NOTE / NOTE Geotechnical properties of a municipal water treatment sludge incorporating a coagulant Brendan C. O’Kelly Abstract: The geotechnical properties of a municipal water treatment sludge from an upland catchment are presented. The gelatinous sludge comprised flocs of mainly quartz, manganoan calcite, and clay-sized organic solids, and incorpo- rated an alum coagulant and an anionic polyelectrolyte. Standard Proctor compaction yielded low bulk density values of 0.95–1.10 t/m 3 and dry density values of 0.12–0.36 t/m 3 (water content is 160%–780%) in line with the low specific gravity of solids value of 1.86. The undrained shear strength and the water content were inversely related on a semi-log plot. The effective stress shear strength parameter values were c’ = 0 and ’ = 398. The consolidation properties were studied using the oedometer, consolidometer, and triaxial apparatus. The material was highly compressible with primary compression index (C c ) values of 2.5–3.7, and primary compression ratio (C* c ) values of 0.20–0.28. The majority of the strain response occurred due to primary consolidation although the material had a very low permeability (coefficient of permeability values decreasing from 2  10 –9 to 5  10 –11 m/s for an effective vertical stress of s’ v = 3–800 kPa). Sec- ondary compression was minor, with a mean secondary compression index (C e ) value of 0.15, and C e /C c = 0.04–0.06. Key words: water treatment sludge, index, geotechnical properties, lagoon, land filling. Re ´sume ´: On pre ´sente les proprie ´te ´s ge ´otechniques de la boue de traitement des eaux municipales provenant d’un captage sur des hautes terres. La boue ge ´latineuse comprenait des flocons principalement de quartz, de calcite manganeux et de so- lides organiques de grosseur argileuse, et incorporait un coagulent d’alun et un polye ´lectrolyte anionique. Un compactage Proctor Standard a donne ´ de faibles valeurs de poids spe ´cifique de 0.95 a ` 1.10 t/m 3 et des valeurs de densite ´ se `che de 0.12 a ` 0.36 t/m 3 (teneur en eau = 160 % a ` 780 %) en ligne avec la faible valeur du poids spe ´cifique des solides de 1.86. La re ´- sistance au cisaillement non draine ´ et la teneur en eau e ´taient inversement relie ´es sur un graphique semi-log. Les valeurs des parame `tres effectifs de re ´sistance au cisaillement e ´taient c’ = 0 et ’ = 398. Les proprie ´te ´s de consolidation ont e ´te ´e ´tu- die ´es au moyen de l’oedome `tre, du consolidome `tre, et de l’appareil triaxial. Le mate ´riau e ´tait fortement compressible avec des valeurs d’ndice de compression primaire (C c ) de 2.5 a ` 3.7, et des valeurs de rapport de compression primaire (C* c ) de 0.20 et 0.28. La plus grande partie de la re ´ponse en de ´formation s’est produite durant la consolidation primaire quoique le mate ´riau a une tre `s faible perme ´abilite ´ (coefficient des valeurs de perme ´abilite ´ de ´croissant de 2  10 –9 a `5  10 –11 m/s pour s’ v =3a ` 800 kPa). La compression secondaire e ´tait mineure, avec un indice de compression secondaire moyen (C e ) d’une valeur de 0.15, et C e /C c = 0.04 a ` 0.06. Mots-cle ´s : Boue de traitement d’eau, indice, proprie ´te ´s ge ´otechniques, lagon, mate ´riau de remblayage. [Traduit par la Re ´daction] Introduction Water treatment sludge (WTS) is the residue of the filter backwash, water softening, chemical coagulation, floccula- tion, and settling processes used in the treatment of potable water (Twort et al. 2000). The WTS material generally con- tains sand, silt, organic matter and microorganisms, chemi- cals (including lime, soda ash, caustic soda, carbon and iron or aluminium salts), and polyelectrolyte coagulant aids that are added during the treatment processes. The coagulants cause the impurities to aggregate into flocs that can be sepa- rated from the water, removing not only the impurities but also the chemical additives, and these processes have been described by Achari and Joshi (1995). Polyelectrolytes are synthetic, long-chained organic molecules (which may be cationic, anionic, or nonionic) that act as a binding agent to increase the inherent shear strength of the flocs. In general, WTS can be categorized as either alum sludge or iron sludge depending on the type of coagulant added. Increasing quantities of WTS are being produced world- wide due to increasing water usage by consumers; the implementation of more stringent regulations on the quality of potable water (e.g., Council of European Communities 1998), and more stringent controls on the quality of waste- water discharges. At present, the principle means of disposal for WTS are thermal treatment and landfilling, either at Received 26 June 2006. Accepted 31 October 2007. Published on the NRC Research Press Web site at cgj.nrc.ca on 29 May 2008. B.C. O’Kelly. Department of Civil, Structural and Environmental Engineering, Museum Building, Trinity College Dublin, Dublin 2, Ireland. (e-mail: bokelly@tcd.ie.). 715 Can. Geotech. J. 45: 715–725 (2008) doi:10.1139/T07-109 # 2008 NRC Canada