What minimum removal of Cryptosporidium did the Interim Enhanced Surface Water Treatment Rule require of systems that filter?
- 2-log (99%)
- 1-log (90%)
- 3-log (99.9%)
- 4-log (99.99%)
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A. 2-log (99%)
Filtered systems must achieve at least 2-log Cryptosporidium removal, which is credited to plants that meet the tightened turbidity limits. The 3-log and 4-log figures are the Giardia and virus requirements from the original rule.
Reference: 40 CFR 141.170 and 141.173 (Interim Enhanced Surface Water Treatment Rule)
A well-operated conventional filtration plant is normally credited with 2.5-log Giardia removal. How much Giardia inactivation must its disinfection step then provide?
- 1.0-log
- 2.5-log
- 3.0-log
- 0.5-log
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D. 0.5-log
The total requirement is 3-log. With 2.5-log credited to coagulation, sedimentation and filtration, the remaining 0.5-log must come from disinfection, and that is the CT the plant has to meet. A direct filtration plant is usually credited with 2.0-log and so needs 1.0-log from disinfection.
Reference: Surface Water Treatment Rule; EPA Guidance Manual for Compliance with the Filtration and Disinfection Requirements (removal credits)
Under the Surface Water Treatment Rule, the disinfectant residual in water entering the distribution system may not be below 0.2 mg/L for more than:
- 1 hour
- 4 hours
- 24 hours
- 15 minutes
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B. 4 hours
The residual at the entry point must be monitored continuously (grab sampling is allowed for small systems) and may not stay below 0.2 mg/L for more than 4 hours. A lapse longer than that is a treatment technique violation. The 15-minute figure relates to how often continuous turbidity readings are recorded, not to this limit.
Reference: 40 CFR 141.72(b)(2) (disinfection requirements for filtered systems)