Chapter 8 - Anaerobic digester biogas upgrading using microalgae

Microalgae-based wastewater treatment plants are low-cost alternatives for recovering nutrients from contaminated effluents through microalgal biomass, which may be subsequently processed into valuable bioproducts and bioenergy. Anaerobic digestion for biogas and biomethane production is the most straightforward and applicable technology for bioenergy recovery. However, pretreatment techniques may be needed to enhance the anaerobic biodegradability of microalgae. To date, very few full-scale systems have been put through, due to acknowledged bottlenecks such as low biomass concentration after conventional harvesting and inefficient processing into valuable products. The aim of this study was to evaluate the anaerobic digestion of pretreated microalgal biomass in a demonstration-scale microalgae biorefinery, and to compare the results obtained with previous research conducted at lab-scale, in order to assess the scalability of this bioprocess. In the lab-scale experiments, real municipal wastewater was treated in high rate algal ponds (2 × 0.47 m3), and harvested microalgal biomass was thickened and digested to produce biogas. It was observed how the methane yield increased by 67% after implementing a thermal pretreatment step (at 75 °C for 10 h), and therefore the very same pretreatment was applied in the demonstration-scale study. In this case, agricultural runoff was treated in semi-closed tubular photobioreactors (3 × 11.7 m3), and harvested microalgal biomass was thickened and thermally pretreated before undergoing the anaerobic digestion to produce biogas. The results showed a VS removal of 70% in the reactor and a methane yield up to 0.24 L CH4/g VS, which were similar to the lab-scale results. Furthermore, photosynthetic biogas upgrading led to the production of biomethane, while the digestate was treated in a constructed wetland to obtain a biofertilizer. In this way, the demonstration-scale plant evidenced the feasibility of recovering resources (biomethane and biofertilizer) from agricultural runoff using microalgae-based systems coupled with anaerobic digestion of the microalgal biomass.

Palm Oil Mill Effluent (POME) is a liquid waste from palm oil processing that holds great potential as a raw material for biogas production through fermentation in an anaerobic digester. Biogas is a form of renewable energy produced through fermentation in anaerobic conditions. This study aims to analyze the effect of fermentation time on the composition of biogas produced from POME. The research was conducted over a period of 30 days using anaerobic fermentation, with 200 liters of POME mixed with 2 probiotic capsules and 8 liters of water in the digester. Biogas composition including CH₄, CO₂, H₂S, and O₂ was analyzed every 5 days during the fermentation period.The results showed that fermentation time had a significant effect on both the production and composition of biogas. The volume and gas content increased over time, with optimal results achieved on the 25th day. On that day, the biogas composition consisted of 53.02% methane (CH₄), 26.32% carbon dioxide (CO₂), 0.054% hydrogen sulfide (H₂S), and 1.74% oxygen (O₂), with a total biogas volume of 62 liters. Based on these results, the 25th day was identified as the most optimal fermentation time for producing biogas from POME.

Challenges and opportunities of lignocellulosic biomass for anaerobic digestion

The absorber for removing hydrogen sulfide and carbon monoxide from natural gas with aqueous solutions of amine was rebuilt by specialists from Kedr–89 Scientific and Industrial Company together with workers from Astrakhan GPP. The essence of the reconstruction was to replace the bubble sieve trays with centrifugal trays and VAKUPAK structured packing. Industrial tests showed that the absorber with the new contact devices ensures output with respect to the gas in normal conditions of: desulfurized: 130,000 m3/h; separated, 196–200,000 m3/h;. These indexes are 8–10% higher than the projected indexes. The hydrogen sulfide content in the treated gas is no greater than 0.0073 g/m3 (admissible: 0.012 g/m3). The maximum carbon dioxide content in the treated gas is 2.68 ppm (standard: 150 ppm).

Dublin's Ringsend WWTP was designed to serve a population of approximately 1.2 million p.e. with a sludge production of 37,000 dry tonnes per year after upgrading to full secondary treatment. Several technical solutions were put forward as part of a design, build, finance and operate (DBFO) competition, with the chosen solution being a proposal by Black and Veatch for a combination of sequencing batch reactor (SBR) technology and anaerobic digestion with Cambi thermal hydrolysis pre-treatment (THP). The THP plant was built by Cambi and handed over to B&V in 2002. The plant is now operated by Celtic Anglian Water. In September 2004 a test was carried out on the mass and energy balance of the plant following 2 years of operation and is detailed in this paper. The process enables digestion at very high dry solids feed and low hydraulic retention time. The plant was built with three digesters of 4250 m3 each and is fed with hydrolysed sludge at 11% DS. There are four no. 1 MW Jenbacher engines operating mainly on biogas. Each pair of engines is fitted with a waste heat boiler with a capacity of one tonne steam per hour. These boilers have sufficient capacity to provide 80% of the steam required for the THP, which in turn provides all the heat for the subsequent digestion in the form of hydrolysed feed. There are two main biogas boilers for top up steam and other uses of the biogas including thermal oxidation of concentrated odours.

Biogas is a valuable product of the anaerobic digestion process, which is used for the generation of renewable energy. The main component of biogas is methane together with impurities like carbon dioxide water vapour, hydrogen sulphide, siloxanes, hydrocarbons, ammonia, oxygen, carbon monoxide and nitrogen. Their presence is undesired, reducing the calorific value of biogas and creating various operational problems on machines. Therefore, multi-stage technologies for their removal are used during the process of transformation of biogas to biomethane. This paper presents the most used physicochemical technologies for biogas upgrading and CO2 removal with the addition of novel findings and further possibilities of their improvements. Compared are their costs, energy requirements, removal efficiency and other parameters to give the reader a more complex view on their competitiveness. The review also includes a comprehensive description of chemotrophic as well as phototrophic methods of hydrogen sulphide treatment with a focus on biological means. Although these methods are not always feasible when compared with physicochemical counterparts, they offer attractive advantages, including lower cost and more environmentally friendly operation with greater possibilities of improvement.

Most natural gases and industrial gas mixtures cannot be used in their original state as fuel or raw material for chemical processes but must generally be purified before further treatment. An important step in gas purification is acid gas removal with emphasis on removal of carbon dioxide and hydrogen sulfide. Generally, chemical or physical absorption processes are applied. One of these processes particularly suitable for low-temperature purification is the Rectisol process.

Since the installation of the first membrane system for CO2removal in the early 1980s, membrane separation has become a proven technology for natural gas purification, such as the removal of carbon dioxide, hydrogen sulfide, heavy hydrocarbons, and nitrogen. Membrane systems can perform the bulk removal of impurities or produce pipeline natural gas. Today, large membrane plants can process up to 1 billion standard cubic feet of natural gas per day (scfd) for CO2removal. This article outlines the advantages of and challenges to membrane processes for natural gas purification, and provides an overview of the advancement in membrane materials, membrane structure, membrane separation performance, module configuration, and process designs for natural gas purification. Furthermore, this article reviews the current commercial membrane systems installed in natural gas processing plants, providing theory and literature references relevant to commercially practiced technology, as well as case design studies derived from actual industrial installations.

Potential of crop residues management for soil organic carbon sequestration in European countries until 2050: a simplified modelling approach

Impact of shear stress and impeller design on the production of biogas in anaerobic digesters

Manure management within the United States is considered one of the key hotspots of greenhouse gas (GHG) emissions. Linked to the formation of gases like methane, carbon dioxide, nitrous oxide, and ammonia, great attention must be paid to this portion of the dairy production chain in order to mitigate negative environmental impacts. The dairy industry has begun implementing a series of interventions throughout the manure management system to address emissions, helping to reduce the volatile fraction of manure through solid separation, improve nutrient management, and capture biogas in anaerobic digesters. Although more work is still needed, especially joint efforts between policy makers and farmers to increase rates of adoption and implementation, great strides have been made to reduce the environmental impact of manure management.

Some wastewater treatment (WWT) facilities are using magnetite in their treatment process, through a technology called BioMag, and stabilizing their wasted sludge using anaerobic digestion (AD) after recovering most of the magnetite for reuse. This research investigated whether this magnetite would benefit the AD process to assess the possibility of magnetite recovery after AD, which could reduce the loading on the recovery equipment as well as operation and maintenance costs of magnetite recovery due to decreased sludge quantities. Because some conflicting results were found in previous studies, which could be due to particle size, or to particle stability, because some studies used synthesized magnetite particles that can be unstable, which may affect the properties and behavior of the magnetite, this study used the same magnetite supplied for the BioMag process. The effects of this magnetite on biogas production, biogas composition, hydrogen sulfide gas (H2S) reduction, and digester effluent were investigated. Results showed an increase in the effluent dissolved iron and a 49% reduction in H2S. No considerable effect was seen on the gas volume and composition.

CO2 removal Demand for a better way to remove carbon dioxide (CO2) from natural gas led to the creation of a new material at Rice University in Houston that does something unprecedented on the molecular scale, and might even change gas processing. For professor James Tour, the fundamental science behind this porous carbon material created in his laboratory “is really profound.” The black powdery material developed by a Rice graduate student, Chih-Chau Hwang, absorbs nearly its weight of carbon dioxide by creating long polymer chains of carbon dioxide at a pressure level that is elevated, but easily attained, in a gas processing facility. For Tour, it is an exciting development because the carbon dioxide polymer structure, which was observed using spectroscopy, is something that he said has never been seen in nature, and only rarely in labs at extreme pressures. But the pioneering chemist recognizes that for those developing new oilfield technologies, what matters is what it can do, not how it might do so. Rice’s patents have been licensed by Apache Corp., which supported research seeking a new material for gas separation for the past 2½ years. On the plus side, it appears to be energy-efficient, can hold a large volume relative to its weight, and is highly selective at grabbing carbon dioxide without also removing some natural gas as well. The investment by the independent oil company reflects the exploration and production industry’s appetite for improved methods that are more compact, cost less to run, and occupy less space. One indication of that is the increased demand for improved membrane-based separation methods, which are far more compact and adaptable to offshore environments than liquid chemical separation methods using amines. Cameron recently began selling a new, more efficient membrane material, called PN-1 multifiber membrane element, which was developed in partnership with Petronas, the Malaysian national oil company. Better carbon dioxide removal is a pressing concern in southeast Asia, where many large gas deposits have not been developed because of carbon dioxide levels that can equal the gas in the ground. Production growth there, and in far-flung regions from Russia to Brazil, will require dealing with higher levels of carbon dioxide and hydrogen sulfide, which often are found together and removed using similar processes.

Strongly basic anion exchange resins containing quaternary ammonium functionality and fluoride or acetate anions were found to remove carbon dioxide and hydrogen sulfide from gas streams. The absorption/desorption isotherms, heats of absorption, and gas separation properties were determined for a series of such resins. The fluoride form of Amberlyst® A-26, for example, absorbed CO2 and H2S reversibly, 0.23 mol CO2/mol F− and 0.24 mol H2S/mol F− at 100 kPa and 22 and 30°C, respectively. Absorption of CO2 was fast compared to its desorption. Characterization by NMR indicated that bicarbonate was formed by reaction of CO2 with F− containing resins. Heats of CO2 absorption by F− Amberlyst® A-26 were pressure dependent and ranged from −5.0 to −3.2 kcal/mole CO2 for pressures of 50 to 1000 kPa. The fluoride and acetate containing resins were effective for removal of CO2 and H2S from gas mixtures. Passage of a gas mixture containing 1% CO2 or 5% H2S through a packed column of F− Amberlyst® A-26 at 22°C reduced the CO2 or H2S concentration to less than 25 ppm. Regeneration of the absorbents was accomplished by purging with inert gas at 50°C. Removal of CO2 from gas streams containing substantial water vapor concentrations was achieved using F− Amberlyst® A-26 resin.

World production and trade of fruits generate losses in the harvest, post-harvest, handling, distribution and consumption phases, corresponding to 6.8% of total production. These residues present high potential as a substrate for the anaerobic digestion process and biogas generation. Thus, the energy valuation of the agro-industrial residues of kiwi production was evaluated by anaerobic digestion, aiming at optimizing the biogas production and its quality. Ten assays were carried out in a batch reactor (500 mL) under mesophilic conditions and varying a number of operational factors: different substrate/inoculum ratios; four distinct values for C: N ratio; inoculum from different digesters; and inoculum collected at different times of the year. The following parameters were used to control and monitor the process: pH, alkalinity, volatile fatty acids (VFA), volatile solids (VS) and chemical oxygen demand (COD). Among the tests performed, the best result obtained for the biogas production corresponded to the use of 2 g of substrate and 98 mL of inoculum of the anaerobic digester of the Wastewater Treatment Plant (WWTP) of Bragança, with addition of 150 mg of bicarbonate leading to a production of 1628 L biogas.kg-1 VS (57% methane). In relation to the biogas quality, the best result was obtained with 20 g of substrate and 380 mL of inoculum from the anaerobic digester sludge of WWTP of Ave (with addition 600 mg of sodium bicarbonate), presenting a value of 85% of CH4, with a production of 464 L biogas.kg-1 VS.