1.10.2 Water clearing up and discoloring by coagulation
Coagulation of admixtures in water is a process of enlargement (agglomeration) of colloidal and hanged up particles due to forces of co-operation between molecules and connection in aggregates (flakes). This process ends by emission of particles that are stucked together because of water. Except of hanged up and colloidal particles, the flakes absorb considerable amount of bacteria and substances, which give color to the water. Dose of coagulants (Al2(SO4)3∙18H2O, FeSO4, FeCl3) is determined depending on muddy water content, water index pH, alkaline of water, time of settling and, as a rule, it is in the range of 25-125 mg/l. Optimal dose of coagulants can be determined more precisely by laboratory experiments. For this the investigated water is poured in cylinder, and coagulants of different dozes are added to it. Those water, that causes the formation of the biggest flakes in the water in 10-15 minutes, and the highest their settling in 30-60 minutes is achieved, is considered to be optimal. If there is no such opportunity, then the dose of coagulants is primarily determined by СНиП 2.04.02-84.
The accordance of natural water alkality to the adopted coagulant dose is checked during water coagulation, and if there is a shortage of it, the alkaline reagents are added: lime or soda. Alkaline reagents are entered in mixers as a solution or lime milk together with coagulants.
For intensification of coagulation, the water sometimes is processed by flocculants – substances that make the coagulation quicker (polyamide or silica acid). During such processing the created flakes are bigger and they settle quicker.
Admixtures’ coagulation in water can be achieved by electric-chemical method: water, which is cleared, is passed through electrolyzer with aluminium (or iron) plates (electrodes), to which the permanent electric current is conducted. From anodes’ plates the aluminium (or iron) transfers into the solution as ions, which are hydrolyzed and create flakes.
Solution preparation at clearing stations is conducted in departments (reagent economy), where special tanks with solutions and dossal advices are located.
Mixing of coagulants’ solutions with cleared water is conducted in special devices – mixers.
Intensive mixing in mixers is achieved due to swirl streams of water by immovable directional surfaces or as a result of mechanical mixing by mixers.
Mixers can be: of tray type – with holes and partitions, and swirl (fig.1.48). The water is tricked into the vertical swirl mixer with speed 1-1.2 m/sec; in the result of sharp decreasing of speed, the swirl streams are created in the conical part, to which the coagulants are given.
Process of mixing finishes in 2-3 minutes before the flakes creation.
The water gets from mixers to the flakes creation chamber, in which the speed of moving is such, that flakes don’t settle.
Duration of water presence in chamber is 20-30 minutes. During this time the process of coagulation creation ends and enough big flakes are created. The most spread flakes’ creation chambers are those with partitions (fig.1.48b), swirl (by the type of swirl mixer – fig. 1.48a) and whirlpool (fig. 1.48в).
