Joseph C. Weissman

Energy production by microbial photosynthesis

The large amounts of microalgae produced in waste treatment ponds during sewage purification are a potential source of methane and fertiliser. If techniques can be developed for the selective cultivation of filamentous or colonial microalgae, harvesting of microalgal biomass could be accomplished by low-cost straining or sedimentation methods.

Screening of algal strains for metal removal capabilities

Eight algal species were tested for their ability to remove five toxic metalsduring 30-min exposures to single-metal (1 mg L-1) solutions at pH7. Efficacy of metal bioremoval varied according to algal species and metal. Al+3 was best removed by the thermophilic blue-green alga(cyanobacterium) Mastigocladus laminosus, Hg+2 and Zn+2 by the thermophilic and acidophilic red alga Cyanidiumcaldarium, and Cd+2 by C. caldarium and the green alga Scenedesmus quadricauda. All of these alga/metal combinations resultedin >90% metal removal. However, none of the eight algal speciesremoved more than 10% of Cr+6. Results indicate that some toxicmetals are more readily removed than others are by algae and that selectionof appropriate strains could potentially enhance bioremoval of specificmetals from wastewater at neutral pH.

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

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 1091−1 d−1 of secondary cooling water containing 100 μg 1−1 of available P and 400 μg 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 conductive 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

In conjunction with a proposed algal cultivation scheme utilizing thermal effluent, twelve Fischerella strains were tested for tolerance to temperatures above and below their growth range. Exposure to 65 °C or 70 °C for 30 min caused bleaching and death of most or all cells. Effects of 60 °C exposure for periods of up to 2 h ranged from undetectable to severe for the various strains. Chlorophyll a content typically decreased 21–22% immediately following 60 °C or 65 °C (1 h) exposure. However, the 60 °C-shocked cultures regained normal Chl a content after 24 h at 45 °C, whereas Chl a in 65 °C-shocked cultures immediately lost visible autofluorescence and was later degraded. Exposure to 15 °C virtually stopped growth of all strains during a 48 h exposure period. Most strains grew as rapidly as 45 °C controls when restored to 45 °C, while a few strains recovered more slowly. Comparison with dark-incubated controls indicated that photooxidative damage did not occur during cold shock. Certain strains exhibited relatively rapid recovery from both heat and cold exposure, thus meeting the temperature tolerance criteria for the proposed algal cultivation process.

BIOPHOTOLYSIS: PROBLEMS AND PROSPECTS

Abstract Biophotolysis is the production of hydrogen from water by sunlight energy using biological systems. Several approaches are possible using either isolated cellular components or algae cultures. Technical and economic considerations restrict practical applications to algae cultures, The only algae system demonstrated to meet the basic requirements of biophotolysis uses nitrogen starved cultures of nitrogen–fixing heterocystous blue–green algae. Photosynthetic bacteria could be used for hydrogen production from wastes. Development of practical biophotolysis systems is limited by the low efficiency of photosynthesis, lack of basic scientific knowledge, and severe economic constraints.

BIOMASS PRODUCTION AND WASTE RECYCLING WITH BLUE-GREEN ALGAE

Abstract Large-scale algae ponds are being used as economical and effective means of sewage treatment. The large amounts of algae biomass produced in these ponds could be harvested and fermented to methane with the residue used for fertilizer. Harvesting of algae from waste treatment ponds is difficult because of the small size of most algae species. Larger size algae can be effectively harvested using slowly rotating fine mesh (25–100 μ) screens equipped with a backwash (microstrainers). In a series of experiments conducted during the summer (1976), Oscillatoria (a filamentous blue-green algae) and Micractinium (a colonial green algae with long spines) were grown outdoors on sewage and maintained as the predominating species in 1,000-liter ponds. The method used to achieve this result involved recycling a fraction of the algae biomass harvested by the microstrainers. In both theory and practice we have demonstrated that species control can be achieved in microbial systems through a selective recycle process. Large-scale algal cultivation will require a variety of different algae species control and harvesting methods. Blue-green algae have special advantages in large-scale cultivation: their filamentous nature allows mechanical harvesting, their capacity for nitrogen fixation makes them desirable for fertilizer production, and their gas vesicles could lower pond mixing requirements. Large-scale algae biomass production and waste recycling systems could be designed which convert sewage and power plant wastes (carbon dioxide and heat) into methane, fertilizer, and reclaimed water.

Practical Considerations in Cyanobacterial Mass Cultivation

Bench-scale tests have been conducted on a process using cyanobacteria to remove nutrients and utilize waste heat in nuclear reactor cooling waters. Based on its tolerance of temperature extremes, Fischerella Strain 113 was selected for semicontinuous culture studies. Cultures maintained with 10 mg L−1 daily productivity, diurnally varying temperature (55 °C to 26–28 °C), 200 μE sec−1 m−2 illumination, and 50% biomass recycling into heated effluent at the beginning of each 12-h light period, removed >95% of NO3 − + NO2 −-N, 71% of NH4 +-N, and 70% of total P. Nutrient removal was not severely impaired under conditions simulating scaled-down reactor operation, increased insolation, or dissolved inorganic carbon (DIC) limitation. Recycling biomass at the end of the light period resulted in slower growth, unimpaired NO3 − + NO2 −-N removal, 38–45% P removal, and no net NH4 + removal. Approximately 80% N removal and diurnally varying P removal (averaging 50–60%) are projected for the full-scale process.