THE CONTROLLED EUTROPHICATION PROCESS: USING MICROALGAE FOR CO_2 UTILIZATION AND AGRICULTURAL FERTILIZER RECYCLING

摘要

In 1960, Oswald and Golueke presented a conceptual techno-economic analysis, "The Biological Transformation of Solar Energy", proposing the use of large-scale raceway ponds to cultivate microalgae on waste nutrients and then to ferment (anaerobic digestion) the algal biomass to methane fuel. The methane was to be converted into electricity, with the CO_2-containing flue gas and the nutrient-containing digester effluents recycled to the ponds to support further 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 CO_2 utilization. However, significant technical challenges have limited the practical application of this technology: the difficulties of maintaining selected algal species in algal mass cultures, the lower-man anticipated biomass productivities and methane yields, and, in particular, the high costs of harvesting the algal biomass. These limitations could be overcome through further research and technology development and by integrating biofuel production with wastewater treatment and nutrient recovery, providing for additional economic and environmental benefits, in particular where relatively large scales are possible. One potential site for such a large-scale integrated process is the Salton Sea in Southern California, a shallow (average 10 m depth) large (900 km~2) inland lake located below sea level and, thus, without an outflow. About 10,000 tons of nitrogen (mostly as nitrate) and phosphorous (as phosphates) are discharged annually into the Salton Sea by three rivers that drain wastewaters from population centers and drainage waters from large tracts of irrigated agriculture. Removal of nutrients from these inflows is required to avoid eutrophication of this large lake with resulting massive algal blooms, fish kills and other environmental impacts. Nutrient capture could, in principle, be accomplished with 1,000 hectares of algal pond systems, assuming a production of 100 tons of dry weight biomass per hectare per year (t/ha/y) and a typical 10% N and 1% P content in the harvested algal biomass. This biomass could then be converted into biofuels with the residual sludge used in agriculture for its fertilizer value. As stated above, a major limitation of such a process is the harvesting of the algal biomass, which is expensive using current technology. The "Partitioned Aquaculture System" (PAS), developed at Clemson University, integrates microalgae production with fish aquaculture in a process that promotes a vigorous algal culture of desirable species and converts these into a sedimentable biomass through the production of fish fecal pellets. An adaptation of the PAS, the "Controlled Eutrophication Process" (CEP), has been designed and is being tested by Kent SeaTech Corp. to achieve the objective of nutrient removal from the Salton Sea influents. The CEP, by producing a combination of valuable outputs, including fuels, fertilizers and fish, could help to affordably manage the Salton Sea environmental quality while also abating substantial quantities of greenhouse gases.