Grease, oil, hydrocarbon residues, heavy metals, and surfactants are all present in car wash wastewater (CWW), which all can have detrimental effects on the environment and human health. This study was designed to assess CWW treatment using an upgraded physical technique combined with a range of conventional and more sustainable coagulants. Physical treatment effectively lowered the oil and grease (O&G) and chemical oxygen demand (COD) of the CWW by 79 ± 15% and 97 ± 1.6%, respectively. Additional treatment was provided using chemical coagulation–flocculation–settling. In jar test studies, humic acid (HA) and alum were found to provide significantly higher turbidity removal, 79.2 ± 3.1% and 69.8 ± 8.0%, respectively, than anionic polyacrylamide (APA), 7.9 ± 5.6% under influent turbidity values from 89 to 1000 NTU. Overall physical/chemical treatment of CWW yielded 97.3 ± 0.8% COD removal, and 99.2 ± 0.4% O&G removal using HA and alum. Due to the numerous problems created when using synthetic coagulants, naturally occurring coagulants that have no impact on human health, such as HA, are highly desirable options. The findings of this study show that treating CWW provides several advantages for sustainable development, health and well-being, and raising public knowledge and support for water reuse.

Nanofluid, the fluid suspensions of a metallic nanoparticle, became a coolant fluid that is used when a promising enhancement in heat transfer is required. In the current study, the characteristics of fluid flow and heat transfer are numerically investigated using different nanofluids (Al₂O₃–H₂O, TiO₂–H₂O, and SiO₂–H₂O) and different micro-channel heat sink (MCHS) configurations (rectangular, triangular, trapezoidal, and circular). In this numerical investigation, the effect of Re number ranged from 890 to 1500, and the effect of nanoparticle concentration ranged from 1% to 7% at constant heat flux q = 10⁶ W/m², and constant fluid inlet temperature of 288 K, were studied. The average heat transfer coefficient, h, and pressure drop, $\Delta p$, are used to quantify the fluid flow and heat transfer characteristics in each MCHS configuration and for each nanoparticle concentration. It is revealed that a better heat transfer coefficient is obtained for Al₂O–H₂O compared with other types of nanoparticles and pure water, such as 8.58% heat transfer coefficient improvement obtained at Re = 1500 and $\mathsf{\phi }=7\%$ more than that of pure water. It is also inferred that the maximum heat transfer coefficient is obtained by the triangular MCHS; however, it has the highest pressure drop because of the lowest hydraulic diameter.