1021-4437/00/4706- $25.00 © 2000 MAIK “Nauka /Interperiodica” 0786 Russian Journal of Plant Physiology, Vol. 47, No. 6, 2000, pp. 786–788. From Fiziologiya Rastenii, Vol. 47, No. 6, 2000, pp. 892–895. Original English Text Copyright © 2000 by Petkov. INTRODUCTION The main requirements for an intensive large-scale cultivation of microalgae include a nutritional medium, a carbon dioxide supply, light, and the proper tempera- ture. Nutritional media are preliminarily balanced, and the elements are proportionally exhausted. On the other hand, the desorption of carbon dioxide from cultivation ponds can present a difficulty. The mathematical description of this process is useful for finding the proper length of the cultivation bed [1–3]. Neverthe- less, the absorption of CO 2 in a horizontal pond or sloped layer cannot be rationally intensified by the application of a model. Carbon dioxide desorption is a natural process, and the models in these cases remain predominantly cognitive and descriptive. As for light, it is normally more than sufficient [4–8]. In the temperate zone, throughout most of the day, water temperature remains too low. This is due to the intensive evapora- tion of water in the relatively dry air. That is why evap- oration is a factor which must be limited and controlled in order to maintain a near-optimum temperature. According to heat waste and maintenance of optimal temperature, the cultivation devices for microalgae could be subdivided as follows: (1) open ponds [9–11], (2) devices without contact between the algal suspen- sion and the cover [12–15], and (3) devices with direct contact between the algal suspension and the wall of the photobioreactor [16–24]. In all these devices, light absorption, gas exchange, heat exchange, and motion of the liquid take place. Having made rational compro- mises to combine all these processes, we can achieve a physiological optimum. Here, we present our experi- ence in the outdoor cultivation of microalgae in a trans- parent counter-current absorber, in which mass exchange, heat exchange, light absorption, and a proper hydrodynamic are combined to insure more acceptable conditions for algal growth. RESULTS AND DISCUSSION Mass Transfer The algal suspension is moved vertically, and min- eral substances are dissolved and reach every single cell. Carbon dioxide, which is supplied in the algal sus- pension, and the oxygen which is given off, are unevenly distributed along the cultivation bed. There- fore, the mass transfer process treats mainly the absorp- tion of CO 2 in the algal suspension and desorption of O 2 from it. Carbon dioxide is a weakly soluble gas, and excessively low concentrations are maintained in the suspension. It has been quickly consumed, which leads to the limitation of photosynthesis. The desorption losses increase when CO 2 is supplied in larger amounts. This is why we have introduced counter-current absorption (figure). Perforated plates of transparent plastic material are situated one over the other at a dis- tance of 20–25% of their width in a metal frame. The frame is made of constructive steel rods 8 mm in diam- eter. In this arrangement, the whole frame is inserted into a transparent plastic sleeve. Carbon dioxide is sup- plied in the middle of the column, and the algal suspen- sion trickles down through the perforated plates. Des- orption losses in such a mass-exchange column are minimized because of a counter current between the gas phase and the great surface of the liquid phase. No hydrostatic pressure is to be overcome, and there is no need for compressors and their maintenance and energy costs. The perforated transparent plates serve as culti- vation areas and perform the function of mass transfer. In the available sloped-layer installations, which are in use in algological practice, there are metal bars. They create hydrodynamic resistance along the course of the flow and enhance the thickness of the layer [9, 10]. In the present column, the side edges of the plates which are 10–12 mm in height perform a similar function. By comparing columns of different sizes, we found that a diameter or width of 0.7 m and height of 2 m are the Absorber Tower as a Photobioreactor for Microalgae* G. D. Petkov Institute of Plant Physiology, Bulgarian Academy of Sciences, Sofia, Bulgaria; e-mail: gpdalg@bgeict.acad.bg Received July 26, 1999 Abstract—Green microalgae were grown under natural light in a photobioreactor similar to a transparent plate absorber. A proper temperature was maintained through the control of evaporation and the minimization of con- vective heat waste. Carbon dioxide desorption was lower in comparison to its level during cultivation in open or covered ponds. A yield of 1 g/l per day and over 100 g/m 2 projection area was achieved. Key words: algae - biomass - photobioreactor * This article was submitted by the author in English.