Electroplating Wastewater: Segregated Collection and Separate Treatment under HJ 2002-2010

1. Establish the general rule first: segregated collection and separate treatment

HJ 2002-2010, Technical specifications for electroplating industry wastewater treatment, was approved by the former Ministry of Environmental Protection on 2010-12-17 and took effect on 2011-03-01 as a first release. It applies to the whole process management of electroplating wastewater treatment projects from scheme selection, engineering design, construction, acceptance to operation, and serves as the technical basis for environmental impact assessment, design and construction of environmental facilities, completion environmental acceptance and post completion operation. Clause 5.1.4 is the master rule of the whole document: electroplating wastewater shall be collected by category and treated according to its character. Pollutants that are monitored at the workshop or production facility outlet shall be collected and treated there; pollutants monitored at the general outlet shall be collected and treated at the general outlet; cyanide containing and chromium containing wastewater shall be collected and treated separately; and filter residues from plating solution filtration and spent plating solutions shall not enter the wastewater collection and treatment facilities. Clause 5.1.7 adds that electroplating sludge is a hazardous waste, to be handed to a qualified organisation for recovery, treatment or disposal, with on site temporary storage complying with GB 18597.

Clause 4.3.1 sets a three tier logic for the design flow: with measured data the design flow may be taken as 110 to 120 percent of the measured value; without measured data, survey data from analogous plants may be used; with neither, the treatment quantity may be estimated as 85 to 95 percent of the total water use of the plating workshop or line. Clause 5.1.8 is a mandatory requirement that is often missed: the wastewater treatment station shall be provided with an emergency accident tank sized to hold 12 to 24 hours of wastewater. Clause 5.1.6 requires an on line monitoring system at the general outlet complying with HJ/T 353, HJ/T 355 and HJ/T 212 - note that these three have been replaced by HJ 353-2019, HJ 355-2019 and HJ 212, so design documents should check the current versions as well.

2. Cyanide containing wastewater: concentration decides the process

Clause 6.2.1.2 routes by concentration: where the cyanide ion concentration is below 50 mg/L the alkaline chlorination process is preferred, and above 50 mg/L electrolysis is preferred. Ozone treatment imposes no limit on influent cyanide concentration, but wastewater containing complexed cyanide is not suitable for ozone. Clause 6.2.1.3 requires that iron and nickel ions be kept out of the cyanide stream, and clause 6.2.1.5 warns that small amounts of CNCl gas may form, requiring sealed ventilated operation with the collected gas treated before discharge through a stack.

2.1 Alkaline chlorination (clause 6.2.2.4)

• The oxidant dose shall be determined by testing; where testing is impossible it is calculated from the mass ratio of cyanide ion to active chlorine: 1 to 3 up to 1 to 4 for first stage oxidation and 1 to 7 up to 1 to 8 for second stage oxidation, with the two stages dosed separately in a 1 to 1 ratio;
• First stage pH 10 to 11 with a reaction time of 10 to 15 minutes; second stage pH 6.5 to 7.0 with a reaction time of 10 to 15 minutes;
• ORP control is permitted: the first stage reaction is essentially complete when ORP reaches 300 mV, and the second stage requires ORP of 650 mV;
• Wastewater temperature preferably 15 to 50 degrees C, with residual chlorine after reaction in the range of 2 to 5 mg/L.

2.2 Ozone oxidation (clause 6.2.3.2)

Theoretical dose mass ratios are m(CN-) to m(O3) of 1 to 1.85 for first stage and 1 to 4.61 for second stage oxidation, with the actual dose necessarily above the theoretical value and determined by experiment. For free cyanide, a contact time of not less than 15 minutes is required for 97 percent removal and not less than 20 minutes for 99 percent removal. pH should be held at 9 to 11, and off gas from the reaction tank must be collected and discharged after alkaline absorption. The text notes that using cuprous ion as a catalyst shortens the reaction time.

2.3 Electrolysis (clause 6.2.4.2)

pH preferably 9 to 10, adjusted with NaOH solution; the NaCl dose may be estimated at 30 to 60 times the cyanide concentration; the net electrode gap of the cell is preferably 20 to 30 cm; the anodic current density is 0.3 to 0.5 A per square decimetre with a cell voltage of 6 to 8.5 V; with air agitation the air rate is 0.1 to 0.5 m3 per minute per cubic metre at a pressure of 0.5 to 1.0 times 10 to the power 5 Pa. A coagulant may be added where the precipitate settles poorly.

3. Chromium containing wastewater: reduce Cr(VI) first, then precipitate Cr(III)

Clause 6.3.1.1 requires chromium containing wastewater to be collected alone without mixing, and states that only after hexavalent chromium has been reduced to trivalent chromium may it be mixed with other heavy metal wastewater. Clause 6.3.1.3 stipulates that for ion exchange treatment of chromium plating rinse water the hexavalent chromium should not exceed 200 mg/L, and that rinse water from black chromium plating and fluoride containing chromium plating is not suitable for ion exchange.

3.1 Sulfite reduction (clause 6.3.2.2 and Table 1)

• For batch treatment the equalisation tank volume is sized on 4 to 8 hours of average hourly flow; continuous systems may use a smaller tank with automatic monitoring and dosing;
• Influent pH preferably 2.5 to 3.0, ORP preferably 230 to 270 mV, reaction time 20 to 30 minutes;
• Dose ratios of Cr(VI) to sulfite: sodium bisulfite theoretical 1 to 3 and practical 1 to 4 to 5; sodium sulfite theoretical 1 to 3.6 and practical 1 to 4 to 5; sodium metabisulfite theoretical 1 to 2.74 and practical 1 to 3.5 to 4;
• After reduction, add alkali to pH 7 to 8 to precipitate trivalent chromium, with a reaction time above 20 minutes and a settling time of 1.0 to 1.5 hours;
• The reaction tank should use mechanical, not air, agitation, and reaction and sedimentation tanks are preferably set at ground level, covered and ventilated.

3.2 Ferrous sulfate and lime process (clause 6.3.3.2 and Table 2)

Banded by hexavalent chromium concentration: below 25 mg/L, adjust pH to 2 to 3 before dosing, dose ratio 1 to 40 to 50, final pH 7.5 to 8.5, mixing only; from 25 to 50 mg/L, ratio 1 to 35 to 40 with 5 to 10 minutes of mixing; from 50 to 100 mg/L, ratio 1 to 30 to 35 with 10 to 20 minutes; above 100 mg/L, ratio 1 to 30 with 20 minutes. Continuous treatment requires a reaction time above 30 minutes and batch treatment 2 to 4 hours. The lime dose is preferably m(Cr(VI)) to m(Ca(OH)2) of 1 to 8 to 15.

3.3 Micro electrolysis and ion exchange

Micro electrolysis (clause 6.3.4.2): continuous operation is permissible at 5 m3/h or more, batch operation is preferred below 5 m3/h; influent pH preferably 2 to 4 and the effluent must be adjusted with alkali to 8 to 9; iron filings must be cleaned of impurities, oil and rust before packing, backwashed periodically with combined air and water, and kept submerged during maintenance or shutdown to prevent oxidation and caking. Ion exchange (clause 6.3.5.2): influent hexavalent chromium preferably not above 200 mg/L; pH into the anion column shall be kept below 5; the anion column regenerant is preferably industrial sodium hydroxide with regenerant and rinse water prepared from demineralised water, and the rinse endpoint pH is 8 to 10; the cation column regenerant is preferably industrial hydrochloric acid with a rinse endpoint pH of 2 to 3; the hexavalent chromium fraction of the resin regeneration eluate must be returned to the equalisation tank.

4. Heavy metal wastewater: treatment sequence and metal specific parameters

Clause 6.4.1.1 fixes an order that must not be reversed: where cyanide is present, remove cyanide first; where hexavalent chromium is present, reduce it to trivalent chromium before removing other heavy metal ions. Clause 6.4.1.2 requires that ion exchange for one class of heavy metal must not be contaminated with other plating streams or floor wash water, with an influent ion concentration preferably not above 200 mg/L, and all backwash and rinse water from filter and exchange columns must return to the electroplating wastewater system. Clause 6.4.1.3 states that reverse osmosis treatment requires disinfection and scale control pretreatment, and that the concentrate must pass through a biological treatment system before discharge.

• Cadmium (clause 6.4.2.1.3): cadmium ion preferably not above 50 mg/L; polyferric sulfate may be used as coagulant with polyacrylamide or iron sulfide as coagulant aid, at a coagulant dose of preferably 40 mg/L; reaction pH around 9, reaction time 10 to 15 minutes, settling time above 30 minutes.
• Nickel (clauses 6.4.3.1, 6.4.3.2.2, 6.4.3.3.2): chemical precipitation with sodium hydroxide at a reaction pH above 9 for not less than 20 minutes with mechanical agitation; ion exchange influent nickel preferably not above 200 mg/L, with cation exchangers put into service in the sodium form; weak acid resin shrinks to a volume ratio of 0.5 to 0.6 when converted from the sodium form to the nickel or hydrogen form, so sufficient freeboard must be left above the resin bed; a filter column is required where influent suspended solids exceed 10 mg/L; for reverse osmosis, two stage or higher counter current rinsing is mandatory, influent residual chlorine must be below 0.1 mg/L (removed by granular activated carbon or sodium bisulfite with ORP monitoring), and the concentrate may be returned directly to the nickel plating bath.
• Copper (clause 6.4.4.1.2): for ion exchange of cyanide copper and copper tin alloy wastewater, total influent cyanide should not exceed 100 mg/L.

5. Sludge and the parameters the standard does not give

Chapter 7 is short on numbers but decisive: clause 7.1.1 restates that electroplating sludge is a hazardous waste, that recovery of the heavy metals it contains takes priority, and that where recovery is not possible it must be transferred under the hazardous waste manifest system to a qualified organisation. Clause 7.2.2 gives moisture contents to be used where no test data exist: 99.5 to 98.0 percent for sludge discharged from the sedimentation tank in the lime process, and 98 to 96 percent after thickening. Clause 7.2.3 requires the hopper wall to make an angle of 55 to 60 degrees with the horizontal for hopper sludge withdrawal, with a separate pipe and valve for each hopper in multi hopper designs. For acid and alkali neutralisation (clause 6.1.4) the dry sludge quantity should be determined by testing, and in the absence of testing may be estimated at 0.1 to 0.25 percent of the treated wastewater volume.

Two caveats must be stated plainly. First, this specification does not give universal volumetric loadings, surface loadings or retention times for the individual unit tanks; the figures quoted above are reaction conditions - pH, ORP, time and dose ratios - and tank volumes and loadings still have to be calculated from flow and water quality. Second, Annex A, which lists the sources, main constituents and concentration ranges of electroplating wastewater, is an informative annex; it may be used for reference only where measured data are absent and cannot substitute for measurement.