Introduction: The “Invisible” Major Source of Pollution in Pretreatment Before Coating
The pretreatment section of the painting workshop (degreasing → water rinse → pickling → surface conditioning → phosphating → water rinse — 6-8 processes)
——consumes >60% of the plant’s total water usage — generates >80% of the plant’s wastewater — >50% of the plant’s exhaust gas
——is the “pollution heart” of the painting factory
. (1) Degreasing waste liquid
——per vehicle produced — degreasing waste liquid >200-500L — COD >10000mg/L ”high-concentration organic wastewater” if directly discharged — the river’s DO (dissolved oxygen) drops sharply from >5mg/L to <1mg/L — fish suffocate — this is the ecological hazard of degreasing wastewater; (2) Phosphating wastewater
——PO₄³⁻ >100-500mg/L — discharged into water bodies — PO₄³⁻ is the ”trigger factor” of eutrophication, algal blooms in water bodies — cyanobacteria — water blooms
——China’s Taihu Lake (2007) ”cyanobacteria crisis” had phosphorous emissions from pretreatment as an important cause (nationwide >tens of thousands of painting lines — total phosphorous emissions >tens of thousands of tons/year); (3) Pickling exhaust gas
——HCl/H₂SO₄ acid mist ”white smoke” >100mg/m³ — health hazards to workshop and surrounding residents (respiratory irritation). The wastewater/exhaust gas treatment of painting pretreatment — is not an environmental project of ”icing on the cake” — it is the “survival bottom line” of legal compliance (pollutant discharge permit — no treatment = shutdown)
and the “green competitiveness” technical barrier (treatment cost — export to EU — REACH/drainage standards stricter)
.
Pre-treatment wastewater and exhaust gas treatment for coating is an environmental protection technical system that uses a three-stage series process of physical (oil skimming/sedimentation/dissolved air flotation/filtration), chemical (demulsification/flocculation/redox/lime precipitation) and biological (AO/MBR/anaerobic) methods—to treat the high-concentration degreasing waste liquid (COD>10000→500→<0.5mg/L), heavy metal-containing wastewater (Zn/Ni<1.0mg/L) and acidic exhaust gas (HCl/H₂SO₄<30mg/m³—NOx60% reuse rate—reducing fresh water consumption + lowering discharge fees).
I. Comparison of Chemical Treatment Technologies for the Three Major Pollution Sources
| Pollution Source | Main Pollutants | Concentration (Influent) | Primary Treatment | Secondary Treatment | Advanced Treatment | Discharge Standard (mg/L) |
|---|---|---|---|---|---|---|
| Degreasing Waste Liquid | COD/Animal and Vegetable Oils/Surfactants | COD>10000/Oil>5000 | Demulsification (Acidification/Calcium Salt) + Skimming——COD<3000 | Flocculation (PAC+PAM) + Sedimentation——COD<1000 | AO/MBR——COD<80 | COD<100 (Grade 1)/SS<70 |
| Phosphating Wastewater | PO₄³⁻/Zn²⁺/Ni²⁺/Mn²⁺ | TP>100-500/Zn>50-200 | Lime Milk Precipitation Ca₃(PO₄)₂+Zn/Ni(OH)₂——TP<5/Zn<5 | Flocculation + Sedimentation——TP<1/Zn<1 | Sand Filtration + Precision Filtration (5μm)——TP<0.5/Zn<0.5 | TP<0.5 (Surface Water Class IV)/Zn<2.0/Ni<1.0 |
| Pickling Exhaust Gas | HCl/H₂SO₄/NOx | >100 (HCl)/>240 (NOx) | Alkali Scrubber Tower (NaOH 5-10%)——HCl<30/H₂SO₄<20 | H₂O₂ Oxidation of NOx (NOx only) + Secondary Alkali Wash——NOx<200 | Activated Carbon Adsorption (Standby)——NOx<100 | HCl<100/H₂SO₄<45/NOx<240 (15m Exhaust Stack) |
FAQ
Q1: Why can “demulsification” of degreasing waste liquor be achieved by acid precipitation (pH<3) but not by alkali?
The emulsified oil in degreasing liquor (1-20μm oil droplets)——formed by anionic surfactants (LAS/SDS——containing -SO₃⁻ or -COO⁻——carrying negative charge) adsorbed on the oil droplet surface——forming an electric double layer——Zeta potential <-30mV——oil droplets repel each other——emulsion stable
——will not separate spontaneously. Acid precipitation——add H₂SO₄ until pH<3——(1) H⁺ reacts with the anionic surfactant's -SO₃⁻ → RSO₃H (uncharged——no surface activity——emulsifying ability lost)——the oil droplets' surface charge becomes zero——Zeta potential → 0——repulsive force disappears
——oil droplets spontaneously coalesce——stratify——float up (oil is lighter than water); (2) H⁺ compresses the electric double layer——further lowering the Zeta potential. Alkali (pH>10)——(1) OH⁻ increases negative charge——Zeta potential becomes more negative——emulsion more stable
. Calcium salts (CaCl₂/Ca(OH)₂)/iron salts——multivalent metal ions Ca²⁺/Fe³⁺ compress the electric double layer + form “insoluble soap” with anionic surfactants (RCOO⁻+Ca²⁺→(RCOO)₂Ca↓)
which equals “consuming” the surfactant——emulsifying power becomes zero——oil droplets coalesce.
Q2: Why is Ca(OH)₂ the most economical phosphorus removal agent in the “lime method” for phosphating wastewater?
Calcium phosphate precipitation——Ca²⁺+PO₄³⁻→Ca₃(PO₄)₂↓(Ksp≈2.0×10⁻²⁹——extremely low solubility——25°C——theoretical TP<0.01mg/L——but in practice due to pH/CO₃²⁻/interference from other ions——TP<0.5mg/L). Ca(OH)₂ (lime)—— phosphorus removal agent, lowest cost (compared to CaCl₂<1000/FeCl₃5-10 times advantage); (2) Lime simultaneously provides OH⁻——adjusts pH to >8.5——heavy metals (Zn/Ni) form hydroxide co-precipitation“phosphorus removal + heavy metal removal——completed in one step”
——simple process; (3) But the sludge volume produced by lime is large (Ca₃(PO₄)₂+CaCO₃+Zn(OH)₂——water-containing sludge >5-10% of treated water volume)
——dewatering and disposal costthe total cost of the lime method——still needs comprehensive evaluation (chemicals + sludge disposal)
. For extremely high PO₄³⁻ concentration (>1000mg/L)——first lime then——reduce TP to <10——then flocculation precipitation——this optimizes sludge volume.
Q3: Why is the removal rate of NOx low with alkaline spray for the “yellow smoke” from NOx pickling waste gas?
NOx in production—(1) NO (colorless—hardly soluble in water—does not react with alkali)—accounts for >90% of NOx; (2) NO₂ (reddish-brown—pungent—reacts with water to form HNO₃—2NO₂+H₂O→HNO₃+HNO₂—acid—absorbed by alkaline solution—NaNO₃—but this reaction requires NO₂/O₂/sufficient contact time
—alkaline spray (contact time <2s—NO₂ absorption rate 30%) in the “buffer tank” before alkaline spray—oxidize NO→NO₂→HNO₃—then alkaline spray—absorption >80%—total—NOx <100mg/m³ “Oxidation + alkaline washing” is currently the mainstream process route for NOx acidic waste gas treatment
; (2) High-temperature catalysis (SNCR/SCR/selective catalytic reduction—>300°C—in pickling waste gas <50°C—not applicable—only for high-temperature boiler waste gas).
Q4: What are the advantages and costs of MBR (Membrane Bioreactor) in coating wastewater treatment?
MBR——replaces the secondary sedimentation tank of the traditional activated sludge process with hollow fiber membranes (pore size <0.1μm/ultrafiltration grade)
——(1) Effluent SS——traditional sedimentation <30mg/L——MBR——<1mg/L——effluent can be directly reused by RO membrane——no longer at discharge standard level——but “reuse-grade” water quality——reuse rate breaks through >60%——becomes possible
；(2) The membrane completely retains microorganisms inside the reactor——MLSS (sludge concentration in bioreactor) increases from 3-5g/L (traditional) to >10-15g/L
——biochemical efficiency >2-3 times——COD<30mg/L——better than first-class discharge (COD1000 yuan/m² (hollow fiber); (2) Membrane fouling (microorganisms/colloids——transmembrane pressure rises——requires periodic chemical cleaning (acid wash + alkali wash——2-4 times per year)——operating cost——total system operating cost >5-8 yuan/m³. MBR is suitable for “coating factories with high discharge standards (surface water Class IV/COD60% water reuse”.
Q5: Wet electrostatic precipitator (WESP) for pickling exhaust gas — difference from ordinary spray tower?
WESP (Wet Electrostatic Precipitator) — (1) High-voltage DC (>30-50kV — discharge electrode/corona electrode) ionizes gas molecules — generates a large amount of electrons and negative ions — electrons attach to acid mist (micro-droplets <1μm) and fine particles — become negatively charged
; (2) Negatively charged particles/droplets move in the electric field — toward the collection electrode (positive electrode/pipe wall) — form a liquid film on the collection electrode — self-flow washing (wet type — self-cleaning — no dust accumulation)
— collected liquid returns to the spray tower. WESP achieves removal efficiency >95% for submicron (<1μm) acid mist and fine particles
— far exceeding ordinary spray towers (<50% for <1μm particles — inertial impaction efficiency extremely low). WESP secondary pollution — high-voltage electricity — O₃ generation (corona produces ozone — odor — requires alkaline solution absorption). WESP “fine treatment”
(final checkpoint — for high emission limits <30mg/m³) “coarse treatment”
(alkaline spray tower — front stage — removes >90% coarse droplets and >95% acidic gas) — WESP treats the remaining 500,000-1,500,000 RMB — used for high-end coating (automotive/aerospace) or near residential areas (sensitive sites).
Q6: Is “Zero Liquid Discharge” (ZLD) for painting wastewater economically feasible?
ZLD (Zero Liquid Discharge / zero discharge—no liquid wastewater discharge—all water reused—solid salt/sludge collected and shipped out)——(1) Pretreatment (demulsification/flocculation/sedimentation/biotreatment/filtration—COD→3000mg/L to >50000mg/L—concentrate—>20% of feed flow)——(3) Evaporation crystallization (MVR/multi-effect evaporation—evaporates water from RO concentrate—condensate reused (reuse rate >95%)—solid salt (mainly NaCl/Na₂SO₄—hazardous waste?—identification—not hazardous—general solid waste—shipped out)—achieves “zero discharge”. Economy of ZLD—RO+MVR—treatment cost >15-25 RMB/m³ (traditional treatment 3-5 times traditional treatment—currently only used for (a) extremely water-scarce/arid areas—no water to draw—forced to recover (Middle East/Xinjiang/Inner Mongolia) and (b) extremely strict discharge standards (COD<20/TP<0.3/—chemical treatment cannot meet standards "forced to reuse" using ZLD to replace discharge)
. General painting plants—wastewater >100m³/d—annual O&M cost of ZLD >1 million RMB—for thin-margin painting factories—economic feasibility needs detailed assessment “not recommended to pursue zero discharge priority ‘compliant discharge + partial reuse (>60%)’
“.
Q7: How to select a dewatering machine for “sludge dewatering” in the lime precipitation method?
Pretreatment sludge (mainly Ca₃(PO₄)₂/Zn(OH)₂/Fe(OH)₃/CaCO₃ — moisture content >98% — fluid slurry state) — dewatering — (1)Plate and frame filter press
— high pressure (>1.0MPa — hydraulic compression) — filter cake moisture 35% — can directly enter subsequent disposal (landfill) — no secondary water generated — but per batch operation — intermittent (requires manual plate pulling — high labor intensity) — suitable for <small-scale lines; (2)Belt filter press
— continuous operation (processing capacity >5t/h) — filter cake moisture large-scale lines (>50m³/d wastewater); (3)Horizontal screw centrifuge
— high speed (>3000rpm — C₃ centrifugal force) > plate frame — moisture 500-1000k RMB — high O&M cost (wear/power consumption — rotor — high power consumption) — suitable for “high-end — fully automatic — unattended” modern painting wastewater station. Sludge dewatering “plate frame = universal — belt = continuous — centrifuge = high-end
Q8: What is the main application scenario of “activated carbon adsorption” for pickling exhaust gas—can it replace alkaline spray scrubbing?
Activated carbon adsorption (granular activated carbon/honeycomb activated carbon—specific surface area >800m²/g—iodine value >800mg/g)—for HCl/H₂SO₄—(1) Activated carbon mainly relies on physical adsorption for acidic gases (van der Waals force)—weak binding—>low adsorption capacity (<5g HCl/100g activated carbon)
—quickly breaks through—frequent replacement/regeneration—extremely high cost
—(2) NOx—activated carbon—oxidation—catalysis—moderate effect (<70%—under high humidity, "water vapor" competes for adsorption—NOx breaks through earlier). Alkaline spray—chemical absorption—NaOH reacts with acidic gases—constant reaction—not easily broken through—cost (alkali solution—consumption <1kg/100m³ exhaust—<3 yuan/100m³). The role of activated carbon is “end-stage polishing” (after alkaline washing—activated carbon—removes residual trace VOCs (brought in by spraying exhaust—adsorption))—rather than replacing alkali solution—the two are used in series
—alkaline spray (acid mist/coarse removal)—activated carbon (VOC/fine removal).
Q9: In painting wastewater, “first-class pollutants” (Ni/Cr/Hg)——must meet standards at the workshop outlet——why can’t they be diluted to meet standards at the total outlet?
First-class pollutants (GB 8978——total mercury/total cadmium/total chromium/hexavalent chromium/total arsenic/total lead/total nickel/total silver——8 types in total——nickel——pretreatment——in nickel-containing phosphating solution——first-class)——(1) First-class pollutants are highly toxicand have low environmental background——any “dilution” will increase the total environmental load——irreversible
——therefore requiredto be treated to meet standards at the source (workshop outlet)
——dilution to meet standards is prohibited (connecting a water pipe at the total outlet——diluting the workshop discharge’s Ni “dilution” <1mg/L——then discharge——this is prohibited); (2) Sampling/monitoring at the workshop outlet——higher frequency“online real-time monitoring” Ni>1mg/L→automatically close the workshop drain valve——ensuring compliance from the workshop step onward
——this is the “highest control level” for first-class pollutants. Nickel-containing phosphating solution——replaced with “nickel-free phosphating solution” (zinc-based/zinc-manganese-based——low or no nickel)——eliminating first-class pollutants at the source——is the trend in painting pretreatment.
Q10: Why is the “emergency pool” at the wastewater station a mandatory standard design?
The pre-treatment line has >1-2 occurrences per year of tank liquid accidental discharge
——Degreasing/phosphating/pickling tanks entire tank (>10-30m³) of high-concentration chemicals discharged all at once
——(1) Its concentration (COD>50000mg/L/TP>5000mg/L/acidity pH<1 is >50-100 times the concentration of normal rinse water——direct entry into the biological system——microorganisms instantly “killed” and the biological system collapses——recovery takes 1-2 months
“The only threat to the biological system——’shock’ from the highest concentration waste liquid——all biological systems are designed with an ‘accident pool/equalization pool’”; (2) Accident pool volume——> capacity of one maximum tank liquid discharge (>50m³) + fire water volume (>2h flow rate)——total volume >100m³——store the accidental discharge temporarily in the emergency pool——then continuously “drip” into the treatment system at an “extremely slow rate” (<10% of normal flow) that microorganisms can tolerate
——This is called “homogenization and equalization”.
Related Reading
Summary
The three major pollutants in pre-treatment prior to coating—degreasing waste liquid (COD>10000: demulsification + flocculation + three-stage biological treatment), phosphating wastewater (TP>500: lime precipitation + flocculation + filtration—TP→<0.5 mg/L / Class I heavy metals—compliance at workshop discharge outlet), and pickling exhaust gas (HCl/NOx: alkaline scrubbing + oxidation)—each have targeted chemical treatment pathways. MBR and WESP are the "high-end weapons" for advanced treatment, and zero liquid discharge (ZLD) is economically feasible only under water scarcity or ultra-stringent discharge standards. Kexin New Materials provides customers with full-process technical support for process design and equipment selection of pre-treatment wastewater and exhaust gas—delivering complete compliant solutions from source to discharge.