Flue gas generated by combustion of natural gas in a boiler was used for outdoor cultivation of Chlorella sp. in a 55 m 2 culture area photobioreactor. A 6 mm thick layer of algal suspension continuously running down the inclined lanes of the bioreactor at 50 cm s −1 was exposed to sunlight. Flue gas containing 6-8% by volume of CO 2 substituted for more costly pure CO 2 as a source of carbon for autotrophic growth of algae. The degree of CO 2 mitigation (flue gas decarbonization) in the algal suspension was 10-50% and decreased with increasing flue gas injection rate into the culture. A dissolved CO 2 partial pressure (pCO 2 ) higher than 0.1 kPa was maintained in the suspension at the end of the 50 m long culture area in order to prevent limitation of algal growth by CO 2 . NO X and CO gases (up to 45 mg m −3 NO X and 3 mg m −3 CO in flue gas) had no negative influence on the growth of the alga. On summer days the following daily net productivities of algae [g (dry weight) m −2 ] were attained in comparative parallel cultures: flue gas = 19.4-22.8; pure CO 2 = 19.1-22.6. Net utilization (η) of the photosynthetically active radiant (PAR) energy was: flue gas = 5.58-6.94%; pure CO 2 = 5.49-6.88%. The mass balance of CO 2 obtained for the flue gas stream and for the algal suspension was included in a mathematical model, which permitted the calculation of optimum flue gas injection rate into the photobioreactor, dependent on the time course of irradiance and culture temperature. It was estimated that about 50% of flue gas decarbonization can be attained in the photobioreactor and 4.4 kg of CO 2 is needed for production of 1 kg (dry weight) algal biomass. A scheme of a combined process of farm unit size is proposed; this includes anaerobic digestion of organic agricultural wastes, production and combustion of biogas, and utilization of flue gas for production of microalgal biomass, which could be used in animal feeds. A preliminary quantitative assessment of the microalgae production is presented.

Two variants of open photobioreactors were operated at surface-to-volume ratios up to 170 m −1 . The mean values for July and September obtained for photobioreactor PB-1 of 224 m 2 culture area (length 28 m, inclination 1.7%, thickness of algal culture layer 6 mm), operated in Tȓebon (49 • N), Czech Republic, were: net areal productivity, P net = 23.5 and 11.1 g dry weight (DW) m −2 d −1 ; net photosynthetic efficiency (based on PAR -Photosynthetic Active Radiation), η = 6.48 and 5.98%. For photobioreactor PB-2 of 100 m 2 culture area (length 100 m, inclination 1.6%, thickness of algal culture layer 8 mm) operated in Southern Greece (Kalamata, 37 • N) the mean values for July and October were: P net = 32.2 and 18.1 g DW m −2 d −1 , η = 5.42 and 6.07%. The growth rate of the alga was practically linear during the fedbatch cultivation regime up to high biomass densities of about 40 g DW L −1 , corresponding to an areal density of 240 g DW m −2 in PB-1 and 320 g DW m −2 in PB-2. Night biomass loss (% of the daylight productivity, P L ) caused by respiration of algal cells were: 9-14% in PB-1; 6.6-10.8% in PB-2. About 70% of supplied CO 2 was utilized by the algae for photosynthesis. The concentration of dissolved oxygen (DO) increased from about 12 mg L −1 at the beginning to about 35 mg L −1 at the end of the 100 m long path of suspension flow in PB-2 at noon on clear summer days. Dissipation of hydraulic energy and some parameters of turbulence in algal suspension on culture area were estimated quantitatively. AbbreviationsA size of the culture area (m 2 ) C O 2 concentration (DO) of dissolved oxygen (g O 2 m −3 ) C O 2 ,0 DO concentration at the beginning of culture area (g O 2 m −3 ) C O 2 ,L DO concentration at the end of culture area (g O 2 m −3 ) C O 2 *

The following bead mills used for disruption of the microalga Chlorella cells were tested: (1) Dyno-Mill ECM-Pilot, grinding chamber volume 1.5 L; KDL-Pilot A, chamber volume 1.4 L; KD 20 S, chamber volume 18.3 L; KD 25 S, chamber volume 26 L of Willy A. Bachofen, Basel, Switzerland, (2) LabStar LS 1, chamber volume 0.6 L of Netzsch, Selb, Germany, (3) MS 18, chamber volume 1.1 L of FrymaKoruma, Neuenburg, Germany. Amount of disrupted cells decreased with increasing Chlorella suspension feed rate and increased up to about 85% of the beads volume in the grinding chamber of the homogenizers. It also increased with agitator speed and number of passes of the algae suspension through the chamber. The optimum beads diameter was 0.3-0.5 mm in the homogenizers Dyno-Mill and LabStar LS 1 and 0.5-0.7 mm in the homogenizer MS 18. While the degree of the cell disruption decreased with increasing cell density in Dyno-Mill and LabStar, the cell disruption in the MS 18 increased. Depending on processing parameters, more than 90% of algae cells were disrupted by passing through the bead mills and bacteria count in algae suspension was reduced to about two orders.

A flue gas originating from a municipal waste incinerator was used as a source of CO(2) for the cultivation of the microalga Chlorella vulgaris, in order to decrease the biomass production costs and to bioremediate CO(2) simultaneously. The utilization of the flue gas containing 10-13% (v/v) CO(2) and 8-10% (v/v) O(2) for the photobioreactor agitation and CO(2) supply was proven to be convenient. The growth rate of algal cultures on the flue gas was even higher when compared with the control culture supplied by a mixture of pure CO(2) and air (11% (v/v) CO(2)). Correspondingly, the CO(2) fixation rate was also higher when using the flue gas (4.4 g CO(2) l(-1) 24 h(-1)) than using the control gas (3.0 g CO(2) l(-1) 24 h(-1)). The toxicological analysis of the biomass produced using untreated flue gas showed only a slight excess of mercury while all the other compounds (other heavy metals, polycyclic aromatic hydrocarbons, polychlorinated dibenzodioxins and dibenzofurans, and polychlorinated biphenyls) were below the limits required by the European Union foodstuff legislation. Fortunately, extending the flue gas treatment prior to the cultivation unit by a simple granulated activated carbon column led to an efficient absorption of gaseous mercury and to the algal biomass composition compliant with all the foodstuff legislation requirements.