9.3 Physical/Chemical Industrial Wastewater Treatment

9.3 Physical/Chemical Industrial Wastewater Treatment

South Carolina licenses Physical/Chemical Wastewater Treatment Operators as a discipline entirely separate from biological wastewater, with its own Trainee through Class A ladder and its own facility groups I-P/C through IV-P/C (Section 1.3). The Board's designated references are Sacramento State's Industrial Waste Treatment and Treatment of Metal Wastestreams. If you are sitting the P/C exam, this material is your core.

The defining characteristic: treatment happens by chemistry and physics, not biology. Wastestreams here are frequently toxic to microorganisms, too concentrated, too variable in pH, or simply not biodegradable.

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1. Equalization and Neutralization

Equalization

Industrial discharge is batch-driven — a plating line dumps a tank, a rinse cycle ends, a shift changes. Without equalization, downstream chemical feed cannot keep up. An equalization basin with adequate mixing dampens both flow and concentration swings and is often the difference between a functioning P/C plant and a chaotic one.

Neutralization

Most subsequent chemistry is pH-dependent, so pH control comes first.

The control challenge: the pH titration curve near neutrality is extremely steep, so a tiny reagent overshoot swings pH several units. Practical control uses multi-stage neutralization — a coarse stage followed by a trim stage — with adequate mixing and probes placed where they see fully mixed liquid, never immediately at the injection point.

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2. Chemical Precipitation of Metals

The workhorse of industrial metals treatment: raise pH so dissolved metal ions form insoluble metal hydroxides that can be coagulated, settled, and dewatered.

$\text{M}^{2+} + 2\text{OH}^- \rightarrow \text{M(OH)}_2 \downarrow$

The critical concept — each metal has its own optimum pH

Amphoteric behavior is the trap. Metals such as zinc, lead, chromium, and aluminum redissolve if pH goes too high — their solubility curve is a U shape with a minimum, not a downward slope. An operator who reasons "more caustic means more removal" will drive zinc back into solution and violate the permit.

Where a wastestream carries several metals with incompatible optima, options are:

• Operate at a compromise pH accepting partial removal of each,
• Use staged precipitation at two different pH values with clarification between, or
• Use sulfide precipitation — metal sulfides are far less soluble than hydroxides across a wider pH range, at the cost of handling sulfide reagents and the risk of hydrogen sulfide generation if pH falls.

Chelating agents such as EDTA, ammonia, and citrate hold metals in solution and defeat hydroxide precipitation entirely. They must be broken with oxidation or handled with sulfide, and the real answer is usually source segregation.

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3. Oxidation and Reduction — The Two Classic Sequences

Hexavalent chromium reduction

Cr⁶⁺ is highly toxic and does not precipitate as a hydroxide. It must first be reduced to Cr³⁺:

1. Lower pH to about 2–3 with sulfuric acid.
2. Add a reducing agent — sodium metabisulfite, sodium bisulfite, or sulfur dioxide.
3. Cr⁶⁺ → Cr³⁺, monitored by ORP (oxidation-reduction potential).
4. Raise pH to about 8–9 to precipitate Cr(OH)₃.
5. Clarify and dewater.

The exam point: reduction happens at LOW pH, precipitation at HIGH pH. Reversing them accomplishes nothing.

Cyanide destruction — alkaline chlorination

1. Raise pH above 10 — this is a safety-critical step.
2. Add chlorine or hypochlorite. Cyanide is oxidized to cyanate (CNO⁻).
3. Optionally continue at pH ~8–8.5 to oxidize cyanate to carbon dioxide and nitrogen.
4. Then proceed to metals precipitation.

Deadly hazard: if pH drops below about 8.5 while cyanide is present, hydrogen cyanide (HCN) gas is released. HCN is rapidly lethal. Alkaline pH must be established before any chlorine is added and maintained throughout, with pH alarms and interlocks. Never store or feed acid and cyanide-bearing waste where a mixing accident is possible, and never mix cyanide and acid wastestreams.

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4. Physical Separation Processes

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5. Residuals

Physical/chemical treatment generates chemical sludge, not biosolids. It is typically thickened, then dewatered by filter press, belt press, or centrifuge. The critical difference from municipal biosolids is regulatory: metal-bearing sludge is frequently a hazardous waste subject to RCRA characterization by toxicity characteristic leaching procedure (TCLP) testing, and disposal costs are far higher. Lime-based precipitation produces substantially more sludge than caustic, which is a real operating cost trade-off. Reducing sludge volume by segregating clean streams at the source is nearly always cheaper than treating and disposing of it.