Digestate is a byproduct of anaerobic digestion, which can be considered waste or a product of potential use for the chemical industry or agriculture. In either case, the digestate must usually be treated prior to being disposed of or valorized. This review describes digestate processing technologies and their specific characteristics. Nutrient recovery and removal from digestate can be achieved through mechanical, physicochemical or biological processes. Available and potential digestate treatment techniques are presented. The complexities of the technologies available, legislation, the agronomical value of the digestate and the economic value of the process mean a decision support tool is required to help managers choose the best digestate processing technology. To ensure adequate analysis, the whole biomethanization project should be integrated in the use of these decision support tools. The objectives and limits of some of the currently available tools are analyzed at the end of this review.

Agro-industrial residues such as bagasse, pomace, municipal residues, vinasse and cheese whey are an environmental problem around the world, mainly due to the huge volumes that are generated because of the food production to satisfy the nutritional needs of the growing world population. Among the above residues, cheese whey has gained special attention because of its high production with a worldwide production of 160 million tons per year. Most of it is discarded in water bodies and land causing damage to the environment due to the high biological oxygen demand caused by its organic matter load. The environmental regulations in developing countries have motivated the development of new processes to treat transform cheese whey into added-value products such as food supplements, cattle feed and food additives. In addition, during the last decade, several processes and technologies have been developed to produce bioenergy through the biotechnological process using cheese whey as a potential feedstock. This review discusses the production of bioethanol, biohydrogen, biomethane and microbial lipid-biodiesel production using cheese whey as a potential substrate.

This strategic review examines the pivotal role of sustainable methodologies in battery recycling and the recovery of critical minerals from waste batteries, emphasizing the need to address existing technical and environmental challenges. Through a systematic analysis, it explores the application of green organic solvents in mineral processing, advocating for establishing eco-friendly techniques aimed at clipping waste and boosting resource utilization. The escalating demand for and shortage of essential minerals including copper, cobalt, lithium, and nickel are comprehensively analyzed and forecasted for 2023, 2030, and 2040. Traditional extraction techniques, including hydrometallurgical, pyrometallurgical, and bio-metallurgical processes, are efficient but pose substantial environmental hazards and contribute to resource scarcity. The concept of green extraction arises as a crucial step towards ecological conservation, integrating sustainable practices to lessen the environmental footprint of mineral extraction. The advancement of green organic solvents, notably ionic liquids and deep eutectic solvents, is examined, highlighting their attributes of minimal toxicity, biodegradability, and superior efficacy, thus presenting great potential in transforming the sector. The emergence of organic solvents such as palm oil, 1-octanol, and Span 80 is recognized, with advantageous low solubility and adaptability to varying temperatures. Kinetic (mainly temperature) data of different deep eutectic solvents are extracted from previous studies and computed with machine learning techniques. The coefficient of determination and mean squared error reveal the accuracy of experimental and computed data. In essence, this study seeks to inspire ongoing efforts to navigate impediments, embrace technological advancements including artificial intelligence, and foster an ethos of environmental stewardship in the sustainable extraction and recycling of critical metals from waste batteries.

Anaerobic digestion using mixed-culture with broader choice of pretreatments for hydrogen (H₂) production was investigated. Pretreatment of wastewater sludge by five methods, such as heat, acid, base, microwave and chloroform was conducted using crude glycerol (CG) as substrate. Results for heat treatment (100 °C for 15 min) showed the highest H₂ production across the pretreatment methods with 15.18 ± 0.26 mmol/L of medium at 30 °C in absence of complex media and nutrient solution. The heat-pretreated inoculum eliminated H₂ consuming bacteria and produced twice as much as H₂ as compared to other pretreatment methods. The fermentation conditions, such as CG concentration (1.23 to 24 g/L), percentage of inoculum size (InS) (1.23% to 24% v/v) along with initial pH (2.98 to 8.02) was tested using central composite design (CCD) with H₂ production as response parameter. The maximum H₂ production of 29.43 ± 0.71 mmol/L obtained at optimum conditions of 20 g/L CG, 20% InS and pH 7. Symbiotic correlation of pH over CG and InS had a significant (p-value: 0.0011) contribution to H₂ production. The mixed-culture possessed better natural acclimatization activity for degrading CG, at substrate inhibition concentration and provided efficient inoculum conditions in comparison to mono- and co-culture systems. The heat pretreatment step used across mixed-culture system is simple, cheap and industrially applicable in comparison to mono-/co-culture systems for H₂ production.