⚗️ Kjeldahl nitrogen in sewage sludge — mineralisation with sulfuric acid with a catalyst, distillation and determination of the ammonia

Physicochemistry PN-EN 13342

Overview

The determination of Kjeldahl nitrogen in sewage sludge is based on the same chemistry as the classical method for water (ISO 5663:1984 — "Water quality — Determination of Kjeldahl nitrogen — Method after mineralization with selenium"). In the ISO method Kjeldahl nitrogen is defined as the sum of organic and ammonium nitrogen; only trivalent negative nitrogen is determined, and nitrogen in the form of azides, azines, azo compounds, hydrazones, nitrites, nitro and nitroso groups, oximes and semicarbazones is not determined quantitatively. The principle: mineralisation of the sample with sulfuric acid in the presence of selenium as a catalyst, conversion of the nitrogen into the ammonium ion, making alkaline and distillation into a solution of boric acid with an indicator, and then determination of the ammonia in the distillate by titration. Selenium was chosen as the catalyst instead of mercury because of concerns about the toxicity of mercury in many countries — the standard notes however that the toxicity of selenium must not be taken lightly either and gives a way of removing the selenium from the residues after the mineralisation.

For sludge, the comparative study of the HORIZONTAL project (the report "Determination of total Phosphorus, total Nitrogen and Nitrogen fractions", 2003) describes EN 13342 as a method of determining Kjeldahl nitrogen in sludge, carried out on a moist sample, without the need for prior drying, with mineralisation with sulfuric acid and a selenium catalyst; the report points out that EN 13342 describes the Kjeldahl determination "without reference to nitrite and nitrate" — that is, without taking nitrite and nitrate nitrogen into account. This matters for the laboratory: if the client expects total nitrogen, the nitrites and nitrates have to be added separately to the Kjeldahl result, and the test report has to state clearly what the determination covered.

Practice: sludge is a difficult matrix — inhomogeneous, with a high content of organic matter and water. The sample has to be homogenised well and a weighed portion taken which is representative of the moist material, and the result related to the dry matter determined in parallel (PN-EN 12880 / PN-EN 15934). With a high content of organic matter the mineralisation takes longer and requires a larger amount of acid; foaming is limited by the addition of an antifoaming agent or by slow heating at the beginning.

Method principle

A weighed portion of homogenised sludge is mineralised with concentrated sulfuric acid in the presence of a catalyst mixture containing selenium, until a clear solution is obtained — the organic and ammonium nitrogen pass into ammonium sulfate. After cooling, the acid digest is diluted with water, made alkaline with an excess of sodium hydroxide and the ammonia released is distilled into a receiver with a solution of boric acid with a mixed indicator; according to ISO 5663 the indicated part of the first distillate is discarded and the rest is collected into a glass vessel. The ammonia in the distillate is determined by titration with a standardised acid (sulfuric or hydrochloric) until the colour of the indicator changes. In parallel a blank is carried out with the same amounts of reagents, and the result is related to the dry matter of the sample determined separately. The residues after the mineralisation containing selenium are collected separately and put through the procedure for removing selenium described in the standard.

Applications

• Sewage sludge intended for agricultural use — nitrogen as a fertilising component in the documentation of the receiver
• Control of the operation of a treatment plant — the nitrogen balance in the excess (surplus) and the digested sludge
• Sludge before it is handed over for management — the requirements of the regulation on municipal sewage sludge
• Biomass and digestate — determination of Kjeldahl nitrogen alongside total phosphorus
• Laboratories determining total nitrogen as the sum of Kjeldahl, nitrites and nitrates (the result has to be supplemented)

Key parameters

Standard

Standard number

PN-EN 13342:2002 (EN 13342:2000 [IDT])

Title (PL)

Charakterystyka osadów ściekowych — Oznaczanie azotu Kjeldahla

Title (EN)

Characterization of sludges — Determination of Kjeldahl nitrogen

Step-by-step procedure

1. Preparation of the sample

   Homogenise the sludge thoroughly; take a weighed portion representative of the moist material (without prior drying, which causes losses of ammonium nitrogen). In parallel take a sample for the determination of the dry matter according to PN-EN 12880 or PN-EN 15934.

2. Mineralisation

   Introduce the weighed portion into a digestion flask, add the catalyst mixture and concentrated sulfuric acid. Heat gently at first (foaming), then vigorously, until the solution becomes clear; continue the heating for the time provided for in the procedure so as to complete the conversion of the organic nitrogen into the ammonium ion.

   🌡 Temperature: boiling of the sulfuric acid

3. Dilution and making alkaline

   Carefully dilute the cooled acid digest with water, transfer it quantitatively into a distillation flask and make it alkaline with an excess of concentrated sodium hydroxide solution — add it slowly down the wall, avoiding losses of ammonia.

4. Distillation

   Distil the ammonia released into a receiver with a solution of boric acid with an indicator; according to ISO 5663 discard the indicated first part of the distillate and collect the rest into a glass vessel. Carry out the distillation until the required volume has been collected.

5. Titration

   Titrate the distillate with the standardised acid until the colour of the mixed indicator changes. Note the volume of titrant used.

6. Blank

   Carry out the whole procedure without the sample, with the same amounts of reagents; subtract the result of the blank from the result of the determination (the water and the reagents are sometimes contaminated with ammonia).

7. Calculation and test report

   Calculate the content of Kjeldahl nitrogen and relate it to the dry matter of the sample. In the test report note expressly that the result covers organic and ammonium nitrogen and does not cover nitrites and nitrates — where total nitrogen is required, carry out separate determinations of both forms.

8. Waste

   Collect the residues after the mineralisation containing selenium separately and put them through the procedure for removing selenium indicated in the standard; do not introduce them into the drains.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Mineralisation with concentrated sulfuric acid at a high temperature — work only under a fume hood, in goggles, a laboratory coat and acid-resistant gloves.
• Selenium and its compounds are toxic — weigh them out carefully, avoid raising dust, collect the residues as hazardous waste.
• Making the acid digest alkaline is strongly exothermic and releases ammonia — add the alkali slowly, with cooling, with the receiver connected.
• The ammonia in the distillate acts as an irritant on the respiratory tract and the eyes — the distillation set must be tight.
• Sewage sludge is infectious material — gloves, avoidance of aerosols, disinfection of the working place after the work.

Frequently asked questions