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The present study intends to evaluate a synergy towards enhanced biogas production by co-digesting municipal sewage sludge (SS) with brewery spent grain (BSG). To execute this, physicochemical and metagenomics analysis was conducted on the sewage sludge substrate. The automatic methane potential test system II (AMPTS II) biochemical methane potential (BMP) batch setup was operated at 35 ± 5 °C, pH range of 6.5–7.5 for 30 days’ digestion time on AMPTS II and 150 days on semi-continuous setup, where the organic loading rate (OLR) was guided by pH and the volatile fatty acids to total alkalinity (VFA/TA) ratio. Metagenomics analysis revealed that Proteobacteria was the most abundant phyla, consisting of hydrolytic and fermentative bacteria. The archaea community of hydrogenotrophic methanogen genus was enriched by methanogens. The highest BMP was obtained with co-digestion of SS and BSG, and 9.65 g/kg of VS. This not only increased biogas production by 104% but also accelerated the biodegradation of organic matters. However, a significant reduction in the biogas yield, from 10.23 NL/day to 2.02 NL/day, was observed in a semi-continuous process. As such, it can be concluded that different species in different types of sludge can synergistically enhance the production of biogas. However, the operating conditions should be optimized and monitored at all times. The anaerobic co-digestion of SS and BSG might be considered as a cost-effective solution that could contribute to the energy self-efficiency of wastewater treatment works (WWTWs) and sustainable waste management. It is recommended to upscale co-digestion of the feed for the pilot biogas plant. This will also go a long way in curtailing and minimizing the impacts of sludge disposal in the environment.

Herein, the alkaline supernatant of a struvite recovery system from municipal wastewater was successfully co-managed with acid mine drainage (AMD). Various ratios (v/v) of AMD to struvite supernatant were examined, and the quality of the passively co-treated effluent and of the generated sludge were examined using state-of-the-art analytical techniques including ICP-OES, FE-SEM/FIB/EDX, XRD, XRF, and FTIR. The optimum ratio was 1:9, where metals and sulphate were largely removed from AMD, i.e., from higher to lower score Fe (~100%) ≥ Pb (~100%) ≥ Ni (99.6%) ≥ Cu (96%) ≥ As (95%) ≥ Al (93.7%) ≥ Zn (92.7%) > Ca (90.5%) > Mn (90%) ≥ Cr (90%) > sulphate (88%) > Mg (85.7%), thus implying that opportunities for mineral recovery could be pursued. The pH of the final effluent was regulated to acceptable discharge levels, i.e., 6.5 instead of 2.2 (AMD) and 10.5 (struvite supernatant), while a notable reduction in the electrical conductivity further implied the attenuation of contaminants. Overall, results suggest the feasibility of the passive co-treatment of these wastewater matrices and that opportunities for direct scaling up exist (e.g., using waste stabilization ponds). Furthermore, apart from the initial recovery of struvite from municipal wastewater, metals could also be recovered from AMD and water could be reclaimed, therefore introducing circular economy and zero liquid discharge in wastewater treatment and management.

The concept of circular economy in wastewater treatment has recently attracted immense interest and this is primarily fueled by the ever-growing interest to minimise ecological footprints of mining activities and metallurgical processes. In light of that, countries such as the Republic of South Africa, China, Australia, and the United States are at the forefront of water pollution due to the generation of notorious acid mine drainage (AMD). The disposal of AMD to different receiving environments constitutes a severe threat to the receiving ecosystem thus calling for prudent intervention to redress the prevailing challenges. Recent research emphasises the employment of wastewater treatment, beneficiation and valorisation. Herein, the techno-economic evaluation of the reclamation of clean water and valuable minerals from AMD using the Magnesite Softening and Reverse Osmosis (MASRO) process was reported. The total capital expenditure (CAPEX) for the plant is ZAR 452,000 (USD 31,103.22) which includes ZAR 110,000 (USD 7569.37) for civil works on a plant area of 100 m2. The operational expenditure (OPEX) for the pilot is 16,550,000 ZAR (South African Rand) or USD 1,138,845.72 in present value terms (10 years plant life). The plant reclaimed drinking water as specified in different water quality standards, guidelines, and specifications, including Fe-based minerals (goethite, magnetite, and hematite), Mg-gypsum, and calcium carbonate. These minerals were verified using state-of-the-art analytical equipment. The recovered valuables will be sold at ZAR 368/kL (USD 25.32), ZAR 1100/t (USD 75.69), and ZAR 2000/t (USD 137.62) for water, gypsum, and limestone, respectively. The project has an NPV of ZAR 60,000 (USD 4128.75) at an IRR of 26%. The payback period for this investment will take 3 years. The total power consumption per day was recorded to be 146.6 kWh, and 103,288 kWh/annum. In conclusion, findings of this work will significantly contribute to improving the sustainability of the mining sector by proposing economically feasible solutions for wastewater streams treatment, beneficiation, and valorisation.

This study describes the AMD impacted environments and critically discusses the effects of AMD, current prediction and prevention methods and treatment technologies. The study further, crit-ically analyses the case studies, gaps in AMD research and the challenges and opportunities of-fered by AMD. The study outlined future technological intervention aiming at a paradigm shift towards reducing the volume of sludge generated as well as the operating cost and improving AMD treatment efficiency. The integrated AMD treatment technologies that result in a holistic approach toward sustainable AMD treatment are a current need. As a result, a sustainable AMD treatment strategy has been made possible due to water reuse and rich resource recoveries such sulphuric acid, rare earth elements, and metals. The cost of AMD treatment can be decreased with the use of recovered water and resources, which is essential for developing a sustainable AMD treatment process.