Static supply of different simulated flue gases for native microalgae cultivation in diluted cow manure digestate.

The application of microalgae to sequester CO2 from flue gases can be an interesting process since it can contribute to mitigate CO2 emission into the atmosphere. One obstacle of such application is the high CO2 concentration in the flue gases, which can lead to low pH in the cultivation medium and hence process failure. This study aims to investigate static CO2 gas supply for microalgae cultivation as a potential alternative that might allow applying different flue gases with different compositions and higher CO2 concentrations. Two sets of experiments were performed. First, the effect of increasing the amount of supplied carbon was tested. In the second experiment, the applicability of such system for different flue gases regarding their oxygen and carbon content was tested. In all experiments, 50 times diluted cow manure digestate was used as a culture medium. By increasing CO2 concentration up to 10% in the supplied air, microalgae growth productivity of 48.7 mg/L/d was achieved. A further improvement of microalgae growth was shown with increasing the gas/culture volume ratio. Microalgae productivity rate increased form 48.7 mg/L/d to 73.5 mg/L/d when the volume of gas increased from 47% to 81% of total volume. Applying CO2 in air (O2 content around 20%) or in N2 (O2 content less than 2%) didn't show any difference regarding inorganic carbon dissolution, pH, ammonium nitrogen removal, CO2 fixation or biomass productivity. Generally, it can be concluded that static gas supply for microalgae cultivation can allow the application of different flue gases from different industries with low or high O2 content and with CO2 concentration as high as 20%. According to our results, a microalgae cultivation system with continuous static gas supply was proposed.

Attempts to alleviate inhibitory factors of anaerobic digestate for enhanced microalgae cultivation and nutrients removal: A review.

Anaerobic digestion is a well-established process that is applied to treat organic wastes and convert the carbon to valuable methane gas as a source of energy. The digestate that comes out as a by-product is of a great challenge due to its high nutrient content that can be toxic in case of improper disposal to the environment. Several attempts have been done to valorize this digestate. Digestate has been considered as an interesting medium to cultivate microalgae. The nutrients available in the digestate, mainly nitrogen and phosphorus, can be an interesting supplement for microalgae growth requirement. The main obstacles of using digestate as a medium to cultivate microalgae are the dark color and the high ammonium-nitrogen concentration. The focus of this review is to discuss in detail the major attempts in research to overcome inhibition and enhance microalgae cultivation in digestate. This review initially discussed the obstacles of digestate as a medium for microalgae cultivation. Different processes to overcome inhibition were discussed including dilution, supplying additional carbon source, favoring mixotrophic cultivation and pretreatment. More emphasis in this review was given to digestate pretreatment. Among the pretreatment methods, filtration, and centrifugation were of the most applied ones. These strategies were found to be effective for turbidity and chromaticity reduction. For ammonium nitrogen removal, ammonia stripping and biological pretreatment methods were found to play a vital role. Adsorption could work both ways depending on the material used. Combining different pretreatment methods as well as including selected microalgae stains were found interesting strategies to facilitate microalgae cultivation with no dilution. This study recommend that more study should investigate the optimization of microalgae cultivation in anaerobic digestate without the need for dilution.

Appropriate conditions for applying NaOH-pretreated two-phase olive milling waste for codigestion with food waste to enhance biogas production.

The high methane gas production potential of two phase olive milling waste (2POMW) makes its application to biogas plants in business an economical process to increase the productivity of the plants. The objective of this study was to investigate the appropriate conditions for the codigestion of NaOH-pretreated 2POMW with food waste. NaOH pretreatment can increase the methane production by increasing the soluble chemical oxygen demand (sCOD), but it may cause inhibition because of higher levels of alkalinity, sodium ion, volatile fatty acids and long chain fatty acids (LCFAs). Therefore, the first experimental phase of this study aimed to investigate the effect of different mixing ratios of 2POMW to food waste. A continuous stirred tank reactor experiment with different mixing ratios of 3%, 4.3%, 5.7% and 8.3% (2POMW: food waste) was conducted. NaOH pretreatment in the range of 6-20% was used. A mixing ratio up to 4.3%, when 10% NaOH pretreatment was used, caused no inhibition and increased methane production by 445.9mL/g-VS(2POMW). For this mixing ratio an additional experimental phase was conducted with the 20% NaOH pretreatment as the 20% NaOH pretreatment had the highest sCOD. The methane gas production was increased by 503.6mL/g-VS(2POMW). However, pH adjustment was required for applying this concentration of the high alkalinity 20% NaOH-pretreated 2POMW. Therefore, we consider using 10% NaOH pretreatment in a mixing ratio of 4.3% to be more applicable. The increase in methane gas production was correlated to the oleic acid concentration inside the reactors. The high oleic acid concentration of 61.8mg/L for the 8.3% mixing ratio was responsible for the strong inhibition. This study showed that adjusting the appropriate mixing ratio of the NaOH-pretreated 2POMW could increase the electricity production of a reactor that regularly receives food waste.