11.2 Physical/Chemical Treatment Processes for Industrial Wastewater

11.2 Physical/Chemical Treatment Processes for Industrial Wastewater

North Carolina's Physical/Chemical needs-to-know covers general operation, laws and regulations, safety, laboratory and sampling, mathematics, and then a long list of unit processes. 08G .0306 sets the classification: a physical process alone is Grade I; a chemical process — including reverse osmosis, electrodialysis, and ultrafiltration — is Grade II. Visitation under 08G .0204(2)(h) is weekly for Grade I and five days per week for Grade II.

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1. Equalization and neutralization

Flow and load equalization is the first unit process in most industrial plants: a mixed, often aerated basin that dampens batch dumps, shift changes, and pH swings so downstream chemistry sees a steady feed. Sizing is based on the diurnal or batch pattern; mixing prevents settling and septicity.

Neutralization adjusts pH toward the range the next process requires:

• Acids: sulfuric (cheap, but forms calcium sulfate scale with lime), hydrochloric (no scale, more corrosive), carbon dioxide (self-limiting near pH 6–7, cannot overshoot).
• Bases: sodium hydroxide (fast, no sludge added, hazardous), lime (cheap, adds sludge, slower), magnesium hydroxide (self-limiting near pH 9, low sludge, safer to handle), soda ash.
• Control: titration curves are steep near neutrality, so single-stage control overshoots. Two- or three-stage neutralization with cascaded pH control is standard for wide-swing waste.

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2. Metal precipitation

Dissolved metals are converted to insoluble hydroxides, sulfides, or carbonates and then removed as solids.

   M(2+)  +  2 OH(-)  ->  M(OH)2 (solid)

The controlling variable is pH, and each metal has its own minimum solubility pH:

Two practical consequences. First, a mixed-metal stream has no single optimum pH, so the operator either selects a compromise or treats sequentially. Second, for amphoteric metals, raising pH too far redissolves the metal — the effluent metal concentration goes back up even though more caustic was added. Sulfide precipitation produces far less soluble metal sulfides and can reach lower effluent concentrations, but it must be carefully controlled to avoid generating hydrogen sulfide gas.

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3. Chromium reduction and cyanide destruction

Hexavalent chromium reduction (a two-step sequence):

1. Lower pH to about 2–3 with acid.
2. Add a reducing agent — sulfur dioxide, sodium bisulfite, or sodium metabisulfite — to convert Cr(VI) to Cr(III), monitored by ORP.
3. Raise pH to 8–9 to precipitate chromium hydroxide.

Alkaline chlorination of cyanide (also two stages):

1. Stage 1: at pH above 10, chlorine oxidizes cyanide to cyanate. High pH is not optional — at lower pH the reaction produces cyanogen chloride, a highly toxic gas.
2. Stage 2: at pH 8–9, additional chlorine oxidizes cyanate to carbon dioxide and nitrogen.

Both sequences are ORP and pH controlled, and both are classic Grade II Physical/Chemical exam material.

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4. Solids and oil separation

Ash sedimentation basins, listed separately in the North Carolina needs-to-know, settle ash sluice water at combustion facilities and are operated like any other settling basin with attention to abrasion and solids removal.

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5. Dissolved constituent removal

• Granular activated carbon (GAC): adsorbs dissolved organics. Vessels are commonly run in series (lead/lag) with an intermediate sampling port so breakthrough on the lead vessel is detected before it reaches the effluent. Spent carbon is regenerated off site or replaced. Carbon vessels can go anaerobic and generate hydrogen sulfide if left stagnant.
• Air stripping: packed towers transfer volatile organics from water to air; efficiency depends on the air-to-water ratio, temperature, and the compound's Henry's constant. Off-gas may require vapor-phase carbon.
• Ion exchange: cation and anion resins remove specific ions; regeneration produces a concentrated waste brine that becomes its own disposal problem.
• Membranes: ultrafiltration for macromolecules and emulsified oil; reverse osmosis for dissolved salts and metals. Both concentrate the contaminant into a reject stream that must be treated or hauled.

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6. Operating discipline in a chemical plant

1. Jar test before you dose. Metal hydroxide chemistry is waste-specific; a jar test with pH stepping identifies the true optimum rather than the textbook value.
2. Trust instruments only after verification. pH and ORP probes drift and foul; calibrate on a schedule and confirm with a bench meter.
3. Respect incompatibilities. Acid and hypochlorite together release chlorine gas; sulfide-bearing waste plus acid releases hydrogen sulfide; cyanide plus acid releases hydrogen cyanide. Segregate storage, label lines, and design for the worst case.
4. Watch the sludge. Metal hydroxide sludge is voluminous and may be a hazardous waste depending on the process and characteristics — which determines the disposal route and cost.
5. Batch treat when in doubt. A batch tank that can be tested, adjusted, and released only when in specification is the most forgiving arrangement for variable waste.

[!WARNING] The overshoot trap. If effluent zinc, lead, or trivalent chromium rises after a caustic increase, do not add more caustic. Those metals are amphoteric: above roughly pH 10.5 they redissolve as soluble complexes. The correct action is to bring pH back down into the minimum solubility window and re-test.