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Carbon-based conductive additives have been studied for their positive effects on anaerobic digestion (AD) using synthetic substrates, but their importance in wastewater sludge digestion has not been sufficiently explored. This research investigated and compared the effects of two conductive materials (graphene and petroleum coke) with and without trace metal supplementation. The results indicated that supplementing reactors with graphene and petroleum coke could significantly improve biogas production. The supplementation of 1 g/L petroleum coke and 2 g/L graphene, without trace metal addition, led to an increase in the biogas production by 19.10 ± 1.04% and 16.97 ± 5.00%, respectively. Thus, it can be concluded that petroleum coke, which is an oil refinery by-product, can be used to enhance biogas production in a similar way to other carbon-based conductive materials that are currently available on the market. Moreover, using petroleum coke and graphene, the average chemical oxygen demand (COD) removal was 42.84 ± 1.23% and 42.80 ± 0.45%, respectively, without the addition of trace metals. On the other hand, supplementation of the reactors with trace elements resulted in a COD removal of 34.65 ± 0.43% and 34.05 ± 0.45% using petroleum coke and graphene, respectively.

Phosphate removal is an important measure to control eutrophication in aquatic environments, as it inhibits algal bloom. Salinity exists in these media along with high phosphate and currently available phosphate removal methods function poorly under this condition. In this study, the main objective is to fabricate a nanocomposite to improve and accelerate phosphate removal from saline solutions. To achieve this goal, Fe3O4/ZnO and a novel nanoadsorbent, Fe3O4/ZnO/CuO, were synthesized. Their characteristics were determined using FE-SEM, EDX, FT-IR, and XRD analyses, and their capability to adsorb phosphate from saline solutions was investigated and compared. The overall results suggest that the trimetallic oxide nanocomposite has a great potential for the efficient removal of phosphate, in comparison with Fe3O4/ZnO. Experiments showed that Fe3O4/ZnO/CuO exhibited a remarkable sorption capacity of 156.35 mg P/g, fast sorption kinetic, strong selectivity for phosphate even in the presence of a high concentration of salinity (60 mg/L), and a wide applicable pH range of 3–6. Furthermore, using Fe3O4/ZnO/CuO, even a low dosage of 0.1 g/L was sufficiently enough to reach an adsorption efficiency of 96.13% within 15 min compared to Fe3O4/ZnO (80.47% within 30 min). Moreover, the pseudo-second-order kinetic model best described the experimental adsorption data for both nanocomposites.