⚗️ Sludge volume after 30 min of settling and the sludge volume index (Mohlman index) — a 1000 ml cylinder, determination of the dry matter and the quotient of the two quantities

Physicochemistry PN-EN 14702-1

Overview

This is the simplest and the most frequently carried out determination in the operation of an activated-sludge plant: a cylinder, a clock and a balance. The introduction to the standard puts the matter plainly — determining the settleability by establishing the proportion of sludge volume is an operational parameter and makes it possible to assess (only within certain limits, admittedly) the dry matter content in the mixture of sludge and water; the value serves for calculating the settling velocity of the sludge, for calculating the sludge volume index and for assessing the working of settlers. The scope covers sludge suspensions originating from the management of rainwater, from municipal wastewater collecting systems, from municipal wastewater treatment plants, from treatment plants for industrial wastewater similar to municipal wastewater (within the meaning of Directive 91/271/EEC) and from waterworks; the method also applies to sludge suspensions of other origin.

What this indicator really says. The sludge volume index (in Polish literature: indeks osadu, the Mohlman index, IO; in English-language literature SVI) is the quotient of the sludge volume and the dry matter concentration in the sludge. The lower its value, the more favourable the settling properties of the sludge. The laboratory instruction of Wrocław University of Science and Technology states that activated sludge with good settling properties has an IO within the range 50–100 cm³/g DM, and that higher values point to so-called sludge bulking, caused most often by filamentous bacteria. A survey of technological parameters in a paper in "Gaz, Woda i Technika Sanitarna" gives 80–120 ml/g DM as the optimum value and explains the mechanism: high IO values follow from the intensive development of filamentous bacteria caused by unfavourable operating conditions, and a large rise in this parameter may lead to the aerobic stabilisation of the sludge, whereby the flocs become smaller and settle worse. The practical sense is this: a rising index is a warning signal of a deterioration of settling in the secondary settler — which is why the determination is carried out routinely and interpreted in a time series, not occasionally.

Which sheet of the series is which document. The series PN-EN 14702 „Charakterystyka osadów ściekowych — Właściwości sedymentacyjne" [Characterisation of sludges — Settling properties] contains no sheet with requirements — all three parts are standards of test methods and differ in what they measure. Part 1 (this entry) describes settleability: free settling for 30 min in a 1000 ml cylinder, the proportion of sludge volume and the sludge volume index. Part 2 describes thickenability: the same 30 min, but in a cylinder of 10 cm diameter and 50 cm measuring height (nominal volume 3500 ml) with a slow stirrer at 1 rev/min — there one measures the stirred sludge volume and the stirred sludge volume index, and the standard notes that the settling properties depend on the concentration of the suspension, so that comparing sludges of different concentration is admissible if a reference concentration is adopted, for which 3.5 g/l has been taken. Part 3, considerably younger, concerns the determination of the zone settling velocity (ZSV). For the ordinary "sludge index" of treatment-plant operation only Part 1 is the right one.

Status and editions. The current edition is PN-EN 14702-1:2008 — the Polish-language version, published on 2 December 2008, 15 pages, price group H, managed by KT 119 for Water Quality — Basic Problems, ICS 13.030.20. It implements EN 14702-1:2006 [IDT] and supersedes PN-EN 14702-1:2006 — the English-language version. The PKN catalogue card for this edition has neither a "Date of withdrawal" field, nor a "Superseded by" field, nor a corrigendum (Ap). Only this one language version has a current card — the English version exists solely as the withdrawn edition of 2006 — so the trap of "the corrigendum hangs only on one version" does not occur here. The other items of the series in the PKN list are: PN-EN 14702-2:2008 — the Polish version (implementing EN 14702-2:2006 [IDT]) and PN-EN 14702-3:2019-04 — the English version (implementing EN 14702-3:2019 [IDT]); the English versions of Parts 1 and 2 of 2006 bear the annotation "Withdrawn and superseded by" the corresponding Polish versions of 2008. It is worth noting a small point which is sometimes confused when the title is copied out: the title of the withdrawn English version of 2006 read „Oznaczanie zdolności opadania (Oznaczanie stosunku objętości osadu do indeksu objętościowego osadu)", whereas the binding Polish version of 2008 has the title „Oznaczanie zdolności opadania osadu (Oznaczanie objętości względnej oraz indeksu objętościowego osadu)" — the second wording is the correct one. EN 14702-1:2006 was prepared by technical committee CEN/TC 308 "Characterisation of sludges", whose secretariat is held by AFNOR; the standard was approved by CEN on 3 February 2006, and conflicting national standards were to be withdrawn at the latest by September 2006.

What this description does not contain. We have not seen a Polish copy of PN-EN 14702-1:2008 — only the title, the scope and the publication data come from the PKN catalogue card. The substantive content is given from the preview of SIST EN 14702-1:2006 — the Slovenian implementation of the same European standard, identical with EN 14702-1:2006 (and therefore with the text which PN-EN 14702-1:2008 implements as IDT) — covering clauses 1–10 in full. We have not seen Annex A (informative) with the results of the validation trials or the bibliography, so we give only those precision data which stand in clause 9 of the main text. The threshold values for interpreting the index (50–100 or 80–120 ml/g) do not come from the standard — the standard gives no limit values at all — but from technological literature, and are marked as such in the text.

Method principle

The sludge volume after settling and the sludge volume index are determined by a 30-minute settling of the sludge suspension. A representative sample of the suspension is taken with a scoop and poured at once into a measuring cylinder up to the 1000 ml mark; after 30 minutes standing in one place, without shaking, the sludge volume is read at the level of the sludge surface, that is at the sludge–water boundary. The sludge volume divided by the volume of the initial sample used gives the proportion of sludge volume in ml/l, and the same volume divided by the dry matter concentration in the sludge, expressed in g/l, gives the sludge volume index in ml/g. Settling takes place solely under the force of gravity, without stirring — in this the method differs from the determination of thickenability according to Part 2 of the standard, where the cylinder is stirred with a slow stirrer.

Applications

• Routine operational control of activated-sludge tanks and secondary settlers in municipal wastewater treatment plants
• Early detection of sludge bulking caused by the development of filamentous bacteria, before the sludge is carried out of the settler
• Assessment of the working of treatment plants for industrial wastewater of a character close to municipal wastewater (within the meaning of Directive 91/271/EEC)
• Sludges from the management of rainwater, from sewer systems and from waterworks
• Choice of the sludge recirculation ratio and estimation of the settling velocity of the sludge for technological calculations
• Comparative examination of sludges before a change of process parameters (sludge age, loading, aeration)

Key parameters

Standard

Standard number

PN-EN 14702-1:2008

Title (PL)

Charakterystyka osadów ściekowych — Właściwości sedymentacyjne — Część 1: Oznaczanie zdolności opadania osadu (Oznaczanie objętości względnej oraz indeksu objętościowego osadu)

Title (EN)

Characterisation of sludges — Settling properties — Part 1: Determination of settleability (Determination of the proportion of sludge volume and sludge volume index)

Step-by-step procedure

1. Checking the conditions before taking the sample

   Establish the temperature of the sample and the ambient temperature. When the difference is greater than 5 °C the standard recommends standing the settling cylinder with the sample in a bucket filled with the liquid of the sample — otherwise convection and the release of gas bubbles will disturb the settling.

   🌡 Temperature: difference sample–surroundings ≤ 5 °C

2. Taking the sample with a scoop

   Take a representative sample of the sludge suspension with a scoop holding 1 l to the brim and pour it at once into a measuring cylinder up to the 1000 ml mark. The use of a scoop of that capacity has a definite purpose: it prevents the sludge from settling in the scoop itself before it is poured out. The mixture of sludge and water must not be shaken too vigorously, so as not to change the course of settling.

3. Settling for 30 minutes

   Leave the cylinder in one place for 30 min, without shaking. Do not move it, do not stir it, do not take samples during that time — the process is disturbed both by the walls of the vessel and by the mutual interaction of the flocs, especially when the proportion of sludge volume is large.

   ⏱ Time: 30 min

4. Reading the sludge volume

   After 30 min read the sludge volume at the level of the sludge surface, that is at the boundary between the sludge and the supernatant water. The reading is unambiguous only when the boundary is distinct — its blurring is in itself information about the state of the sludge.

5. Deciding on dilution

   If the sludge volume exceeds 250 ml/l, the determination has to be repeated on a diluted sample. For the dilution use the supernatant of the sludge suspension or water from the settler outlet, in a volume ratio of 1:1, 1:2 or 1:3. Homogenise the diluted sample gently — by turning the closed cylinder upside down twice.

6. Repeating the settling for the diluted sample

   Repeat the 30 min settling and the reading. In the calculations multiply the sludge volume read off by the dilution factor 2, 3 or 4 — corresponding to the dilution 1:1, 1:2 and 1:3. The test report states the dilution at which the value first fell below 250 ml/l.

   ⏱ Time: 30 min

7. Duplicate

   The determination is carried out in duplicate. The sludge volume which enters the formula for the sludge volume index is the mean of at least two measurements.

8. Determining the dry matter concentration

   If the concentration of total dissolved matter is less than 10 % of the total matter — determine the mass concentration of dry matter in the sludge (g/l) according to EN 12880. If it is greater than 10 % — according to EN 872. The standard also notes that when the content of dissolved matter is small and negligible in relation to the dry matter content, the total matter content is to be used in the calculations.

9. Calculating the proportion of sludge volume and the index

   The proportion of sludge volume V_s in ml/l is obtained as the sludge volume after settling divided by the volume of the initial sample used. The sludge volume index is calculated from the formula I_sv = V_s / ρ_T, where ρ_T is the dry matter concentration in g/l. The proportion of sludge volume is given rounded to 10 ml/l, the index — to 1 ml/g. The standard gives examples of the wording: „Objętość względna osadu po sedymentacji 180 ml/l" [proportion of sludge volume after settling 180 ml/l] for the original sample, „510 ml/l (170 ml po trzykrotnym rozcieńczeniu)" [510 ml/l (170 ml after a threefold dilution)] for a diluted sample, and „Indeks objętościowy osadu 145 ml/g" [sludge volume index 145 ml/g].

10. Test report

    In the test report give: the reference to the standard, all the information needed for the full identification of the sludge sample, the details of the preparation of the sample (e.g. the dilution), the results calculated according to clause 8, and any details not specified in the standard or optional and any other factor which may have influenced the results.

11. Use of the result in operation

    Interpret the result in a time series, not singly. Technological literature takes it that sludge with good settling properties has an index of 50–100 cm³/g DM (the Wrocław University of Science and Technology laboratory instruction), and that the optimum values lie within the range 80–120 ml/g DM; higher values point to sludge bulking, caused most often by filamentous bacteria developing under unfavourable operating conditions. The standard itself gives no thresholds — the limit values are a technological finding, not a normative one.

Required equipment and apparatus

Health and safety (OHS)

• Activated sludge suspension and raw sludge are infectious material — work in gloves, avoid aerosol when pouring from the scoop, do not touch the face, and disinfect hands and vessels after work.
• Taking a sample from an aeration tank or from a settler channel takes place over an open basin — use a scoop on a handle, do not lean over the edge, and apply fall protection in accordance with the instructions for the works.
• In channels and closed chambers hydrogen sulfide and methane may accumulate and the oxygen concentration is sometimes lowered — such places are entered only under a confined-space working procedure, with measurement of the atmosphere and a standby person.
• Return the sludge from the sample to the process after the determination or dispose of it as waste — do not pour it into the laboratory sink, where it blocks traps and colonises the installation.
• A 1000 ml cylinder filled with suspension is heavy and unstable — stand it on the bench away from the edge and from thoroughfares, because knocking it over interrupts the determination and spills infectious material.

Frequently asked questions