In an effort keep nutrients from reaching the Chesapeake Bay where they may contribute to algae blooms, the Fruitland Waste-water Treatment Plant has gotten into the farming business. Its crop is algae. The plant, located just outside Salisbury, MD, pipes a portion of its wastewater through an algae "scrubber" that soaks up most of the nutrients in the effluent. The algae is then "harvested" and composted. "Millions of years ago, the Earth's biosphere had learned that the best way to clean up water is to use algae," said Walter Adey, director of the Smithsonian Institution's Marine Systems Laboratory, who developed the system. "All we are doing is going back to that."
The $250,000 experiment is one of several projects supported by the Maryland Department of the Environment to test cost-effective technologies that remove nutrients in treatment plant effluent. So far, the scrubber has been highly effective since it began operation this summer. It removes most of the inorganic nitrogen from the 75,000 gallons of treated effluent - about 15 percent of the total - it receives from the treatment plant daily.
It does so at about one quarter of the cost of more traditional methods of controlling nitrogen at wastewater treatment plants. Further, Adey said, the protein-rich algae produced by the "algal turf scrubber" may eventually be marketable, perhaps as a livestock food additive. "It's a tremendously valuable product," Adey said. "Treating sewerage can literally be a money-making industry." Indeed, a Houston-based company, Aquatic Bioenhancement Systems, has acquired the rights to the Smithsonian's patent for the process and is using it at a giant aquaculture facility in Texas. Cycling water from the fish ponds through the scrubbers cleans and adds oxygen to the water before it is returned to the ponds. Nutrients from the ponds grow algae - the base of the aquatic food chain - which is processed as fish food and returned to the ponds.
"We produce protein at a lower cost and more effectively than any system in the world," said Richard Purgason, who heads Aquatic Bioenhancement Systems. But Purgason said the technique may ultimately have other uses. He said runoff from animal feedlots may someday be pumped through algal turf scrubbers to not only reduce pollution, but to produce food for the livestock.
"You bring in nitrogen and phosphorus and you export protein," Purgason said. Adey developed the algal scrubber in the late 1970s. After studying coral reefs for years, he wanted to create a model that simulated real-world reef conditions. Typically, Adey said, natural, planktonic algae acts to "scrub" nitrogen from the water. The algae are then eaten or die and sink to the bottom before the water reaches the reef. That helps keep the water clear and nutrient poor so that corals can calcify and compete with larger, reef algae. Normally, Adey said, a model would use bacteria to filter the water, but that didn't work in this case because the bacteria also filtered out the larval stages of many reef organisms and lowered oxygen levels, making it impossible to reproduce a functioning ecosystem. The solution was to build his own algae-scrubbing system to remove the nutrients. The model proved successful, and later became the basis for a coral reef exhibit in the Smithsonian Institution's Museum of Natural History.
Adey saw the potential for real-world applications as well, so the Smithsonian patented the process in 1982. Several years ago, the process was incorporated at a wastewater treatment plant in Patterson, CA. At the Fruitland facility, about 15 percent of the plant's 500,000-gallons-a-day discharge is piped to the scrubber, which consists of 10 parallel, 100-yard raceways. All the effluent flows down the first raceway, then is pumped to the top of the second raceway, and so on until it flows down all 10. The amount of algae grown gradually diminishes from the first raceway to the last. When harvested, the first raceway alone may yield 75-80 gallons of algae, while the last three combined may produce only about 60 gallons. Inorganic nitrogen content coming into the scrubber, Adey said, is about 20 parts per million. The scrubber can reduce that to 3-4 ppm and - on brightsunny days that are particularly conducive to algae growth - even lesser amounts. The state generally seeks an 8 ppm concentration or less from traditional nitrogen-removal technologies used at treatment plants.
Adey said the system should be able to operate year-round because the water temperature coming out of the treatment plant is warm enough to allow algae growth even in the winter. The scrubber removes nitrogen until it runs out of carbon, a natural fuel the algae also needs to grow. If a carbon source such as carbon dioxide were added, Adey said, the scrubber could theoretically remove almost all of the nitrogen. In the coral reef model, he said, the nitrogen was reduced to about .014 ppm. About every five days, Chris Luckett, the project's manager, backs up a pickup truck to each raceway and uses a winch to roll down the screen the algae grows on. As he does so, the algae fall into a container. He then takes the algae to a greenhouse constructed near the raceway and spreads it on tables to dry. When dried, it goes to a local composter for use.
"Right now, we're giving it away," Luckett said. "But at least we're not having to pay to have it hauled away." In the long term, Adey said, a better solution is for the algae to be incorporated into a marketable product, whether it is animal feed, fertilizer, alcohol or another biochemical.
"The potential is obviously there," Adey said. "When you have a 40 percent protein product in what would eventually be hundreds of thousands of tons, to throw it away is criminal. It's more valuable than what comes out of most farms."