Using the natural cycles of plant and animal life instead of chemicals and mechanical systems to process wastewater holds a great deal of attraction. Conventional sewage treatment systems already rely on bacteria to do much of the work, but these organisms perform very restricted functions within a system that is generally mechanistic. Ecological engineering brings more highly evolved flora and fauna into the equation, providing an attractive and educational alternative for wastewater treatment.
Since the late 1980s about two dozen ecological wastewater treatment plants have been built, ranging from small systems serving individual schools, to medium-sized municipal systems serving several thousand households, to waste treatment plants for industries designed to treat specialized waste-water flows.
These systems are commonly known by the now-trademarked name Solar Aquatics™ or the term “living machines.”
What is ecological wastewater treatment?
All sewage treatment plants rely on living organisms to break down biological and chemical wastes. In conventional treatment plants, concentrated bacteria serve to partially decompose wastes in a series of aerated pools. Various chemicals are then used to precipitate out sludge and disinfect the effluent.
Ecological wastewater treatment plants function in ways that are fundamentally similar but use highly varied ecosystems with a wide range of organisms—algae, aquatic plants, marsh plants, worms, crustaceans, mollusks, and vertebrates—supporting the populations of bacteria that break down wastes and eliminate nutrients from the wastewater. This ecological community makes the systems more stable in the face of sudden doses of heavily contaminated water, known as shock flows, according to Steve Fluck, a process chemist at one such facility.
The principles of ecological wastewater treatment were developed by Dr. John Todd, a Woods Hole biologist who founded the New Alchemy Institute in 1969 and Ocean Arks International (OAI) in 1982. The nonprofit OAI is carrying out a broad range of living machine demonstration projects in different parts of the world. Meanwhile, Todd’s ideas are being pursued commercially by two different companies—Ecological Engineering Associates (EEA) of Marion, Massachusetts, which owns the tradename Solar Aquatics and certain proprietary aspects of Todd’s original treatment technology, and Living Technology, Inc. (LTI) in Burlington, Vermont, which is specializing in wastewater treatment for food processors and other industrial applications.
Though systems differ widely in their specifics, the schematic below provides a typical example of how an ecological wastewater treatment plant works.
Raw sewage flows into the system from an equalization tank, where grit is settled out and the wastewater is “seeded” with bacteria to jump-start biological activity. This stage can be part of the ecological system, or part of a larger, conventional plant from which the ecological system draws sewage.
Depending on the strength of the wastewater, anaerobic reactors may or may not be used for preliminary processing. From here, the wastewater flows through a series of aerated reactor tanks or silos. These tanks are connected in series, with wastewater flowing by gravity pressure from one to the next. Usually there are several parallel trains of these in-series tanks to increase retention time and provide redundancy in case of a malfunction. Each of the tanks provides a mini-ecosystem with a layer of floating aquatic plants at the top (often supported by mesh netting). The extensive root systems of these plants extend down into the water and provide habitat for the bacteria, which do most of the actual work in breaking down wastes. The tanks also support algae, small crustaceans, snails, and higher animals. The tanks later in the purification series are typically designed for an ecology of higher organisms, including vertebrates.
Clear or translucent plastic tanks are the hallmark (and most significant patent) of the Solar Aquatics plants designed by EEA—Todd calls it “using light in three dimensions.” In part to avoid patent conflicts, plants designed by LTI have larger
opaque tanks made of enameled steel, rigid non-chlorine plastic (polypropylene or polyethylene), or concrete. The tanks in LTI’s new 80,000 gallon-per-day (300,000 l/day) municipal sewage treatment plant in South Burlington, Vermont are 14 ft (4.25 m) deep and 17–18 ft (5.2–5.5 m) across and have artificial support structures for plants to facilitate ecosystem development deep underwater.
Following processing in this series of tanks, the water flows into a clarifier tank where solids are settled out as sludge. Some of these solids may be removed and used to seed the incoming sewage with bacteria; the rest is removed for storage, composting, or land application.
In EEA plants, the clarified water flows into a subsurface-flow constructed wetland (passing through a sand filter first in some facilities). The wetland maintains an anoxic environment in which anaerobic bacteria remove nitrogen from the water.
The constructed wetland is very similar to the subsurface-flow constructed wetlands described in an earlier
EBN article on alternatives to conventional on-site wastewater treatment (
Vol. 3, No. 2), except that in cold climates they are typically built in greenhouses to keep them warm enough for optimal biological activity year-round.
At LTI facilities, the constructed wetlands are partially or totally replaced by a new, more compact system to provide this denitrification: an
ecological fluidized bed. This system, recently developed by Todd and LTI, is essentially a submerged biofilter that does the job of the wetland in a much smaller area at a much lower cost.
The ecological fluidized beds are one example of a gradual shift in the technology, according to Bob Bastian, an environmental scientist with the U.S. EPA and project manager for a forthcoming report on ecological wastewater systems. “In many cases they have made changes from the original ecological engineering systems, making this a much more conventional design to make it more compact and efficient,” Bastian says. John Todd is no longer directly affiliated with EEA, but he is on Living Technologies’ board of directors. Todd presents this evolution in a different light, saying that, LTI has an excellent team of engineers, which “is focused on bringing the ecology part of ecological engineering into the language and workplace of mainstream civil engineering.”
Steve Fluck acknowledges that some aspects of the living machines function in fairly conventional ways, but argues that later stages are fundamentally different with their reliance on more complete ecosystems. From the constructed wetland or fluidized bed, water may undergo disinfection with ultraviolet (UV) light or another system prior to discharge into surface waters or into groundwater through an infiltration bed.
Performance of ecological wastewater systems
Proponents of Solar Aquatics and living machines describe their systems as more effective than conventional wastewater treatment for most criteria. An independent assessment of these systems for a forthcoming U.S. EPA report on ecological wastewater systems, however, is less generous. Consultant Sherwood Reed, P.E., author of the EPA report, feels that functionally these systems are at best comparable to conventional systems on most counts. “There are no wastewater parameters which I am aware of that are removed more effectively in their systems,” Reed states.
The performance of wastewater treatment can be measured in several different ways. BOD, or
biochemical oxygen demand, is one of the most common measures of purity. This is a measure of how much oxygen will be removed in the process of decomposing (oxidizing) organic matter in the water, measured in milligrams per liter over a specified number of days. BOD5 refers to that oxygen demand over five days. The higher the BOD, the greater the damage to natural ecosystems, which depend on dissolved oxygen in water. Typical residential wastewater has a BOD5 level of 200 to 290 mg/l, according to the U.S. Environmental Protection Agency (EPA), but levels may be considerably higher if wastes from garbage disposals are included. Ecological waste-water systems are generally effective at reducing BOD and the related but more persistent
chemical oxygen demand (COD).
Another measure of performance is nutrient removal. Wastewater contains high levels of both nitrogen and phosphorous. Nitrogen removal occurs through a biological process of nitrification and denitrification. In the first stage, aerobic nitrifying bacteria convert ammonia into nitrite and nitrate. In the second stage, anaerobic denitrifying bacteria convert nitrate into molecular nitrogen (N2) and release it into the atmosphere. This stage requires some carbon, but nearly all the naturally occurring carbon has been eliminated from the flow by this point. Ecological system designers are experimenting with various low-tech sources of added carbon, but to date the only cost-effective and reliable solution has been to use the same chemical process used at conventional sewage treatment plants—the addition of methanol. Fluck argues that they are using the methanol only until more organic sources of carbon are developed, and to establish that these systems can outperform conventional treatment when it comes to nitrogen removal.
Phosphorous removal has proven even more difficult than nitrogen removal for ecological systems. Natural uptake by plants can only remove up to 60% to 70% of the occurring phosphorous, according to Shaw. To achieve further reductions ecological systems would have to use conventional chemical precipitators, such as ferric chloride or polymeric compounds. They rarely do so, however, preferring to leave relatively high phosphorous levels in the effluent. This weakness in nutrient removal is not critical if the outflow goes into the ocean or soil, but it could be a problem for systems that feed into surface waters.
Other measures of wastewater treatment plant performance include removal of Coliform bacteria (which are considered indicator species for pathogenic bacteria and viruses), water clarity (turbidity), odor, and heavy metal concentrations. With the exception of a few specific compounds, ecological wastewater systems are generally effective for all these contaminants, according to current operating experience.
As with most innovative new technologies, whether they succeed in penetrating the market often comes down to a matter of cost. If they can compete economically and do as well or better in terms of performance, the companies engineering and building these systems should be able to prosper. Both initial system costs and lifecycle costs, including operating and maintenance expenses, are important considerations.
Construction costs of ecological wastewater treatment plants have been highly varied. A 25,000 gallon-per-day (95,000 l/d) ecological wastewater plant designed by EEA and currently nearing completion in Ashfield, Massachusetts is costing a hefty $2.2 million. Because the technology is not yet “endorsed” by EPA, all sorts of obstacles had to be cleared in permitting it, and numerous redundancies had to be incorporated into the plant to provide for successful operation if a portion of the plant fails, since this will be the sole wastewater treatment plant for the town. These factors drove up the price.
A similar system recently built near Halifax, Nova Scotia with a design flow of 18,000 gpd (68,000 l/d), came in at $600,000 (Canadian) and was completed in a fraction of the time required for the Ashfield system.
This Bear River plant won out over conventional wastewater treatment technologies in competitive bidding, because the treatment plant could be located right in the center of a densely populated area, while the conventional plant would have necessitated several miles’ separation from the town center (because of odor). Even though the ecological wastewater plant was itself more expensive than the conventional plant, the savings achieved by not having to pipe the sewage an extra two miles brought the total package price lower.
An 80,000 gpd (300,000 l/d) plant completed this past February in South Burlington, Vermont by LTI came in at a total cost of $640,000. This plant cost so much less than the Ashfield system because it is a demonstration project working in conjunction with a much larger conventional facility—all pretreatment functions, such as screening and grit removal, and post-treatment functions, including discharge piping, disinfection, and outfall, are provided by that facility.
Operating costs for wastewater systems include labor, energy, and chemicals. John Todd estimates that labor costs for ecological systems should be comparable to those for conventional systems. Energy use in both conventional and ecological systems is primarily to power the pumps that circulate air through the tanks. Reed estimates that energy and chemical requirements for each type of system should be similar. Shaw of LTI reports that they are finding 25% reductions in energy costs, however, and that chemical costs of ecological systems should also be lower.
Studies done for the EPA report suggest that on a 20-year lifecycle basis, current ecological systems are comparable in cost to conventional wastewater treatment only for quantities of 50,000 gallons per day (180,000 l/day) or less, according to Reed. According to this analysis, at larger sizes the multiple greenhouses and tanks required become more expensive to build than the single large chambers used in conventional systems. System proponents contend that lower operating costs make ecological wastewater systems a less expensive choice in many cases.
Moving forward with ecological wastewater treatment
Ecological wastewater treatment is a different approach for treating wastewater that flies in the face of an inherently conservative civil engineering profession. To succeed on a municipal level, proponents of the technology have to convince multiple layers of government bureaucrats on local, regional, state, and sometimes even federal levels that the systems will work and can be operated economically. Bidding and contracting typically require cumbersome, drawn-out procedures that make innovation difficult at best. Given these challenges, the fact that systems have been built at all may be remarkable. Some plants, such as the facility in Ashfield, Massachusetts, have taken as long as five years to move through from concept to operation. These stumbling blocks are one of the reasons that Living Technologies, Inc. has focused more on industrial facilities, where decision making and contracting can proceed more quickly.
Ecological wastewater treatment is currently proceeding largely in the demonstration phase. The systems being installed tend to vary considerably from one another as the researchers continually modify and improve the technology. As performance data is collected from these systems over the coming year or two, certain design aspects are likely to emerge as preferable to others. Eventually, simple, turn-key plant designs may become available that can be specified and contracted out fairly quickly. Once that stage is reached, assuming that performance matches expectations, the potential for more rapid implementation of the technology should be very good.
Because ecological wastewater treatment plants operate odor-free and can be esthetically pleasing, they are unlikely to generate as much NIMBY (not-in-my-backyard) opposition, and they can be situated in populated areas.
The fact that they can be integrated into an attractive landscape gives them a strong advantage for some food-processing and industrial sites—its positive experience with a living machines in Henderson, Nevada and Waco, Texas has spurred the M&M/Mars Company to order three additional systems for plants in other parts of the world. These advantages should help them attain significant market penetration.
On the other hand, possible relaxation of water pollution standards and loss of federal support for new water treatment plants could be major obstacles to greater use of these systems. Unless effective ecological means of removing phosphorous can be found, ecological systems will be forced to depend on more conventional technologies before releasing treated effluent into surface waters.
Another obstacle that may inhibit greater penetration of this technology is the real or perceived infighting that has been occurring among the technology developers. Both EEA and LTI have sought to commercialize ideas of Dr. Todd, and Todd maintains an interest in both companies. But proprietary aspects of ecological wastewater systems threaten to prevent the ideas from making it into the mainstream, because even the terminology has to differ from one company to the other due to trademarks and patents. The technology used to be widely known as Solar Aquatics, but this name is now owned by EEA, along with a patent for use of clear or translucent solar tanks. LTI has its Living Machine terminology with opaque tanks (which they claim to have found work just as well as the translucent ones), and they have patented their Ecological Fluidized Bed concept, though company president Michael Shaw told
EBN the company willingly shares the technology.
EEA and LTI should recognize that each needs the other to be successful if they want their ideas to be widely implemented. Confusion that is generated in the marketplace by inconsistent terminology and differences in technology based on legalities rather than differences in performance is not healthy. With greater cooperation, ecological wastewater treatment stands a good chance of establishing itself firmly in conventional practice.