San Diego Showdown: IPR vs. Purple Pipes

This is the fourth of a 5-part blog series examining the nature of our local water supply and how to increase the reliability of our supplies now and into the future.

Today’s match-up features two contenders, both aimed at solving San Diego’s water crisis.  

In the first corner, the “purple pipe system” is looking to continue its reign in San Diego.  San Diego currently reuses a small fraction of its sewage for irrigation.  This recycled water is distributed through a separate purple pipe system.  Because the water is non-potable, it is not fit for human consumption.

In the second corner, the up-and-coming “Indirect Potable Reuse” (IPR) is looking to solve San Diego’s water problems.  In scientific terms, IPR is a process to treat wastewater and sewage using advanced technology to produce potable water fit for human consumption.  Essentially, we would be drinking purified sewage.  Right now, you are probably cringing at the thought of drinking recycled wastewater; I know I did.  But then I did some research, and I found out that the water produced from IPR is actually superior to our existing water supply.  How is this possible? 

First, advanced water technology removes any remaining solids through microfiltration.  Next, reverse osmosis is used to eliminate viruses, bacteria, pharmaceuticals, and other microbes.  The water is then disinfected by UV light and hydrogen peroxide.  Finally, it is added to groundwater or surface water reservoirs where it is further purified by natural processes.  Once drawn from the groundwater or reservoir, the recycled water goes through the standard water purification process all drinking water undergoes to meet EPA standards.  Once this IPR-produced water is fit for consumption, it is distributed through the existing drinking water infrastructure.  Now that doesn’t sound so bad, does it?

Round 1:  Costs

The cost of producing one acre-foot of water with IPR ranges from $1,200-$1,800.  The purple pipe system ranges from $1,600-$2,600 per acre-foot.  

Purple pipe recycled water cannot be added to the existing drinking water infrastructure, so it requires a separate pipe system which costs about $2 million per mile to build.  It also requires homes and businesses to be plumbed with two sets of pipes—one for recycled water and one for potable water.  This is beginning to sound expensive!  

Although the purification process of IPR sounds expensive, the City of San Diego estimates that implementing IPR would be cheaper than expanding the purple pipe system.  This is because IPR negates the need for a separate water infrastructure and would maximize the use of the available recycled water supply.  

IPR – 1; Purple Pipes – 0

Round 2:  Energy

The energy intensity of the IPR process is higher than that of the recycled water in purple pipes.  Compared to non-potable recycled water, IPR generates a higher carbon footprint.  However, IPR uses significantly less energy than other potential water sources in San Diego, such as desalination or imported water.

IPR – 1; Purple Pipes – 1

Round 3:  Environmental Impact

By using recycled wastewater, IPR reduces the amount of waste flowing to the Point Loma Treatment Plant.  In doing so, IPR reduces the amount of potentially harmful pollutants being released into the ocean from the Point Loma Plant’s effluent.    

Purple pipe recycled water does have some red flags.  Particularly, the use of non-potable recycled water can lead to the accumulation of byproducts over time in the irrigated soil.

IPR – 2; Purple Pipes – 1

Round 4:  Water Quality/Safety

Studies show that water produced through IPR treatment processes contains fewer contaminants than our existing treated imported water supply.  Further, a study performed by the National Research Council concluded that there were no significant health risks as a result of IPR.  
Because the water in the purple pipes is not treated to the point that it is drinkable, it contains pathogens and harmful chemicals. Simply stated, the consequences of ingesting non-potable recycled water can be severe.      

IPR – 3; Purple Pipes – 1

After four hard fought rounds, IPR has dominated the ring, proving that it would be a strong, viable addition to San Diego’s arsenal for fighting the water crisis.  

Still think “from toilet to tap” sounds less than appetizing, or has your mind changed?  Tell us what you think!  

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5 Responses for San Diego Showdown: IPR vs. Purple Pipes

  1. We are here presented with the proverbial false choice wothout consideration of a third choice, on site water recyccling. While local regulators seem intent on maintaining the staus quo and focus on potable water, they ignore the fact that most of out potable water transported and cleansed at enormous cost is used for purposes that do not require potable water, toilets, irrigation for example. If we were to take all 180 mgd we throw away daily and grow a bamboo forest on top of the maturing landfill we would transevaporate that water and create a whole new bamboo based local industry. If allowed individual homes with a 600 square foot greenhouse would transevaporate 300 plus gallons a day a substantial portion of which could be condensed and reused. Indeed just take a look at for using salt or briney water to grow things or to recycle water on site. Our engineering/regulatory industry does not seem to want to change the ways things are down, big pipes, lots of steel and cement pump that water and wastewtaer around the town and get rid of that storm water, to the beach with it, rather than create a water shed that uses the water given to it. Sorry you pushed my button

  2. Rick Row says:

    Why does the purple pipe system cost $2 million per mile to build. That seems high. What is the breakdown? And what is the source of this information? Thanks

  3. Bobbi Beglau says:

    What is the cost per acre foot of water produced? What is the environmenal footprint if you take water away from our creeks? How many people want to use what presious little free time they have messing with processing sewage? I think most people are happy to flush it and forget about it. I like the idea of having prefessionals test my water before I use it.

  4. Kate Hailey says:

    Hi Bobbi, the recycled water from IPR does not take water away from creeks, and the amount of time spent wouldn’t increase to process the waste water. The facility and processes already exist to produce water through the purple pipe system, but the purple pipe water has not been treated to drinking water standards. IPR would make the water drinkable. Professionals treat the water at every stage in the process, ensuring that it meets those drinking water standards. This project provides valuable jobs to San Diegans and provides the region with a more reliable source of drinking water, rather than having to import water from Northern California or the Colorado River. In fact, tests have shown that the water produced through our IPR facility is cleaner than the water we import (this blog post highlights those findings http://localhost/sdcoastkeeper/blog/san-diego-water-supply/item/229-ultra-purified-recycled-water-cleaner-than-imported-water.html).

    Thanks for taking the time to ask these important questions!

  5. Bobbi says:

    Thank you. I support water repurification. My coments were intended for Norman’s post.