1. Types of Solid-liquid Separation
Water purification technologies are classified into several methods based on the fundamental principle of each purification process. Among them, the process to separate the components producing turbidity in water from raw water is solid-liquid separation. In the presence of less soluble components in raw water, high quality treated water can be obtained by separating the components producing turbidity properly. The basic technologies of solid-liquid separation include sedimentation and filtration.
Sedimentation is the method to precipitate the components producing turbidity in water. Larger particle components can be separated by natural sedimentation, while smaller particle components are difficult to precipitate naturally because they suspend stably in water. Due to this, sedimentation treatment is conducted after coagulation-flocculation pretreatment for particle enlargement (coagulation-sedimentation Fig. 1). The removal rate of the components producing turbidity by a sequence of processes is approximately 90%.
Filtration is the method to stay the components producing turbidity in filter beds composed of filter media such as sand. Filtration is classified into slow and rapid filtrations.
Slow filtration is the method to stay the components producing turbidity on biological membranes produced on the filter bed surface, oxidize inorganic substances and remove biodegradation substances by decomposition.
Rapid filtration is the method to remove by adsorbing the components producing turbidity transported into filter beds to the particle surfaces of filter media (Fig. 2). That is to say, it is the method based on coagulation of the components producing turbidity and filter media particles. Coagulation process for influent water is vital. The removal rate of the components producing turbidity reaches approximately 99%. This is the more accurate separation method than coagulation-sedimentation.
Recently membrane filtration technology has widely been employed. This is the method to separate the components producing turbidity by fine cores formed on membrane surface. 100% separation of the components producing turbidity is expected when grain size is over the membrane pore diameter.
Fig. 1 Diagram of coagulation-sedimentation
Fig. 2 Diagram of filtration
2. Principle of coagulation
As mentioned above, coagulation process is vital for sedimentation and rapid filtration treatment. Coagulation indicates the process that a coagulant neutralizes the surface charge of the components producing turbidity suspending stably in water. This is done by electrical repulsion. At the same time, the components producing turbidity are conglomerated (Fig. 3). That is, the function that neutralizes surface charge and simultaneously conglomerates the components producing turbidity neutralized is required for a coagulant. Injection ratio, rapid mixing after injection and existence form of coagulation in water are important requirements to be considered for coagulants to play the said roles. Among them, existence form depends greatly on pH.
Although the mechanism of coagulation is clarified, it is difficult to decipher theoretically the injection ratio, driver operational factor, and optimal pH because it is impossible to comprehend all amounts and quality of the components producing turbidity which control them with time. Therefore, calculation in the actual processes is necessary based on prediction of optimal coagulation conditions by implementing the jar test periodically(Fig. 4).
Fig. 3 Diagram of coagulation
Fig. 4 Jar test
3. Types and characteristics of coagulant
Metal salts such as aluminum and iron are generally used as a coagulant. Also, after the surface charge of the components producing turbidity is neutralized, polymer coagulant can be used to promote cross-linking and enhance flock strength. This case is called coagulation aid. As hereinafter defined, the proper selection of a coagulant responding to water quality of local raw water is the most important because every coagulant has its own characteristic.
(1) Aluminum coagulant
Aluminum sulfate is widely used as an aluminum coagulant. Although the applicable range of pH is small, superior quality treated water is obtained when it is used under the optimal coagulation conditions. In Japan, polyaluminum chloride, which has a wider applicable range of pH than aluminum sulfate is used. However, the amount of aluminum in treated water is likely to rise because the effect of coagulation is significantly reduced when polyaluminum chloride is used for raw water with high water temperature and pH.
(2) Iron coagulant
Ferric chloride and ferric sulfate are used as an iron coagulant. Ferric chloride and ferric sulfate can precipitate easily because their applicable range of pH is wide and flock intensity produced is large. However, much attention must be paid to a corrosion of the distribution system because their optimal pH is lower than an aluminum coagulant.
(3) Polymer coagulant
Based on the types of surface charge, polymer coagulant is classified into the three following types: cationic, anionic and nonionic. Polymer coagulant is often used as a coagulation aid. There is a polymer coagulant which shows coagulation performance on its own. For instance, cationic polymer coagulant works for coagulation of organic matter or colloid by itself. However, much attention must be paid to some toxic polymer coagulants.
4. Selection of solid-liquid separation
In the case of few components producing turbidity in raw water, filtration alone can conduct solid-liquid separation. On the other hand, in the presence of many components producing turbidity in raw water, coagulation-sedimentation as a pretreatment must be conducted. If light density components such as algae are included in raw water, “dissolved air floatation”, which adsorbs the components producing turbidity to minute bubbles produced in water and then floats them, is sometimes used first.
In short, when solid-liquid separation is selected, advantageous processes to separate high concentration are installed first and finer separation processes like filtration must be subsequently used.