Sewage Treatment Plant Blooms
Flowers and profits are potential offshoots of biological technique for handling waste water. PILOT GREENHOUSE PROJECT
THE greenhouse sits at the base of the ski slopes, just below the condominiums and across the road from the sports center. It is a modest structure that for two years has housed a significant experiment: one that promises to convert sewage treatment into a pleasant and perhaps profitable undertaking. Sales of plants (or their cut flowers) that purify the sewage passing through the greenhouse would lower costs dramatically. That is the expectation of Michael J. Shaw, whose Advanced Greenhouse Systems company of Burlington, Vt., recently began studying the horticultural possibilities of biological sewage treatment. The system was developed by John Todd of the Ocean Arks International research corporation, in cooperation with Ecological Engineering Associates (EEA), both of Falmouth, Mass.
Dr. Todd's ``Solar Aquatics'' waste-water treatment system duplicates, under controlled conditions, the natural purification process of freshwater marshes in the wild. In effect, a slow-moving stream is compressed, concertina-like, into the confined space of a greenhouse so that a relatively small area can be made to do the work of a much larger one in nature. And, where natural marshes shut up shop, so to speak, in more northern latitudes during winter, the sun-warmed temperatures of a solar greenhouse make the Todd system operable year-round.
The system works by circulating the waste water through a series of ecosystems - raceways containing groups of plants ranging from willows to bulrushes, including watercress, the blue-flowered forget-me-not, and aquatic ferns such as floating duckweed. ``All water-loving plants do it best,'' says Dr. Susan Peters, who heads up EEA. ``They are pre-adjusted to having their feet wet.'' But, as she points out, conventional hydroponics has shown that many other plant species can adjust to ``wet feet.'' Several garden varieties, on floating beds, including mums, marigolds, and tomatoes, have adapted to the Sugarbush trials.
Along with various plant species, bacteria, algae, zooplankton, fish, and crustacea such as snails and freshwater clams, living on and among the plant roots, all play a part in the purification process.
As Susanna McIllwane, who operates the Sugarbush pilot project, explains it, the treatment process takes place in four stages over approximately five days.
1.On the first day air is bubbled through the effluent as it enters the greenhouse. At the same time, seven strains of bacteria are added. In the oxygen-enriched effluent some strains break down soluble organic chemicals into carbon dioxide and water. Others degrade proteins, fats, and starches into compounds useful to naturally occurring microorganisms further down stream.
2.During the first two days nutrients are broken down and taken up by nitrifying bacteria, green algae, and the floating plants. Snails and other zooplankton graze on the algae and begin the process of sludge digestion.
3.In the third and fourth days plants further downstream, with their root masses reaching into the water, continue the rapid removal of nutrients from the waste stream. Large populations of grazing zooplankton, which inhabit these root-filters, eat the suspended solids.
4.On the final two days, solids are filtered out in a sand and stone substrate, remaining nitrates are reduced to nitrogen gas and water, and certain pathogenic bacteria are destroyed by the action of several marsh plants, including bulrushes, reeds, and cattails. In the end, water emerges that meets the tough Vermont standards for release into a trout stream.
A few years back the state came up with an ultimatum to the ski resort. Improve your waste-water treatment system or be closed down. The resort responded with two pilot treatment plants, Todd's solar system and a breakpoint chlorine system. Where the former uses sunlight and natural organisms to purify the waste, the latter adds sodium hydroxide to the effluent and then injects it with chlorine gas to convert the ammonia to harmless nitrogen. Excess chlorine is finally taken out by the addition of sulfur dioxide and the remaining waste passed through an activated carbon filter.
During the trial period, workers from the breakpoint system would often eat lunch in the greenhouse. No one ever ate in the almost windowless chemical plant surrounded by bags of sodium hydroxide stamped with the warning ``Danger, caustic'' and with a gas mask at the ready in case a ``chlorine emergency'' should occur.
But, the plant does meet Vermont's standards for clean water, and because the solar system had not proved itself through a tough Vermont winter, when the deadline arrived, Sugarbush authorities went with the established technology.
The greenhouse system has since proved that it, too, can meet the Vermont standards during winter. Consequently, Providence, R.I., has elected to put up a solar aquatic system, with a capacity of 15 to 20 thousand gallons a day. Construction is currently under way and operation is schedule to begin by mid June.
Meanwhile the 3,000-gallons-a-day plant at Sugarbush continues to operate on an experimental basis, this time to see if it can be made into an income-generating facility.
Ecological Engineering's Dr. Peterson makes it plain that her company designs the biological systems to function ``as straight treatment plants. People must realize that,'' she says. ``If any additional values come out of them, then that's great, but that isn't their primary purpose.''
On the other hand, Michael Shaw sees income generation as a distinctly possible secondary purpose. ``We can't be certain just yet,'' he says, ``but we intend to find out. Sewage sludge is loaded with the very nutrients that plants thrive on,'' he points out. To this end he is experimenting with house plants, raising seedlings and rooting cuttings. Cut flowers have distinct commercial possibilities, he notes, including spring-flowering bulbs and roses. Early trials appear ``promising,'' he says, noting that ``paperwhites have done particularly well.''
If the trials prove out, the possibility exists that towns might one day subcontract part of their sewage treatment needs to greenhouse operators. One futuristic scenario: Sugarbush-sized treatment plants atop high-rise office complexes, producing cut flowers for the offices and pristine water for the fountains and interior landscaping in the shopping mall.