WATER TREATMENT
< 6.5 , whereas at pH > 8.5 dissociation is virtually complete .
Real-world conditions
The reality is slightly more nuanced than the above would suggest , however . Other factors , particularly temperature and ionic strength , will impact both the product stability and the pH range at which HOCl predominates .
In practice , when either HOCl or OCl- is used for their biocidal properties , water samples are tested for ‘ free chlorine ’ content . The measured value will include all the dissolved chlorine gas , HOCl and OCl- anions . As the test does not differentiate between the species , it is not a true indicator of the biocidal efficacy of the water .
For this reason the UK Health & Safety Executive ( HSE ) document , HSG274 Part 1 , The Control of Legionella Bacteria in Evaporative Cooling Systems , now requires that a combination of free chlorine concentration and pH are used to determine if the concentration of HOCl is sufficient for microbiological control during routine operation .
Increased concentrations of free chlorine , or alternative disinfectants , are recommended where the pH is above 8 . Similarly , for domestic water systems PD855468:2015 ‘ Guide to the flushing and disinfection of services supplying water for domestic use within buildings and their curtilages ’. This provides details of required extensions to contact times for disinfection of systems with a pH above 7.6 .
Challenges in use
Traditionally , active chlorine for water treatment is supplied as a sodium hypochlorite solution ( bleach ). This is not only corrosive but also harmful to the environment . In addition , the high pH of the product can be a factor in scale formation at the dosing point .
The corrosive nature of OClsolutions , particularly to metals like stainless steel and aluminium , can limit their use in certain applications .
Genox systems come in various sizes
OCl- solutions can cause discoloration and degradation of fabrics , plastics and other materials , further limiting their applicability in sensitive environments .
The alkaline pH of the OClproducts can also increase the pH of the treated water , resulting in a higher proportion of the less efficacious OCl- being present . HOCl , on the other hand , can be supplied as a solution under the trade name Ultralox .
This product is prepared in such a way that the ionic strength is reduced as far as possible , thereby enhancing the stability of the solution . In this form , the solution is far less corrosive and has a better safety profile than sodium hypochlorite .
Hypochlorous acid can also be generated onsite without the use of hazardous chemicals through the electrolysis of brine solution on demand . The electrolysis reaction can be depicted as : 2NaCl ( aq ) + 2H 2
O → 2Na + ( aq ) +
2OH- ( aq ) + Cl 2 ( g ) + H 2
( g ) This generates the free chlorine :
Cl 2
( g ) + H 2 O ( l ) → HOCl + HCl
Cl 2
( g ) + 2OH- ( aq ) → OCl- ( aq ) + Cl- ( aq ) + H 2
O ( l ) depending on pH
The electrolysis process takes place in a specialised generator , such as Genox , and the resulting biocidal solution can be easily introduced into the water system by controlling it with a water meter and sensor . This negates the need for any mixing or handling of hazardous chemicals .
Different options
The various routes to obtain HOCl in a solution will depend on the system , which is being treated , as well as the ability to handle hazardous chemicals on site .
Genox systems ( pictured ) are available in a range of sizes that can cover hot and cold-water services in all buildings . Typically , Ultralox is preferred for smaller systems . Systems using electrolytic generation , known as Hyprolyser , are also available for swimming pools and spas .
It is well known that HOCl has more bactericidal activity than OCl- , which is generally attributed to its ability to penetrate cell walls more easily . There are numerous studies showing its efficacy as an antimicrobial agent .
HOCl will be the predominant form of free chlorine at pHs in the neutral to slightly acidic pH ranges . HOCl can be added to systems either as Ultralox , by use of Genox generators ( electrolysis ) or by the addition of OCl- ( given suitable pH ). Using Ultralox or Genox is preferable when considering the safety profile of the various options . ●
Kathy Hood
TECHNICAL REGULATORY LEAD
MARLOWE ENVIRONMENTAL k + 44 1327 871967 J kathy . hood @ wcs-group . co . uk j www . marlowe-es . com
NOV / DEC 2024 SPECCHEMONLINE . COM
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