Abstract: As human populations have expanded, Earth’s atmosphere and natural waters have become dumps for agricultural and industrial wastes. Remediation methods of the last half century have been largely unsuccessful. In many US watersheds, surface waters are eutrophic, and coastal water bodies, such as the Chesapeake Bay and the Gulf of Mexico, have become increasingly hypoxic. The algal turf scrubber (ATS) is an engineered system for flowing pulsed wastewaters over sloping surfaces with attached, naturally seeded filamentous algae. This treatment has been demonstrated for tertiary sewage, farm wastes, streams, and large aquaculture systems; rates as large as 40 million to 80 million liters per day (lpd) are routine. Whole-river-cleaning systems of 12 billion lpd are in development. The algal biomass, produced at rates 5 to 10 times those of other types of landbased agriculture, can be fermented, and significant research and development efforts to produce ethanol, butanol, and methane are under way. Unlike with algal photobioreactor systems, the cost of producing biofuels from the cleaning of wastewaters by ATS can be quite low.
During the late 1980s, it was determined that agriculturally derived nutrients, principally Phosphorous (P), were seriously affecting the Florida Everglades. In the search for a landscape-scale technology for removing that P from farm run-off, the South Florida Water Management District screened two-dozen technologies and selected nine for further study. Managed, constructed wetlands were eventually selected as the most suitable technology after a decadelong comparison. In an economic analysis published in 2005, Sano and colleagues,... normalized data from the S-154 ATS test plant as a 23-ha facility over a 50-year operation. It was determined that such an ATS system could remove P for $24 per kg. The ATS cost, per unit of P removed, was about one-third of the least expensive equivalent constructed wetlands module.
In late 2005, the engineering firm Hazen and Sawyer ... revaluated the S-154 data, with and without pumping and algal harvesting costs. They evaluated several scenarios for the algal biomass, including “giving it away.” Using the data for 0.5 mg per L P influent concentration and including one-half pumping costs (for river floodplain operation) and a discount rate of 5.375%, the firm’s figures provide a cost of $28 per kilogram P. Assuming this number (Florida construction and labor costs) to be higher than the US average and allowing for a broad range of value in the algal biomass, including energy value, the basic nutrient scrubbing task was accomplished for $24 per kg (with N removed at the same time, for the dollars already invested). Therefore, N and P were removed at a cost of approximately $1.50 and $22.40 per kg, respectively. When the production of the ATS plant is normalized for the lower light and temperatures in the center of the country (e.g., in St. Louis, Missouri), the cost is roughly 20% of the average cost of nutrient removal as it was published by the Chesapeake Bay Commission in 2004 (CBC 2004). Because these analyses attributed all costs to P, the relative costs of the two nutrients are distributed according to the CBC mean proportions.
More widely promoted in recent years has been the closed photobioreactor concept, in which selected or genetically engineered monocultures of algae are grown in an interconnected array of clear tubes or bags (Carvalho et al. 2006, Ugwu et al. 2008). Such algal culture is carried out in greenhouses, using a wide range of proprietary technologies to optimize photosynthesis. Greenfuels Technologies has reported a three-month mean rate of production of 98 g per m2 per day in a pilot operation linked to an Arizona Public Service power plant. On 12 December 2007, Vertigro Joint Venture issued a press release reporting a three-month average algal production in a pilot photobioreactor at El Paso, Texas, of 102 metric tons per ha per year (138 g per m2 per day). Although the economics of such operations remain largely unknown, the infrastructure required clearly suggests very high costs if the key environmental and culturally pristine conditions requisite to high production are to be met. Recent estimates of algal biomass production costs for photo bioreactors are about $3.50 per kg (Chisti 2007). Although carbon dioxide sequestration is clearly a favorable feature of this methodology, carbon capture can be only a minor economic element in such a high-cost endeavor.
In 1998, the chemist David Ramey improved the 90-year old acetone-butanol-ethanol industrial fermentation. Ramey (1998) used two separate species of the anaerobic bacteria Clostridium in a two-step fermentation process, followed by a physical concentration process that produced a 90% butanol product plus hydrogen gas as a byproduct. In a 2004 report to the US Department of Energy, he described a continuous production plant of 185 L per week from corn and dairy wastes and proposed plans for expansion to multimillion-gallon production. Researchers from the University of Western Michigan have analyzed the Taylor Creek algal biomass and produced a preliminary plan for producing butanol (from carbohydrates) from the algal product (table 1). Using the current cost data for a 580-ha, 11-billion-lpd ATS system designed to clean the Suwannee River in Florida, and applying that study to a similar plant in the center of the country, we calculated that the algal biomass substrate available for energy conversion would cost about $0.75 per kg. This compares with recent estimates to produce microalgal biomass using photobioreactors of $3.50 per kg (Chisti 2007), as was noted above.
Although the photobioreactor biomass is estimated to have higher oil content than ATS algal biomass, and therefore to have lower refining costs, the ultimate price of the biofuel produced by the two methods is likely to be about the same: between $1.60 and $2.70 per L ($6 to $10 per gallon). Therefore, growing algae using ATS solely to produce energy—even at large, efficiently operated facilities on river floodplains where pumping costs and energy input are minimal—is not likely to be a profit-making endeavor and would be highly sensitive to the price of crude oil. On the other hand, ATS algae provide a much larger potential for bioenergy supply than corn and soy because of their high productivity. In addition, the value in the nutrient removal process, given as credits or bankable dollars—even at a fraction of the cost of current removal in the Chesapeake Bay watershed, for example—would cover the cost of construction, operations, and maintenance, and still leave a significant profit margin. The recovered oil and butanol would be byproducts available at the cost of refining, very likely at 20% to 30% of current fuel prices. Because the processed biomass would produce a balanced fertilizer, this would provide an additional return. Perhaps most important, the energy product would have little sensitivity to the global price of crude oil.
by Walter H. Adey, Patrick C. Kangas, and Walter Mulbry
Volume 61, Issue Number 6; June, 2011; pages 434-441
Keywords: algae, biofuel, ecological engineering, nitrogen, phosphorus