🌿 Aqua regia digestion of sewage sludge before the determination of trace elements and phosphorus

Environment PN-EN 13346

Overview

What this standard does and what it does not. It is not a method of determining elements but a method of SAMPLE PREPARATION: digestion of the sludge with aqua regia, after which the measurement is made by a spectrometric technique. The catalogue card describes the scope thus: "Four methods are given for the extraction of trace elements and phosphorus from sewage sludge and the products of its treatment using aqua regia, differing in the manner of heating. The solution obtained may be used for the determination of As, Cd, Cr, Cu, Hg, Ni, Pb, Se, Zn and P by spectrometric techniques".

STATUS — checked 11 September 2026. The card of PN-EN 13346:2002 carries the annotation "Withdrawn" and the date of withdrawal 01-07-2016; it has NO "Superseded by" field. The absence of that field means nothing either way, so we looked for a successor by the SUBJECT of the test and not by the field — and the chain turned out to be longer than a single link: PN-EN 13346:2002 — WITHDRAWN 01-07-2016, card with no field naming a successor → PN-EN 16174:2012 (English version), „Osady ściekowe, uzdatnione bioodpady oraz gleba — Roztwarzanie frakcji pierwiastków rozpuszczalnych w wodzie królewskiej" [the Polish title from the PKN card; in English: "Sludge, treated biowaste and soil — Digestion of aqua regia soluble fractions of elements"] — WITHDRAWN 30-03-2022, likewise with no field naming a successor → PN-EN ISO 54321:2021-07 (English version), „Gleba, uzdatnione bioodpady, osady ściekowe oraz odpady — Roztwarzanie frakcji pierwiastków rozpuszczalnych w wodzie królewskiej" [the Polish title from the PKN card; in English: "Soil, treated biowaste, sludge and waste — Digestion of aqua regia soluble fractions of elements"], IN FORCE (published 02-07-2021, 52 pages, TC 191, implementing EN ISO 54321:2021 [IDT] and ISO 54321:2020 [IDT]). THE DATES MATTER HERE MORE THAN THE ORDER ITSELF: almost SIX YEARS passed between the withdrawal of 13346 and the withdrawal of 16174, during which the intermediate link was a standard in force. A laboratory that moved from 13346 to 16174 in 2018 acted correctly at the time, and only since 30-03-2022 has it stood on a withdrawn document. Without the dates this looks like one jump, whereas there were two, six years apart. THE WIDENING OF THE SCOPE ALSO HAPPENED IN TWO STEPS, not in the last one: PN-EN 13346 covered SEWAGE SLUDGE ALONE; PN-EN 16174 added treated biowaste and SOIL; PN-EN ISO 54321 added WASTE. Half the widening therefore happened in the link we call intermediate — and that is the second reason why we give it as a separate source: it is not a stopping place but the point at which the standard changed its subject. THE LANGUAGE VERSION OF THE SUCCESSOR: PN-EN ISO 54321:2021-07 exists ONLY in the English version — a query to the PKN catalogue for "54321" returns one item and it is the English edition (checked 11 September 2026). A laboratory moving to the successor will therefore read the procedure in English and translate the terms itself for its quality system documentation; there is no Polish translation to look for. NONE of these three cards declares succession — the arrows "→" used above are OUR ordering by subject of testing, not a record from the catalogue. The sentence "PN-EN ISO 54321 supersedes PN-EN 13346" therefore has no backing in the catalogue and we do not state it — we state only that the catalogue names no successor and that the document in force on the same subject is 54321, and we base that on a comparison of the titles on the two cards, which we give in the sources.

Trails rejected — we write them out so that no one walks this road a second time. PN-EN 16173:2012 (in force) concerns digestion with NITRIC acid, not aqua regia. PN-EN 13657:2006 (in force) concerns aqua regia, but its subject is WASTE, not sludge. Both documents have either a matching number or a matching fragment of the title, and content that does not match.

What the legislation says — and here the answer is other than one might expect. The Regulation of the Minister of the Environment of 6 February 2015 on municipal sewage sludge (Journal of Laws 2015 item 257) INVOKES NO STANDARD: the string "PN-" does not occur in the whole document. The reference methods for testing sludge (Annex No 4) and soil (Annex No 5) are described in words — for example the content of calcium and magnesium: "mineralisation with a mixture of acids and determination by a titrimetric method or by atomic spectrometry, or atomic emission spectrometry with inductively coupled plasma (ICP-OES)". THE LEGAL POSITION OF THIS ACT — checked 11 September 2026. The 2015 regulation was amended by the Regulation of the Minister of Climate and Environment of 31 December 2021 (Journal of Laws 2022 item 89); the amendments took effect on 14-01-2026, and the act bears today the title "on the USE of municipal sewage sludge". The amendment also gave a new wording to the WHOLE of Annex No 4 — the method quoted above for calcium and magnesium reads differently in it in TWO places: mass spectrometry with inductively coupled plasma (ICP-MS) was added, and "atomic emission spectrometry" was replaced by "OPTICAL emission spectrometry" — even though the abbreviation ICP-OES remained the same. THE PROPOSITION OF THIS PARAGRAPH REMAINS TRUE IN BOTH WORDINGS, HOWEVER: we checked the string "PN-" both in the 2015 text and in the new annex — in neither does it occur even once. For a laboratory this means that the withdrawal of PN-EN 13346 does not collide with the legal basis, because the legal basis never named a standard — it describes an operation. Moving to PN-EN ISO 54321:2021-07 is therefore a decision of the laboratory and not a matter of a change in the legislation.

THE CHANGE IS ALREADY VISIBLE IN THE TITLE, and the title goes into the report. PN-EN 13346 is called "Determination of trace elements and phosphorus", and PN-EN 16174 and PN-EN ISO 54321 — "Digestion of aqua regia soluble FRACTIONS of elements". The proviso about the fraction is therefore not an addition in the text of the standard but part of its title: a laboratory that moves to the successor "because it is the same thing" changes the substance of what it declares to the client in a report quoting that title.

A note on the scope of the successor: PN-EN ISO 54321 is not a simple rewriting of the old standard. Its scope is WIDER (soil, treated biowaste, sewage sludge and waste — not sludge alone) and it gives TWO digestion methods, whereas PN-EN 13346 gave four, differing in the manner of heating. The card of 54321 adds a proviso that is not present with 13346: "Digestion with aqua regia will not necessarily bring about complete dissolution of the sample. The concentrations of the constituents extracted may not necessarily reflect their total content in the sample, but represent the metals soluble in aqua regia under such conditions as during the performance of the test" (quoted from the Polish-language PKN card; the Polish wording contains twice the misspelling „nie koniecznie" for „niekoniecznie" — so in the card) — that is, the result is by definition dependent on the method and is not a total content.

What this entry does not contain. The working steps of PN-EN 13346 itself the entry DOES NOT GIVE, because we did not have the text of the standard — PKN supplies it for a fee and we did not obtain the preview (checked 11 September 2026). We therefore do not give: which of the four extraction methods is recommended, what the manners of heating and the times are, the ratio of sample mass to the volume of aqua regia, the way of separating the solution from the residue, or the conditions for storing the solution. The legislation that describes this operation does so in one sentence and likewise gives none of these parameters. A laboratory implementing the method must buy the standard — today rather PN-EN ISO 54321:2021-07 than the withdrawn 13346.

Method principle

A weighed sample of sludge is subjected to the action of aqua regia — a mixture of nitric(V) acid and hydrochloric acid — at elevated temperature, as a result of which the trace elements and phosphorus pass into solution. The standard gives four variants differing in the MANNER OF HEATING. The solution obtained is separated from the undissolved residue and in it As, Cd, Cr, Cu, Hg, Ni, Pb, Se, Zn and P are determined by spectrometric techniques. The result is not the total content of the element in the sludge but the content of the fraction soluble in aqua regia under the conditions of the test — this proviso stands expressly in the scope of the superseding standard (PN-EN ISO 54321).

Applications

• Testing municipal sewage sludge before its agricultural or environmental use
• Determination of metals in sludge intended for land reclamation
• Control of sludge from industrial treatment plants for the content of trace elements
• Testing the products of sludge treatment — composts and stabilised sludge
• Preparation of the sample for ICP-OES, ICP-MS, AAS and CV-AAS (mercury) determinations

Key parameters

Standard

Standard number

PN-EN 13346:2002 (EN 13346:2000 [IDT])

Title (PL)

Charakterystyka osadów ściekowych -- Oznaczanie pierwiastków śladowych i fosforu -- Metody ekstrakcji wodą królewską

Title (EN)

Characterization of sludges — Determination of trace elements and phosphorus — Aqua regia extraction methods (tytuł angielski odtworzony z polskiego — karta katalogowa PKN podaje wyłącznie polski)

Step-by-step procedure

1. Preparation of the sludge sample (a step from the SCOPE of the standard on the PKN catalogue card — not from the text of the standard)

   Dry and homogenise the sample, weigh out the test portion and determine the dry matter. WE GIVE NEITHER THE SIZE OF THE TEST PORTION NOR THE DRYING CONDITIONS — they are set by the standard, whose text we did not have.

2. Choice of one of the four extraction variants (the variants named in the SCOPE from the PKN catalogue card)

   The standard gives four methods differing in the manner of heating. WHICH VARIANT TO USE WHEN — we do not give, because that is settled by the text of the standard.

3. Digestion with aqua regia (the direction of the operation from the SCOPE of the standard; the card gives no parameters)

   Subject the test portion to the action of aqua regia at elevated temperature, in accordance with the variant chosen. WE GIVE NEITHER THE RATIO OF THE ACID VOLUMES, NOR THE TEMPERATURE, NOR THE TIME — they are in the standard.

4. Separation of the solution from the residue (the direction of the operation from the SCOPE of the standard; the card gives no parameters)

   Separate the solution from the undissolved residue. (Its composition — described in the literature as chiefly silicate — is not given by the catalogue card and we do not assert it.) WE GIVE NEITHER THE METHOD OF SEPARATION NOR THE PORE SIZE OF THE FILTER.

5. Determination of the elements in the solution (the list of elements from the SCOPE of the standard on the PKN catalogue card)

   Determine As, Cd, Cr, Cu, Hg, Ni, Pb, Se, Zn and P by spectrometric techniques. The determination is described by separate standards — EN 13346 ends with the preparation of the solution.

6. Conversion to dry matter and the report (the requirement to state the variant is our recommendation following from the existence of four variants, not a quotation from the standard)

   Convert the result to the dry matter of the sludge. IN THE REPORT STATE WHICH OF THE FOUR EXTRACTION VARIANTS WAS USED — the four variants differ in the manner of heating, so the entry "aqua regia extraction to PN-EN 13346" alone does not say how the test was carried out. State also that the result concerns the fraction soluble in aqua regia and not the total content.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Aqua regia is strongly corrosive and oxidising; prepare it only under a fume hood, adding the hydrochloric acid to the nitric acid in the order given IN THE METHOD USED — we do not know the order from the text of PN-EN 13346, because we did not have the text of the standard.
• During digestion nitrogen oxides and chlorine are evolved — work only with sound extraction, with a face shield and acid-resistant gloves.
• Open vessels containing hot solution only after cooling; with microwave digestion observe the pressures admissible for the vessels.
• Sewage sludge is material presenting a biological hazard — before drying and milling apply the protective measures proper to infectious material.
• Mercury in sludge is volatile — do not dry samples intended for the determination of mercury at high temperature without checking whether the standard permits it.

Frequently asked questions