Automated pH monitoring and dosing system

01

Set the pH target and size the neutralisation tanks

In the US, 40 CFR 403.5(b)(2) prohibits introducing into a publicly owned treatment works pollutants that cause corrosive structural damage and, in any case, discharges with pH lower than 5.0 unless the works is specifically designed to accommodate them; local limits and permits can add further requirements, so the outlet target comes from the permit for the site. [1]

Faanes and Skogestad (NTNU, Computers and Chemical Engineering 28, 2004) give control as a reason for carrying out neutralisation in a series of mixing tanks with gradual concentration change. Delays from mixing, measurement and valve or pump response limit how fast feedback can act; they assume an effective delay of 10 s in each tank. For a flow of 0.01 m³/s and a disturbance gain of 2.5 × 10⁶ (strong acid to pH 7 ± 1), their reference total volume falls from 250,000 m³ with one tank to 316 m³ with two, 40.7 m³ with three, 15.9 m³ with four and 9.51 m³ with five; they recommend identical tanks with a total volume of twice this reference value and advise keeping measurement delay as small as possible. [2]

Editorial calculation: doubling the paper's reference values gives about 632 m³ for two tanks and 81 m³ for three at 10 L/s; 81 m³ at 0.01 m³/s is a residence time of about 8,100 s (2.25 h). The numbers apply to the paper's strong acid case; ask the designer to calculate the disturbance gain, delay and tank count for your stream, buffering and reagent. [2]

Editorial note: weak acids, bases and buffering chemicals change the titration curve; give the supplier a titration curve of the real stream with the chosen reagent.

Give the supplier the stream flow (normal and peak), inlet pH range and how fast it changes, titration curve with the chosen reagent, outlet pH limit and permit, reagents and concentrations, temperature, solids and fouling, tank sizes available, hazardous area, power supply, outputs and data logging needed, and wetted-material requirements.

02

Check where the outlet set point may move

The outlet set point comes from the discharge route. For a US sewer connection the federal floor is pH 5.0. [1] For direct discharge, Meng and Jiao (Xiamen University, The Innovation, 2025) reviewed effluent standards across Asia, the Americas, Europe and the Global South and report lower pH limits set at 6.0, or lower in some instances, against natural seawater at about pH 8.1. They calculate that wastewater at pH 6.0 holds about 158 times more hydrogen ions than seawater at pH 8.2, and they propose a minimum effluent pH above 8.0 for marine outfalls, which would cut hydrogen ion concentration about 100-fold relative to pH 6.0. [4]

Editorial note: the proposal is a policy perspective, not an adopted rule, but it shows why a dosing system should allow the set point and alarm limits to be changed without hardware changes. In Faanes and Skogestad's method, the required volume per tank is proportional to flow, loop delay and the scaled disturbance gain kd raised to the power 1/n, and kd scales the outlet by the maximum tolerated effluent concentration, so ask the supplier to re-run tank sizing for any new set point. [4] [2]

03

Choose the controller, dosing pump and outputs

Hanna Instruments' BL7916 pH controller and pump combines measurement and dosing in one unit: 0.00–14.00 pH range, 0.01 pH resolution, ±0.01 pH accuracy, 10¹² Ω input impedance, proportional dosing selectable for acid or base that slows the pump as pH approaches the set point to avoid over-dosing, an alarm contact that operates when pH is more than 2 pH units from the set point, auxiliary contacts for a mixer or priming pump, and an isolated 4–20 mA recorder output; listed pump flow rates run from 13.3 down to 7.8 L/h, at 0–50 °C and up to 95% RH. [3]

Faanes and Skogestad show that local PI or PID control in each tank can hold the outlet within limits when the tanks are sized for the loop delay; they prefer adding a single reagent type where possible to save equipment, which requires a nominal reagent flow and different pH set points in each tank. [2]

Editorial note: the pump's listed flow falls across its five values, so ask the supplier for the flow at your injection back-pressure and confirm that the maximum reagent demand at peak flow and worst-case inlet pH stays inside the pump's range.

04

Sourcing the controller, pump and tanks

No dosing-system maker is linked to this page yet. The Hanna BL7916 figures cited above describe one controller-and-pump unit; the mixing tanks, mixers and reagent storage listed below are separate items to specify. [3] [2]

05

Electrode, controller, pump, tanks and recorder output

The mixing tanks are the row the Faanes and Skogestad sizing changes; the instrument rows follow the Hanna unit. [2] [3]

06

The outlet pH limit drives tank count and pump flow

Tank count and loop delay come from one NTNU worked case at a single flow and disturbance, so ask for sizing on your own flow and titration curve. [2]

07

Send the stream, limit and site data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Stream: [flow; inlet pH range; titration curve]
Limit: [outlet pH; permit]
Reagent: [type; concentration]
Tanks: [number; volume; mixers]
Control: [mode; alarms; outputs]
Site: [power; area; materials]

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