Marin water district digs into recycled water costs
Officials at the Marin Municipal Water District said potential projects come with a high cost and lots of complexities.
It doesn’t look like wastewater will be turned into tap water in Marin County any time soon.
California regulators approved new rules in December allowing water agencies to purify wastewater and put it back into the pipes that carry drinking water to homes, schools and businesses.
Officials at the Marin Municipal Water District said potential projects come with a high cost and lots of complexities.
“Where we stand is we look forward to continuing to monitor the regulations and larger agencies,” said Lucy Croy, water quality manager.
With that said, members of the district board said they are interested in pursuing expansion of its purple pipe system that delivers recycled water for such purposes as irrigation, toilet flushing and industrial cooling.
The district board and staff held a discussion last month on the latest cost estimates, water yields and challenges of expanding its recycled water system, including opportunities for turning sewage into drinking water.
“In summary, expansion of purple pipe is definitely possible,” Croy said.
However, she said, “It’s capital intensive, and from what we’ve seen it’s supply limited.”
The district, which serves 191,000 residents in central and southern Marin, launched a study into new water sources in 2022 after facing potential reservoir depletion from the drought. Rains in late 2021 nearly refilled the district’s reservoirs, giving the county’s largest water supplier more time to study the costs and benefits of potential new sources of supply.
The resulting study is the water supply roadmap that was approved last year, which, in addition to increasing reservoir storage, explores expanded recycled water opportunities.
The district’s recycled water program provides more than 700 acre-feet of recycled water to 330 customers in the Terra Linda area of San Rafael. The district was one of the first California agencies to recycle wastewater, pioneering the use for car washes, commercial laundries and more.
The district operates approximately 25 miles of recycled water pipeline, three pump stations and two storage tanks and a treated water clear well with a combined storage capacity of 1.7 million gallons. The Las Gallinas Valley Sanitary District provides the recycled water.
At a board presentation on March 19, Croy said district staff conducted a recycled water feasibility study in 2000. Those findings have been refreshed with new estimates, she said.
There are several expansion opportunities to increase recycled water use, including through infill — which is adding new customers within the existing recycled water system.
Other opportunities include expanding the purple pipe system throughout the county, and partnering with other agencies to create a new recycled water system.
The expansion was broken down into 21 phases.
One option would phase 10 projects throughout San Rafael, including in the Canal and Peacock Gap neighborhoods. In total, the phased approach is expected to yield 345 acre-feet of recycled water distributed through 24 miles of pipe.
However, it would cost an estimated $60.6 million, plus an annual cost of nearly $11,000 per acre-foot to operate.
Expansion for project phases 11 through 21 in the areas of San Quentin, Corte Madera, Larkspur and Greenbrae would yield 602 acre-feet of water through 17 miles of pipe at a cost of $64.7 million. It would cost about $7,000 per acre-foot annually.
Staff said that rather than this phased approach, the district could do a “project-based” expansion of the system, where it can pick and choose individual projects.
Board members seemed interested in exploring an option for Peacock Gap, which was a project that would yield the most recycled water on its own at 285 acre-feet. That project is estimated at $26.7 million, with an annual expense of $6,355 per acre-foot annually.
Board members were also interested in a partnership with Central Marin Sanitation Agency for an $11.4 million expansion in the San Quentin area and a $4.3 million project in coordination with the Sewerage Agency of Southern Marin.
“I do think there are some further explorations that we could have here, and I would like us to make some efforts to continue our recycled water work,” said Jed Smith, a board member.
When it comes to making drinking water, Croy said, officials want to monitor the new state regulations and see how wastewater treatment technology might evolve. Croy presented possible projects for indirect potable reuse, or IPR projects, and direct potable reuse, or DPR.
IPR is where recycled water is used to replenish drinking water supplies either through groundwater injection or placement in surface water reservoirs.
One project being considered would collect wastewater from Las Gallinas and the Sewerage Agency of Southern Marin and carry it to a treatment facility at the Central Marin Sanitation Agency to be purified.
The purified water would be distributed through 28 miles of pipelines and four new pump stations to Kent Lake, one of the district’s reservoirs.
The annual yield is estimated at 7,840 acre-feet of water. The project would cost $452 million. It would cost about $4,500 per acre-foot of water to operate annually.
However, because reservoirs have been spilling in recent years, the treatment method would need to operate intermittently. The startup and shutdown operations drive up the expected cost to $13,512 per acre-foot, staff said.
When it comes to direct potable reuse projects, the purified water would be either placed in a raw water supply immediately upstream of a water treatment plant, or directly into a public water system.
These types of projects posed the same problems: high costs, complex treatments and coordination among agencies. One project that would transfer purified water to Bon Tempe Lake would cost about cost $433 million. Continuous annual operation is estimated at $5,100 per acre-foot, while intermittent operation is about $15,000.
Another DPR option that would treat water at Central Marin Sanitation Agency is estimated at $124 million. Continuous operation would be about $3,500 annually, while intermittent operation is about $10,686.
Board member Monty Schmitt said he believes the DPR method of treating sewage for drinking water is part of the future. He said he is interested in staff taking a deeper dive into the costs estimates and funding opportunities of the projects.
“If grant funding would substantially cover the costs of capital construction, that could really change these numbers,” Schmitt said.
Board member Matt Samson said the discussion around turning wastewater to tap water is directly related to storage capacity.
“I think we can start looking at some possible synergies that would start making sense for that, especially when it’s dry all winter and we have plenty of storage to be able to fill up,” Samson said.
