What Is an Inorganic Flocculant?
A flocculant is an agent that combines small suspended particles in water to form coarse particles (floc) or to enlarge (coarsen) the floc that has formed.
Flocculants are classified into two types: inorganic flocculants and polymeric flocculants. Inorganic flocculants are used to form flocs and can be broadly classified into aluminum-based and iron-based flocculants, such as polyaluminum chloride (PAC) and iron chloride, which are well known.
Unlike inorganic flocculants, polymeric flocculants are added to coarsen the formed flocs rather than to form the flocs. Therefore, they are also called flocculants. Typical flocculants are hydrolyzed polyacrylamide.
Uses of Inorganic Flocculants
As mentioned above, the addition of inorganic flocculant produces flocs of suspended particles in water.
Among the suspended particles, fine particles with diameters of 1 nm to 1 µm are difficult to separate and remove by ordinary sedimentation or sand filtration processes. Inorganic Flocculants increase the apparent size of particles, making it possible to apply these treatment methods to particles that would normally be difficult to separate.
Inorganic flocculants are used to remove suspended particles from sewage in the treatment of water and wastewater. Since each coagulant has different operating conditions, such as pH and water temperature, the most appropriate agent must be selected according to the water quality and composition of the water to be treated.
Principle of Inorganic Flocculants
The surface of suspended particles in water has a negative charge. These negatively charged particles repel each other and thus disperse without settling.
On the other hand, the aluminum and iron ions in inorganic flocculant are positively charged. By adding these ions, which have opposite charges to the suspended particles, the surface charges of the particles can be neutralized and canceled. When the repulsive force is thus reduced and becomes less than the inter-particle attraction, the particles bond with each other and flocs are formed.
The speed at which the floc settles is proportional to the square of the particles. In other words, the larger the coarser and larger the particle diameter, the faster the floc settles.
The addition of inorganic flocculant produces flocs with relatively small particle diameters. Therefore, in order to promote particle coarsening, a polymeric flocculant is sometimes used in addition to the Inorganic Flocculant.
When a polymer flocculant is added, the flocs are cross-linked by the flocculant. This cross-linking action causes coarsening of the flocs, which increases the sedimentation rate and thus separation efficiency.
Selection of Inorganic Flocculants
Inorganic flocculants are often used to purify water, but there is a general unsuitability between the quality of the raw water and the type of flocculant, as well as a kind of compatibility that can only be seen during actual operation. Inorganic flocculant selection requires a variety of perspectives.
Inorganic flocculants can be categorized into aluminum-based, iron-based, chlorine-based, and sulfur-based flocculants, and calcium-based flocculants.
Aluminum-based flocculants are often used to soften industrial water or to purify water that does not contain heavy metals, such as tap water, with the image of further purifying clean water with little contamination. Iron-based coagulants are often used to purify water containing heavy metals and are used to purify wastewater that is difficult to treat. In either case, there is no general distinction between the two, and the choice is made based on the treatment test of raw water and the running cost.
Chlorine and sulfur systems affect the final wastewater sludge treatment method. Chlorinated sludge cannot be treated for composting. Also, sulfur systems may generate foul odors depending on the quality of the raw water, which will affect the choice of treatment destination. If the sludge does not contain chlorine and does not generate a foul odor, the number of treatment sites can be expanded, but this will result in a penalty to the recipient and an increase in treatment costs. In addition, depending on the composition of the sludge, it may be possible to create value, including selling it. At steel mills, the majority of wastewater treatment sludge is rich in iron, so it is recycled as a raw material for iron. In selecting an inorganic flocculant, it is important to consider the running cost, including the destination of the sludge, in addition to purifying water.
Recycled Inorganic Flocculants
Iron chloride solution is used for etching and surface treatment of metals, and its waste solution is sold as recycled products after the metal content is removed by the manufacturer. Inorganic Flocculant is also used for wastewater treatment because it is more reasonably priced than new aluminum flocculant.