Flocculation coagulation, slow

The experimental stability ratio is defined generally as the rate of fast flocculation divided by the rate of slow coagulation (see, e.g., Section 7.8 in R. J. Hunter, op. cit.1). Equation 6.70 is a mathematical interpretation of this definition in terms of experimentally accessible quantities related to floccule size. 

Flocculation is the agglomeration of charge-neutralized particles into larger particles. Unlike coagulation, rapid mixing is not required for flocculation to take place. It typically occurs in the reaction chamber or "slow mix" zone of the clarifier. 

Polymeric forms of the coagulants (e.g., polyaluminum chloride) are rather useful because they require lower alkalinity consumption and the concomitant lower production of sludge. Once the colloids are destabilized, slow stirring can also help them merge and form relatively large aggregates called floes (hence the name flocculation) that are separated from water by settlement and filtration. In some cases, these floes can be attached to purposefully generated gas bubbles and separated by flotation. 

Conventional treatment, which includes coagulation, flocculation, clarification (sedimentation or flotation), and filtration, along with disinfection, can achieve 99.9% inactivation of Giardia cysts and 99.99% inactivation of enteric viruses when properly designed and operated. Direct filtration, slow sand filtration, and diatomaceous earth filtration systems, each combined with disinfection, have also achieved these reductions. 

A typical metal precipitation system is shown in Figure 5. Some systems have equalization basins to equalize flow into the treatment system, along with a prereaction tank for pH adjustment. The central part of the treatment plant is the rapid mixing tank with a detention time of between 1 and 5 minutes, where coagulating chemicals are added and a slow mix tank or flocculation tank with a detention time of 20 to 30 minutes for the agglomeration of metal precipitates or floes. Organic polymers may be added to aid in the flocculation. The precipitates are settled out in a settling tank or clarification tank with typical overflow rates... 

The stabihty of the latexes was determined by determining the critical coagulation concentration (ccc) using CaClj. Although the CCC was low (0.0175-0.05 mol dm ), it was higher than that for the latex prepared without surfactant The subsequent addition of INUTEC SPl resulted in a large increase in the CCC, as illustrated in Figure 17.2, which shows log W-log C curves (where W is the ratio between the fast flocculation rate constant to the slow flocculation rate constant, referred to as the stability ratio) at various additions of INUTEC SPl . 

The kinetic theory of fast irreversible coagulation was developed by von Smoluchowski. Later the theory was extended to the case of slow and reversible coagulation. In any case of coagulation (flocculation), the general set of kinetic equations reads... 

Protozoa may become another problem of water treatment. Usually, removal or inactivation of Entamoeba has been the central problem, but recent investigations also concerns Giardia [43]. Proper coagulation, flocculation, sedimentation, and filtration can remove up to 99% of protozoan cysts slow sand filtration can remove Giardia cysts [16]. Improper operation of treatment facilities can cause outbreaks of water-borne illnesses caused by protozoa [44]. Chlorination and disinfectants such as chloramines and ozone can further reduce levels of protozoa [45-47]. 

Usually, in a conventional coagulation-filtration process for arsenic removal, the addition of the coagulant is followed by a short rapid-mix step followed by a slow-mix step for flocculation. Flocculation is usually followed by sedimentation and filtration. To do away with the multitude of steps required in the conventional process, the authors designed and tested a simplified iron coagulation-direct mi-croflltration process, as described below (3,4). 

Coagulation-flocculation ineffective, supernatant murky pH > 10/wrong dosage of alum or coagulant/pH < 4/increase in concentration of particles in feed/rpm of reels in flocculation basin too slow/feed temperature <12 °C/rpm of reels in flocculation basin too fast. 

Now a colloid may be termed stable when it does not flocculate, say, in a week or a month, which means that the time of flocculation should be longer than 10 sec. Consequently the ratio IV between rapid and slow coagulation will have to surpass 10 for diluted and 10 for very concentrated sols, to give them a reasonable stability. 

Sometimes slow filtration is used without previous coagulation. This is generally practiced with water that does not ctmtain much suspended matter. If the water is loaded (periodically or permanently) with clay particles in suspension, pretreatment by coagulation-flocculation is necessary. Previous adequate oxidation of the water, in this case preozonization producing biodegradable and metabolizable orgairic derivatives issuing from disstdved substances, can be favorable because of the biochemical activity in slow filters. 

Many of the particles found in water have low densities close to or even less than those of water. Particles less dense than water have a tendency to rise to the surface from which they can be skimmed off, but this is often a slow and incomplete process. The removal of such particles can be aided by dissolved air flotation in which small air bubbles are formed that attach to particles causing them to float. As shown in Figure 5.2, flotation of particles with air can be accompanied by coagulation of the particles as aided by a coagulant. Water supersaturated with air under pressure is released to the bottom of a tank where bubbles are formed in a layer of milky (white) water. Bubble formation accompanied by flocculation of the particles entrains bubbles in the floe, which floats to the surface where it is skimmed off. 

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