Sludge solids concentration: Which are the limits ?

P. Ginisty*, IFTS - Institute of Filtration & Techniques of Separation, France; J.B. Kopp, Sewage Sludge Treatment Consulting KBKopp, Germany; A.K. Melsa, L. Spinosa, International Organization for Standardization - ISO TC275/WG6 Working Group

Sludge is produced by wastewater treatment and is in the form of a suspension with a water content of 95-99%. After thickening, sludge has still a water content of 92-97% and needs dewatering. The main objective of dewatering is to produce sludge that can be more easily transported and safely handled, valorized or disposed of. One of the greatest problems of parameters and tests usually adopted for sludge characterization is that they are generally specific to the method of treatment but not able to give fundamental information on the sludge in question (Spinosa, 2013). In particular, for the evaluation of performance of several sludge management operations, an important role is played by basic parameters, such as particle and floc size distribution, rheological properties and water distribution. The latter parameter gives important information on the availability of different types of water to be removed according to the type and intensity of their bonding to the solids. Scientific papers (Vesilind 1997,; Kopp 2000,; Vaxelaire, 2004) try to classify the different types of water introducing some terms, such as free water, bulk water, interstitial water, flock water, internal water, capillary bound water, vicinal water, surface water, water of hydration, intracellular water, physical or chemical bound water…., but most people seem to use these mentioned terms interchangeably and debates are quite confusing as most of the types of water proposed are difficult to be precisely defined and to quantified.

In the framework of ISO/TC 275 (International Standardization Committee devoted to sludge management) work, definition of terms needs to be clarified and a simpler classification is proposed with 2 main categories...

....The paper proposes to illustrate different concepts by pointing out experimental results obtained with a given wastewater sewage sludge: drainage tests, floc shearing/compression tests, dryness limit and thermogravimetric methods and will show how much the modification of floc structure can change the state of water in sludge and the kinetics and amount of free water removable by thickening and dewatering processes. Comparison of these values to dryness obtained at pilot and full scale centrifuge decanter will be made...

Session: L16 - Dewaterability of Sludges
Day: 24 October 2019
Time: 13:00 - 14:15 h

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