Analysis on the characteristics, process and process of electroplating wastewater treatment

1 Pollution characteristics of wastewater in electroplating industry

The wastewater quality of the electroplating industry is complex. The wastewater contains heavy metal ions such as chromium, zinc, copper, nickel and cadmium, as well as acids, alkalis, cyanides and other highly toxic impurities. The industry wastewater has the following characteristics:

(1) The composition is complex, and the pollutants can be divided into two categories: inorganic pollutants and organic pollutants.

(2) The variation of water quality is large, and the types of wastewater in each production wastewater are various, and the coefficient of variation of CODcr is large.

(3) The wastewater is highly toxic and contains a large amount of heavy metal ions. If it is discharged without treatment, it will cause great pollution to the surrounding water body.

2 Determination of the process plan

The production wastewater of a company mainly comes from the acid and alkali treatment of the plated parts before plating and the rinsing after plating. In addition, a certain amount of waste acid is discharged regularly.

(1) Pretreatment of production wastewater

Removal of 1Cr6+

At present, the chromium-containing electroplating wastewater is mainly treated by a redox-precipitation process.

Redox method refers to the oxidation or reduction of toxic substances in wastewater into non-toxic or low-toxic substances by using strong oxidants or strong reducing agents. The hexavalent chromium in the electroplating wastewater mainly exists in the form of CrO42-, and exists in the form of Cr2O72- under acidic conditions. The reduction reaction occurs under the action of ferrous ions, and the reduction reaction is faster. The chromium after the reduction exists in the form of precipitation of Cr(OH)3 under alkaline conditions, and the obtained sludge is a mixture of trivalent chromium and iron hydroxide. The hexavalent chromium is reduced by ferrous sulfate. Considering that the reduction reaction is not complete, the amount of ferrous sulfate used in the actual operation is 2.5-3 times the theoretical calculation amount, so the amount of sludge is large.

The specific process is as follows:

Ferrous sulfate

↓

Electroplating wastewater→reduction reaction→PH neutralization→flocculation sedimentation→standard discharge

The basic principle is:

Cr2O72-+6Fe2++14H+=2Cr3++6Fe3++7H2O

Cr3++3OH-=Cr(OH)3↓

It can be seen from the above process that since the reduction of hexavalent chromium by ferrous sulfate is carried out under relatively acidic conditions, and the amount of sludge generated is large, it is also difficult to increase sludge disposal.

2. Removal of other metal ions

Cr6+ in electroplating wastewater exceeds national emission standards, and it also contains a large amount of metal ions such as Zn2+, Cu2+, Ni2+, Fe2+, etc. Therefore, the method of aeration oxidation under alkaline conditions can not only achieve the PH value, but also Effectively remove heavy metal ions from wastewater, the principle is:

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↓

The pH is first adjusted to overbased. Since the zinc ions start to precipitate at pH=6.4, respectively, it can be completely precipitated (2.0mg/l) at pH=9.3, and dissolves at pH=10.5. Therefore, it is divided into two-stage reaction, and the pH of the primary reaction cell must be controlled at 9.5. -10 range.

In the first-order reaction, Fe3+ ions can completely precipitate when PH=4.1, Cu2+ ions form a basic salt precipitate when PH=5.0, PH=7.2 can completely precipitate, Cr3+ ions start to precipitate at pH=4.9, and PH=6.8 It can be completely precipitated and begins to dissolve at pH=12. Since Ni2+ ions start to precipitate at pH=7.7, they can be completely precipitated at pH=10.5 (1.0mg/l), so Ni2+ and Fe2+ cannot be completely precipitated in the first-order reaction, so a secondary reaction is required in the second-stage aeration oxidation reaction. In the middle, 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 passed the test, but because the wastewater contains organic substances such as additives, the CODcr in the wastewater exceeds the standard. (The CODcr in the wastewater is partly produced by ferrous iron and partly by organic matter) After the two-stage precipitation, the CODcr value is about 200 mg/l, and the national standard is 100 mg/l. Therefore, after the two-stage precipitation pretreatment, the wastewater is treated by aerobic biochemical method to make it meet the national standard.

Electroplating additives are mainly divided into leveling agents, stress relieving agents, surfactants, brighteners, auxiliary brighteners, etc., mainly aldehydes, coumarins, saccharin and decomposition products, most of which are biodegradable substances. The aerobic biological treatment process is divided into: activated sludge method and biofilm method. The activated sludge process includes SBR and its improved type, AB method, etc.; biofilm method has contact oxidation method, biological filter and the like. Among them, SBR and its improved and biological contact oxidation method are the hottest processes in the international sewage biochemical treatment.

1, SBR and its improved

The SBR method is an abbreviation of the sequence batch type activated sludge method, and is an activated sludge water treatment technology which is operated by intermittent aeration method, and is also called a sequencing activated sludge method.

SBR wastewater treatment technology and its improved type are different from traditional sewage treatment technologies. It uses a time division operation instead of a spatial segmentation operation, an unsteady biochemical reaction instead of a steady-state biochemical reaction, and an ideal precipitation instead of a dynamic precipitation. It combines both sequential and intermittent operation on the run. The SBR method is a method in which an activated sludge treatment process is carried out in a single reaction tank, and the unit operation and time of the sewage treatment are continuously processed.

The SBR reaction tank is provided with a partition wall, and the reaction tank is divided into a pre-reaction zone and a main reaction zone, and the bottom of the wall is communicated with the hole. The influent, reaction, sedimentation, drowning and idle processes of each cycle are carried out in the same pool. The SBR process greatly simplifies the sewage treatment structure compared with other treatment processes.

a. During the aeration period, due to the aeration system supplying oxygen to the reaction tank, organic pollutants are oxidatively decomposed by microorganisms, and NH3-N is converted into NO3-N by nitrifying bacteria.

b. Stop the aeration during the sedimentation period, and separate the muddy water. At the same time, the microorganisms use the remaining dissolved oxygen in the water to oxidize and decompose. The reaction tank gradually transforms from an aerobic state to an anoxic state, and denitrification reaction begins.

c. After the sedimentation in the drowning period, the supernatant is drained from the water, and the water level in the pool gradually decreases. At this time, the reaction tank gradually transitions to an anaerobic state, and denitrification continues.

d. During the idle period of 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, the SBR process and its improved type have the following characteristics:

a, the process is simple, eliminating the precipitation, secondary sedimentation tank, sludge return and sludge reflux equipment.

b. The area is saved, which is 20%-30% lower than the common aeration method.

c. Low operating cost, high degree of automation control and convenient management. The oxygen absorption rate is high, and the running cost is 25%.

d. High processing efficiency, reliable operation stability, strong impact resistance, and good effluent quality.

e, nitrogen and phosphorus removal effect is good.

f, sludge sedimentation is good.

2. Biological contact oxidation method

The biological contact oxidation process is a biofilm process in which an activated sludge process is applied between biological filters. The contact oxidation tank is provided with a filler, and some microorganisms are fixed in the form of biofilm on the surface of the filler, and some are flocculent suspension growth and water. At this stage, the biological contact oxidation method is to set up a filler in the pool, so that the oxygenated sewage is immersed in all the fillers, and flows through at a certain speed. The filler is covered with biofilm, and there is also a certain activated sludge in the sewage. The sewage is in contact with the biofilm and the activated sludge, and the sewage is purified under the action of microorganisms. It can be said that the biological contact oxidation method is a biological treatment method which has the dual efficacy of activated sludge and biofilm between the activated sludge method and the biofilm method.

The biological contact oxidation process has the following characteristics:

a. The volume of the biological contact oxidation method is high, and the equipment of the same size and volume has a short processing time, high processing capacity, and saves floor space, which is less than the conventional aeration method.

b. The operation cost is saved, the degree of automation control is high, and the management is convenient. The oxygen absorption rate is high and no additional chemicals are needed.

c. The treatment efficiency is high, and the effluent water quality is good and stable. Under the impact of poison and PH value, the biofilm is less affected and recovers quickly.

d. It has stable operation stability and strong resistance to load and impact.

e, can effectively prevent sludge expansion, and can fully exert its characteristics of high decomposition and oxidation.

Biofilter process description:

The waste acid is periodically discharged into the waste acid pool. After the integrated electroplating wastewater and waste acid enter the regulation tank, the pump is lifted into the reduction tank, and the reducing agent is added by the dosing pump to reduce the hexavalent chromium to trivalent chromium under acidic conditions. After the reaction, it is left to enter the reaction cell, and the alkali reaction is carried out by the dosing pump. Most of the divalent iron ions, ferric ions, trivalent chromium, zinc ions, nickel ions and copper ions in the wastewater form a precipitate under alkaline conditions. The reacted wastewater enters the 1# sedimentation tank through the guide tube for precipitation separation.

The supernatant liquid flows into the aeration oxidation tank, further aeroxidizes, oxidizes the remaining ferrous ions to ferric ions, and forms iron hydroxide precipitate under alkaline conditions, and precipitates in the 2# sedimentation tank. After clearing the pH, the supernatant is removed from the CODcr by the contact oxidation biochemical pool and then enters the 3# sedimentation tank. The supernatant liquid is discharged after flowing into the clear water pool.

The sludge in each sedimentation tank enters the sludge concentration tank for concentration. After concentration, the sludge is injected into the filter press by high pressure pump, and the sludge is used as hazardous waste after being dried, and sent to the solid waste center for disposal.

Processing technology features:

a. Pretreatment of sewage by physical and chemical methods, effectively removing most of the heavy metals in the sewage, reducing toxicity and enhancing the biodegradability of the sewage.

b. Biochemical treatment adopts contact oxidation treatment process to effectively remove CODcr and reduce investment and operating costs.

c. The waste acid adopts the “regular discharge, daily treatment” treatment method, which avoids the excessive change of the water quality of the adjustment tank and affects the subsequent treatment process. The salt in the waste acid is high. If the treatment is one-time, the water body will have a sharp CL-concentration in a short time. Ascending, causing inhibition or even death of activated sludge.

d. Try to adopt gravity self-flow method to reduce the pump power, and use a reliable and efficient brand to reduce the consumption of chemicals. Through the above various methods, the operating cost of the sewage treatment system can be reduced to a large extent.