A relatively detailed analysis of an algal wastewater treatment-energy production process was carried out by Benemann et al. (1977) as part of a larger study that examined a system integrating wastewater algal ponds with tree biomass production. The so-called “Photosynthetic Energy Factory” (InterTechnology Solar Corporation 1978) was to use the effluents of a waste treatment pond system to fertilize short-rotation trees for fuel farming. In turn, the power plant burning the woody biomass would provide CO2 for the algal ponds.

A design of the algal pond subsystem was carried out by Benemann et al. (1978) for a typical municipal community of 50,000 people, generating approximately 18,000 m3 of municipal wastewater per day. The assumption was that algal biomass would be grown up to the N growth potential of the wastewater, containing 65 mg/L of useable N (as organic N and ammonia). This required recycling about 5 to 7 tons of CO2 per day from the power plant to the algal ponds. A temperate site with an average insolation of about 15 GJ/m2/d was assumed, with a solar conversion efficiency averaging only 2.7% of visible light (about 1.35% of total solar), somewhat higher in winter than summer. This is considerably lower than current assumptions.

This study, for the first time, took into consideration monthly variations in temperature, insolation and other parameters. Algal harvesting was assumed to be with microstrainers (this analysis was carried out while this option was still being investigated, see Section III. A.3.). This report also carried out the first, though preliminary, analysis of the mixing power required for such large algal ponds and of the transfer requirements for CO2 to the algal culture. A 160-ha algal pond system was required to treat this wastewater flow year-round. This was about three times larger than a conventional oxidation pond system. Costs were projected to be competitive with conventional wastewater treatment systems.

Energy outputs were twice the energy inputs, based on digester gas production and requirements for pumping the wastewater, mixing the ponds, etc. The overall economics were very favorable because of the wastewater treatment credits.

Although this concept appeared favorable, in practice the relatively small scale of the locally available municipal wastes could supply only a small fraction of fertilizer needs for the very large (> 10,000 ha) energy plantations being projected. It does, however, point to the potential of this technology in wastewater treatment.

I Publications:

Benemann, J. R.; Koopman, B. L.; Baker, D.; Goebel, R.; Oswald, W. J. (1977) “Preliminary design of the algae pond subsystem of the photosynthesis energy factory.” Final Report to Inter-Technology Solar Corp., Sanitary Eng. Res. Lab., Univ. of Calif.-Berkeley.

InterTechnology/Solar Corp. (1978) “The photosynthesis energy factory: analysis, synthesis and demonstration.” U. S. DOE HCP/T3548-01.