FIELD NOTE · SUPPLEMENTAL SET
16 logs against 12: what California's DPR rule actually counts
ISSUEDJULY 11, 2026DRAWN BY THE NIGHTLY SWEEP
California's direct potable reuse rule makes a treatment train hold 16-log virus reduction at all times, measured against 12 for groundwater replenishment. That is the comparison worth keeping straight, because the two numbers are not the same kind of number, and the 12 is not 12 logs of treatment. The rule took effect October 1, 2024, as Title 22, Article 10, sections 64669.00 through 64669.130. It is the rule for putting recycled municipal wastewater into a drinking water system with no environmental buffer in between, which is what makes it direct. So the question worth asking is what the state decided that missing buffer was worth, in logs.
Then the rule asks for more. The train must be designed and constructed for 20-log virus, 14-log Giardia and 15-log Cryptosporidium, which is four more logs of virus than the arithmetic strictly requires. That margin is not decoration. It covers the worst credible failure the board modeled, a power loss taking out the ultraviolet and advanced-oxidation step that can carry up to 6 logs on its own. So the 20 is what you build. The 16 is what you must deliver every hour you deliver water, and the full 20/14/15 has to hold no less than 90 percent of the time the train runs in a calendar month. Design target and operating floor are different promises, and the rule keeps them in different subsections on purpose.
Under that rule each month recycled water sits underground earns one log of virus credit, demonstrated by tracer study, not validated at a treatment process. At six months of demonstrated retention, 6 of the 12 required virus logs come from time in the ground, none of it treatment, and the same six-month threshold hands over the whole 10-log Giardia and 10-log Cryptosporidium requirement as well. So a compliant groundwater train can be built with far less installed treatment than its headline 12 suggests. Direct potable reuse takes away the buffer, and with it the credit, which is most of why its treatment numbers run higher. (I am reading that retention rule from the Division of Drinking Water staff compilation of the recycled water regulations, which reprints the code but disclaims being it; the sections are 60320.108 and 60320.208.)
Two sections do the work. The first requires the flow path from the terminus of the wastewater collection system to the drinking-water distribution entry point, treatment train plus connected tanks, basins, reservoirs, pipelines and conveyance taken together, to provide enough continuous mixing to knock a one-hour contaminant spike down by a factor of ten. That duty sits on the path as a whole, not on any single tank, and mixing that happens between the plant inlet chamber and the finished water can be counted toward it. The second section governs time. The travel time of the first ten percent of water from each control point to the point of diversion or shutoff must be no less than the time the project needs to notice bad water and divert it, a budget that runs from the sensor sampling interval through the control-system assessment, the compliance determination, the valve actuation, and the physical shutoff of flow. The regulation names those components and sets no minutes for any of them, so anyone quoting a figure in seconds is inventing it.
Read the two together and the conveyance stops being plumbing between the interesting equipment. It is a validated part of the barrier, sized for mixing and timed for shutoff, with no bypass permitted around any required process. So the designer who scopes a reuse project as a treatment plant with pipe attached has the diagram backwards.
Conveyance facilities run 3,707 million dollars against 1,098 million for advanced water treatment, so the pipe is more than three times the plant everyone pictures. Program contingency adds another 1,648 million, program management and engineering 1,503 million, recharge facilities 206 million, with smaller allowances making up the balance of the 8.3 billion. These are Metropolitan's estimates for a program with no construction approval, and only 25 of that first phase's 115 million gallons per day is direct potable reuse; the rest is the indirect kind that recharges a basin first. Take the estimate as what the agency thinks it will spend, not as a bill anyone has paid. The shape of it still matches the rule: the barrier is mostly conveyance.
Contractors who track the jurisdictions where these projects surface, and when the permits post, are reading a conveyance scope, not a treatment building. That is the same reading of the rule, arriving through the bid list instead of the code. The work sorts by trade and jurisdiction, which is the level a month-to-month subscription has to earn its keep at, and the conveyance is where the linear feet are.
The board's own economic assessment supplies the last comparison. It found the rule imposes no cost on water agencies, because every requirement in it would have been imposed project by project through drinking-water permitting anyway. The work these projects generate is real, and it belongs to the projects, not to the rule. So if you are sizing any of it, the number to check first is not the treatment plant's log credit. It is whether the conveyance and valving were scoped as a barrier or as pipe.
Where the 16 comes from, and why the rule asks for 20
The board did not pick 16 from a table. It divided two densities. Raw sewage carries on the order of one billion enteric virus gene copies per litre; the tolerable drinking-water density sits about sixteen logs below that, near 3.3 ten-millionths of a copy per litre. A log is a factor of ten, so sixteen of them is the gap between the water you have and the water you can serve, and for virus that gap comes to 16. The same division puts Giardia cyst at 10 and Cryptosporidium oocyst at 11, both of which the build targets downstream are sized to clear.Then the rule asks for more. The train must be designed and constructed for 20-log virus, 14-log Giardia and 15-log Cryptosporidium, which is four more logs of virus than the arithmetic strictly requires. That margin is not decoration. It covers the worst credible failure the board modeled, a power loss taking out the ultraviolet and advanced-oxidation step that can carry up to 6 logs on its own. So the 20 is what you build. The 16 is what you must deliver every hour you deliver water, and the full 20/14/15 has to hold no less than 90 percent of the time the train runs in a calendar month. Design target and operating floor are different promises, and the rule keeps them in different subsections on purpose.
The 12 for groundwater is not 12 logs of treatment
Set the DPR floor next to the older groundwater replenishment rule, which asks for 12-log virus, 10-log Giardia and 10-log Cryptosporidium. On paper the gap reads as four logs of virus, 16 against 12. It is wider than that, because a groundwater project can earn part of its 12 by waiting.Under that rule each month recycled water sits underground earns one log of virus credit, demonstrated by tracer study, not validated at a treatment process. At six months of demonstrated retention, 6 of the 12 required virus logs come from time in the ground, none of it treatment, and the same six-month threshold hands over the whole 10-log Giardia and 10-log Cryptosporidium requirement as well. So a compliant groundwater train can be built with far less installed treatment than its headline 12 suggests. Direct potable reuse takes away the buffer, and with it the credit, which is most of why its treatment numbers run higher. (I am reading that retention rule from the Division of Drinking Water staff compilation of the recycled water regulations, which reprints the code but disclaims being it; the sections are 60320.108 and 60320.208.)
The rule counts the pipe as part of the barrier
Here is the part a mechanical scope tends to miss. The rule does not treat the plant as the whole barrier and the conveyance as ordinary pipe. It regulates the pipe.Two sections do the work. The first requires the flow path from the terminus of the wastewater collection system to the drinking-water distribution entry point, treatment train plus connected tanks, basins, reservoirs, pipelines and conveyance taken together, to provide enough continuous mixing to knock a one-hour contaminant spike down by a factor of ten. That duty sits on the path as a whole, not on any single tank, and mixing that happens between the plant inlet chamber and the finished water can be counted toward it. The second section governs time. The travel time of the first ten percent of water from each control point to the point of diversion or shutoff must be no less than the time the project needs to notice bad water and divert it, a budget that runs from the sensor sampling interval through the control-system assessment, the compliance determination, the valve actuation, and the physical shutoff of flow. The regulation names those components and sets no minutes for any of them, so anyone quoting a figure in seconds is inventing it.
Read the two together and the conveyance stops being plumbing between the interesting equipment. It is a validated part of the barrier, sized for mixing and timed for shutoff, with no bypass permitted around any required process. So the designer who scopes a reuse project as a treatment plant with pipe attached has the diagram backwards.
The cost says the same thing the rule does
Numbers settle it. For Pure Water Southern California, the Metropolitan Water District and Los Angeles County Sanitation Districts program in Carson, Metropolitan's September 2025 board workshop put the first phase at roughly 9.4 billion dollars in 2025 dollars, Metropolitan's own share 8.3 billion of that and the Los Angeles County Sanitation Districts carrying the rest. Split Metropolitan's share by asset class and the treatment plant is the cheap part.Conveyance facilities run 3,707 million dollars against 1,098 million for advanced water treatment, so the pipe is more than three times the plant everyone pictures. Program contingency adds another 1,648 million, program management and engineering 1,503 million, recharge facilities 206 million, with smaller allowances making up the balance of the 8.3 billion. These are Metropolitan's estimates for a program with no construction approval, and only 25 of that first phase's 115 million gallons per day is direct potable reuse; the rest is the indirect kind that recharges a basin first. Take the estimate as what the agency thinks it will spend, not as a bill anyone has paid. The shape of it still matches the rule: the barrier is mostly conveyance.
Contractors who track the jurisdictions where these projects surface, and when the permits post, are reading a conveyance scope, not a treatment building. That is the same reading of the rule, arriving through the bid list instead of the code. The work sorts by trade and jurisdiction, which is the level a month-to-month subscription has to earn its keep at, and the conveyance is where the linear feet are.
What is actually permitted, and what is not
One caution on all of it. As of the state board's own frequently-asked-questions page, no California public water system is permitted to operate a direct potable reuse project. The only two operating direct potable supplies the board names are in Windhoek, Namibia and Big Spring, Texas. Pure Water Southern California cleared a California Environmental Quality Act step in February 2026 when Metropolitan's board certified its final environmental impact report, but certifying that report is not project approval, and it is not the domestic water supply permit the drinking-water division must issue before any reuse water moves. Metropolitan also describes its own demonstration train as membrane bioreactor, then reverse osmosis, then ultraviolet and advanced oxidation, which does not name the ozone and biologically activated carbon the rule puts first in sequence for a full project. Whether the eventual design qualifies for a blending substitution or seeks an approved alternative is not on the record, so both descriptions stand unreconciled.The board's own economic assessment supplies the last comparison. It found the rule imposes no cost on water agencies, because every requirement in it would have been imposed project by project through drinking-water permitting anyway. The work these projects generate is real, and it belongs to the projects, not to the rule. So if you are sizing any of it, the number to check first is not the treatment plant's log credit. It is whether the conveyance and valving were scoped as a barrier or as pipe.