Freeze-Thaw Conditioning of Activated Sludge: Effect of Monovalent, Divalent, and Trivalent Cations

PHYSICAL, chemical, and biological properties of activated sludge are influenced by changes in ionic strength and ionic composition of sludge. Many studies have investigated the effects of various cations on sludge characteristics [e.g. 1–6] and concluded that cations, particularly divalent cations, play a crucial role in the formation of flocs, influence floc properties such as size, density, and strength, and directly affect sludge settleability and dewaterability. On the other hand, few studies have investigated the effect of cations on freeze-thaw conditioning of sludges [7–11]. Vesilind et al. [8] hypothesized that increased compression of double layer in the presence of sodium chloride may promote particle aggregation during freeze-thaw conditioning which would lead to better dewaterability. They found no significant improvement in sludge dewaterability, and concluded double layer compression is not an influential mechanism in freeze-thaw treatment. Chu et al [9] also studied the effect of sodium chloride on freeze-thaw conditioning and they concluded presence of sodium chloride retarded the gross migration of sludge particles, and thus reduced the effectiveness of freeze-thaw conditioning. Volkhin and Zolotavin [7] suggested that complete freezing of ferric hydroxide coagulants can only be achieved at freezing temperatures lower than the eutectic temperature of the water/electrolyte system, and higher freezing temperatures would cause incomplete freezing of coagulants which would reduce the freeze-thaw effectiveness. Jean et al. [11] investigated the formation of eutectics in clay slurry and activated sludge and reported that eutectic peak disappears in activated sludge possibly due to the presence of particles. They also reported significant improvement in sludge dewaterability after freeze-thaw conditioning even if the freezing temperature was higher than the eutectic temperature. Addition of cations directly affects activated sludge properties and the freezing conditions. Depending on the type of the cation added, floc size, shape, and density thus sludge dewaterability and settleability changes. In addition, cations lower the freezing temperature which would change the freezing conditions altogether. Some of these changes may improve, and others may hinder the effectiveness of freeze-thaw conditioning. For example, monovalent cations (such as Na+ and K+) may replace divalent cations (Ca+2 and Mg+2) which are known to aid in flocculation by forming cation bridges in the floc structure [3, 4, 12]. Replacement of cations is likely to decrease the average floc size and improve freeze-thaw conditioning. On the other hand, *Author to whom correspondence should be addressed. E-mail: [email protected]. ABSTRACT: This study investigates the effect of monovalent (Na+ and K+), divalent (Ca+2 and Mg+2), and trivalent (Al+3 and Fe+3) cations on freeze-thaw conditioning of activated sludge. Effects of cations on sludge characteristics and bioflocculation have been studied heavily, but few studies have investigated the effect of cations on freeze-thaw conditioning of sludges. This study evaluated the changes in the dewaterability, settleability, turbidity, and solids content of freeze-thawed activated sludge aggregates after the addition of NaCl, KCl, CaCl2, MgCl2, FeCl3, and AlCl3. The results showed that addition of cations does not improve the freeze-thaw conditioning of activated sludge. The improvement in the dewaterability and settleability of activated sludge after freeze-thaw conditioning decreases with increasing concentrations of cations, and the type of cation seems to be a factor in determining the overall effectiveness of freeze-thaw conditioning.