Algae that are grown in wastewater treatment lagoons could be an important substrate for biofuel production; however, the low C/N ratio of algae is not conducive to anaerobic digestion of algae with economically attractive methane production rates. Increasing the C/N ratio in anaerobic, laboratory scale, batch reactors by blending algal biomass with sodium acetate resulted i increased methane production rates as the C/N ratio increased. The highest amount of methane was produced when the C/N was 21/1. When the C/N was 24/1, the biogas production rate decreased. Batch experiments were done to evaluate the effect of optimizing the C/N ratio on methane production from algae and to identify the most essential information needed to conduct research on co-digestion of algal biomass using the continuous, high-rate, up-flow anaerobic sludge blanket (UASB) reactor system. Based on the results obtained from batch reactor experiments, anaerobic co-digestion of algal biomass, obtained by continuous centrifugation from the Logan City, Utah, 5th stage wastewater treatment lagoon, and sodium acetate was conducted using laboratory scale UASB reactors with the C/N ratio in the feedstock adjusted to 21/1. Duplicate, 34 L UASB reactor systems were built of poly(methyl methacrylate). Both reactors were seeded with 11 L of anaerobic sediment from the 3rd stage lagoon. The pH of the feedstock was adjusted to the neutral range. The feedstock was initially introduced at a low organic loading rate of 0.9 g/L.d with a hydraulic retention time (HRT) of 7.2 days and then increased up to 5.4 g/L.d and a HRT of 5.5 days. These organic loading rates corresponded to an initial influent chemical oxygen demand (COD) of 6.25 g/L and increased to 27.2 g/L. Methane production increased from 270 mL/g to 349 mL/g COD biodegraded. COD removal efficiency was 80% and biogas methane composition was 90% at steady state. Algal biomass contributed 33-50% of the COD in the feed stock depending on the COD of the algae paste from centrifugation. The shortest HRT at which steady state was not affected was 5.5 days. At lower HRT all monitored parameters showed a slight decrease after the 75th day of operation.

Increasing demand for energy coupled with concerns over limited fossil fuel reserves and apprehensions over their contributions to greenhouse gas emissions have made the search for low carbon energy sources a high priority. Algal biomass could serve as an alternative source of renewable biofuels. Research efforts to date have primarily focused on the production of algal biofuels through lipid extraction, which involves high temperature and high pressure, resulting in an energy intensive process. In this research, the use of algal biomass as a supplementary feedstock to anaerobic digesters for the production of methane gas is evaluated. To test the potential of algal biomass as a supplementary feedstock, labscale anaerobic digesters are set-up. The methane gas production of various combinations of thickened waste activated sludge (TWAS) and algal biomass is investigated. Chlorella vulgaris (C. vulgaris) is used as representative microalgae. In addition, the effects of operational parameters, such as biomass loading, temperature and alkalinity, on biogas production are investigated. The results show that the biogas production for all biomass loading combinations of C. vulgaris and TWAS ranged from 0.47-0.57 mL per mg volatile solids (VS) digested. On average, VS and chemical oxygen demand (COD) were reduced 48 and 38%, respectively, at 35°C. Average total coliform (TC) and fecal coliform (FC) concentrations of 6.3x104 and 1.0x104 CFU per gram of total solids (TS), respectively, were measured in the digested waste at 35°C. Thus, the residual meets the USEPA requirements for pathogen reduction (FC 2x106 CFU per g TS) and vector attraction reduction ( 38% reduction in VS) for land application. The total nitrogen and phosphorus content of the residual was determined to be in the range of 9-17% as N and 3-7% as P (7-16% as P2O5), respectively, revealing its potential value as a fertilizer. It was also observed that decreased digestion temperatures resulted in lower biogas yields, while initial alkalinity in digesters did not appear to affect biogas production. From the results of the research, it can be inferred that algae can be co-digested with wastewater sludge, or by itself, to produce methane gas at wastewater treatment plants (WWTPs). This suggests that algae can be utilized as an energy source through anaerobic co-digestion with wastewater sludge. This is significant because algae can be grown with the nutrient and CO2 contained in waste streams at WWTPS, thereby minimizing the release of nutrients and effluent water to the environment. This reduced nutrient load results in treatment cost savings, while the reduction in effluent discharge decreases environmental pollution.

Recent advances in technology to recover bioenergy from various feedstocks make them suitable alternatives to fossil fuel. This book contains several scientific discussions regarding microbes involved in biogas production, the anaerobic digestion process, their operation, and application for sustainable development. The book provides in-depth information about anaerobic digestion for researchers and graduate students. The editor sincerely thanks all the contributors, whose efforts have brought this book to fruition.

Anaerobic digestion (AD) is one of the oldest biotechnological processes and originally referred to biomass degradation under anoxic conditions in both natural and engineered systems. It has been used for decades to treat various waste streams and to produce methane-rich biogas as an important energy carrier, and it has become a major player in electrical power production. AD is a popular, mature technology, and our knowledge about the influencing process parameters as well as about the diverse microbial communities involved in the process has increased dramatically over the last few decades. To avoid competition with food and feed production, the AD feedstock spectrum has constantly been extended to waste products either rich in recalcitrant lignocellulose or containing inhibitory substances such as ammonia, which requires application of various pre-treatments or specific management of the microbial resources. Extending the definition of AD, it can also convert gases rich in hydrogen and carbon dioxide into methane that can substitute natural gas, which opens new opportunities by a direct link to traditional petrochemistry. Furthermore, AD can be coupled with emerging biotechnological applications, such as microbial electrochemical technologies or the production of medium-chain fatty acids by anaerobic fermentation. Ultimately, because of the wide range of applications, AD is still a very vital field in science. This Special Issue highlights some key topics of this research field.

The combination of rising energy consumption in the U.S. and sustained growth of developing countries has made clear the importance of developing an energy source that is renewable and minimizes greenhouse gas emissions. The use of algae as an energy source can satisfy both of these criteria, but the current focus on developing it as a biofuel requires a significant amount of energy input, making it not yet economically feasible. This research combines a promising energy source with a decades-old wastewater treatment technology to generate biogas by combining the anaerobic digestion of algae and wastewater sludge. Bench-scale anaerobic digesters were setup with various proportions of the microalgae Scenedesmus quadricuada and thickened waste activated sludge (TWAS) and their biogas production was evaluated. In addition, the effects of operational parameters, such as temperature and alkalinity, on biogas production and residual characteristics were investigated. Biogas production for the various algae and TWAS combinations ranged from 0.46 to 0.72 mL per mg of volatile solids (VS) digested, while VS and chemical oxygen demand (COD) were reduced on average, 47 and 50%, respectively, at 35°C. Total coliform (TC) and fecal coliform (FC) concentrations saw at least a one log reduction after digestion, allowing the digestant to meet the USEPA requirements for classification as a Class B biosolid and its use in certain land applications. The digestant had nitrogen and phosphorous levels in the range of 5 to 19% as N and 5 to 15% as P, respectively, putting it in the range of commercial fertilizer levels. It was also determined that decreasing digestion temperatures from 35°C produced significantly less biogas, while adjusting the amount of initial alkalinity in digesters did not have a significant effect on biogas production. From these results, anaerobically digesting algae along with wastewater sludge can be utilized as a feasible method to harness the energy potential of algae. Although some of this potential remains locked up in the undigested portion, its synergy with wastewater treatment plants (WWTPs) cannot be overstated. Growing algae using existing waste streams at WWTPs such as CO2 and effluent wastewater highlights this technology's ability to transform waste into a valuable commodity without enormous new infrastructure investment..

Some terms, such as eco-friendly, circular economy and green technologies, have remained in our vocabulary, because the truth is that mankind is altering the planet to put its own subsistence at risk. Besides, for rationalization in the consumption of raw materials and energy, the recycling of waste through efficient and sustainable processes forms the backbone of the paradigm of a sustainable industry. One of the most relevant technologies for the new productive model is anaerobic digestion. Historically, anaerobic digestion has been developed in the field of urban wastes and wastewater treatments, but in the new challenge, its role is more relevant. Anaerobic digestion is a technologically mature biological treatment, which joins bioenergy production with the efficient removal of contaminants. This issue provides a specialized, but broad in scope, overview of the possibilities of the anaerobic digestion of lignocellulosic biomass (mainly forestry and agricultural wastes), which is expected to be a more promising substrate for the development of biorefineries. Its conversion to bioenergy through anaerobic digestion must solve some troubles: the complex lignocellulosic structure needs to be deconstructed by pretreatments and a co-substrate may need to be added to improve the biological process. Ten selected works advance this proposal into the future.

Anaerobic Reactors is the forth volume in the series Biological Wastewater Treatment. The fundamentals of anaerobic treatment are presented in detail, including its applicability, microbiology, biochemistry and main reactor configurations. Two reactor types are analysed in more detail, namely anaerobic filters and especially UASB (upflow anaerobic sludge blanket) reactors. Particular attention is also devoted to the post-treatment of the effluents from the anaerobic reactors. The book presents in a clear and informative way the main concepts, working principles, expected removal efficiencies, design criteria, design examples, construction aspects and operational guidelines for anaerobic reactors. About the series: The series is based on a highly acclaimed set of best selling textbooks. This international version is comprised by six textbooks giving a state-of-the-art presentation of the science and technology of biological wastewater treatment. Other titles in the series are: Volume 1: Waste Stabilisation Ponds; Volume 2: Basic Principles of Wastewater Treatment; Volume 3: Waste Stabilization Ponds; Volume 5: Activated Sludge and Aerobic Biofilm Reactors; Volume 6: Sludge Treatment and Disposal