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Concentration (ppm) 0.1 to 3 3 to 10


10 to 50


50 to 100 100 to 300


300 to 500 500 to 1000 > 1000 Table 1. H2 S health effects at different concentrations. An issue with all chemicals


is that to introduce them to a collection system, a bulk quantity must be stored nearby. To ensure that chemicals are always available for treatment, continued deliveries to the bulk storage tank must be made. To avoidadverse effects to the environment, engineered controls, such as secondary containment and leak monitoring, must be designed, implemented and maintained. Ideally, a successful treatment of wastewater odour and corrosion would:


• End sulfide production • Quickly eliminate sulfides that are present


• Bring about no additional hazard to life or the environment


• Do no harm to the collection system


• Create no additional challenges downstream


In addition,the treatment solution must be cost-effective. One answer is introducing ozone and oxygen into wastewatersystems to control odour and corrosion.


Ozone has long been used in


water treatment, dating back to at least the late 19th century, primarily for the disinfection and


Summer 2020 polishing of drinking water.4 In


Europe, ozone treatment of water is a common process. 5O zone’s environmental sustainability and relative safety versus chemical systems have established it as a favoured current and future technology. The controlled use of ozone as a treatment does not produce harmful byproducts that could contaminate or damagethe environment or ecology. Typically, the only byproducts from its reaction are O2


and inert oxides.


In recent years, interest in its use to treat wastewater has led to the development of new and sustainable (green) technology for odour and corrosion control in wastewater collectionsystems. Ozone is a naturally occurring


form of atmospheric oxygen. Instead of two oxygen atoms it has three, represented by its chemical formula O3


. This third


oxygen atom makes it a highly reactive molecule and a strong oxidizing agent, the fourth highest overall after atomic fluorine, the hydroxyl radical and atomic oxygen. Ozone can be generated by exciting a flow of oxygen with sufficient electrical or optical energy. This will cause a certain amount of oxygen atoms to split


and recombine with other O2 molecules (see below).


302 + Energy > 203


The remainder of the stream is left as oxygen.The concentration is limited to this range because of the following reaction.


Under typical treatment conditions, using a relatively pure oxygen stream and a corona discharge chamber that uses a high-voltage electrical arc, this reaction can produce up to 9 to 12 percentage by weight (wt%) ozone,6 although typically the output is in 1 to 9 wt% ozone.7


202 > 303


As ozone concentrations rise above this concentration, this destruction reaction becomes more frequent, returning greater quantities to O2


and maintaining


this equilibrium. This instability is also the reason ozone cannot be stored and must be generated immediately before application. Because of its extreme instability and highly oxidizing nature, ozone is powerful and indiscriminate in terms of reactivity with other chemical species. Ozone has been shown as an effective treatment for the destruction of volatile organic compounds; removal of metals,


total suspended solids and organic carbon; and significantly reducing chemical oxygen demand. In freshwater, the half-life of ozone is typically 10 to 20 minutes, but in wastewater, ozone has been documented as being entirely consumed within 8.6 seconds.8


is because of the extreme amount of potential reactants present in wastewater including H2 simple structure of H2


S.The S makes it an


easy target for oxidation by ozone. Ozone’s unique structure also tends to create free radicals, chemical species that have unbonded electrons making them highly reactive, especially in water. Not only is the benefit of ozone’s direct reaction with different chemical species realized, but also as part of these reactions, additional free radicals, which can be even more reactive than ozone, can form. Additionally, radicals tend to create additional radicals as they react, in a free radical chain reaction. These additional reactions are indirect effects of ozone.5


With the


source of ozone generation being ambient air, it is the ultimate in sustainable and green chemical treatment. The current technology for producing ozone has benefitted from more than 45 years of


51


Physiological effect Odour threshold Offensive odour


Headache Nausea Throat and eye irritation


Eye injury


Conjunctivitis Respiratory tract irritation Olfactory paralysis


Pulmonary oedema Imminent threat to life


Strong nervous system stimulation Apnea


Immediate collapse with respiratory paralysis


Water Treatment


This


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