Stormwater capture, treatment and recharge for urban water supply
Dr. Richard Luthy presents the latest research on stormwater treatment systems can be used to recharge groundwater
Traditionally, the approach to stormwater management viewed urban runoff as a flood management problem in which stormwaters need to be conveyed as quickly as possible from urban areas to waterways in order to protect public safety and property. Consequently, stormwater has been seen as a problem, and not a resource. In recent years, urban stromwater runoff management has been receiving more attention as drought has put more pressure on water supplies, and municipal governments have been held increasingly responsible for pollutants washed from urban areas within their jurisdictions that is discharged into waterways.
But what if we could capture that stormwater, treat it, and use it to recharge groundwater? In this presentation from the National Water Research Institute, Dr. Richard Luthy, a professor of civil and environmental engineering and the Director of the National Science Foundation’s Engineering Research Center for re-inventing the nation’s urban water infrastructure (renuwit.org), talks about stormwater capture, treatment, and recharge, and the idea of using stormwater as part of our urban water supply.
In the past, stormwater has been managed for flood control, began Dr. Richard Luthy. In Los Angeles and Orange County, there were catastrophic floods in 1938 and after that, the stream channels were widened and hardened, creating the concrete rivers we have today. Those concrete rivers do a good job of carrying stormwater to the coast, but along with that, it carries pollutants to the beaches.
“If we could do more in terms of stormwater management, capture, and use, that could help augment the urban water supply, and at the same time, it would provide other community benefits, like cleaning the beaches,” he said. “Just as you could reuse waste water, so could we think about urban runoff, and the urban runoff could become part of our water supply for non-potable or possibly potable water use, and also could be used to recharge our aquifers, and then from there, find its way back into our water supply system.”
In October of 2014, Governor Brown signed a stormwater capture bill. “If you read the language, it says that within the Bay Area and Southern California, there’s the opportunity to capture more than 600,000 acre-feet of stormwater and put that to good use,” he said, presenting a pie chart of LA DWP water supplies. “It shows the situation as of today where half the water for Los Angeles imported. Now the plan for the future under Mayor Villiaragosa was to reduce dependency on imported water by half, and that gives greater reliability and greater water security. How is that achieved? One of the solutions is stormwater capture, and on this drawing, it says the stormwater would be 4% of the urban water supply, and in a moment I’ll show you that it could really be much more than that. It could be 3, 4, maybe even 5 times that if plans come into being in this century.”
The goal was 2035 under Mayor Villiaragosa, but in response to drought, Mayor Garcetti moved it up 10 years to 2025. “In other words, not that far away,” he noted.
Los Angeles, like other cities, has been looking at stormwater management and has prepared a stormwater capture master plan, Mr. Luthy said. He presented a slide from the plan, noting that it shows the present state with regard to capturing stormwater into spreading basins. “The spreading basins are located a little bit inland near the base of the foothills, and they are shown in blue; the green is the incidental recharge. On the left, you see that with today’s systems, we’re capturing about 100,000 acre-feet.”
Now what might the future be? “There are different scenarios here,” he said. “There’s a conservative future, there’s an aggressive future, and there are futures in between, but the point is that with the conservative future, the amount of stormwater we capture could actually double. It could go to 200,000 acre-feet and if we’re aggressive – if we have the political will and the financing and the like – that number could become 300,000 acre-feet (or close to it). Now this is for the city of Los Angeles. What’s the water demand for the city? Maybe in the future 700,000 acre-feet a year, so the opportunity here is for stormwater to do much more than 4%. It could be as much as 20% or a quarter or something like that. So this is for me an important motivational slide about what’s the opportunity there for us.”
In a Mediterranean climate such as California’s, it rains in the winter, but we need the water in the dry months, so what kind of system might we envision? “We need a big rain barrel, is what we need. … This is the stormwater capture tank in Hollywood Hills put together by an environmental group, and you can see this is a major construction for 200,000 gallons of water. Storage is a challenge with seasonal rainfall patterns, so there are two parts: there is a centralized part and a decentralized part. There are good reasons to do decentralized stormwater capture. The reasons are here with the Tree People’s headquarters, its aspirational; they give tour groups there on sustainable practices and the like, and it’s good to do it. Also, in the city it takes stormwater into neighborhoods, and that’s very important too so a neighborhood could see an example of beneficial capture of stormwater.”
“But to make a big difference, we’re going to have larger systems,” Dr. Luthy said, presenting a schematic and explaining how it might work. “Stormwater goes into a capture basin, and that capture basin would rise and fall with the level of the stormwater, but it allows you at this capture basin to settle solids, and to have some photolysis occur; then in a metered way and a controlled way, it could go through filtering operations that are passive that may contain a sand and ion filter in a trickly filter mode for removal of phosphorous and heavy metals. Then a filter to remove nitrate, and then some geomedia like oxides of manganese and biochar for a final polishing step.”
“One of our hypotheses is that a system like this is where you have a sequence of filters, each will perform better than one separately,” he said. “We also think that it is a system like this that will remove the contaminants from the urban hardscape.”
He presented an artist’s rendition of a stormwater capture system being planned for Los Angeles in the Sun Valley neighborhood just north of the Burbank Airport. “It doesn’t look like an engineer’s schematic; it looks like a park, and that’s a great public feature,” he said. “The neighborhood is one that really doesn’t have such urban amenities, and so what’s being done here is that a former sand and gravel quarry was bought by the city and the flood control district, and that 45-acre site is being converted into a stormwater capture recharge system, and along the way, they will create hiking paths and basketball courts and playfields and that sort of thing.”
This system will provide 900 acre-feet per year, so it’s a much larger volume of water. “This is the Rory M. Shaw wetlands park, and so this is what a capture, treatment, and recharge system could look like. Here is shown the detention basin, the wetlands, and the water that flows to the wetlands to the lower right corner, and from there, we would put in some additional filters here and then we pump that over to a groundwater recharge facility.”
The artist sketches are great, but they don’t tell the whole story, Dr. Luthy said. “The area that’s drained includes a lot of automobile salvage yards, and industrial activities like the cement plants and that kind of thing, so the water that’s coming into that basin, it just drains all these salvage yards and the like,” he said.
“We know there are contaminants there, and they’ve been measured, so when we think about urban stormwater, we have the contaminants, the pathogens, the urban use biocides, and also the vehicle related compounds – oils and that kind of thing you’d expect.”
These are anti-corrosive agents, compounds from automobile tires and brakes, and other things. “These suite of compounds are not the ones you find in wastewater treatment; I would say they are unique to stormwater, so we need to have treatments systems that can deal with the metals and deal with these kinds of organics, and also with the nutrients as well.”
Mr. Luthy presented a graphic of the standard design for a stormwater filter that’s referred to as best management practice. “There is about a foot and a half of spec soil with some gravel at the bottom,” he said. “The thing about these systems is that they have given inconsistent performance, and they are not designed to a certain quality, they are designed to a specification, you can see the specification there.”
Dr. Luthy noted that this type of system is not likely to work with contaminants which are highly soluble. “If you go to the best management practice database and looked at how well these systems work, there’s a lot of inconsistency,” he said. “These are data from here looking at a lot of data from different systems, looking at three examples here, going from left to right. In the left there’s removal of pathogens, in the middle there’s no removal, and then in the third, there’s actually some generation of pathogens for some reason. So this is not a very good state of affairs.”
They have been working on designing filters which use different kinds of geomedia such as biochar and iron filings to see how well they work for removal of pathogens, nutrients, and trace organics. Dr. Luthy presented the results for tests for removal of E. coli using biochar; circled in red is Sonoma biochar. “This performs quite well; more than 2 log removal of E. coli in our laboratory tests. It also works in the presence of natural organic matter, and it works best with the wet and dry cycles – the kind of things that you would expect to see in a stormwater capture system. So biochar can help a lot with pathogen removal. Now we have to try this out in the field, but these results are very encouraging.”
For removing nitrate, they have been studying the use of woodchip biofilters. “The idea here is you pass water through a bed of woodchips, the oxygen is depleted, and fungus in the reactor will slowly eat away at the woodchips and give a little bit of cellulosic material – not very much, a few milligrams, maybe – and then that’s used by the denitrifying microorganisms to reduce nitrate,” he explained.
“The way this works then, some data here, where you see two different flow rates, 1 centimeter and 2 centimeters an hour of nitrate removal; in one case 10 milligrams per liter easily or maybe 5 milligrams at the higher flow rate; also you see the removal of dissolved oxygen and the removal of dissolved organic carbon from the wood, so our challenge in the lab is there is lots of this kind of data, and then how do you put that together into an integrated model to be able to predict the performance of one of these woodchip reactors? Our reactors in the lab have been operating now for well over a year and a half, and these woodchips will last a number of years once they are set up.”
Lastly, Dr. Luthy addressed the removal of trace organics. They have been studying manganese oxides. “The story is that if you imagine 50 cm of sand-coated manganese oxide and if you’ve gone through these pre-treatment steps, you may have just a little bit of oxidizable organic matter left, maybe 50 micrograms to a couple hundred micrograms per liter, and the estimate is that would have a life for a number of years. … The point is that you won’t have to change these media very often, and you would do maintenance on then anyway, maybe on a five year cycle.”
He then presented a schematic showing how these systems would be put together. “The schematic shows the capture treatment and recharge system here with a settling pond and trickling water over the sand and iron filings mixture for the phosphorous and metals removal, then up flows to the woodchip reactor for nitrate removal, and then through the biochar manganese system for the polishing and then into the groundwater, so we’re recharging groundwater here,” he said.
He added that automated controls would be used so that they can use weather forecasting to help anticipate when storms may come and what rate they might have to put water through these filters.
Dr. Luthy said they’ve been working on testing these systems in the field with a project with Sonoma County Water Agency where they are testing these filters with biochar, woodchips, and other geomedia and organic matter to see how well it works. He noted they would be setting up another similar system in Southern California.
So what are the opportunities for coupling recycled water with stormwater? Dr. Luthy presented a graphic on how it might work. “On the far left you can see advanced water purification facility making high quality water that then can be put into a spreading basin and then from the spreading basin, that would percolate in the ground and it gets pumped out as part of the water supply,” he said. “This in fact is being done from the Donald C. Truman Reclamation Plant in putting water over to the Hansen Spreading Grounds and the plan is to expand that to the Pacoima spreading grounds and then also maybe bring some other water over there as well.”
“The advantage of this is that we already have the infrastructure in place for the spreading grounds; we already have the infrastructure in place for pumping that water out of the ground and having it become part of our city’s water supply,” he said. “There’s a great opportunity here to try and marry these two systems, the water reuse and the stormwater recharge systems.
He presented a picture of the Pacoima Spreading Grounds, noting this is what they look like most of the year. “They are not doing a lot, and you could imagine that this is a place where I could pump recycled water and it could percolate into the ground and now it becomes part of the urban water supply. So we’d like to understand how this might actually work and that requires we have some decision support tools.”
Dr. Luthy presented a graphic from the Metropolitan Water District’s Water Stewardship and Planning Report from September 2015. “What they are looking at is taking water from this coastal plant, that’s the joint water pollution control plant, and then pumping it inland to different spreading grounds. Those of you who live down here, you know these differences are quite large. Maybe to Rio Hondo it might be 15 miles, near the Santa Fe, it’s closer to 30 miles, Rio Hondo is 150 feet up hill, Santa Fe is 500 feet uphill, so is this the best way? … What the challenge is then you are pumping that water back uphill and you have to do all the trenching and piping for that.”
A student of his has been studying how such a system would be optimized. The blue dots are the spreading basins with the size of the dot indicating the estimate of excess capacity, and shown in red are where recycled water would be available. “There’s the Hyperion Plant at the coast; they don’t show the joint water pollution control plant here, but the plant one in the center, that red circle with the X, that’s a proposed Metro Plant; the Bureau of Sanitation has been thinking about a satellite treatment plant for capturing water for reuse. It wouldn’t have to be a full wastewater treatment plant, a satellite plant.”
Dr. Luthy’s student is considering the best arrangement with the different options. “In the upper left is the Hanson Spreading Grounds, and then down there at the bottom is the Hyperion Plant, and then another example of a possible new metro plant, so what it would take to get the water over to the Rio Hondo Spreading Grounds. From the Hyperion, it’s about 20 miles and 200 feet uphill. From a possible Metro Satellite Plant, it’s 10 miles but it’s downhill 50 feet. Which is better here? How would you cost this out?”
Fortunately, they can look to the system that was put in to take water from the Tillman Plant over to the Hanson Spreading Grounds to get current information about construction costs, trenching costs, piping costs, and other expenses. “Our goal here is to figure out what kind of arrangements that might work here and then think about the rest of the county,” he said. “If you look at the Tillman Reclamation Plant to the Hansen Spreading Grounds, the conveyance there is 10 miles and 200 feet. The purple is the cost for the conveyance and the light pinkish is the cost of the treatment, so what is the cost here for the water? Well it might be $750 an acre-foot. Metropolitan’s water costs a lot more than that.”
“Now it might be that in the spreading grounds, you wouldn’t be putting water in them for maybe a couple of months during the year, during the wet time in the winter, if we anticipate an El Nino event, so maybe you don’t put water there for three months, so how does that affect the cost?” he said. “It doesn’t affect the cost very much if the plants were idle for a couple of months, but what is interesting here is to think back to that diagram for the Metropolitan Water District and pumping not ten miles, but maybe 20 miles or maybe 30miles and going large distances up hill and what will happen then is that purple part is going to become a bigger and more important part of the cost.”
“I think this is a very rich problem,” Dr. Luthy said. “It’s complex, there isn’t a straightforward answer here, and that’s what we’re studying.”
Dr. Luthy then gave his take home messages:
“One, that the urban stormwater can make a really significant difference and contribute to our water supplies, and I hope I was able to show you by example there at the Rory M. Shaw Wetlands that if we do it a lot, it can contribute to urban amenities as well,” he said. “But also when we look at the types of contaminants we have from the urban hardscape, we need to have design experience with how these treatment systems might work, so that we can be sure that when we put that water in the ground, we’re not causing a groundwater contamination problem. And then lastly, we need these decision support tools so we can understand how these stormwater recharge systems might work with other components of our urban water infrastructure.”
“And with that … “
Audience question: “We talked earlier this morning about climate change and the need for water. The rivers no longer flow to the sea, yet water sustainability we usually define as the water required for people and for aquatic biota. I like this idea of using urban stormwater, but how do we factor in the fact that the estuaries no longer get any fresh water?”
“That’s a good question and where would that water come from,” Dr. Luthy replied. “Eventually that water would find it’s way back into the estuary, but it wouldn’t’ be in a surge, so there are some dynamics there. I think a different question is what is special about the big cities in California and why can I stand up here and say let’s capture stormwater? That’s because there are no water rights issues. Now I realize that’s not the same thing as ecosystem services and water for ecosystems, but a problem in inland areas is that there’s someone else who has a claimant to that stormwater, but here on the coast, it’s just a lost resource. I think too it’s a question of that water that goes into the estuary, let’s look at Santa Monica Bay. You have a lot of pollutants going down to Santa Monica Bay, so we aren’t maybe doing the ecosystem services with the way the systems are today, so this maybe can help address that too.”
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