I run a wastewater treatment plant that is subject to the risk management program regulations in 40 CFR Part 68 for a covered process containing chlorine. A chlorine scrubber system at the plant is designed to prevent any possible releases from reaching a toxic endpoint offsite. How does this active mitigation system affect my selection and analysis of an alternative release scenario?

Although only passive mitigation systems (not active) may be considered in the analysis of the worst-case release scenario (40 CFR §68.25(g)), an active mitigation systems (e.g., chlorine scrubbers), also may be considered in the analysis of alternative release scenarios as long as the system is capable of withstanding the event that triggered the release and would still be functional (40 CFR §68.28(d)). When selecting the alternative release scenario, the owner or operator must consider the five-year accident history and failure scenarios identified in the hazard review or process hazard analysis (40 CFR §68.28(e)). Additionally, the owner or operator must select an alternative release scenario that is more likely to occur than the worst-case scenario, and that will reach an endpoint offsite, unless no such scenario exists (40 CFR §68.28(d)). If consideration of active mitigation systems at the stationary source results in no release scenarios that reach an endpoint offsite, then the owner or operator still must analyze and document an alternative release scenario, even if its endpoint will be reached within the boundaries of the facility.