Roles of coagulant species and mechanisms on floc characteristics and filterability

Coagulation and flocculation are an essential part of drinking water treatment as well as wastewater treatment. Coagulation and flocculation are essential processes in various disciplines. In potable water treatment, clarification of water using coagulating agents has been practiced from ancient times. As early as 2000 BC the Egyptians used almonds smeared around vessels to clarify river water. The use of alum as a coagulant by the Romans was mentioned in around 77 AD. By 1757, alum was being used for coagulation in municipal water treatment in England. 

About Coagulants and Polymers

Primary coagulants: Primary coagulants are used to cause particles to become destabilized and begin to clump together (California State University, 1994). Examples of primary coagulants are metallic salts, such as aluminum sulfate (referred to as alum), ferric sulfate, and ferric chloride. Cationic polymers may also be used as primary coagulants.

2. Coagulant Aids and Enhanced Coagulants: Coagulant aids and enhanced coagulants add density to slow-settling floc and help maintain floc formation (California State University, 1994). Organic polymers, such as polyaluminum hydroxychloride (PACl), are typically used to enhance coagulation in combination with a primary coagulant. The advantage of these organic polymers is that they have a high positive charge and are much more effective at small dosages. Even though they may be more expensive, a smaller amount may be needed, thereby saving money. Organic polymers also typically produce less sludge.

The commonly used metal coagulants fall into two general categories: those based on aluminum and those based on iron. The aluminum coagulants include aluminum sulfate, aluminium trichloride powderand sodium aluminate. The iron coagulants include ferric sulfate, ferrous sulfate, ferric chloride and ferric chloride sulfate. Other chemicals used as coagulants include hydrated lime and magnesium carbonate.

Important mechanisms relating to polymers during treatment include electrostatic and bridging effects. Figure 1 shows schematic stages in the bridging mechanism. Polymers are available in various forms including solutions, powders or beads, oil or water-based emulsions, and the Mannich types. The polymer charge density influences the configuration in solution: for a given molecular weight, increasing charge density stretches the polymer chains through increasing electrostatic repulsion between charged units, thereby increasing the viscosity of the polymer solution.

Roles of coagulants on floc characteristics and filterability

In full scale water treatment operation, the rapid filtration process, as the last step of solid-liquid separation, is largely influenced by floc characteristics. In this study, aluminium sulphate (alum) and nano-Al13 were investigated to understand the influence of coagulant species on the formation and filterability of flocs. At neutral pH, it was found that nano-Al13, a high MW polymer, showed better floc filterability than alum. This is because of the densely compacted and well-distributed size flocs from nano-Al13, even though floc sizes of alum were generally bigger. Al specie distributions of the two coagulants at different pH levels were compared by using electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) to further elucidate the reasons for the superiority of nano-Al13 in floc filterability. The floc regrowth ability of nano-Al13 at high shear rates (200 rpm and 300 rpm) was much better than at low shear and better than any shear applied to alum and the flocs after breakage at 200 rpm and 300 rpm also showed better filterability than other conditions.