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

Summary

Air pollution impacts the quality of life of all organisms. Here, we describe the use of microalgae biotechnology for the treatment of biogas (simultaneous removal of carbon dioxide and hydrogen sulfide) and the production of biomethane through semi-industrial open high-rate algal ponds and subsequent analysis of treatment efficiency, pH, dissolved oxygen, and microalgae growth.

Abstract

In recent years, a number of technologies have emerged to purify biogas into biomethane. This purification entails a reduction in the concentration of polluting gases such as carbon dioxide and hydrogen sulfide to increase the content of methane. In this study, we used a microalgal cultivation technology to treat and purify biogas produced from organic waste from the swine industry to obtain ready-to-use biomethane. For cultivation and purification, two 22.2 m³ open-pond photobioreactors coupled with an absorption-desorption column system were set up in San Juan de los Lagos, Mexico. Several recirculation liquid/biogas ratios (L/G) were tested to obtain the highest removal efficiencies; other parameters, such as pH, dissolved oxygen (DO), temperature, and biomass growth, were measured. The most efficient L/Gs were 1.6 and 2.5, resulting in a treated biogas effluent with a composition of 6.8%vol and 6.6%vol in CO₂, respectively, and removal efficiencies for H₂S up to 98.9%, as well as maintaining O₂ contamination values of less than 2%vol. We found that pH greatly determines CO₂ removal, more so than L/G, during cultivation because of its participation in the photosynthetic process of microalgae and its ability to vary pH when solubilized due to its acidic nature. DO, and temperature oscillated as expected from the light-dark natural cycles of photosynthesis and the time of day, respectively. Biomass growth varied with CO₂ and nutrient feeding as well as reactor harvesting; however, the trend remained primed for growth.

Introduction

In recent years, several technologies have emerged to purify biogas to biomethane, promoting its use as non-fossil fuel, therefore mitigating undesairable methane emissions¹. Air pollution is a problem that affects most of the world's population, particularly in urbanized areas; ultimately, around 92% of the world's population breathe polluted air². In Latin America, the air pollution rates are mostly created by the use of fuels, whereby in 2014, 48% of the air pollution was brought on by the electricity and heat production sector³.

In the last decade, more and more studies on the relationship between pollutants in the air and the increase in mortality rates have been proposed, arguing that there is a strong correlation between both datasets, particularly in children populations.

As a way to avoid the continuation of air pollution, several strategies have been proposed; one of these is the use of renewable energy sources, including wind turbines and photovoltaic cells, which diminish the CO₂ release into the atmosphere^4,5. Another renewable energy source comes from biogas, a byproduct of the anaerobic digestion of organic matter, produced along with a liquid organic digestate⁶. This gas is composed of a mixture of gasses, and their proportions depend on the source of organic matter used for anaerobic digestion (sewage sludge, cattle manure, or agro-industrial biowaste). Generally, these proportions are CH₄ (53%-70%vol), CO₂ (30%-47%vol), N₂ (0%-3%vol), H₂O (5%-10%vol), O₂ (0%-1%vol), H₂S (0-10,000 ppmv), NH₃ (0-100 ppmv), hydrocarbons (0-200 mg/m³) and siloxanes (0-41 mg/m³)^7,8,9, where the scientific community is interested in the methane gas since this is the renewable energetic component of the mixture.

However, biogas cannot be simply burned as obtained because the byproducts of the reaction can be harmful and contaminant; this raises the need to treat and purify the mixture to increase the percentage of methane and decrease the rest, essentially converting it into biomethane¹⁰. This process is also known as upgrading. Even though, currently, there are commercial technologies for this treatment, these technologies have several economic and environmental drawbacks^11,12,13. For example, systems with activated carbon and water washing (ACF-WS), pressure water washing (PWS), gas permeation (GPHR), and pressure swing adsorption (PSA) present some economic or other drawbacks of environmental impact. A viable alternative (Figure 1) is the use of biological systems such as those that combine microalgae and bacteria grown in photobioreactors; some advantages include the simplicity of design and operation, the low operating costs, and its environmentally friendly operations and byproducts^10,13,14. When biogas is purified to biomethane, the latter can be used as a substitute for natural gas, and the digestate can be implemented as a source of nutrients to support microalgae growth in the system¹⁰.

A method widely used in this upgrading procedure is the growth of microalgae in open raceway photoreactors coupled with an absorption column due to the lower operation costs and the minimal investment capital needed⁶. The most used type of raceway reactor for this application is the high-rate algal pond (HRAP), which is a shallow raceway pond where the circulation of the algal broth occurs via a low-power paddle wheel¹⁴. These reactors need large areas for their installation and are very susceptible to contamination if used in outdoor conditions; in biogas purification processes, it is advised to use alkaline conditions (pH > 9.5) and the use of algal species that thrive in higher pH levels to enhance the removal of CO₂ and H₂S while avoiding contamination^15,16.

This research aimed to determine the biogas treatment efficiencies and final production of biomethane using HRAP photobioreactors coupled with an absorption-desorption column system and a microalgae consortium.

Protocol

1. System set-up

NOTE: A piping and instrumentation diagram (P&ID) of the system described in this protocol is shown in Figure 2.

1. Reactor set-up
   1. Prepare the ground by leveling and compacting it to improve reactor stability.
   2. On an open field, dig two elongated holes and 3 m from the end, further dig a 3 m² and 1 m deep hole (known as an aeration well).
   3. Place two HRAPs (Figure 3) within the space on geomembrane-covered metal supports. Each reactor must have an operating capacity of 22.2 m³.
   4. Place an air pump per reactor of 1728.42 watts (2.35 hp) close to the point of the HRAPs where the aeration wells were dug.
   5. Fix a paddle wheel (moved by a 1103.24 watts [1.5 hp] electric motor) across the reactor to promote contact between biomass and media.
2. Gas treatment set-up (Figure 4)
   1. Build the desorption column with a 6" polyvinyl chloride (PVC) tube, where the inlet current enters 2 m from the covered top, and the outlet current flows from the bottom (Figure 2).
   2. Set up the absorption tank (V_t: 2.55 m³), where the gaseous inlet (non-treated biogas) current is bubbled from the bottom through 11 diffuser tubes and comes from the anaerobic digester through a 4" PVC pipeline passing through a biogas blower, a 1" rotameter and a sampling port, while the liquid comes from the media recirculation after the desorption column on the bottom of the tank. The liquid outlet is located on the side of the tank. It transports the CO₂-enriched media to the level-control column, and the gas exits from the outlet at the top of the tank, which is connected with a 1" PVC pipeline to conduct obtained biomethane to a burner for its continuous combustion (Figure 2).
   3. Connect the absorption tank to the desorption column through a 4" PVC tube, passing through a sampling port between both operations (Figure 2).
   4. Build the level-control column with a 6" PVC tube where the inlet is located at the bottom. It has two outlets (controlled with butterfly valves), depending on the needs of the system; the first one is located at a height of 2.5 m and the second one at 3 m from the ground (Figure 2).
   5. Connect the HRAP photobioreactors through a 2" PVC pipeline to the 6" desorption column, passing through a recirculation centrifugal pump (1103.24 watts [1.5 hp]) and a 1" rotameter (Figure 2).
   6. Connect the level-control column through a 4" PVC tube to a schedule 40 PVC tube, passing through a sampling port. Next, connect it to a portion of flexible PVC tubing, followed by another schedule 40 PVC tube, and finally, a 4" PVC tube, which opens to the HRAP photobioreactors (Figure 2).
   7. Set up the bypass of the desorption column with 2" PVC pipeline and connect it to the main tube before the sampling port (Figure 2).

2. Functional testing of the system

1. Recirculation centrifugal pump (1103.24 watts [1.5 hp])
   1. To determine the maximum flow rate of the pump, prime the interior for at least 10 min to avoid air suction and start it up at 230 V and 1 phase.
   2. Test the recirculation flow by letting it flow through the 1" rotameter.
2. Biogas bubbling system
   1. To determine the force required to bubble at least an air column equivalent to 200 mbar, test at least 3 blowers with different powers (485.52 watts [0.66 hp], 1838.74 watts [2.5 hp], and 3309.74 watts [4.5 hp]) by bubbling air into the absorption tank.
   2. Visually verify the size and distribution reached by the air bubbles inside the tank. Under the operating conditions described here, the predicted average diameter of the bubbles is 3 mm.

3. Inoculation and growth under indoor conditions

1. Transfer a pure strain of Arthrospira maxima from agar plates to 15 mL of aqueous mineral medium¹⁷ (NaHCO₃ [10 g/L], Na₃PO₄ ·12H₂O [0.033 g/L], NaNO₃ [0.185 g/L], MgSO₄ ·7H₂O [0.014 g/L], FeSO₄ ·7H₂O [0.0008 g/L], NaCl [0.4 g/L]).
2. Scale up the culture to 500 mL flasks with innocuous Jourdan aqueous medium, using 100% of flask volume, and let it grow in 12 h light/ 12 h dark photoperiods using light emitting diode (LED) lamps with surface mount device (SMD) 2835 providing while-cold light at 2000 lm and under continuous mixing by air bubbling (0.3 L/min or 0.6 vvm). (step lasting around 1 month).
3. Continue the scaling-up process by adding 20% of the previous volume to the new volume until 50 L are reached.
4. Adapt the culture to natural light conditions of operation and Jourdan culture media in a greenhouse in 50 L transparent sacks (step lasting around 2 months).
5. Continue scaling in these conditions up to 5 m³ HRAP photobioreactors (step lasting around 2 months).

4. Operational start of the system under outdoor conditions

1. Add the full volume of these 5 m³ HRAP photobioreactors to HRAPs photobioreactors of 13 m³ located outdoors and fill the rest of the volume with Jourdan culture medium. Start mixing through a paddle wheel at a speed of 30 cm/s, cultivating in batch mode for 15 days or until it reaches 0.7 g/L (step lasting around 1 month).
2. Once growth reaches 0.7 g/L, transfer the volume to the operating 22.2 m³ HRAP, fill the rest with Jourdan media, and set the paddle wheel at a speed of 30 cm/s. Let the biomass grow until it reaches 0.7 g/L and a pH of 10; once these conditions are met, start sampling and harvesting, if needed.
3. 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 m³/h after 2 h to provide inorganic carbon to the culture. Pay attention to the pH since it must remain above 9.
   NOTE: Before recirculating the media through the absorption tank, prime the centrifugal pump described above.
4. Nutrient addition: Monitor nutrient conditions weekly through harvesting and the overall nitrogen balance assuming steady state calculated as shown:
   M_NaNO3 = (M_Biomass x 0.10)/0.12 [g]
   Where:
   M_NaNO3 = Sodium nitrate mass [g]
   M_Biomass = Harvested biomass [g]
   1.10: Mass yield of nitrogen/biomass¹⁶ [g/g]
   1.12: Mass fraction of nitrogen in sodium nitrate [g/g]
5. With the nitrogen balance results, reformulate the Jourdan media to add the proportional amount of Na₃PO₄·12H₂O, MgSO₄·7H₂O, and FeSO₄·7H₂O. Do not add more sodium bicarbonate or sodium chloride.
   NOTE: Dissolve the nutrients in clean water before adding them to the reactors.
6. Monitor water evaporation and add weekly if needed.

5. Sampling and analysis

1. Biogas
   1. Sample the biogas from the sampling outlet before the absorption tank and from the sampling outlet after the tank by connecting a 10 L polyvinyl fluoride bag to the outlet with a flexible tube of appropriate diameter; place each one in separate polyvinyl fluoride bags.
   2. Calibrate the portable gas analyzer by setting the pressure transductor to zero and waiting for stabilization. Do this by pressing Start, then Next, and connecting a clear tube and a yellow tube as instructed by the analyzer. Press Next and finally, Gas Readings.
   3. Connect each sample contained within the polyvinyl fluoride bags to the analyzer, press Next and measure the CH₄, CO₂, O₂ and H₂S concentrations as %vol from both points of the system.
   4. Determine the volumetric recirculation liquid/biogas ratio (L/G) by dividing the liquid recirculation flow by the biogas production flow. Compute the corresponding gas flow (m³/h) that presents the highest efficiency of CO₂ and H₂S removal.
2. Online measuring of system conditions (pH, dissolved oxygen, temperature)
   1. Calibrate all sensors according to the specifications of the manufacturer.
   2. Place a pH sensor, a dissolved oxygen (DO) sensor, and a temperature sensor in the liquid of each HRAP.
      NOTE: For brand and specifications for each of the sensors, review the Table of Materials file.
   3. Connect the pH and DO sensors to a data-acquisition device consisting of a 1.4 GHz 64-bit quad-core processor connected to a portable screen that stores a pre-made Python program written in Integrated Development and Learning Environment (IDLE) 2.7.
      1. Open the program through the screen and indicate the time intervals to store each data point (in this case, every 2 min).
      2. Create a spreadsheet where the program will automatically store the data it collects.
      3. Click on the button that reads ON, indicating it is ready to start storing data.
      4. To stop the data acquisition, click on the button that reads OFF.
      5. To download the information, insert a universal serial bus (USB) and import the spreadsheet.
   4. Connect the temperature sensor to a thermo-recorder to store the data recorded during the experiments.
3. Short exploratory tests
   1. Determine the most efficient L/G
      1. Regulate the incoming biogas flow to select the L/G value to be tested (0.5, 1, 1.5, 1.6, 2, 2.5, 3.3, 3.4).
      2. Measure the pH and the inlet and outlet concentrations of each gas (CH₄, CO₂, H₂S, O₂, N₂) at the start and every 15 min for an hour (60 min), using the instruments described previously.
      3. Determine the most efficient L/G by comparing the outlet values and choose the one most convenient according to the needs of the experiment.
   2. Relationship between L/G, pH and CO₂
      1. Choose at least two L/G's to compare.
      2. For each L/G, measure the pH and the inlet and outlet concentrations of CO₂, and of H₂S, O₂, and N₂ as a control at the start, every 15 min for 60 min, and then every hour for a total of 5 h, using the instruments described previously.
      3. Calculate the CO₂ removal percentages using the equation:
         %CO₂ removal = ((CO₂in - CO₂out)/(CO₂in)) x 100
      4. Graph the results and compare the behavior of the pH and CO₂ for each of the L/G's tested.
4. Calibration curve to correlate biomass weight per liter of culture versus absorbance at 750 nm¹⁸
   1. Sample the algae culture to try and get an absorbance of 1.0. If the culture has an absorbance below 1.0, extract water by filtration (0.45 µm filter) from a culture sample. If the absorbance is greater than 1, it can be decreased by adding a fresh culture medium.
   2. Prepare five algae cell suspensions using the sample and add fresh culture medium, in volume/volume (V/V) percentage: 100%, 80%, 60%, 40%, and 20%.
   3. Measure and record the absorbance at 750 nm of the five solutions with a spectrophotometer using plastic cuvettes, where the fresh culture medium is the blank.
   4. Determine the biomass weight per liter of culture of every suspension by filtering 10 mL through a previously weighed 0.45 µm filter and drying the sample in a silica desiccator for 24 h and later 48 h to ensure a constant weight. Repeat this step for each of the five solutions.
      NOTE: A higher temperature (above 60 °C) is not recommended for drying due to the loss of certain key compounds that could volatilize and change the sample's weight.
   5. Once confirming the weight, calculate the biomass concentration within the reactor with the equation:
      ​Biomass concentration = (Biomass weight - filter weight) x 1000/Filtered volume [g/L]
   6. Make a linear regression of the biomass weight data in grams per liter of culture as a function of the absorbance measured at 750 nm using a spreadsheet or any other software. The linear regression coefficient should be greater than 0.95; otherwise, the curve is not useful, and the protocol should be repeated.
      NOTE: It is described as biomass weight and not as dry weight as most methods because the drying method used does not allow for full removal of water in the sample, leaving a water content of less than 5%¹⁹.
5. Biomass growth
   1. Monitor the reactors every day. Take a 1 L sample from the halfway point between the paddlewheel and its return from each culture and bring it to the laboratory.
   2. Check colony growth and purity of the culture under the microscope.
   3. Measure and record the absorbance at 750 nm of the samples with a spectrophotometer, where the fresh culture medium is the blank.
   4. Compare with the calibration curve to obtain the estimated biomass weight in grams per liter.
   5. Record the growth of each raceway reactor.
6. Biomass production - harvesting
   1. Monitor the reactors every day. If biomass growth rises above 0.7 g/L during sampling, harvesting is needed.
   2. Alternating between both HRAPs, place a polyester mesh on top of a section at one end of the reactor and place an end of a flexible PVC tube within the flow of the liquid so that the other end drains the liquid on top of the mesh.
   3. Drain between 4500 L to 7500 L (depending on the biomass saturation of the reactor) onto the mesh, maintaining a continuous flow back to the corresponding HRAP. The biomass will be retained on the mesh.
   4. To harvest, remove the mesh from the top of the reactor and place it on a different surface to scrape the biomass off and place it into a funnel.
   5. Push the biomass through the funnel to create elongated shapes on top of a clean and dry mesh; set the mesh in a warm, covered room (34-36 °C) for 48-72 h.
   6. Once dry, remove the biomass from the mesh and weigh it. Calculate the biomass harvested concentration in g/L with these equations:
      Volume of drained liquid = Pump flow rate x Drain time [L]
      Biomass harvested concentration = Biomass weight of harvested biomass/Volume of drained liquid [g/L]

Results

Following the protocol, the system was built, tested, and inoculated. The conditions were measured and stored, and the samples were taken and analyzed. The protocol was performed a year, starting in October 2019 and lasting until October 2020. It is important to mention that from here onwards, the HRAPs will be referred to as RT3 and RT4.

Biomethane productivity
In order to determine the conditions that promote the highest H₂S and CO₂ removal and, c...

Discussion

Throughout the years, this algal technology has been tested and used as an alternative to the harsh and expensive physicochemical techniques to purify biogas. Particularly, the Arthrospira genus is widely used for this specific purpose, along with Chlorella. There are few methodologies, however, that are made on a semi-industrial scale, which adds value to this procedure.

It is critical to maintain lower O₂ concentrations by using the proper L/G ratio; however, thi...

Materials

Name	Company	Catalog Number	Comments
1" rotameter	CICLOTEC	N/A
1" rotameter	GPI	A10-LMA100IA1
Absorption tank	EFISA	Made under previous design
Air blower (2.35 HP)	Elmo Rietschle	2BH11007AH01
Biogas blower (2 HP)	Elmo Rietschle	2BH11007AH01
Biogas composition measure	Geotech	BIOGAS 5000
Data-acquisition device	LabJack Co.	U3-LV
Diffuser tubes	Aero-Tube	C3060AR
DO sensor	Applisens	Z10023525
Dodecahydrated trisodium phosphate	Quimica PIMA	N/A	Fertilizer grade (greenhouse and experior use)
Dodecahydrated trisodium phosphate	Fermont	35963	Analytical grade (Used in cultures inside the laboratory)
Durapore membrane (45 µm)	MerckMillipore	HVLP04700
Electric motor 1.5 HP	Weg	00158ET3ERS56C
Ferrous sulfate heptahydrate	Agroquimica Samet	N/A	Fertilizer grade (greenhouse and experior use)
Ferrous sulfate heptahydrate	Fermont	63593	Analytical grade (Used in cultures inside the laboratory)
Geomembrane	GEOSINCERE	N/A
Magnesium sulfate heptahydrate	Tepeyac	N/A	Fertilizer grade (greenhouse and experior use)
Magnesium sulfate heptahydrate	Fermont	63623	Analytical grade (Used in cultures inside the laboratory)
Paddle wheel	GSI	Made under previous design
pH sensor	Van London pHoenix	715-772-0041
Portable screen	Rasspberry	Pi 3 B+
Recirculation centrifugal pump (1.5 HP)	Aquapak	ALY 15
Sodium bicarbonate	Industria del alcali	N/A	Fertilizer grade (greenhouse and experior use)
Sodium bicarbonate	Fermont	12903	Analytical grade (Used in cultures inside the laboratory)
Sodium chloride	Sal Colima	N/A	Fertilizer grade (greenhouse and experior use)
Sodium chloride	Fermont	24912	Analytical grade (Used in cultures inside the laboratory)
Sodium nitrate	Vitraquim	N/A	Fertilizer grade (greenhouse and experior use)
Sodium nitrate	Fermont	41903	Analytical grade (Used in cultures inside the laboratory)
Storing program (pH, DO)	Python Software Foundation	Python IDLE 2.7
Tedlar bags	SKC Inc.	232-25
Temperature recorder	T&D	TR-52i
UV-Vis Spectrophotometer	ThermoFisher Scientific instrument	GENESYS 10S
Vacuum pump	EVAR	EV-40