Expert Q&A with Michael Ogden

Advocating for natural systems may have seemed an unlikely path for an engineer with a B.S.C.E. from the University of California Berkeley and an M.B.A. in mathematical methods and finance from the University of Chicago. However, when Michael combined the mathematical rigor and systems training of his education with his passion for fly fishing, his career in making clean water began. Since then he has led the engineering design and project management of over 600 water infrastructure projects, using natural treatment systems for the treatment of wastewater, stormwater, and septage. These projects have included municipal, decentralized and on-site wastewater collection, treatment, disposal and reuse systems in 44 states, Canada, Mexico, China, Afghanistan, Australia, El Salvador, Panama, Costa Rica, Bahamas, Cambodia, India, and Fiji. We were honored to chat with Michael about the design and use of constructed wetlands for wastewater treatment.

There seems to be a growing awareness of aging infrastructure in the U.S. Besides their age and condition, what are some other, perhaps lesser-known down sides of conventional wastewater treatment systems?

You cannot divorce the collection system from the treatment system.  In many cities, it is not just aging wastewater treatment plants, but 100-year-old sewer systems. It may be possible to upgrade a municipal wastewater treatment plant, but the sewer lines are all under city streets. How are you going to repair them all?

For the benefit of readers who may not know much about wastewater, can you describe the general composition of domestic wastewater? If you had a one-liter bucket of typical domestic wastewater, how much of is water and how much is “waste?”

Interestingly, the total amount of organic material, which is measured using the term “biological oxygen demand (BOD), is less than one thousandth the total volume of wastewater. It’s insignificant in terms of quantity. “Waste” is less than ¾ of a gram of measurable organic compounds per liter.

But think about what is in that ¾ of a gram. Think of lists of chemicals in the household products we use. Think of things like copper, lead, zinc, and other metals that leach out of plumbing. People pour gasoline down their garage sinks. There are 1,200 compounds on the EPA’s list of toxic compounds, and all of them—at one time or another—have ended up in our sewer systems.

So although the quantity of the waste -¾ of a gram per liter—is seemingly so small, it is so harmful to our water bodies. Think of a city the size of Baltimore. That city is dumping over 306 million liters/day.

How did constructed wetlands come to be used to improve the quality of wastewater?

The original work was done in the 1950s by a research biologist named Kathe Seidel at the Max Planck Institute in Germany. The Ruhr River valley was Germany’s most industrialized area. In the 1950s, it was very highly polluted because the wastewater treatment plants were not able to deal with all of the pollutants produced by all of the industry. Dr. Seidel observed that the wetlands along the riparian area of the river were actually providing some form of treatment. Biologists knew that wetlands served some function in settling solids and recycling nutrients, but Dr. Seidel noted that they actually did much more than that. [Dr. Seidel demonstrated the effectiveness of naturally growing plants in breaking down ballast substances, transposing toxic into non-toxic substances, destroying pathogenic bacteria, viruses and worm eggs, removing heavy metals, cleaning oil spills, removing salt, neutralizing pH, enriching with oxygen, transforming waste water into drinking water and replenishing groundwater — all with photosynthesis as the primary energy source. (Sustainable Sanitation Alliance)] She looked at 273 different species of plants, and settled on three families of plants that are found worldwide and that we still use today in treating wastewater: reeds, cattails, and bulrushes.

Dr. Seidel’s work subsequently spread through Europe and made its way to the U.S. The original research has been greatly improved upon. We now have a huge volume of publications and research, and the treatment wetland is a now a worldwide technology.

What are constructed wetlands, and how do they work?

We use the terms “constructed wetlands” or “treatment wetlands” to distinguish them from natural wetlands because they don’t have the same evolutionary history [and diversity]. There are three kinds of constructed wetlands: subsurface flow, where water flows through a gravel or sand media under the surface; surface flow, where water flows around plants as it does in a natural wetland; and vertical flow, where water flows down through a planted sand and soil media.

Vertical flow systems can be used without any form of primary treatment. The raw wastewater simply floods into the vertical flow wetlands, percolates through the soil, sand, and gravel, and drains out as clean water at the bottom.  Horizontal flow wetlands usually require some kind of primary treatment to limit the solids coming into the wetland to prevent the pores of the gravel from becoming clogged.

In the U.S., a constructed wetland is generally not a stand-alone treatment. It usually includes some kind of primary treatment. Primary treatment often uses gravity to settle the organic solids that are in the wastewater.  In primary treatment tanks relying on gravity, wastewater is introduced into a large tank, typically with a minimum retention time of two hours. 50% or more of the organic solids settle with the remaining solid and soluble compounds flowing to the next treatment step.  For small communities of less than say 50,000 persons, the next treatment would ideally be one of the three types of wetlands. (Note, however, that vertical flow systems in France do not typically use any form of primary treatment.)

All three types of constructed wetlands rely primarily on the microbial community that is able to use the organic compounds–and some inorganic compounds such as nitrate, ammonia, and hydrogen sulfide–in wastewater as food. Bacteria convert those complex compounds into simple compounds such as carbon dioxide, water, hydrochloric acid, etc.  The plants’ role is to provide substrate for the microbial community with their roots, stems and surfaces. Over 99% of the bacteria and microorganisms involved in wastewater treatment, whether in mechanical or natural systems, are attached growth microorganisms that need to be affixed to some surface. Plant root hairs, for example, provide huge surface area for bacteria to grow. On a single root hair, there may be three separate layers of bacteria. There may be as much as 10,000 miles of fine root hair in a mature bulrush or cattail for every square meter of surface.

Bacteria are very mutable. They grow and die in 15 or 20 minutes. If you expose bacteria to a toxic compound like toluene [a highly carcinogenic solvent used to clean machine parts], 99.99% of the bacteria might die. But those that remain have a genetic adaptation to this compound, and they will continue to reproduce and eventually they will be able to metabolize it. In a constructed wetland, we use that mutability to our advantage. We create an ecology in which bacteria is going to metabolize or sequester almost everything that comes their way.

What are important factors to consider when deciding when to go with a surface flow, subsurface flow, or vertical flow wetland?

The collection system is very important in determining what technology to use and how much land you need. Our conventional sewer systems, which have been around since 2500 B.C., have the difficult task of moving both solids and liquids. Around 1960, a Harvard professor of civil engineering named Matthew Gordon Fair came up with the idea to keep the solids in a [primary treatment] tank at the house/building. If you do that, trash and most organic solids remain in the tank, so now you essentially only have to convey water to the next treatment element. Now you can use a small diameter collection system (a low pressure or gravity water line) which greatly reduces the size of the sewer line. The resulting water and soluble organic compounds can easily be treated by a wetlands system. Like that primary treatment tank, the wetlands are passive. The sewer lines don’t have manholes and don’t need to be cleaned anymore, and the trash, which is so often a problem in conventional systems, stays in the tank at the home. So now we no longer need trash collection systems at the wastewater treatment plant.

As we eliminate machinery we move towards an ideal system – one in which there is no machinery to repair or maintain and no electricity needed to run the machines. Wetlands are passive. They don’t need people to intervene in the natural processes. Maintenance is reduced to removing weeds and occasionally thinning plants. The primary treatment tanks at each home will need the digested solids pumped out every few years

What happens when those primary treatment tanks at the house reach capacity?

In those interceptor tanks, anaerobic digestion is taking place. It’s the same biology that takes place in the bottom of a pond. The total solids end up getting reduced by a significant amount. The solids can accumulate, and every 7-10 years, a volume (typically about a third of the tank) must be pumped out and taken to a reed bed or regional wastewater plant for additional treatment. A reed bed, which is a lower-tech alternative to the wastewater plant, is basically a vertical flow wetland planted with reeds, which provides the final treatment and creates a stabilized humic material which you can land apply.

Is that what “sludge” is?

Sludge may be settled organic solids from primary settling tanks or aerobically or anaerobically digested solids. The organic anaerobically digested solids from residential septic tanks (aka interceptor tanks) is called “septage” as opposed to sludge. It is an anaerobic solid that consists primarily of dead bacterial cells. The soluble organic material, the stuff that doesn’t get digested and converted to bacterial cells, such as sugars and soluble cellulose and longer chained carbon molecules, will leave the tank and be readily digestible by the microbial community in the wetlands.

Many conventional wastewater treatment plant rely on aeration systems, which basically blow air bubbles into wastewater or rely on splasher type mixers. These very high energy systems are designed to get oxygen into the wastewater to support the aerobic bacteria. The bacteria grow, respiring CO2, and die and settle out as sludge. The idea with conventional wastewater treatment plants was that if you convert all of organic stuff into dead bacterial cells, it can settle, and you could then make sludge and land apply it. But you also send lots of carbon dioxide into the atmosphere, not only as part of the bacterial respiration, but also as CO2 from fossil fuels used to generate the electricity.

In wetland systems, those soluble compounds are converted into plant material with some CO2 and methane released to the atmosphere. Wetlands however are net carbon sequestering systems. The real atmosphere carbon savings are in the passive nature of the treatment – no electricity required.

Let’s talk about that energy. How do constructed wetlands compare with conventional wastewater treatment plants when it comes to the amount of energy it takes to treat a liter of water?

Provided you are not on dead flat ground [and require pumping], it doesn’t take any energy at all to treat wastewater using an interceptor tank, a small diameter collection system, and a wetland. Gravity will move the water.

Conventional treatment systems are based upon this concept of blowing bubbles into the water and that takes a lot of energy. Typically, a wetland system that incorporates passive primary treatment and includes a recirculating sand filter to meet advanced wastewater treatment standards, requires only about 10% of the energy a conventional wastewater system would require to do the same job. Wetlands are essentially solar powered systems; the sun is what ultimately drives them.

You have said that the cost of operating and maintaining a conventional wastewater treatment plant will exceed the original capital cost several times over its life cycle, which you say is typically 20 to 50 years. How do the lifecycle and maintenance costs of constructed wetlands compare to those of conventional wastewater treatment plants?

They are a great deal less. We have found that the cost of construction for wetlands is about 80% of that of a conventional system, but that includes the land cost. The big saving is in the energy cost, which is about 10% of what it would be for a conventional system. The overall lifecycle cost [of a constructed wetland system] usually come out to be about a third of the cost of a conventional wastewater system. It can be much less, depending factor such as the discharge requirements or whether we are doing land application.

So if you have the right site, and you are looking at energy, maintenance, and cost, treatment wetlands seem like a no-brainer. But it seems like the only down side to constructed wetlands might be the time required to treat the water. Is that true, and if so, does that matter?

A conventional system will take about eight hours to treat the water, while constructed wetlands typically take 5-6 days. We know from research done in Hawaii that estrogen, which is not metabolized in a conventional WWT plant, is metabolized in a wetland system. Estrogen is a pretty complex molecule, and the concentrations are so low to begin with that you just need more time [to break it down]. The same is true for antibiotics.

I would imagine that treatment wetlands are a good solution for communities in developing countries that may lack access to (or the resources to construct) a conventional wastewater treatment plant. Is that true? If so, can you share an example?

Many people may not be aware that constructed wetlands treating wastewater do not smell. How do you explain that?

In the subsurface flow wetlands, the flow of the water is below the surface. In surface flow wetlands, odor causing molecules in the water rise in the water column and run into floating aquatic plants such as duckweed (Lemna spp.) which colonizes the surface. The underside of the duckweed is colonized with bacteria which utilize these compounds as food. Odor producing compounds become food or energy sources for the bacteria; nature wastes very little.

What is the range of plants available for constructed wetlands? Are there any limitations, in terms of plants you know do not work well in these systems?

One of our earliest treatment wetlands, at a conference center Santa Fe, New Mexico, was planted with cattails and bulrushes. The owner added daffodils, and it worked fine. The daffodils come up first, before the cattails. Remember, attached growth microorganisms in that root structure are down in the gravel, and the roots of the cattails remain even when the leaves are removed in the fall (to allow the daffodils to be seen in the spring).

We like to work with landscape architects. There are all kinds of wetland plants that don’t necessarily assist in the treatment process but can be planted along the edges of the wetland [to add beauty, create habitat, or make the wetland blend into the natural landscape]. We have done all kinds of things to make the wetlands look more like a natural landscape. For example, we created a treatment wetland at the Desert Living Center in Las Vegas that people have used as a backdrop for their wedding pictures. They have no idea that they are standing in front of a system that is treating wastewater from the Center’s 8,000 daily visitors.

It is hard to make a machine look good, but working with good landscape architects, you can create some stunning systems. For example, we worked with Studio Hansen Roberts to create a beautiful backwash recovery wetlands for the Woodland Park Zoo’s Humboldt Penguin Exhibit that won an award from the American Zoological Association!

Have constructed wetlands become commonplace enough that they are now accepted in most locations, or are there still regulatory hurdles to putting them in place in some places in the U.S.?

At the county level, it can still be very challenging, but generally speaking, once you get into the state regulatory agencies, there is no problem.

What are the considerations for constructed wetlands that designers should address with clients at the outset of the design process?

The very first is siting: what is the best location for the treatment system and where we can we reuse the clean water? We don’t want to discharge into the waters of the U.S. Soil conditions are important and ideally, we want to put the water to use in the building or in irrigating the landscape. In siting the disposal field, we may need to determine the best field we can irrigate to create a wildflower meadow, for example. Once we have a conceptual design and layout, then we go talk to the regulatory agency and make sure they are happy with what we are considering doing.

Reviews of your 1999 book with Craig Campbell, Constructed Wetlands in the Sustainable Landscape indicate that it was one of the first to integrate and draw attention to the additional benefits of constructed wetlands: recreation, education, wildlife habitat, and aesthetics. Has awareness of these benefits and demand for these additional benefits grown since you published that book?

One of the biggest things we discovered as a result of that book is that landscape architects saw constructed wetlands as a marvelous addition to the aesthetics of any site improvements. They could see these systems as what they were: fascinating plant communities and a wonderful amenity to the landscape. They also appreciated the habitat that comes with that. After we helped the Desert Living Center convert a detention basin into a stormwater wetland, the number of bird species visiting the site went from two to 200. The study was done by a wildlife biologist.

Have there been any documented increase of wildlife in wastewater treatment wetlands from what was there before?

Operators, especially those working with larger constructed wetlands, often tell me that they went from being wastewater treatment operators to game managers!

What are some of the common mistakes people make when trying to create constructed wetlands to treat wastewater?

There are two for subsurface flow wetlands. The first is that people do not get the length-to-width ratio right. People make the wetlands longer than wider, and do not make adequate preparations for total solids coming into the system. For guidance on that, you need to refer to the engineering textbooks. The best is Small and Decentralized Wastewater Systems by Ronald Crites and George Tchobanoglous. This book has many options, not just treatment wetlands. Another good resource is new book, Treatment Wetlands, Second Edition, by Robert H. Kadlec and Scott Wallace. Those books are the best. The second mistake is not paying attention to the porosity and associated hydraulic conductivity of the gravel. Guidance for that can also be found in the textbooks.