What characteristic of ozone contributes to its necessity for on-site generation?

The characteristic of ozone that makes on-site generation necessary is its instability during storage. Ozone is a molecule consisting of three oxygen atoms, and it is highly reactive and unstable. This instability means that ozone cannot be stored safely for long periods; it tends to decompose back into diatomic oxygen (O2) fairly quickly when contained.

For this reason, generating ozone on-site, typically using an ozone generator, is the most practical approach for water treatment facilities. By producing ozone as needed, plants can ensure that they are using fresh, effective ozone for disinfection, instead of dealing with potential degradation or loss of potency in stored ozone.

While the other options touch on aspects related to water treatment, they do not directly influence the necessity of generating ozone on-site in the same way that instability during storage does. High cost can vary depending on production methods, large volume requirements relate to storage of the treated water rather than ozone itself, and environmental restrictions, while important, do not inherently dictate the feasibility of ozone storage like its instability does.