Development of energy saving illumination system for Arthrospira (Spirulina) platensis cultivation in raceway pond

Abstract

Microalgal cultivation in open ponds is more preferable than in the photobioreactors, particularly in the commercial large-scale cultivation because of the low capital cost and ease of maintenance. However, the one major limitation of open pond cultivation is the low light penetrability, which is limited the pond depth to be only about 25-30 cm and the requirement of large land area for large scale cultivation is inevitable. In this study, the illumination system improvement for cultivation in the high depth open pond cultivation under real environment with solar light as sole light source was focused and aimed to increase the illumination and the photosynthesis efficiencies for the higher depth pond cultivation. The clear plastic cylindrical column and the clear plastic drinking bottle, the respectively so-called light scattering column (LSC) and light scattering bottle (LSB), were used as the illumination enhancement equipment for the study. The two open tank photobioreactors (OT-PBR) were built for the preliminary tests. In the main study, two pilot scale race way ponds with the size of 40cm x 150cm and the depth of 55 cm or equivalent to the volume capacity of about 300 L were fabricated for comparative studies. Both LSC and LSB were dipped vertically in the OT-PBR and race way ponds with partly above and partly under the pond or tank levels. The sunlight collided with the top parts of LSC or LSB then transmitted through the LSC or LSB and scattered to the deeper pond level. The illumination intensities at various pond depths were measured using the calibrated light dependent resister and the algal growth of both control and testing ponds were investigated. The incoming sunlight intensity was daily continuous measured with an auto-data accumulation to the Windows PC and was used for analysis of the photosynthesis efficiency. In this study, Arthrospira (Spirulina) platensis AARL C005 obtained from the Applied Algal Research Laboratory Chiang Mai University was cultured in the CMU02 medium. The illumination efficiency increased significantly after installation of LSC and LSB at all depth of both OT-PBR and race way ponds experiments. Results from the OTPBR cultivation in the pond depth of 55 cm using the LSC with a diameter of 6 cm and a height of 40 cm found that the optimum submerged height and emerged height were 30 cm and 10 cm respectively. In raceway pond cultivations with LSC, the cultivation efficiency including the net energy ratio was found to be improved satisfactorily. The increase of the biomass productivity of 69.8% and 48.5%, the improvement of the net energy ratio of 59.8% and 41.7%, and the photosynthesis efficiency of 59.9% and 42.1% were respectively observed for the OT-PBR and race way ponds cultivations. Result from macronutrients analysis in the A. platensis biomass found that the protein composition was rather high with an amount of 62.7% which is comparable to that of other cultivations using nutrient rich medium such as Zarrouk medium under the controlled environment. The final results from raceway pond cultivations using the 35 cm extended length LSB and the 20 cm length single bottle LSB found that the increase of the biomass productivity of 56.7% and 33.7%, the improvement of the net energy ratio of 57.6% and 30.6%, and the photosynthesis efficiency of 54.9% and 31.3% were respectively obtained and those values were closed to the theoritical values as found in the normal depth race way ponds. In term of economic cost evaluation of energy simple payback period from cultivation using single bottle LSB found that payback from the higher biomass production can cover the cost of both bottle manufacturing and transportation with in 2.5 yeas excluding energy from refuse derived fuel (RDF) and 1.3 years including energy from RDF. As a result, the commercial benefit could be improved by about 300% or more and the payback period to capital cost could be reduced by about 70 % when compared with that of the regular cultivation. LSCs and LSBs used in this study were successfully and practically applied for illumination enhancement of microalgal cultivation. In particular, LSBs were collected from used materials, which can extend the consumption stage of plastic bottle waste, including the more energy and water savings in accordance with the experimental results. The achievements from this study clearly indicate the contribution for sustainable environment conservation.