by Daniel Wickham
Conventional wastewater treatment is based on a relatively simple premise: First, remove as much of the organic waste as possible through settlement and filtration; second, convert the soluble organic matter into biological tissue that can be removed by physical means; and finally, destroy the rest through oxidation to carbon dioxide.
While wastewater technology based on microbial treatment has done much to purify our waters, it has done so at a cost. While we usually acknowledge that cost in dollars, it can also be seen in its effect on global atmospheric carbon balance.
In 1991, US water treatment systems collected some 35 billion gallons of wastewater each day, requiring some 72.8 million pounds of oxygen to oxidize the organic material in the wastewater. About one-third of the organic load goes to anaerobic digesters. Stabilizing the remaining soluble fraction in aeration basins takes about 48 million pounds of oxygen and some 26 million kilowatts (kw) of electric power.
On average, 1.5 pounds of CO2 are produced for each kw used. Just supplying the power to operate the aeration basins generates 19,500 tons of CO2 each day - 7,117,500 tons a year. Supplying the power to oxidize the sulfur and nitrogen in the wastewater, along with pumping and other costs, generates another 40,000 tons of CO2 per day - 14,600,000 tons per year.
Ironically, the purpose of all this electricity is to create more CO2 through the oxidation of the organic carbon in the waste stream.
Virtually all of the 72 million pounds of oxygen eventually is converted to CO2, resulting in 97 million pounds of CO2. The aeration basins receive about two-thirds of that - 65 million pounds per day or 11,862,500 tons per year. Add that to the 14,600,000 tons released by the electricity and you have conventional aerobic treatment of domestic waste releasing over 26.5 million tons of CO2 into the atmosphere every year.
Adding in the industrial sewage treatment systems that oxidize the vast quantities of organic waste from food processing, pharmaceuticals, petroleum and such would conservatively raise the impact of conventional aerobic treatment to more than 50 million tons of CO2 per year (a huge amount but still only probably 2-3 percent of the total US releases).
A New Paradigm
The premise behind conventional secondary treatment is the conversion of soluble organic matter to CO2 and biological cells, which can be physically removed from the wastewater. But who ever said that bacteria were the only organisms capable of such a feat?
About 25 years ago a gentlemen in Willits, California named Ed Burton developed a novel concept to treat wastewater. Knowing that trees love wastewater, he proposed planting trees right over a leach device so that roots could invade the system. Distribution pipes allowed wastewater to pass through the system and be released directly to tree root zone.
He installed a small forest at a wastewater treatment plant in Martinez, California in the late 1970s. This system still functions, providing unequivocal proof of the success of the technology. None of the units have ever clogged and the associated trees have shown spectacular growth rates. Redwoods planted with the units grew to 40 feet tall in as little as 9 years.
The wastewater at Martinez is secondarily treated so the full advantage of using untreated effluent was never gained. Burton grew trees with effluent coming directly from his home septic tank with equal success.
The fundamental treatment concept is identical to conventional secondary treatment: Conversion of soluble organic matter into cellular biomass. However, instead of growing a noxious, potentially pathogenic bacterial sludge that has to be disposed of at great expense, we obtain biomass in the form of valuable tree products. In areas without significant heavy metal content in their sewage, irrigated tree farms provide a constructive alternative to convential treatment plants.
Forestry right now is still at the hunter-gatherer stage for most of the industry. While industrial tree farming exists, silviculturalists have never had access to unlimited supplies of nutrient-rich water for irrigation.
The effluent from a typical 20 million gallon per day treatment plant serving about 100,000 people could be distributed to a plantation of redwoods of approximately 800 acres planted at 200 trees per acre. The growth rate of redwood irrigated with this nutrient-rich water would result in a standing inventory of timber of about 8 billion board feet in 60 years, or about 133 million board feet per year.
At a $1 per board foot for redwood, the city in question could earn an increase in asset value of its wastewater treatment system of $133 million dollars every year. Conventional treatment plants simply depreciate in value. Concrete does not grow. A living treatment infrastructure such as a wastewater forest, however, increases in capacity and growth of the system is genetically pre-programmed.
One could grow 1,400,000 acres of trees in US wastewater plantations. Within 60 years, the amount of timber produced with such a system rises to the staggering quantity of 28 trillion board feet, or 460 billion board feet per year. Each board foot contains about 2 pounds of cellulose which draws about 3 pounds of CO2 from the atmosphere for its creation. These forests therefore would remove about 690 million tons of CO2 from the atmosphere each year.
Add that to the 50 million tons that the now unneeded aeration basins no longer release into the atmosphere and you get a net reduction of 740 million tons of CO2 per year - almost 15% of the total US release of CO2. And trees will tie up the carbon for centuries or even millennia so the yearly savings can compound themselves. Once a wastewater forest is planted and grown for a time it will sustain itself even if you stop irrigating it. You can now move the wastewater to a new plantation and remove yet more CO2.
New Hope for Mexico
The US has already built most of its infrastructure according to the old model. Mexico, however, like many developing countries has just begun building its wastewater treatment infrastructure. Over the next 50 years such countries will spend billions of dollars for the infrastructure to collect and treat their wastewater. Unlike the US they have the chance to do it correctly.
The 1997 conference on global warming in Kyoto introduced the concept of carbon dioxide credits. I like to imagine a "carbon dollar" that can be traded. But, as with paper dollars, a carbon dollar needs a bank to store it in. The wastewater plantation can be that bank. Mexico could invite the US - the worlds largest carbon dioxide emitter - to build its carbon dollar bank using Mexico's wastewater. What better way to finance the creation of Mexico's infrastructure?
The amount of credit for each tree could be worked on a sliding scale depending on the final use of the wood product. If left as forest habitat and unharvested the trees would get the maximum credit. If harvested for construction lumber, it would get the next level of credit because such wood will still tie up the carbon for many decades.
Wood-based paper products, with a shorter cycle, would get a lesser credit. The lowest credit would be for firewood. It still would get a credit, however, because every BTU generated in that fashion prevents CO2 releases from fossil fuels so there is no net CO2 gain.
Cut CO2 Not Trees
The US, which produces 25 percent of the world's CO2, could reduce its CO2 emissions by 15 percent. Wastewater plantations on a worldwide basis have the potential to offset current CO2 emissions entirely.
Beyond the CO2 emissions or the profitability of such systems is an even more important consideration - the inherent ecological value of forests. A forest represents the most significant buffer that the earth's surface can have.
Western Australia cut its forests down years back and found that the soil water table moved to the surface. Without trees, the soil dried out and water began to evaporate from the surface. In the process salt was left behind and the entire region was converted into a desolate salt desert.
The only other proposal focusing on the positive aspect of removing CO2 from the atmosphere on a global scale that I have seen was one floated by John Martin at Moss Landing Marine Lab. Martin proposed adding iron to the open ocean to stimulate huge algae blooms that would remove CO2. Experiments have shown that significant stimulation can occur, but most marine biologists think that the potential outcomes of such a project are even less predictable and potentially more damaging than our current CO2 buildup.
The fact that such a simple change in one of our major industries can compound in so many ways makes a strong argument for re-analyzing the basic premises in all our industries.
© 1998 by Daniel Wickham. Daniel Wickham Ph.D., is a marine biologist with the IOS Corporation [4080 Heather Lane, Sebastopol, CA 95472, ios@interx.net]
State Forester Treats Waste with Trees
US - North Carolina State University Professor of Forestry Douglas Frederick is using trees to reduce harmful runoff from municipal, agricultural and industrial wastewater.
Frederick [frederick@cfr.cfr.ncsu.edu] sprays chlorinated wastewater on fields of hardwood and pine trees at five sites in eastern North Carolina. He has found that between 60 percent and 90 percent of the nitrogen and phosphorus in the wastewater is removed in the process. Some of the nutrients are taken up and stored in the trees; other nutrients are used or tied up by microorganisms in the top 10 inches of soil near the trees' roots.
"Fewer nutrients end up as contaminants in groundwater and surface waters, and because the wastewater acts as a fertilizer, the sprayed trees grow much faster than they would normally," Frederick says. "At the site in Edenton, we have trees that have grown 60 feet in just eight years."
Frederick's tree plantations, most of which cover several hundred acres, are located on the grounds of existing municipal or industrial wastewater treatment facilities.
At the municipal sites, the system is used as a secondary treatment process to remove nutrients left in the wastewater after the primary treatment process - chlorination and separation of solids - has taken place.
Tree plantations treat wastewater for about 60 cents per 1,000 gallons - about one-half to one-third the cost of traditional secondary treatment methods.
The town and Edenton asked Frederick to install a sewage forest after tests showed the towns' conventional treatment programs weren't in compliance with government standards.
"Everything's working fine now. The analyses we're getting show we are in compliance, and about the only heavy maintenance we have is mowing the grass around the trees," says Jimmy Patterson, Edenton's wastewater manager.
"We may find that a system using both constructed wetlands and spray applications on adjacent forests will provide the most efficient treatment," Frederick says.
- North Carolina State University, [www2.ncsu.edu]
The Wrong Way to Do It
US - Residents of Dallas, Oregon want to block a municipal plan to cleanse wastewater with a grove of poplar saplings. Dallas, which currently pours 2 million gallons of treated sewage into nearby Rickreall Creek every day, wants to use hybrid poplars to absorb 3,000 gallons of that effluent.
While poplars safely metabolize ammonia and other nitrogen compounds, they leave most heavy metals and salts in the soil.
An existing sewage forest in the town of Woodburn, 25 miles away, is so successful that scientists have traveled all the way from Argentina and Holland to observe it.
The effluent used in Woodburn comes in the form of treated water from the city sewage plant. The wastewater destined for the Dallas poplars, however, comes straight from a computer circuit board factory. According to the Associated Press, the wastewater contains "salts and heavy metals such as lead, cadmium, copper and possibly cyanide."
Soil scientists admit that these contaminants could leach into the groundwater if the soil acidity is not carefully balanced. Such concerns have driven Dallas residents to file lawsuits claiming that local officials have broken land-use laws by siting the experimental sewage plant on prime farmland.
Sewage Forests: Cleaning Water and Cooling the Planet