Enhanced wastewater treatment plays a pivotal role in preserving ecological equilibrium and ensuring human well-being. Traditional wastewater treatment methods face challenges in effectively removing emerging contaminants, such as pharmaceuticals, personal care products, and microplastics. This has led to the exploration of alternative materials for wastewater treatment, and one such class of materials that has gained significant attention is bismuth oxyhalides (BiOX). This article aims to provide an overview of the emerging opportunities of BiOX in wastewater treatment. BiOX materials possess unique properties, including excellent photocatalytic activity, chemical stability, and low toxicity, making them promising candidates for various wastewater treatment applications. The paper emphasizes recent advancements in the synthesis and characterization of diverse BiOX nanostructures, including BiOCl, BiOBr, and BiOI and their performance in removing organic pollutants, heavy metals, and microorganisms from wastewater. The review also discusses the mechanisms underlying the photocatalytic activity of BiOX, including the generation of reactive oxygen species and the role of different crystal facets. Moreover, it explores strategies for enhancing the photocatalytic performance of BiOX through modifications such as doping, heterostructure formation, and co-catalyst loading. Furthermore, this paper addresses the challenges and limitations associated with BiOX-based wastewater treatment, including material stability, reusability, and cost-effectiveness. It also identifies future research directions and potential applications of BiOX in emerging fields such as wastewater treatment in resource-limited settings and the removal of emerging contaminants. In conclusion, BiOX materials present promising opportunities for advanced wastewater treatment due to their exceptional photocatalytic properties. However, further research is needed to overcome the current challenges and optimize the performance of BiOX-based systems for practical applications in real-world wastewater treatment scenarios.

Organic wastes can be converted to value-added products using biotransformation technologies since waste disposal and the depletion of fossil fuels are major concerns. There has been a great deal of environmental pollution caused by the extraction of palm oil, which produces a large number of by-products, along with the industry’s development and economic progress. It is important to address the issue of palm oil mill effluent (POME) in order to reduce its negative environmental effects. By regenerating energy from renewable sources, such as organic residues, anaerobic digestion (AD) is considered one of the most effective technologies for organic waste treatment. Since biomass is converted to energy through inputs and outputs, questions have been raised regarding the sustainability of bioenergy pathways. Defining sustainable disposal alternatives for organic residues can be achieved by evaluating the environmental benefits associated with biogas utilization, and digestate treatment.

Biohythane has been gaining a lot of attention lately, due to its gaseous composition of biohydrogen and biomethane. In this way, biogas production from POME can be evaluated and ensured to be sustainable through a holistic and comprehensive environmental tool, such as a life cycle assessment (LCA). By providing environmentally relevant information, LCA and carbon footprint analysis proves to be an appropriate tool for supporting future investment decisions in sustainable bioenergy production.

This study aims to conduct a life-cycle assessment (LCA) on the efficiency of biohydrogen production from palm oil mill effluent (POME) in laboratory-scale up-flow anaerobic sludge fixed-film (UASFF) and continuous stirred-tank reactor (CSTR). LCA is a tool used to determine the environmental performances of products, processes, or services, through their production, distribution, usage, maintenance, and disposal stage. It is a systematic set of procedures developed to compile, examine, and evaluate the material and energy balance of the system by converting those inputs and outputs to associate with the potential environmental impact that is directly attributable to the operation of a product or service system throughout its entire life cycle. SimaPro software (version 9) was chosen to carry out the LCA of biohydrogen production under the ISO 14040 Standard. The life cycle inventory (LCI) data from Ecoinvent database (version 3.6) libraries were used. The study covered the analysis of the cradle-to-gate system boundary, which contains the raw material and energy acquisition. The assessment was done based on the functional unit of 1 kg biohydrogen production. The POME input to the system is considered a waste product that carries zero environmental loads. The potential of the avoided burden of POME utilization was also not considered in the system boundary. The system boundaries of the biohydrogen production include the inputs: POME, electricity usage (pumps, water bath and stirrer), molasses as fermentation stimulant, and output: emission of biogas (H2, and CO2), and effluent described within the boundary. The density and specific heat of POME used for the analysis are 1007.7 kg/m3 and 4374.89 j/kg. The density of molasses, hydrogen and carbon dioxide gases are 1124.6 kg/m3, 0.08988 kg/m3 and 1.87 kg/m3. The energy conversion factors of 1kWh equal to 3.6 MJ. The inventory analysis was conducted by collecting and calculating the input-output data, which consist of energy flows and material used as defined in the system boundary. The inventory data were collected for the CSTR reactor and the UASFF reactor with both on 24h HRT. The amount of energy usage for both reactors are collected using power meters installed. For CSTR, the electricity consumption for water bath, stirrer and pumps were measured, while, for UASFF, the electricity usage was measured for water bath and pump. The water bath and pump(s) in the UASFF reactor consumed higher amounts of energy compared to the CSTR reactor due to the operating volume of POME in UASFF on average (1.9L) being higher than CSTR (4.2L). The impact assessment results show that in the 24h HRT system, CSTR performed slightly (7%) better than UASFF in terms of carbon footprint (kg CO2e) and energy consumption (MJ) per kilograms of biohydrogen produced. It is also noticed that the water bath was the main contributor to carbon footprint and energy consumption in both reactors with CSTR at an average of 61% and UASFF at 66%. While the pump and stirrer (CSTR only) come second and third. The usage of molasses produced a negative values of carbon footprint due to its carbon sequestration capabilities during its life cycle. The biohydrogen production from CSTR reactor showed a better performance than UASFF reactor in term of carbon footprint and energy consumption. 

Biohydrogen production from palm oil mill effluent (POME) through dark fermentation process was evaluated in a two different lab scale reactor configuration namely up-flow anaerobic sludge blanket fixed-film reactor (UASFF) and a continuous stirred tank reactor (CSTR). The effect of different organic loading rate (OLR) and hydraulic retention time (HRT) on hydrogen yield and volumetric hydrogen production rate (VHPR) was investigated. The data on the UASFF reactor is based on the start-up study published research data by the authors undertaken from 2021. In that study, UASFF reactor operated at large OLRs of 5-80 g COD/L.d and HRTs of 6-24h, and CSTR reactor at OLRs of 15-100 g COD/L.d and HRTs of 6-24h. The UASFF achieved a VHPR of 1.9-4.8 ± 0.1 L H2/L-d and COD removal of 35% while the CSTR had a VHPR of  1.8-4.5 ± 0.1 L H2/L-d and COD removal of 30%. The results indicated the change of bacterial community diversity over the operation in the UASFF reactor, in which Clostridium sensu stricto 1 and Lactobacillus species contributed to hydrogen fermentation. While the CSTR resulted in higher population of Clostridium sensu stricto 12 but also includes Lactobacillus sp. that mainly contribute to hydrogen production. Both Clostridium sensu stricto 1 (C. acetobutyricum) and Clostridium sensu stricto 12 (C. tyrobutyricum) are known to be producers of hydrogen as a byproduct of both respective metabolic pathways that converts carbon sources into acetic and butyric acid. Previous research has also shown that depending on the operating conditions and parameters of the fermentation process, Clostridium sensu stricto 12 can be converted into Clostridium sensu stricto 1. The similar performance of the UASFF and the CSTR can be explained by the similarity in the microbial community composition, which shows high prevalence of hydrogen-producing bacteria, Clostridium sensu stricto 1 in the UASFF and Clostridium sensu stricto 12 in the CSTR. The difference in COD removal could be due to the method operation between the two reactors, as mixing capabilities of UASFF (through up-flow velocity and recirculation rate from a settling tank to the main UASB reactor) is well-established and known to be efficient as compared to CSTR. However, CSTR can be easily modified to optimize mixing capabilities with changes in tank size, stirrer type, and also stirring speed, not to mention maintenance for CSTR is relatively simple and does not involve complex techniques. 

Greenhouse gas emissions reduction and climate change mitigation are among the biggest challenges facing societies right now and in the future. The increasing population has caused agriculture and food waste to become major issues affecting the environment and climate. Therefore, a renewable energy source, such as biogas, is a good alternative to replace fossil fuels. It is possible to produce biogas from a wide range of organic waste streams and by-products of industrial processes.  Anaerobic digestion (AD) of organic waste is an efficient means of producing fossil free energy with synergistic sustainable waste management. Nevertheless, it requires a circular economy and an integrated waste-management system. The integration of AD and pyrolysis is not only for treating agricultural and food wastes and prospects for biogas production but also could be a sustainable approach towards providing nutrients and resources. Moreover, addition of the by-product of the pyrolysis viz. biochar in AD processes could stabilize and increasing the quality and quantity of biogas production, improving soil fertility and promote the circular bioeconomy. This study aims to critically assesses the available strategies for enhancing biogas production and to explore the synergistic effects of integrated AD and pyrolysis of agro-food waste that will promote the concept of a circular bioeconomy to reduce the negative impact on the environment including carbon footprints, which encourage the scientists, industrialists, and policy makers to explore integrated concepts for using organic waste in biofuel production.