⚗️ BOD5 in undiluted samples — measurement of dissolved oxygen before and after 5 days of incubation at 20 °C (oxygen probe)

Physicochemistry PN-EN 1899-2

Overview

PN-EN 1899-2:2002 (which implements EN 1899-2:1998, based on a modified ISO 5815:1989) describes the determination of the biochemical oxygen demand after n days without diluting the sample; part 1 (PN-EN 1899-1:2002) is the method of dilution and seeding with the addition of allylthiourea. The BOD value is an important parameter of water management: it defines the quality of the water and the efficiency of the biological treatment of wastewater, it is needed when designing and sizing treatment plants, and it is routinely determined at the inlet and the outlet of a treatment plant. In the methods according to EN 1899-1 and EN 1899-2 the BOD value is the difference between two measurements of dissolved oxygen, made with an oxygen probe before and after a 5-day incubation period; the samples have to be kept at 20 °C for 5 days. The regulation of the Minister of Maritime Economy and Inland Navigation of 12 July 2019 (Dz.U. 2019 poz. 1311 [Journal of Laws 2019, item 1311]) names PN-EN 1899-1 and PN-EN 1899-2 as the reference methods for BOD5 in wastewater.

In the PWr (Wrocław University of Science and Technology) textbook "Chemia wody. Monitoring i metody oznaczeń" [Water chemistry. Monitoring and methods of determination] the determination of BOD5 in undiluted water according to PN-EN 1899-2:2002 is described as follows: the amount of dissolved oxygen is determined before the five-day incubation at 20 °C and after it has ended; the BOD5 value is the difference — the oxygen consumed by the micro-organisms for the mineralisation of the organic compounds. Heavily polluted samples (e.g. wastewater) are examined after dilution with dilution water (part 1). Samples are taken into clean 120 cm³ glass bottles with a ground-glass stopper and are determined directly after sampling because of the biochemical changes that occur. For this method the manufacturers offer laboratory oxygen meters with a BODn program and self-stirring oxygen probes for the bottles, as well as thermostatic cabinets (20 °C ± 0.5 K); in the respirometric extension of the method for undiluted samples the oxygen consumption is measured through the drop of pressure in a closed bottle.

Method principle

Two bottles of 120 cm³ with a ground-glass stopper are filled with the water under test, which is introduced through a siphon from the bottom of the bottle up to the middle of the neck, so that no air bubble is formed once the stopper is in place. In one bottle the dissolved oxygen is determined at once, the other is set aside for five days and thermostatted at 20 °C. After the incubation the oxygen is determined — its content should not be lower than 2 mg/dm³. BOD5 (mg O₂/dm³) is the difference between the oxygen concentrations before and after the incubation; the oxygen is measured with an oxygen probe (electrochemical method, e.g. a self-stirring probe in the bottle) or by chemical methods. When the expected BOD value exceeds the available oxygen reserve, the sample is diluted with dilution water in accordance with part 1 of the standard (distilled water with a phosphate buffer, magnesium sulfate, calcium chloride and iron(III) chloride; the pH of the sample is adjusted to 7.2 with a 0.5 mol/dm³ solution of sulfuric acid or a 1 mol/dm³ solution of sodium hydroxide); the result is then calculated taking into account the oxygen consumed by the dilution water itself: BOD5 = [(a − b) − (c − d)·M/1000]·1000/m, where a, b are the oxygen in the diluted sample before and after the incubation, c, d the oxygen in the dilution water before and after the incubation, M the amount of dilution water and m the amount of the water under test in 1 dm³ of the dilution.

Applications

• Treated wastewater at the outlet of a treatment plant and surface waters of low BOD — samples without dilution
• Control of the efficiency of the biological treatment of wastewater (inlet/outlet) — the reference method under the 2019 regulation (Dz.U. 2019 poz. 1311)
• Design and sizing of wastewater treatment equipment (organic load)
• Waterworks and treatment plant laboratories — measurement with an oxygen probe with a BODn program and incubation in a thermostatic cabinet at 20 °C
• Heavily polluted samples — after dilution with dilution water (the method of part 1 of the standard)

Key parameters

Standard

Standard number

PN-EN 1899-2:2002 (EN 1899-2:1998; ISO 5815:1989 MOD)

Title (PL)

Jakość wody — Oznaczanie biochemicznego zapotrzebowania tlenu po n dniach (BZTn) — Część 2: Metoda do próbek nierozcieńczonych

Title (EN)

Water quality — Determination of biochemical oxygen demand after n days (BODn) — Part 2: Method for undiluted samples

Step-by-step procedure

1. Sampling

   Take the sample into clean 120 cm³ glass bottles with a ground-glass stopper; carry out the determination directly after sampling.

2. Filling the bottles

   Fill two bottles with the water under test through a siphon, introducing it from the bottom up to the middle of the neck so that no air bubble is formed once the stopper is in place.

3. Initial oxygen

   In the first bottle determine the dissolved oxygen at once (with an oxygen probe or chemically).

4. Incubation

   Set the second bottle aside for five days in a thermostatic cabinet at 20 °C.

5. Oxygen after incubation

   After 5 days determine the oxygen; its content should not be lower than 2 mg/dm³ — otherwise repeat the determination with dilution (part 1).

6. Calculation

   BOD5 = oxygen before the incubation − oxygen after the incubation (mg O₂/dm³); for diluted samples apply the formula with the correction for the dilution water. Report the result rounded to one decimal place.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Raw wastewater may contain pathogens — gloves, hand washing, disinfection of the glassware
• Sulfuric acid and sodium hydroxide for the pH adjustment — goggles and gloves
• Glass bottles filled up to the stopper — take care when closing them (breakage)

Frequently asked questions