degree of automation control and convenient management. The absorption rate of oxygen is high and no additional agent is needed.
c. The treatment efficiency is high, and the effluent quality is good and stable. Under the impact of poisons and pH value, the biofilm is less affected and recovers quickly.
d. Reliable operation stability and strong load impact resistance.
e. It can effectively prevent sludge bulking and give full play to its high decomposition and oxidation capacity.
Description of Biofilter Process Flow:
The waste acid is regularly discharged into the waste acid tank. After the comprehensive electroplating wastewater and waste acid enter the regulating tank, they are lifted into the reduction tank by the pump, and the reducing agent is added by the dosing pump to reduce the hexavalent chromium into trivalent chromium under acidic conditions. After the reaction, they are left in the reaction tank, and the alkali reaction is added by the dosing pump. Most of the divalent iron ions, trivalent iron ions, trivalent chromium, zinc ions, nickel ions and copper ions in the wastewater form precipitation under alkaline conditions The waste water after reaction enters 1# sedimentation tank through guide tube for sedimentation and separation.
The supernatant flows into the aeration oxidation tank by itself for further aeration and oxidation to oxidize the remaining ferrous ions into ferric ions and generate ferric hydroxide precipitation under alkaline conditions. Precipitation and separation are carried out in the 2# sedimentation tank. After pH correction, the supernatant flows into the 3# sedimentation tank after removing CODCr in the contact oxidation biochemical tank. The supernatant flows into the clean water tank by itself and then meets the standard for discharge.
The sludge from each sedimentation tank enters the sludge concentration tank for concentration. After concentration, the sludge is injected into the filter press by the high-pressure pump for pressure filtration. After drying, the sludge is sent to the solid waste Center for disposal as hazardous waste.
Technical characteristics of treatment process
a. The physicochemical method is used to pretreat the sewage, which can effectively remove most of the heavy metals in the sewage, reduce the toxicity and enhance the biodegradability of the sewage.
b. Contact oxidation process is adopted for biochemical treatment to effectively remove CODcr and reduce investment and operation cost.
c. The waste acid adopts the treatment method of "regular discharge and daily treatment", which avoids the excessive change of water quality in the regulating tank from affecting the subsequent treatment process. The salt content in the waste acid is high. If treated at one time, the Cl - concentration in the water body will rise sharply in a short time, which will inhibit or even die the activated sludge.
d. Gravity gravity flow shall be adopted as far as possible to reduce the power of the machine and pump. Reliable and efficient brands shall be selected for dosing, so as to reduce the consumption of chemicals. Through the above methods, the operation cost of sewage treatment system can be greatly reduced.