Geotecnia

715

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 incorporated an alum coagulant and ananionic polyelectrolyte. Standard Proctor compaction yielded low bulk density values of 0.95–1.10 t/m3 and dry density values of 0.12–0.36 t/m3 (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 (Cc) 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 ofpermeability values decreasing from 2 Â 10–9 to 5 Â 10–11 m/s for an effective vertical stress of s’v = 3–800 kPa). Secondary compression was minor, with a mean secondary compression index (Ce) value of 0.15, and Ce/Cc = 0.04–0.06. Key words: water treatment sludge, index, geotechnical properties, lagoon, land filling. ´ ´´ ´ ´ ´ Resume : On presente les proprietes geotechniques de la boue detraitement des eaux municipales provenant d’un captage ´ sur des hautes terres. La boue gelatineuse 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 polyelectrolyte anionique. Un compactage ´ ´ ` ´ ` Proctor Standard a donne de faibles valeurs de poids specifique de 0.95 a 1.10 t/m3 et desvaleurs de densite seche de 0.12 ` ` ´ ´ a 0.36 t/m3 (teneur en eau = 160 % a 780 %) en ligne avec la faible valeur du poids specifique des solides de 1.86. La re´ ´ ´ sistance au cisaillement non draine et la teneur en eau etaient inversement reliees sur un graphique semi-log. Les valeurs ` ´ ´ ´´ ´´ ´ des parametres effectifs de resistance au cisaillement etaient c’ = 0 et ’ = 398. Lesproprietes de consolidation ont ete etu´ ` ` ´ ´ diees au moyen de l’oedometre, du consolidometre, et de l’appareil triaxial. Le materiau etait fortement compressible avec ` des valeurs d’ndice de compression primaire (Cc) 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 reponse en deformation s’est produite durant la consolidationprimaire quoique le ´ ` ´ ´ ´ ´ ´ ` materiau a une tres faible permeabilite (coefficient des valeurs de permeabilite decroissant de 2 Â 10–9 a 5 Â 10–11 m/s ` ´ pour s’v = 3 a 800 kPa). La compression secondaire etait mineure, avec un indice de compression secondaire moyen (Ce) ` d’une valeur de 0.15, et Ce/Cc = 0.04 a 0.06. ´´ ´ ´ ´ Mots-cles : Boue de traitement d’eau, indice, proprietesgeotechniques, lagon, materiau de remblayage. ´ [Traduit par la Redaction]

Introduction
Water treatment sludge (WTS) is the residue of the filter backwash, water softening, chemical coagulation, flocculation, and settling processes used in the treatment of potable water (Twort et al. 2000). The WTS material generally contains sand, silt, organic matter and microorganisms, chemicals (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
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,...