11.3 Surface Water Treatment Rules & Log Removal Credit

A Layered Set of Rules

The Surface Water Treatment Rule and Amendments is listed by name in the treatment blueprint. It is not one rule but a stack, each layer adding a requirement without repealing the one below:

Two universal SWTR residual requirements operators must know:

• A detectable disinfectant residual must be maintained in the distribution system, or an HPC of ≤ 500 CFU/mL may be used in lieu of a detectable residual.
• The residual entering the distribution system may not be undetectable for more than 4 hours.

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Log Removal, Log Inactivation, and Total Log Reduction

$\text{Total Log Reduction} = \text{Log Removal (physical)} + \text{Log Inactivation (disinfection)}$

Log removal is credit for physically taking organisms out - coagulation, sedimentation, filtration, membranes. Log inactivation is credit for killing or disabling them - chlorine, ozone, chlorine dioxide, UV. A log is a factor of ten:

Filtration Removal Credits

What Disinfection Must Then Supply

[!IMPORTANT] This is the single most useful table in the whole SWTR topic. A direct filtration plant needs twice the Giardia inactivation and much more virus inactivation than a conventional plant treating the same water, because it gets less physical credit. That is why so many California direct filtration plants run higher chlorine residuals or added ozone or UV. When a question compares two plants with identical CT, the direct filtration plant is the one closer to non-compliance.

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Turbidity Performance Standards

Turbidity is the day-to-day proxy for physical removal, and the standards are performance-based rather than health-based:

Individual Filter Effluent (IFE) Monitoring

Under IESWTR/LT1, each individual filter must be monitored continuously, with readings recorded at least every 15 minutes. Reporting triggers:

• IFE turbidity > 1.0 NTU in two consecutive 15-minute readings - report and, if it recurs in three consecutive months, conduct a filter self-assessment
• IFE turbidity > 0.5 NTU in two consecutive readings at the end of the first four hours after the filter returns to service
• IFE turbidity > 2.0 NTU in two consecutive readings, in two consecutive months - a comprehensive performance evaluation (CPE) by the State or a third party

[!TIP] The rule targets filter ripening deliberately. The 0.5 NTU four-hour trigger exists because the worst water a filter produces is in the first minutes after backwash, which is also when Cryptosporidium is most likely to pass. Filter-to-waste during ripening is the standard operational answer.

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LT2 and Bin Classification

LT2 required systems to monitor source water for Cryptosporidium (large systems by direct measurement; small systems by E. coli screening first) and then be assigned to a bin:

Additional credit is earned from the microbial toolbox: watershed control, bank filtration, second-stage filtration, membranes, bag/cartridge filters, combined filter performance, presedimentation, roughing filters, ozone, chlorine dioxide, and UV - which is the most common answer because UV delivers large Cryptosporidium credit at low dose.

LT2 also addressed uncovered finished water reservoirs: systems had to cover them or treat the discharge to achieve 4-log virus, 3-log Giardia, and 2-log Cryptosporidium inactivation. This is why California utilities spent the 2010s covering or replacing open finished-water reservoirs.

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Putting It Together

A conventional plant treats a Bin 2 source.

• Giardia: 3-log required; filtration gives 2.5-log; disinfection must supply 0.5-log.
• Virus: 4-log required; filtration gives 2.0-log; disinfection must supply 2.0-log.
• Cryptosporidium: 2-log from conventional filtration plus 1.0-log additional for Bin 2 = 3.0-log total. Chlorine provides essentially no Cryptosporidium credit at practical CT, so the plant must add UV or ozone to earn the extra log.

That single scenario explains most California plant upgrades of the past two decades: UV was added not for Giardia or viruses, but for Cryptosporidium bin credit.