Break point chlorination

Hypochlorous acid reacts very rapidly and quantitatively with a slight excess of free ammonia forming monochloramine, NH2CI, which reacts at a slower rate with additional HOCl forming dichloramine, NHCI2. Trichloramine is formed when three moles of HOCl are added per mole of ammonia between pH 3—4 (100). Hypochlorous acid in the form of chlorine or hypochlorite is used in water treatments to oxidize ammonia by the process of break-point chlorination, which is based on formation of unstable dichloramine. The instabiHty of NHCI2 is caused by presence of HOCl and NCl (101,102). The reaction is most rapid at a pH of about 7.5 (103). Other nitrogen compounds such as urea, creatinine, and amino acids are also oxidized by hypochlorous acid, but at slower rates. Unstable iV-chloro compounds are intermediates in deammination of amino acids (104,105). 

Break-point chlorination (alternatives are ozone and chlorine dioxide)... 

In the case of strongly polluted surface water, chlorination is the first purification step and is carried out after removal of any coarse foreign matter. Sufficient chlorine is added to ensure a free chlorine concentration of ca. 0.2 to 0.5 mg/L in the water after treatment (break-point chlorination). Chlorine reacts with water forming hydrochloric acid and the hypochlorite anion, depending upon the pH. 

In the Federal Republic of Germany ozonization, as pre-ozonization - a post ozonization step being inserted before flocculation (see Section 1.1.2.2) - has largely supplanted break-point chlorination. 

About 50 to 150 g TOC/m (TOC = total organic carbon) of organic carbon are on average removed from water per day. This value is higher, if the water is not break-point chlorinated (see Section 1.1.2.1) or is pretreated with ozone. 

Together the three compounds contribute to what is known as the ree residual chlorine concentration. When chlorine is added, it reacts with organic matter and ammonia to form organochlorine compounds and chloramines [50.5]. So much chlorine has to be added that a break-point chlorination is obtained. Above the break-point the bactericidal effect is considered good and the viricidal effect moderate. 

What is Break-point Chlorination What is its significance ... 

Break-point chlorination is chlorination of a sample to the extent that all the ammonia present in the sample is converted into N2 or its higher states (like... 

The amount of the combined chlorine residuals thus decreases along the curve CD. Point D indicates almost complete decomposition of chloramines. At this stage, known as Break-point Chlorination, there is marked decrease in the residual chlorine ( a minimum in the curve). A higher dosage of chlorine appears as free chlorine residuals and total chlorine residuals start increasing. This is shown by the part DE of the curve. 

A chlorine dosage higher than Break-point chlorination means that the chlorine demand of all the chlorine-reactable materials has been completely met with and free chlorine residuals are available for bactericidal action. It also signifies complete decomposition of NH3, removal of colouring material and modification of taste and odour of the water sample. 

Intensive physical and chemical treatment, extended treatment and disinfection e.g. chlorination to break-point, coagulation, flocculation, decantation, filtration, adsorption (activated carbon), disinfection (ozone, final chlorination). 

In comparison with the free ClOOCl molecule, the photodissociation mechanism and outcome do not change noticeably we find very little production of CIO, and mainly we have the almost simultaneous but asymmetric breaking of both Cl — O bonds. Since the chlorine atoms of the adsorbed molecule are found to point towards the ice surface, upon photodissociation their recoil pushes them slightly inwards, while an 02 molecule is projected away. 

When the system is stopped to be loaded with raw stock, it receives air with humidity, which breaks down silicon tetrachloride into hydrogen chloride and gel of silicon acid, as well as self-inflammable silanes. Calcium chloride (the melting point is 772 °C, the boiling point is 1600 °C) remains in the chlorinator and combines with pieces of unreacted ferrosilicon to form cinder, which is accumulated in the furnace and loaded off with graphite. 

It is of esjx cial importance to know the potential interval within which one or several distinct reactions take place. The determination of this depends upon the change in potential which the presence of a depolarizer produces as opposed to an electrolyte containing no depolarizer. For example, if it is desired to learn if chlorine derivatives of phenol can be prepared at the anode by electrolysis of a hydrochloric-acid solution of phenol, then the point of decomposition of the chlorine ion, in combination with the hydrogen electrode, is found at LSI volts in a l/i n-hydrochloric-acid solution. If phenol is added to this solution, the break in the curve occurs already at 0.1) volt.2 Therefore the span in potential, within which the reaction for the formation of chlorine derivatives of phenol must take place, lies between 0.9 and 1.3 volts. In thin manner Dony-H6nault, among others, determined the decomposition potential of the OH ions, in combination with the hydrogen electrode, in dilute sulphuric-acid solution both without and with the addition of ethyl alcohol. He found... 

Trioxane is a stable, cyclic trimer of formaldehyde. It has chloroform like odor, and is a crystalline solid with a melting point of 64 Celsius, and a boiling point of 114.5 Celsius. It sublimes readily and is very soluble in water, acetone, alcohol, ether, and chlorinated hydrocarbon solvents. Trioxane forms an azeotrope when distilled with water, boiling at 91 Celsius, and containing 70% trioxane by weight. Trioxane slowly depoly merizes when treated with acids, and in the absence of water, it breaks down to monomeric formaldehyde when treated with acids. Trioxane is inert to alkalies. It is commercially available. 

Beyond point B, the chloro-organic compounds and organic chloramines break down. Also, at this range of chlorine dosage, the monochloramine starts to convert to the dichloramine, but, at the same time, it also decomposes into the nitrogen gas and, possibly, other gases as well. These decomposition reactions were addressed previonsly. 

The breakpoint reactions only break down the decomposable fractions of the respective substances. All the nondecomposables will remain after the breakpoint. This will include, among other nondecomposables, the residual organic chloramines, residual chloro-organic compounds, and residual ammonia chloramines. As we have learned, the trichloramine fraction that comes from ammonia chloramines has to be very small at this point to be of value as a disinfectant. All the substances that could interfere with disinfection and all decomposables wonld have already been destroyed, therefore, any amount of chlorine applied beyond the breakpoint will appear as free chlorine residual. 

The chlorine absorber is enclosed by a jacket through which a hot or cold fluid can be pumped to maintain the absorber contents at any desired temperature. A pressure relief pipe emerging from a point near the top of the absorber is equipped with a rupture disk, a thin metal membrane designed to break if the absorber pressure rises above a certain value. If this happens, gas flows from the absorber through the relief pipe to a surge tank, lowering the pressure in the absorber while containing the released gas. 

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