Water Treatment: Clarification, Filtration, Disinfection, and Softening
Water Clarification
Clarifiers of American origin act by carrying out two operations simultaneously on the same device, in addition to proceeding with the automatic removal of sludge. The flocculant and the raw water are introduced through the top and make contact with preformed sludge kept in suspension by a horizontal vane agitator, P, driven by a variable speed motor, M.
Water Filtration
Filtering aims to retain the particles in suspension that the water carries, both from the raw water and originating in a prior process of flocculation. The filtering is done by passing water across a porous material. This material acts in two main ways: a mechanical one, retaining the solid particles in the water, and a biological one, absorbing colloidal material in the water by the action of microorganisms. These microorganisms serve as the filter medium and secrete substances that exert a coagulating action on the water.
There are two types of filters: slow and fast. Slow filters have a filtration rate of approximately 5 to 10 cubic meters per day, while rapid filtration has a characteristic speed of 5-25 cubic meters per hour. Slow filtration is more rapid and complete; it can address both mechanical and biological aspects. Rapid filtration, due to its high speed, only acts on the mechanical aspect. Different characteristics of the filter must be taken into account, including the choice of filter material, particle size, and characteristics of materials used, usually gravel and sand.
Water Disinfection
Most of the water, although previously treated, is generally contaminated by germs and bacteria. If a slow filter is used, a large area can achieve a very important cleansing. Yet, in most cases, the treatment must be completed with effective sterilization to assure total disinfection of the water. The main methods used industrially for disinfection are:
- Chlorination: the addition of chlorine and its derivatives and compounds.
- Ozonation: using ozone.
- Ultraviolet light.
Chlorination
The oxidizing action of chlorine destroys organic matter, making chlorine and its derivatives the most commonly used reagents for water sterilization. If used as chlorogas, it is dissolved in water, which produces hydrochloric acid and hypochlorous acid. The decomposition of the latter yields hydrochloric acid and oxygen. Hypochlorites, especially sodium and calcium, are used for their direct action in the form of dust on the water to be treated. Chlorine and nitrogen compounds, like chloramines, are widely used for removing tastes from water. Chlorine peroxide is also used in solution, through the action of chlorine or acid on sodium hypochlorite. The use of chlorine gas carries the risk of leakage, with the attendant problems of toxicity. Therefore, it is more convenient to use hypochlorite.
Water Softening
Water hardness produces various drawbacks, such as scale formation in pipes and water heaters, with a corresponding decrease in their sections and, consequently, a decrease in flow. It also causes increased spending on soaps and detergents, further weakening the fibers of tissues and leading to different kinds of smells when calcium-magnesium salts are deposited in tissues. The use of water softening plants is not just a convenience but also has a major economic impact. The estimated savings on soap and detergents is three to four times greater than the cost of the softening treatment. Another type of savings is that there would be no need to separate the facilities to use soft water.
Methods of Water Softening
There are three main current methods of water softening:
- The soda-lime process.
- The sodium cation exchange cycle.
- The two-step softening process, which is a combination of lime and cationic exchange with the sodium cycle.
Process with Cold Lime and Soda
This process may use calcium hydroxide (hydrated lime or lime) for the reduction of hardness due to carbonates or by adding sodium carbonate, also known as washing soda or soda ash. If only calcium hydroxide is used, the process is known as a “cold lime” process. If lime and soda are used, the process is known as “lime soda in cold.” Besides these products, the process uses small quantities of other products such as alum, aluminum sulfate, and iron salts. These additives act as coagulants on excessively fine precipitates, ensuring good settlement of the precipitate formed and thus promoting further softened water filtration. The precipitates that will be produced are calcium carbonate and magnesium hydroxide.
Water Softening Plants
The water treatment plants for the municipal cold lime process mainly include the following equipment:
- Dosing equipment, allowing the addition of both reagents and coagulants in appropriate doses.
- Softening units, in which softening and coagulation reactions take place, with the settlement of most of the precipitates.
- Filters that remove the last traces of the precipitate before the water is used.
- A regulation reservoir where treated water is accumulated and from which it is pumped into the system for use.
Types of Water Softeners for the Cold Lime Process
Types of continuous processes currently used include three classes:
- Suspended beds
- Conventional
- Catalytic
Suspended Beds
In this case, the soft water is filtered through a layer of precipitates that have previously been formed. In conventional equipment, such filtration is not carried out since the precipitates that form are collected at the bottom of the reactor. In treatment units with suspended beds, the raw water and reagents are mixed in one compartment of the precipitator. Soft water exits at the bottom of this compartment, causing the water to seep up through the bed of pellets that has been previously formed. This upward seepage has many advantages, such as a smaller equipment size, clarity of the effluent, more effective use of added reagents, and a faster softening reaction.
Conventional
In conventional softeners, hard water and additives are sent down a central tube within the softener, then gently rise towards the exit. Most of the precipitates formed are deposited on the bottom and periodically withdrawn.
Catalytic
In this case, the water is treated with lime as it ascends through a cyclone-shaped reactor and a bed that acts as a catalyst. This bed is normally composed of calcite ground in different sizes. Calcium carbonate formed in the treatment is deposited on the granules that form the catalytic bed, causing it to slowly increase in size. At periodic intervals, coarse particles are retained in the bottom of the reactor, adding a new charge of catalyst. Water is collected at the top. It has the disadvantage that it cannot be used to remove magnesium hardness.