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pH neutralization and chemical feed

Paste the lab titration curve or use the theoretical method to see how much chemical to feed, what flow the metering pump needs and what it costs each year to keep pH within the limits.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

Preliminary estimate
Wastewater
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Neutralizing chemical
—Commercial chemical use
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Titration curve and target pH

Wastewater pH versus reagent dose, in meq per liter of wastewater. Dashed lines: country discharge range and target pH.

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Annual cost versus target pH

What it costs to neutralize to each pH with the chemical and price entered. Aiming beyond what is needed raises cost.

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Method, assumptions and references

Equations used

Titration curve: dose (eq/L of wastewater) = mL of titrant at target pH × normality ÷ sample volume (mL); mL at target pH by linear interpolation between readings
Theoretical (strong acid and base): acidity = 10⁻ᵖᴴ − 10^(pH − 14); dose = |acidity(initial pH) − acidity(target pH)|; pH after dosing: [H⁺] = (a + √(a² + 4 Kw)) ÷ 2
Pure chemical (kg/h) = Q (m³/h) · dose (eq/L) · equivalent weight (g/eq): NaOH 40.00; Ca(OH)₂ 37.05; H₂SO₄ 49.04; HCl 36.46; commercial = pure ÷ strength; volume = commercial ÷ density
Lime slurry: density = 1 ÷ ((1 − x) + x ÷ 2.24), x = Ca(OH)₂ mass fraction; annual cost = commercial (kg/h) · hours · price

Assumptions and limits

  • The titrated sample represents the wastewater; lab and plant chemicals react the same per equivalent (titrate with the chemical you will feed). Constant flow and composition: with swings, use an equalization tank and automatic control.
  • The theoretical method ignores alkalinity, weak acids, metals and CO₂; lime dissolves slowly and may need 10 to 30 min of reaction time.
  • Discharge ranges: NPDES permits (6 to 9) in the U.S.; CONAMA 430/2011 (5 to 9) in Brazil; Real Decreto 849/1986 (5.5 to 9.5) in Spain.

Standards and technical documents

  • APHA, AWWA, WEF. Standard Methods for the Examination of Water and Wastewater, 2310 Acidity and 2320 Alkalinity
  • Metcalf & Eddy / AECOM. Wastewater Engineering: Treatment and Resource Recovery, 5th ed., McGraw-Hill, 2014
  • Eckenfelder, W. W. Industrial Water Pollution Control, 3rd ed., McGraw-Hill, 2000
  • Shinskey, F. G. pH and pIon Control in Process and Waste Streams, Wiley, 1973
  • Perry's Chemical Engineers' Handbook, 9th ed.: densities of acids and caustic

U.S. regulations

  • EPA 40 CFR 403.5(b)(2): no discharge to a POTW below pH 5.0 unless designed for it; local pretreatment limits
  • EPA 40 CFR 401.17: pH effluent limits under continuous monitoring; NPDES permits under 40 CFR 122
  • EPA 40 CFR 261.22: RCRA corrosivity characteristic (pH ≤ 2 or ≥ 12.5); elementary neutralization units
  • OSHA 29 CFR 1910.1200 (Hazard Communication), 1910.132 (PPE) and 1910.151(c) (eyewash and shower)

Frequently asked questions

How do I calculate caustic dose for wastewater neutralization?

Titrate a 100 mL sample with 0.1 N caustic, recording pH. If pH 7.5 occurs at 9.33 mL, the dose is 9.33 × 0.1 ÷ 100 = 9.33 meq/L, or 373 g of NaOH per m³ (3.1 lb per 1,000 gal). For 100 gpm that is about 18.7 lb/h of pure NaOH, or about 900 lb/d of 50% caustic.

Why not calculate from initial pH alone?

pH measures only free H⁺. Wastewater at pH 2.8 has 1.6 meq/L of free acidity, but weak acids, metal salts and buffers can call for 9 meq/L or more. The titration curve shows the real dose.

What pH can I discharge?

NPDES permits usually require 6.0 to 9.0. Discharges to a POTW below pH 5.0 are prohibited by 40 CFR 403.5(b)(2) unless the plant is designed for them, and local limits often apply; waste at pH 2 or below, or 12.5 or above, is RCRA corrosive.

Caustic or lime: which is cheaper?

Lime usually costs less per equivalent, but it makes more sludge, scales lines and reacts slowly. Caustic is clean and fast; at small flows the cost difference matters little.