Combining coagulation, softening and flocculation to dispose reverse osmosis retentates

Utilization of lime-softening sludge to obtain calcium nitrate

“…The traditional and the cheapest method of reducing the carbonate hardness of water is considered its treatment with a solution of calcium hydroxide Са(OH) 2 , the so-called lime-softening. Lime-softening along with simultaneous coagulation is also used to dispose of liquid water treatment waste [2]. As a result of the process, the sediment is formed, namely lime-softening sludge.…”

Combining lime softening with alum coagulation for hard Ghrib dam water conventional treatment

“…This elevation is due to the replacement of Ca 2+ and Mg 2+ cations by Na + cation as is well-known for softening (Arrigo et al, 2007). The chemical reactions describing these phenomena are grouped as follows (Ordóñez et al, 2012;Wang et al, 2005;Abdessemed and Nezzal, 2008 …”

“…Hardness can be measured by the reaction of polyvalent metal ions in a water sample with a chelating agent such as ethylenediaminetetraacetic acid and is expressed as the equivalent concentration of calcium carbonate (CaCO3) (Rodier, 2009;Gabrielli et al, 2006). Water hardness may be evaluated by determining separately the concentration of each element of the hardness and expressing their sum as an equivalent amount of CaCO3 (Ordóñez et al, 2012;Fang et al, 2013;Nanda et al, 2010).…”

“…Because of the restricted solubility of CaCO3 and Mg(OH)2, during chemical precipitation process, zero hardness water cannot be achieved (Yildiz et al, 2003). On the other hand, classical softening plants performing soft water treatment comprise several unit operations like coagulation/flocculation (Ordóñez et al, 2012;Ghernaout et al, 2009Cheng et al, 2017;Yan et al, 2009), sedimentation, recarbonation and filtration (Wang et al, 2005). Sedimentation and filtration units are employed for separating particulate matters like CaCO3 and Mg(OH)2 (Yildiz et al, 2003;Comstock et al, 2011;Vahedi and Gorczyca, 2011).…”

“…Lime softening is also recognized by the U.S. Environmental Protection Agency (EPA) as the best applied technique for arsenic, barium, beryllium, chromium, copper, fluoride, lead, mercury, cadmium, nickel, and radionuclides (Brastad and He, 2013;Kweon and Lawler, 2004). Nevertheless, the disadvantages of lime softening comprise the formation of a highvolume lime sludge stream (Brastad and He, 2013), and the required use of chemicals such as quick lime, coagulants (iron or aluminum based), soda ash, and acids for adjusting the pH (Ordóñez et al, 2012;Brastad and He, 2013).…”