The effect is of monooxochloric acid or its anion is due to liberation of oxigen atom due to solar light.    ClO- --------> Cl-   + O.  In acid mediuim Cl2 evolves which is also disinfectant.

1) HOCl is stronger disinfectant than OCl-

2) Optimal pH for formation of HOCl is between pH 2 and pH of 7.5 

3) Below pH 6, Cl2 gas will form and pretty much escape (although it is also disinfectant; it only does a very small job and you lose it as a gas)

4) above pH 8.5, OCl- will form and no disinfection would occur as HOCl is zero.

Conclusion: Optimal Disinfection pH is 6.0 where 95% of the chlorine species is HOCl; whereas at pH 8.5, HOCl is only 10% and OCl- is 90%

Note that at pH < 4, there is a formation of the toxic Cl2 gas, and that is mainly why it is avoided to work at pH 4-6, and it is safely decided to work at pH=6

http://www.aquaox.net/uploads/hocl-ocl_.pdf

Ali answer is right.

What about disinfection by products? Does the pH have any effect on the formation of DBPs since some of the answers suggest that disinfection is ineffective at certain pH. 

Thanks a lot Ali , Luis and Isaac ..  I am very sorry for the late reply... 

The pH effect of DBPs formation is expected to be significant because of different reaction kinetics of Cl2 and HOCl reacting with the present organic matter.

Thanks a lot Dr Ali Farhat 

• Some additional information related DBPs I found on the internet ...
• Water chemistry - formation of TTHMs in relation to HAA5s change with pH. HAA5 is more likely to form at low pH, and TTHM is more likely to form at high pH.
• Bromide - elevated levels can lead to increased formation of brominated DBPs.
• - https://www.tceq.texas.gov/drinkingwater/chemicals/dbp

Trihalomethanes (THMs) are chemical compounds in which three of the four hydrogen atoms of methane (CH4) are replaced by halogen atoms. Many trihalomethanes find uses in industry as solvents or refrigerants. THMs are also environmental pollutants, and many are considered carcinogenic. Trihalomethanes with all the same halogen atoms are called haloforms. Several of these are easy to prepare through the haloform reaction.

Trihalomethanes were the first new drinking water regulation EPA issued after passage of the 1974 Safe Drinking Water Act. The agency had the responsibility to produce all of the supporting information, and in quite considerable detail, and use that information, be it toxicology, analytical chemistry, occurrence, treatment technology, costs, economic impact, to craft its regulation.

- https://en.wikipedia.org/wiki/Trihalomethane

Disinfection by-products (DBPs) result from chemical reactions between organic and inorganic matter in water with chemical treatment agents during the water disinfection process (source: Wikipedia)

Chlorination disinfection byproducts (DBPs)

Chlorinated disinfection agents such as chlorine and chloramine are strong oxidizing agents introduced into water in order to destroy pathogenic microbes, to oxidize taste/odor-forming compounds, and to form a disinfectant residual so water can reach the consumer tap safe from microbial contamination. These disinfectants may react with naturally present fulvic and humic acids, amino acids, and other natural organic matter, as well as iodide and bromide ions, to produce a range of DBPs such as the trihalomethanes (THMs), haloacetic acids (HAAs), bromate, and chlorite (which are regulated in the US), and so-called "emerging" DBPs such as halonitromethanes, haloacetonitriles, haloamides, halofuranones, iodo-acids such as iodoacetic acid, iodo-THMs (iodotrihalomethanes), nitrosamines, and others.

Chloramine has become a popular disinfectant in the US, and it has been found to produce N-nitrosodimethylamine (NDMA), which is a possible human carcinogen, as well as highly genotoxic iodinated DBPs, such as iodoacetic acid, when iodide is present in source waters.[1][2]

Residual chlorine (and other disinfectants) may also react further within the distribution network—both by further reactions with dissolved natural organic matter and with biofilms present in the pipes. In addition to being highly influenced by the types of organic and inorganic matter in the source water, the different species and concentrations of DBPs vary according to the type of disinfectant used, the dose of disinfectant, the concentration of natural organic matter and bromide/iodide, the time since dosing (i.e. water age), temperature, and pH of the water.[3]

Swimming pools using chlorine have been found to contain trihalomethanes, although generally they are below current EU standard for drinking water (100 micrograms per litre).[4] Concentrations of trihalomethanes (mainly chloroform) of up to 0.43 ppm have been measured.[5] In addition, trichloramine has been detected in the air above swimming pools,[6] and it is suspected in the increased asthma observed in elite swimmers. Trichloramine is formed by the reaction of urea (from urine and sweat) with chlorine and gives the indoor swimming pool its distinctive odor.

Byproducts from non-chlorinated disinfectants

Several powerful oxidizing agents are used in disinfecting and treating drinking water, and many of these also cause the formation of DBPs. Ozone, for example, produces ketones, carboxylic acids, and aldehydes, including formaldehyde. Bromide in source waters can be converted by ozone into bromate, a potent carcinogen that is regulated in the United States, as well as other brominated DBPs.[1]

As regulations are tightened on established DBPs such as THMs and HAAs, drinking water treatment plants may switch to alternative disinfection methods. This change will alter the distribution of classes of DBP's.[1]

Occurrence

DBPs are present in most drinking water supplies that have been subject to chlorination, chloramination, ozonation, or treatment with chlorine dioxide. Many hundreds of DBPs exist in treated drinking water and at least 600 have been identified.[1][7] The low levels of many of these DBPs, coupled with the analytical costs in testing water samples for them, means that in practice only a handful of DBPs are actually monitored. Increasingly it is recognized that the genotoxicities and cytotoxicities of many of the DBPs not subject to regulatory monitoring, (particularly iodinated, nitrogenous DBPs) are comparatively much higher than those DBPs commonly monitored in the developed world (THMs and HAAs).[1][2][8]

Health effects

Epidemiological studies have looked at the associations between exposure to DBPs in drinking water with cancers, adverse birth outcomes and birth defects. Meta-analyses and pooled analyses of these studies have demonstrated consistent associations for bladder cancer[9][10] and for babies being born small for gestational age,[11] but not for congenital anomalies (birth defects).[12] Early-term miscarriages have also been reported in some studies.[13][14] The exact putative agent remains unknown, however, in the epidemiological studies since the number of DBPs in a water sample are high and exposure surrogates such as monitoring data of a specific by-product (often total trihalomethanes) are used in lieu of more detailed exposure assessment. The World Health Organization has stated that "the risk of death from pathogens is at least 100 to 1000 times greater than the risk of cancer from disinfection by-products (DBPs)" {and} the "risk of illness from pathogens is at least 10 000 to 1 million times greater than the risk of cancer from DBPs".[15]

Regulation and monitoring

The United States Environmental Protection Agency has set Maximum Contaminant Levels (MCLs) for bromate, chlorite, haloacetic acids and total trihalomethanes (TTHMs).[16] In Europe, the level of TTHMs has been set at 100 micrograms per litre, and the level for bromate to 10 micrograms per litre, under the Drinking Water Directive.[17] No guideline values have been set for HAAs in Europe. The World Health Organization has established guidelines for several DBPs, including bromate, bromodichloromethane, chlorate, chlorite, chloroacetic acid, chloroform, cyanogen chloride, dibromoacetonitrile, dibromochloromethane, dichloroacetic acid, dichloroacetonitrile, NDMA, and trichloroacetic acid.

- https://en.wikipedia.org/wiki/Disinfection_by-product