Hamish Robert Mackey

Greywater reuse offers a sustainable approach to alleviating freshwater scarcity, but concerns about treatment reliability limit its adoption. This study investigates a two-stage green wall system for greywater treatment, integrating vegetated units planted with Carex secta and vertical flow post-treatment columns (PTCs) packed with waste-derived adsorbents—calcinated kina (sea urchin) shell, ZnCl ₂ -activated apricot stone biochar, and natural zeolite. The system’s performance was evaluated for removal of organic matter (COD, TOC), nutrients (ammonia, nitrate, nitrite, phosphate), and xenobiotics (anionic surfactants, aromatic compounds) under varying hydraulic and pollutant loads. Substrate characterization, plant health assessment, microbial analysis, and physicochemical water quality monitoring were used to elucidate plant–microbe–media interactions. PTCs effectively polished residual pollutants after partial removal in green wall units, achieving removal rates of 34% of phosphate, 99.8% of ammonia, 95% of surfactants, and 25%–37% of aromatic compounds (UV ₍₂₉₄₎ ). While nitrate leached from green wall units (−34%), PTCs mitigated this by removing 64% of nitrate. TOC and COD were further reduced by 17% and 14%, resulting in overall removal efficiencies of 95% and 96%, respectively, across the two-stage system. Under stress conditions of 2-week drying followed by high pollutant loading, PTCs maintained pollutant removal, whereas green wall units showed reduced nutrient removal. Across the planted green wall, ammonia increased from 0.02 to 1.63 mg/L and phosphate from 3.42 to 4.44 mg/L. The non-planted control deteriorated more strongly, with ammonia increasing to 8.88 mg/L and phosphate to 8.32 mg/L. After stress recovery, vegetated units rapidly regained treatment efficiency, while controls released ammonia tenfold and phosphate twofold. This novel integrated treatment design demonstrates a scalable, low-maintenance solution for effective on-site greywater treatment and reuse.

This study highlights the promising role of machine learning in optimizing spent coffee ground (SCG) valorization, particularly through pyrolysis-based conversion to bioenergy. With the global shift toward sustainable energy, integrating diverse technologies and renewable resources is crucial. Coffee, one of the most widely consumed beverages, produces large quantities of SCGs during brewing, posing both environmental and economic challenges. The study investigates a pyrolysis system using the Pyrolysis Interactive Simulator (PIS) V2.0 to model key parameters such as temperature and heating rate to predict the composition of pyrolysis products (biochar, bio-oil, and syngas). Experimental validation supports the model’s biochar predictions. The environmental impact of the process is assessed using Life Cycle Assessment (LCA) tools, revealing that blending and drying at different temperatures require similar energy inputs, while pyrolysis energy demand increases by 19% as temperature rises from 300 °C to 600 °C. The model shows that pyrolysis temperature significantly influences the energy yields of bio-oil, biochar, and syngas. The economic analysis forecasts annual profits of $14–22 million USD in markets such as Qatar, the USA, the UK, and China. In particular, at 600 °C, the Chinese market demonstrates a return on investment (ROI) of 29%, with a payback period of 3.4 years, indicating strong commercial viability. The study also reveals that higher pyrolysis temperatures increase the carbon, water, and land footprints for bio-oil and syngas, while reducing them for biochar. Overall, this work leverages simulation software and machine learning to optimize the pyrolysis process, offering valuable insights for efficient SCG valorization. Graphical Abstract

Purple phototrophic bacteria (PPB) offer a sustainable approach for biological wastewater treatment while simultaneously producing valuable by-products such as polyhydroxyalkanoates (PHAs). This study investigates the effects of continuous light wavelengths over a two-stage nutrient reduction setup on PHA accumulation in a mixed PPB culture grown on fuel synthesis wastewater (FSW). The first stage promoted biomass production under nutrient availability, while the second stage targeted the enhancement of PHA accumulation through nitrogen (N) or phosphorus (P) reduction. Biomass growth remained stable under P reduction but significantly increased under N reduction. The results showed that organics removal efficiency decreased under nutrient reduction, particularly under P reduction, while N reduction conditions enhanced P uptake from the media. Maximum PHA accumulation reached 12.6% CDW under N reduction and 10.0% CDW under P reduction. Light type played a dominant role, with a full-spectrum light that included ultraviolet (UV) and infrared (IR) promoting the highest PHA accumulation, whereas white light with far-red wavelengths (700–770 nm) enhanced biomass growth. These findings highlight the potential of optimizing light conditions and nutrient availability to enhance PHA biosynthesis, paving the way for improved bioplastic production from wastewater streams.

With increasing urbanization, greywater, generated from domestic activities like bathing and washing, is a marginal water resource that can be valorized to meet irrigation demands and overcome water scarcity. This review paper delves into the characteristics and variability in greywater quality, highlighting the benefits of greywater reuse, such as water conservation, nutrient supply, and potential cost savings, as well as challenges like pathogen contamination and salinity buildup. Various treatment methods, including physical, chemical, and biological processes, are discussed concerning their effective use for irrigation. This paper explores the integration of greywater irrigation with vegetated walls, an innovative urban greening solution that offers numerous environmental and social benefits. The types of vegetated walls, their irrigation requirements, and studies involving greywater application in these systems are discussed. By synthesizing the current knowledge, this review article provides a comprehensive understanding of enhancing urban sustainability through the valorization of an otherwise wasted resource.

Sustainable crop production supports food security by mitigating water and nutrient stress from manures by excessively drained water. Cow manure is a good nutrient resource to enhance soil fertility and plant growth but requires a suitable amender to reduce the nutrients loss by leaching. Worldwide more than 20,400 metric tonnes of pistachios were consumed where roughly 30% of the weight of the nut is the shell which could be a good feedstock for biochar production. Therefore, this study was conducted to produce biochar from pistachio shell by pyrolysis process at three different temperatures 350 to 550 °C and based on various properties characterization, the biochar at 450 °C was amended with 0% (control), 2%, 4% and 8% to the cow manure for eggplant growth (Solanum melongena). Out of all fractions of biochar application, 2% biochar has virtuous performance to increase 5.63 ± 1.45 cm of plant height, 1.33 ± 4.79 cm leaf length and 1.90 ± 4.43 cm leaf width over the control. The leaf chlorophyl content and plant biomass were also significantly (p = 0.02) increased compared to the control. The lower fraction of biochar application showed almost similar water retention capacity with control (p > 0.11) due to the better plant growth. Based on the findings of this study, it is proposed that lower fraction of biochar application with manure is a constructive solution to improve soil quality, reduce nutrient loss and eggplant growth. Under the circular economy and frameworks of sustainability, pistachio shell biochar production has a high legislative attention on valorizing nutshell wastes. Graphical Abstract