**3. Coagulation – flocculation**

Coagulation – flocculation is a physical-chemical process of metal ions removal from water and wastewater, which consist of three steps (**Figure 3**). In the first step, a coagulation agent is introduced under vigorous stirring having the role of colloidal

**Figure 3.** *Scheme of the coagulation-flocculation system.*

*Heavy Metals Removal from Water and Wastewater DOI: http://dx.doi.org/10.5772/intechopen.110228*


### **Table 2.**

*The efficiency of various coagulation/flocculation processes applied to different wastewater treatment.*

substance destabilization. Vigorous stirring facilitates the contact between the metal ions and the coagulant. In the second step, under gentle stirring, a flocculant is introduced, which has the role of inducing the union of destabilized particles with the aim of forming large flocs, easily separable. The collision between particles and the growth of flocs is facilitated by the gentle stirring of the reaction mass. In the third stage, the resulting sludge and clarified effluents are separated. Separation can be done by settling, flotation, or filtration [2, 3].

The first and most intensive coagulants used in water treatment are those based on Fe3+ and Al3+, such as ferric sulfate—Fe2(SO4)3∙ 8H2O; ferrous sulfate—FeSO4∙ 7H2O; ferric chloride—FeCl3; aluminum sulfate—(Al2SO4)3∙ 14H2O; aluminum chloride— AlCl3∙ 6H2O; poly aluminum chloride—(Al(OH)1.5(SO4)0.125Cl1.25)n; and sodium aluminate—NaAlO2 [28–33]. Among the two trivalent metal ions, coagulants based on Al3+ are preferred because red coloring is avoided, as can happen when using coagulants based on Fe3+. In both cases, a subsequent pH adjustment is needed, and a great dose of coagulant is required for efficient flocculation. The main disadvantage of using inorganic coagulants is a large amount of resulted sludge and its toxic nature, which leads to serious health problems. For this reason, researchers have turned their attention to the use of organic coagulants, especially polymers [34–37]. Even if the polymers are user-friendly, they do not require pH adjustment, and the obtained flocs are large and easily separated, their high price minimizes their use on a large scale. Thus, an alternative to the drawbacks of synthetic polymers is the use of natural polymers, which are found in abundance and are environmental-friendly, due to their biodegradable property [38–40]. But this characteristic, biodegradability, limits their applications, because the formed flocs present instability and loss of strength in time, the natural polymers presenting a shorter lifetime of the active compound. Coagulation–flocculation cannot achieve a 100% heavy metals removal efficiency; therefore, these techniques must be followed, or combined with other treatment processes. **Table 2** summarizes the methodology and the results of various studies of single or combined coagulation/flocculation processes applied on different waste waters with their metal ions removal efficiencies.

Coagulation/flocculation processes could be applied as primary or tertiary treatment of different types of water. The operating parameters (such as coagulant/flocculant dosage, pH, stirring speed, stirring time, settling time, and temperature) present a significant influence on the metal ions removal efficiency. In order to improve the obtained performance, in some cases the coagulation/flocculation process is combined with different treatment methods.
