sludge properties

factors that establish the nature of the sludge

Section sludge examination details the main analytical protocols for the following properties:

• DM = dry matter content (105°C): total suspended solids and soluble matter;
• Suspended solids (105°C): total suspended solids. Major criterion that has to be taken into consideration when designing the sludge system;
• Fire losses 105° - 180°C: salt crystallisation water, water bound to hydroxides, some extremely volatile and combustible matter (oils, cellulose);
• Fire losses 105° - 250°C: it is assumed that all the cellulose has been burnt;
• Fire losses 105° - 550°C: the most classic analysis. Some so-called volatile solids (VS) is often assimilated to organic matter (OM). With some sludge, there can be differences (see sludge examination). These differences are generally expressed in the form of the following ratio:

• Fire losses 555° - 900°C: mainly CO₂ from carbonate breakdown.
• Weighted elementary composition (especially for organic sludge):

1. CHONS : Of interest for stabilisation systems, agricultural recycling and incineration (calorific value). The analysis of glucids, lipids, proteins, albeit difficult, is undoubtedly of value.
2. Metals (Fe, Aℓ, Mg, Cd, Hg, Zn, Cu, Cr, Pb, Ni…).
3. Silica and granulometric analysis.
4. Calcium salts (carbonates and sulphates).
5. Potassium.
6. Phosphates.
7. PCB and PAH and any other toxic products occurring;

• temperature, pH, oxidation-reduction potentiel (EH), conductivity;
• alkalinity (TAC in CaCO₃ equivalents) and VFA (volatile acids in Acetic Acid equivalents);
• fats (usually expressed as HES: hexane extractible substances);
• fibre content (cellulose): normally via a 500 m (occasionally 350 m) strainer;
• pathogenic agents: Enterovirus, salmonella, viable helminth eggs and others depending on utilisa­tions (coliforms, e-coli…);
• salinity, chlorides: important when selecting the equipment;
• amounts of water bound in with suspended solids: using thermo-gravimetric measurements for a rough concept of limit dry solids content;
• quality of sludge interstitial water: BOD₅, COD, PT, NK, NH₄, pH, M-alk., salinity. This information will allow us to evaluate the returns to the plant inlet created by the sludge system.

factors that establish sludge structure

Sludge structure can be identified by the following factors:

• apparent viscosity in relation to rheological behaviour: slimy suspensions are non-Newtonian liquids. The figure found will depend on the type of rheometer used (coaxial cylinder rheometers, screw rheometers…) and on the strain (shear energy) applied.

Therefore, when stating a sludge’s viscosity, the type of rheometer used and the strain applied must also be specified.

• penetrometer test: this test provides a comparison index used to estimate the more or less paste-like structure of a dewatered cake;
• slump test: this is an important factor for sludge storage design. Sludge is regarded as "solid" when, piled up to a 1 m height, it forms an angle of at least 30° with the ground;
• granulometry: for thermally dried sludge.

Please note that it is more difficult to estimate the proportion of particles that are smaller than 500 µm. In effect, the attrition created by the sieves used produces fines which are extra to the initial sample.

factors used to identify the behaviour of sludge undergoing dewatering

The behaviour of sludge undergoing dewatering can be characterised by the following factors:

• thickening capability: see drainability tests and sludge thickening;
• characterisation of filtrability, compressibility, limit dry solids content and centrifugability : see sludge examination, centrifugation and filter press.