⚗️ pH of sewage sludge — potentiometric measurement with a glass electrode in liquid sludge and in a suspension of dewatered sludge

Physicochemistry PN-EN 12176

Overview

The pH of sludge decides the hygienisation with lime, the solubility of metals and the course of the digestion, and for that reason it is one of the first determinations in every set of tests of sludge. PN-EN 12176 describes the potentiometric measurement; the standard has been withdrawn by PKN, and its role is taken over by ISO 10390:2021 (the pH of soil, sludge and treated biowaste in a 1:5 suspension) and ISO 10523:2008 (the pH of water and of liquid sludge).

The physical basis is the same in both approaches. ISO 10523 formulates it thus: "The determination of the pH value is based on measuring the potential difference of an electrochemical cell using a suitable pH meter", and adds that the pH of the sample depends on the temperature because of the dissociation equilibrium, and for that reason the temperature is always given together with the result. The scope of ISO 10523 covers pH from 2 to 12 at an ionic strength below I = 0.3 mol/kg and concerns among other things liquid sludge.

For solid or dewatered material a suspension is used. ISO 10390:2021 (extended in this edition precisely to sludge and treated biowaste) describes the measurement of pH in the range 2–12 with a glass electrode in a 1:5 (volume fraction) suspension of the sample in water (pH in H₂O), in a solution of potassium chloride 1 mol/l (pH in KCl) or in a solution of calcium chloride 0.01 mol/l. The suspension is prepared from the test portion and five times the volume of the chosen solution, and the pH is measured with a pH meter after the instrument has been calibrated.

Practical notes: the result depends on the solution used — the pH in KCl and in CaCl₂ is usually lower than the pH in water, so the test report has to state the variant unambiguously. The electrode in sludge suspensions has to be cleaned often (a film of organic substances and fats falsifies the reading), and the calibration buffers must cover the expected range. The measurement is carried out as soon as possible after sampling — the pH of sludge changes with the progress of the digestion and the escape of ammonia.

Method principle

The determination of pH consists in measuring the potential difference of an electrochemical cell composed of a glass indicator electrode and a reference electrode, with the use of a pH meter. Because the dissociation equilibrium depends on the temperature, the pH of the sample is a function of the temperature and is always given together with the temperature of the measurement. Liquid sludge is measured directly, under conditions close to those of water (pH range 2–12, ionic strength below 0.3 mol/kg). Dewatered sludge or solid material is prepared as a suspension: the test portion is covered with five times the volume of water, of a solution of potassium chloride 1 mol/l or of calcium chloride 0.01 mol/l, stirred for an established time and the pH is measured in the suspension. Before the measurement the pH meter is calibrated with buffer solutions covering the expected pH range, and the electrode is rinsed between measurements. The result is given to one decimal place together with a statement of the solution in which the suspension was made and of the temperature at which the pH was measured.

Applications

• Hygienisation of sludge with lime — control of reaching and maintaining the required pH
• Methane digestion — pH as an indicator of the acidification of the chamber and of the risk of the process stopping
• Sludge intended for agricultural use — the pH influences the availability of metals and the suitability for the soil
• Dewatered sludge and screenings — measurement in a 1:5 suspension according to ISO 10390
• Control of conformity with the conditions of the handing over of the sludge to the receiver (the documentation of the testing of the sludge)

Key parameters

Standard

Standard number

PN-EN 12176:2004 (EN 12176:1998)

Title (PL)

Charakterystyka osadów ściekowych — Oznaczanie wartości pH

Title (EN)

Characterization of sludge — Determination of pH value

Step-by-step procedure

1. Sampling and preparation

   Take a representative sample of the sludge and measure the pH as soon as possible after sampling. Mix liquid sludge before the measurement; homogenise dewatered sludge and prepare it for making the suspension.

2. Calibration of the pH meter

   Calibrate the instrument with buffer solutions covering the expected pH range, at a temperature close to the temperature of the samples. Note the slope of the characteristic of the electrode — a fall below the value recommended by the manufacturer means that the electrode has to be regenerated or replaced.

3. Liquid sludge — direct measurement

   Immerse the electrode in the mixed sample, read the pH after the reading has stabilised and at the same time note the temperature.

4. Dewatered sludge — 1:5 suspension

   Measure out the test portion and cover it with five times the volume of the chosen solution (water, KCl 1 mol/l or CaCl₂ 0.01 mol/l), stir it for the established time and leave it to reach equilibrium in accordance with the procedure.

5. Measurement in the suspension

   Immerse the electrode in the suspension (with continuous or interrupted stirring — in accordance with the procedure adopted, consistently for all the samples) and read the pH after the reading has stabilised, together with the temperature.

6. Rinsing and checking

   Between measurements rinse the electrode with water and dry it with filter paper without rubbing the membrane. After a series of measurements in sludge clean the electrode with a cleaning solution. Check the calibration periodically with a control buffer.

7. Recording of the result

   Give the pH to one decimal place, the temperature of the measurement and the variant of the preparation (direct measurement in liquid sludge or a 1:5 suspension in water, KCl or CaCl₂) — without this information the result is not comparable.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Sewage sludge is infectious material — gloves, a laboratory coat, avoidance of aerosols and of contact with the face; disinfection of the working place after the work.
• Sludge after hygienisation with lime is strongly alkaline and corrosive to the skin and the eyes — protect the eyes when stirring.
• Glass electrodes are fragile — a cracked membrane threatens injury and contact with the electrolyte.
• Use cleaning solutions with hydrochloric acid under a fume hood, in accordance with the instructions of the manufacturer of the electrode.
• Do not store the electrode in distilled water — the washing out of the electrolyte shortens its life and spoils the measurement.

Frequently asked questions