Microalgae for CO2 Utilization & Agricultural Fertilizer Recycling

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The Controlled Eutrophication Process: Using Microalgae for CO2 Utilization and Agricultural Fertilizer Recycling.

Abstract

In 1960, Oswald and Golueke [1] presented a conceptual techno-economic analysis, "The Biological Transformation of Solar Energy", proposing the use of large-scale raceway ponds to cultivate microalgal on wastewater nutrients and then to anaerobically ferment the algal biomass to methane fuel. The methane was to be converted into electricity, with the CO2-containing flue gas recycled to the ponds to support algal production. Over the past forty years a great deal of research has been carried out on this and similar concepts for microalgae fuels production and CO2 utilization. However, major technical challenges have limited the practical application of this technology: the difficulties of maintaining selected algal species in large-scale production systems, the lower-than anticipated biomass productivities and methane yields, and the high costs of harvesting the algal biomass and of the overall process. These limitations can, however, be overcome by applying such processes where additional economic benefits, such as wastewater treatment or nutrient recovery, are available and where relatively large systems (> 100 hectares) can be deployed, allowing economics of scale.

One such site is the Salton Sea in Southern California, into which over 10,000 tons of nitrogen and phosphate fertilizers are discharged annually by three small rivers draining large tracts of irrigated agriculture. Removal of nutrients from these inflows is required to avoid eutrophication of this large (some 900 km2), shallow, inland sea, with resulting massive algal blooms, fish kills and other environmental impacts. Nutrient capture could be accomplished with some 1,000 hectares of algal pond systems, with the algal biomass harvested and converted into fuels and the residual sludge recycled to agriculture for its fertilizer value. A techno-economic analysis of this process, based on nutrient removal defraying a fraction of the costs, suggests that such a process could mitigate several hundred thousand tons of fossil CO2 emissions at below $10/ton of CO2-C equivalent.

Introduction - Microalgae Applications in Environmental Protection

Microalgae ponds have been utilized for several decades for the treatment of municipal and other wastewaters, with the microalgae mainly providing dissolved oxygen for bacterial decomposition of the organic wastes [2]. The major limitations in this technology are the relatively low loadings that can be applied per unit area-time, increasing land area requirements, and the high cost of removing the algal cells from the pond effluents, using chemical flocculation or other means. High-rate ponds, with channels and mechanically mixed, were introduced a half century ago [3], and allow much higher loadings than the standard unmixed "facultative" ponds.

However, high rate ponds also exhibit higher algal cell densities, making algae removal, harvesting, a requirement for wide-scale applications. Paddle wheel-mixing provides a controllable and flexible mixing regime than pumps, and allows managing the pond culture to promote algal cells that tend to flocculate and settle.

However, it has not been possible to demonstrate such a "bioflocculation" processes with the high reliability required for algal harvesting in municipal wastewater treatment. Thus, in current designs, high rate ponds are followed by large settling or "maturation" ponds, and often the effluents from the ponds are used for irrigation, ground water recharge, or similar applications. Development of more intensive, smaller footprint, microalgae wastewater treatment processes based on high rate pond technology and low-cost algal harvesting remains an R&D challenge.
One process that accomplishes this goal is the Partitioned Aquaculture System (PAS), being applied as the "Controlled Eutrophication Process" at the Salton Sea, as described more fully below.

Microalgae have also been used extensively studied in other environmental applications. The removal of heavy metals from wastewaters has been extensively studied and some actual applications with immobilized algae were reported, though these could not compete commercially with ion exchange resins [8]. The removal of residual nutrients from wastewaters, so-called "tertiary treatment", specifically N and P, has also been studied with a variety of processes, from attached algal cultures to controlled algalculture in cooling reservoirs [9, 10]. Microalgae are excellent for nutrient removal processes, as they exhibit high contents of N and P, about 10 and 1% respectively on a dry weight basis, several-fold that of higher plants. Also, microalgae cultures are able to reduce residual concentrations of these nutrients to vanishingly low levels and allow for a significant variability in N:P ratios, from about 3 to 30 N for each P, on a weight basis, depending on limiting nutrient.

Finally, microalgae cultures have been proposed for some years as a method for fixation of CO2 and production of biofuels, of interest in greenhouse gas mitigation. The first conceptual development of this idea was by Oswald and Golueke in 1960 [1] who described a large-scale system with dozens of large (40 hectare) high rate ponds, with the algae grown, the biomass harvested by a simple flocculation-settling step, and the concentrated algal sludge anaerobically digested to produce biogas (methane and CO2). The biogas would be used to generate electricity and the flue gas CO2, along with the nutrients in the digester effluent, used to grow more algae. Make-up water and nutrients (C, N, P, etc.) would be provided from wastewaters.

A preliminary engineering-cost analysis suggested power production costs similar to those projected for nuclear energy. A more detailed, study-level, design and engineering analysis of this concept was carried out by Benemann et al. in 1978 [11], who concluded that with favorable assumptions (low-cost harvesting, high productivities), such systems could produce biogas competitively with then projected fossil fuel prices.

Since the early 1980's, the U.S. R&D effort in microalgae biofuels production has centered on the DOEsponsored "Aquatic Species Program", which aimed at producing algal oils for production of biodiesel. As part of this effort, several rather more detailed engineering design and cost analysis studies were carried out, during the 1980's, again with many favorable assumptions in particular very high productivities. A quarter hectare pilot plant operated in New Mexico, demonstrating the feasibility of outdoor microalgae cultivation on saline waters and efficient CO2 capture. The relatively long-term nature of the R&D required for such dedicated energy production processes, among other factors, led to the wind-down of the Aquatic Species Program in the mid 1990's. In Japan a very much larger government-sponsored program, involving many private companies, on microalgae biofixation of CO2 was carried out during the 1990's. This program focused on mainly closed photobioreactors, including optical fiber systems, for fixation of CO2 and co- production of high value products [16]. Currently in the U.S. similar concepts are being developed with U.S. Dept. of Energy support [17, 18]. In Japan, also during the 1990's, electric utility companies carried out additional R&D programs, including cultivation of microalgae on seawater and actual power plant flue gas CO2 in small ponds inside greenhouses [19], as well as projects on biological H2 production by fermentation of microalgae biomass [20]. Microalgae biofixation R&D continues in Japan, though at a lower level of intensity [21, 22, 23]. Most recently an "International Network on Microalgae Biofixation of CO2 and Greenhouse Gas Abatement" was formed, to foment and coordinate R&D activities in his field.

Microalgae biofixation of CO2 and greenhouse gas abatement requires open pond systems and must include other co-products or services, such as wastewater treatment or fertilizer recycling. One example is the use of microalgae to remove N and P from agricultural drainage waters, such as at the Salton Sea, described herein.

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