GrokSurf's San Diego

What should we call it? Toilet-to-tap? Purified sewage? Purified recycled water? I think it depends on whether you’re feeling disdain, hold-your-nose neutrality, or support.

“It” is Indirect Potable Reuse, or IPR, a process of purifying wastewater to the point where it’s at least as clean as the raw water that the city imports before it’s treated further for drinking.

I’m a supporter.

On Saturday I had a disagreement on twitter with Rob Davis, a reporter from Voice of San Diego. I suggested that his use of the term “purified sewage” makes it sound nearly as bad as “toilet-to-tap” used by opponents. I said sewage sounds too much like shit. He replied (“it is shit. proponents need to embrace that fact; otherwise it seems like they’re trying to hide a basic fact.“). Well yes, it’s a component of wastewater but why emphasize it? We never did settle that disagreement, and I lost the Voice of San Diego’s CEO from my twitter follower list after that.

The next day, Rob posted an item entitled”Guide to Purified Sewage” on Voice of San Diego. It’s a Q&A that describes Indirect Potable Reuse “in a nutshell”. Here’s a snippet:

The chatty Q&A is spruced up by the liberal use of the word “sewage” (40 times altogether, no doubt to help us “embrace that fact”).

[June 16: Sadly, almost two months later, the Voice of San Diego continues to use “recycled sewage” when talking about the IPR process]

Here’s how I would describe the process in a nutshell without rubbing your nose in sewage:

Let’s start with a definition. What San Diego is considering is Planned Indirect Potable Water Reuse (IPR). The National Research Council defines this term as “the purposeful augmentation of a water supply source with reclaimed water derived from treated municipal wastewater. The water receives additional treatment prior to distribution.”

Presently, the City’s Water Reuse Demonstration Project is conducting a scientific study to assess the viability of using the IPR process to augment raw water supplies that San Diego imports from Northern California and the Colorado River. The idea is to blend highly purified water with raw water entering the San Vicente Reservoir where it is stored before eventually going to the Alvarado Water Treatment Plant prior to distribution as drinking water.

A few other water districts in the country have already implemented an IPR program as part of their potable water supply, including the Orange County Water District. One difference, though, is that OC’s program uses the water to replenish their groundwater (which is pumped for potable use), whereas San Diego is looking at augmenting water in the San Vicente Reservoir where it would age for a specified period of time before being used. The City’s study will need to address that difference.

Routine wastewater treatment includes:

• preliminary screening and grit removal
• primary treatment involving gravity sedimentation (removes slightly more than 1/2 suspended solids and about 1/3 biochemical oxygen demand (BOD) organic material, plus some nutrients, pathogenic organisms, and toxic organic compounds)
• secondary treatment (biological process to remove 95% of BOD plus heavy metals and other organic compounds)
• tertiary treatment (removes other contaminants such as pathogens)

Additional purification treatment is then done to allow the wastewater to be reclaimed for potable purposes, summarized in San Diego’s study documents as

• post tertiary treatment
• membrane filtration
• reverse osmosis
• advanced oxidation/disinfection

Testing of water after it receives this advanced treatment indicates that it is typically of higher quality than the raw water now imported by the city; nevertheless, the IPR Study was considered necessary to scrutinize the water quality and other issues even more closely.

There are no national standards for indirect potable reuse. However, from everything I’ve read, the City’s IPR Study certainly takes into account the recommendation of the National Research Council’s Issues in Potable Reuse: “The major recommendation of this report is that water agencies considering potable reuse fully evaluate the potential public health impacts from the microbial pathogens and chemical contaminants found or likely to be found in treated wastewater through special microbiological, chemical, toxicological, and epidemiological studies, monitoring programs, risk assessments, and system reliability assessments” (the full report can be read online at the link above).

For more background, see the City’s Overview of Water Reuse Opportunities and Public Health Protection.

I’m a supporter of IPR because the amount of water the City can import is guaranteed to shrink for a variety of reasons, while our demand for more water continues to grow. We’re approaching the limit to how much can be saved through conservation. Groundwater resources are extremely limited. In theory IPR can produce significantly large amounts of potable water. It increases the reliability of our water supply, it reduces our wastewater outflow into the Pacific, and it’s less expensive than desalination. Plus, we all know but don’t discuss the fact that we’re downstream from communities (including Las Vegas) that empty their treated wastewater into our incoming water. That’s called “unplanned indirect potable reuse.” At least with the planned variety, there are additional built-in safeguards.

There’s a nutshell for you. If you’re interested in more, I’m following developments on the City’s project on this page.