Cultivation of algae and nutrient removal in a waste heat utilization process

Abstract

A process providing a beneficial use for waste heat and excess nutrients in the cooling waters of nuclear reactors and fossil-fueled power generating plants has been developed. The process involves the cultivation of selected strains of thermotolerant microalgae in heated discharge waters and the subsequent harvesting of the algal biomass for nutrient removal, recovery of energy and fertilizer, and extraction of high value products. The design of such a process is presented for a large cooling reservoir receiving a discharge of 109 1 -1 d -1 of secondary cooling water containing 100 ig 1 -1 of available P and 400 Itg 1-1 of available N. Based on this nutrient load, with a 1% P content in the algal biomass and a productivity of 10 g m -2 d -1, a 100 ha region would be needed for the process. Hydraulic barriers (submerged plastic curtains) would isolate the 100 ha algal production area "cultivation zone" in the influent end of the reservoir to create a hydraulic and thermal environment conducive to the selective growth of filamentous, thermotolerant, nitrogen-fixing, blue-green algae. The algal culture would be inoculated into the thermal plume and harvested near the distal barrier of the cultivation zone with rotating, backwashed, fine mesh screens ("microstrainers"). A portion of the harvested biomass would be recycled to the inoculation site to maintain a dense culture. This process could mitigate both thermal and nutrient loadings on receiving bodies of water.

Exposure of Fischerella [Mastigocladus] to high and low temperature extremes: strain evaluation for a thermal mitigation process

“…It is of interest to determine not only the permissible (maximum, minimum, optimum) growth temperatures, but also the ability of specific strains to tolerate extremes that do not allow growth. Although the present study was inspired by a need to determine such limits as part of the development of an algal cultivation process (Wilde et al, 1991), the data may be of broader physiological and ecological interest.…”

“…A conceptual process to mitigate such combined thermal/nutrient impacts was recently described by Wilde et al (1991). It involves the use of the thermal effluents to cultivate filamentous, thermotolerant cyanobacteria, followed by harvesting of the cyanobacterial biomass to remove the nutrients responsible for eutrophication.…”

Systems and economic analysis of microalgae ponds for conversion of CO{sub 2} to biomass. Final report

“…Although nature currently mitigates about half of current anthropogenic net C02 emissions, through absorption into oceans and storage in forests, the future of these natural processes is uncertain, and they may fail us when we most need them during the next century, when the expected, unavoidable, increased fossil fuel utilization will greatly increase the risks of major adverse impacts. And, it should be further recognized that even a modest reduction in C02 greatly reduces the risks of adverse climatic impacts (see discussion in Benemann, 1991). All these arguments point to the need for initiating risk reduction now, using the least expensive options (energy conservation, preservation of C in the biota) while developing other mitigation technologies for the long-term.…”

Microalgal cultivation with waste streams and metabolic constraints to triacylglycerides accumulation for biofuel production

“…Studies examining the effects of temperature on microalgae production in outdoor cultivation systems demonstrated that heated pond systems performed better (with higher growth yields) than unheated systems (up to strain specific maximum temperatures) . Higher temperatures generally increase microalgal growth rates and extend their growing season in temperate and cold climates, such as in Canada . As such, microalgal cultivation using waste heat from the cooling waters of fossil fuel powered electricity‐generating plants could be beneficial.…”