Biogas Purification through the use of a Microalgae-Bacterial System in Semi-Industrial High Rate Algal Ponds

Our research focuses on developing sustainable and viable microalgae biotechnology for biomethane obtention. We show with that it was possible to scale the biogas filter up to some industrial scale under other conditions. The process of scaling is aimed at treating larger flows of biogas in smaller facilities with the aim of making it more economically viable.

Additionally, the process seeks to promote operating conditions that can lead to greater production of high value by-products extracted from microalgal biomass. Through a full-scale process exposed to outdoor conditions, we have discovered various species of microalgae that enable us to operate the microalgae filter efficiently. We have also identified the optimal operating conditions that lead to a higher volume concentration of biomethane.

We were able to successfully scale up the microalgal filter to full-scale operations and identify the optimal operating conditions for photobioreactors and microalgae filter to achieve methane concentrations of up to 85%volume. The use of other species of microalgae under the same operating conditions could improve the performance efficiency of the microalgae filter. In spring/summer, the increase in temperature will help the efficiency of the system.

To begin, transfer a pure strain of Arthrospira maxima from agar plates to 15 milliliters of aqueous mineral medium. Using Jordan aqueous medium, scale up the culture to 500 milliliter flasks. Grow it for 12-hour light and dark periods using LED lamps with surface-mount device 2835, providing white cold light and ensuring continuous mixing by air bubbling.

To continue the scaling process, add 20%of the previous culture volume to the new volume until 50 liters are reached. Adapt the culture to natural light conditions and Jordan culture media in a greenhouse in 50-liter transparent sacks. Add the full volume of the five cubic meters HRAP photobioreactors to 13 cubic meter HRAPs located outdoors.

Fill the rest of the volume with Jordan culture medium. Mix the components with a paddle wheel and cultivate the culture in batch mode for 15 days or until it reaches 0.7 grams per liter. Once growth reaches 0.7 grams per liter, transfer the volume to the operating 22.2 cubic meters HRAP.

Fill the rest with Jordan media and set the paddle wheel. Let the biomass grow until it reaches 0.7 grams per liter and a pH of 10. Start the liquid recirculation from the HRAP photobioreactor to the absorption tank at variable flow to increase biomass productivity.

Begin biogas bubbling at an average flow of 3.5 cubic meters per hour after two hours to provide inorganic carbon to the culture. Monitor the pH, ensuring it remains above nine. To begin, cultivate Arthrospira maxima from agar plates to 15 milliliters of aqueous mineral medium and scale up the culture to 22.2 cubic meters HRAP.

Once the desired biomass is attained, use a flexible tube to connect a 10-liter polyvinyl fluoride bag to the sampling outlet before and after the absorption tank. Connect each sample contained within the polyvinyl fluoride bags to the analyzer. Press the next button and measure the methane, carbon dioxide, oxygen, and hydrogen sulfide concentrations as percent volume from both points of the system.

To determine the volumetric recirculation liquid to biogas ratio, divide the liquid recirculation flow by the biogas production flow. Compute the corresponding gas flow that presents the highest efficiency of carbon dioxide and hydrogen sulfide removal. Place a pH, dissolved oxygen, and temperature sensor in the liquid of each HRAP.

Regulate the incoming biogas flow to select the liquid-to-gas value to be tested at the start and every 15 minutes for an hour. Measure the pH of the culture and the inlet and outlet concentrations of each gas. Determine the most efficient liquid-to-gas ratio by comparing the outlet values and choose the one most convenient according to the needs of the experiment.

Choose at least two liquid-to-gas ratios to compare the relation with pH and carbon dioxide. For each liquid-to-gas ratio, measure the pH of the culture and the inlet and outlet concentrations of carbon dioxide, hydrogen sulfide, oxygen, and nitrogen as a control. Calculate the carbon dioxide removal percentages using the given equation.

Graph the results and compare the behavior of the pH and carbon dioxide for each of the liquid-to-gas ratios tested. Monitor the reactors daily and take a one-liter sample from the halfway point between the paddle wheel and its return from each culture. After transporting the sample to the laboratory, check the colony growth and purity of the culture under the microscope.

Measure and record the absorbance at 750 nanometers of the samples with the spectrophotometer using the fresh culture medium as the blank. Compare the absorbance with the calibration curve to estimate the biomass weight in grams per liter. Record the growth of each raceway reactor.

Biogas purification had increased efficacy in higher liquid-to-gas ratios, maintaining removal efficiencies at or above 98%for hydrogen sulfide. Dissolved oxygen levels rose during the day due to photosynthesis by microalgae but fell at night due to halted photosynthesis and increased respiration. With the less carbon dioxide dissolved, the pH level increased, and it decreased when less carbon dioxide was removed from the liquid.

The removal percentage of carbon dioxide and the pH at a ratio of 1.58 were remarkably less stable and much lower than the ones recorded for the ratio of 1.64.