Alternative
Wastewater
Treatment:

Advanced
Integrated
Pond
Systems

 

Alternative Wastewater Treatment: Advanced Integrated Pond Systems

Advanced in concept and simple in design, a new wastewater treatment technology may offer a solution for communities beset by intensifying cost constraints and water quality regulations.

Why not build a sewage treatment facility that uses much less energy than a conventional one and produces no odors, and one whose construction, operation, and maintenance costs are also dramatically lower? This question may occur to many who have visited the Wastewater Treatment and Relocation Plant in St. Helena, California, particularly to visitors from communities feeling pressure from federal and state environmental regulations.

Based on the concept of Advanced Integrated Pond (AIP) systems, the St. Helena plant marks a radical departure from conventional wastewater treatment thinking. In conventional plants, for example, aeration often consumes 60% or more of the electrical energy used in wastewater treatment. In contrast, microalgae in an AIP system provide dissolved oxygen through photosynthesis, substantially reducing electrical consumption. Not surprisingly, these systems are optimal for sunbelt communities.

Construction and Maintenance Costs

Good financial sense begins with facility costs. Because solar aerated ponds are built of formed earth rather than of reinforced concrete, they cost about 100 times less to build per cubic foot of containment than do conventional treatment plant reactors. The total pond area needed is much larger than that needed for a conventional plant, but ponds should still cost only one-third to one-half as much to build, according to William Oswald, who designed St. Helena’s system in the early 1960s. Oswald is a professor emeritus at the University of California, Berkeley (UC-Berkeley) and inventor of the AIP system.

Proponents of the technology believe that maintenance costs for the new plants are also lower because such plants minimize the use of mechanical equipment and require a smaller inventory of spare parts and supplies. Operation costs are reduced because the plants can be run with smaller staffs.

Another important advantage of AIP plants is the small amount of sludge they produce. In these ponds, sludge ferments until nothing is left but a small volume of residue. For example, during 27 years of operation, St. Helena’s wastewater treatment plant has never had to remove residue. A recent measurement at St. Helena showed that in nearly 3 decades, less than 1 meter (3.28 feet) of residue had accumulated at the bottom of the deep digester pit. This represents a substantial benefit in terms of meeting environmental regulations for residue disposal.

Energy Costs

A properly designed AIP plant should consume about one-quarter to one-fifth the energy of a conventional mechanical wastewater treatment plant. This translates directly into cost savings. One significant source of savings lies in using solar energy rather than electrical energy for aeration. Conventional plants aerate by using electrical energy to blow or mix air bubbles into the wastewater. In an AIP system, algae use solar energy and photosynthesis to supersaturate the wastewater with the oxygen that microbes need to break down waste.

"For people who have always thought in terms of conventional treatment, it’s hard to understand that you can aerate without any mechanical system," says Oswald. "Using mechanical aeration, you need about 1 kilowatt-hour of electricity for each kilogram of dissolved oxygen. In an AIP system in a good climate, you get around 20 kilograms (44 pounds) of oxygen per kilowatt-hour, because your energy is essentially free. That energy is solar energy."

St. Helena’s plant still uses more energy than an optimal, up-to-date AIP plant would require. That’s because St. Helena’s plant, designed 30 years ago, uses conventional pumps to circulate water in the pond where aeration takes place. Calculations that now show the five-to-one energy advantage of an AIP plant are based on designs using paddle wheels for circulation. Paddle wheels are now a proven technology commonly used in commercial algae-growing operations. Paddle wheel circulation has been incorporated in an AIP system that UC-Berkeley is designing for a St. Helena-sized wastewater treatment plant in California’s Central Valley. St. Helena is also considering a conversion to paddle wheels.

While aeration by algae and solar energy can greatly reduce the electricity consumed by an AIP plant, another source of energy, electricity-generation through combustion of methane, could eliminate electrical power costs completely. Methane can be produced by fermenting algae harvested from the plant’s settling pond.

Conventional plants typically install large tanks known as digesters, in which sludge and effluent solids ferment to produce methane. In an AIP plant, methane from natural fermentation in the digester pit could be captured at the surface of the facultative pond. Developing a good commercial methane capture system for AIP systems is under way. The Environmental Engineering and Health Science Laboratory (UC-Berkeley) in Richmond, California, is working on that development with funds from the California Energy Commission and the California Institute for Energy Efficiency.

Wastewater: A Resource, Not a Waste

As a result of state and federal regulation, wastewater treatment managers are seeing tougher regulations affecting the quality and handling of sludge and effluent from their plants. AIP systems not only produce less sludge, they can also produce cleaner effluent for every dollar spent on treatment.

"Our system breaks down toxic substances," says Bailey Green, a UC-Berkeley scientist who manages the AIP pilot plant at the Richmond laboratory. "By input-output analysis, we know that halogenated organic compounds are biodegraded in large part."

Regulators may put stricter limits on parasites that cannot escape the fermentation pit of an AIP system. In addition, most of the heavy metals in sewage are precipitated and remain trapped in the facultative pond’s digester pit.

Further, nutrients such as nitrogen and phosphorus can damage aquatic ecosystems into which effluent may be discharged. AIP plants are better than conventional plants at removing these nutrients. Nitrogen removal occurs in the digestion phase in the facultative pond. In addition, nitrogen and phosphorus are taken up and contained by algae in the high-rate pond. Oswald champions the use of algae harvested from AIP plants as fertilizer because the nutrients contained in algae would be released more slowly than would the water-soluble forms in chemical fertilizers and thus be less likely to return to lakes and streams in runoff.

AIP systems also exceed conventional primary and secondary treatment plants at killing pathogens because of natural disinfection by high alkalinity and ultraviolet (UV) exposure. Without any other treatment, effluent from a four-pond AIP plant should be sufficient to meet the most recent World Health Organization recommendations for irrigation water, according to Oswald. The St. Helena plant is highlighting the beneficial reuse of its reclaimed water by growing pumpkins, corn, melons, flowers, roses, and more than 0.8 hectares (2 acres) of wine grapes.

Still, not even the most enthusiastic proponents of AIP systems claim that the basic four-stage system of ponds alone can produce effluent meeting standards for drinking water or for "unrestricted" uses such as swimming pools and irrigation of public parks. An AIP system, like other treatment concepts, can only achieve these goals by additional treatment (e.g., disinfection, filtration, and solids removal).

"But the question in meeting increasingly stringent regulations is cost," says Green. "If you’ve saved 200% to 300% of your costs on the front end, with some degree of tertiary and quaternary treatment in the bargain, you may have money to put into UV disinfection or dissolved air flotation and still come out ahead of the game."

Conclusion

The St. Helena plant has demonstrated the AIP concept for nearly 30 years. More than 85 hybrid AIP plants are now employing elements of the AIP concept in the United States and other countries. Most of them, like St. Helena’s plant, produce very little sludge. Many of them use a combination of mechanical and solar aeration, a practice that still requires less electricity than does conventional treatment and less land area than does a system such as St. Helena’s.

As the benefits for AIP systems become more well known, the acceptance of this low-cost treatment concept is likely to grow. In a political climate of intensifying regulation and a fiscal climate in which construction, maintenance, and operation costs are increasingly important, the AIP concept may prove to be the ideal technology for use by many local wastewater managers.

DOE Regional Support Offices

The DOE Office of Energy Efficiency and Renewable Energy reaches out to the states and private industry through a network of regional support offices. Contact your DOE regional support office for information on energy efficiency and renewable energy technologies.

This document was produced for the U.S. Department of Energy (DOE) by the National Renewable Energy Laboratory, a DOE national laboratory. The document was produced by the Technical Information Program, under the DOE Office of Energy Efficiency and Renewable Energy.

DOE/CH10093-246 DE93018228

For More Information

National Association of Towns and Townships, 1522 K Street, NW, Suite 730 Washington, DC 20005; (202) 737-5200 Treat It Right, A Local Official’s Guide to Small Town Wastewater Treatment

Marty Oldford, Director of Public Works City of St. Helena, 1480 Main Street, St. Helena, CA 94574; (707) 963-2741

Lorne E. Swanson, Swanson International Engineering Inc., 1320 Arnold Drive, Suite 169, Martinez, CA 94553; (925) 228-5801 Fax (925) 228-5804; E-mail swanint@ix.netcom.com

Alexander Horne,University of California, Berkeley Environmental Engineering and Health Sciences Laboratory, 1301 South 46th Street, Richmond, CA 94804;(510) 642-1089

http://www.waterandwastewater.com