Circular Economy in Environmental Engineering: Material Flow Analysis, Closed-Loop Recovery, and Industrial Symbiosis
The Circular Economy (CE) framework in environmental engineering replaces traditional linear "take-make-dispose" models with closed-loop systems that prioritize resource recovery, material regeneration, and waste elimination. By integrating mass-balance accounting, industrial symbiosis, and eco-efficiency metrics, civil infrastructure is transformed into a regenerative system where solid, liquid, and energetic waste streams serve as secondary raw materials. Material flow and systemic resource efficiency across a circular economy node are quantified using Material Flow Analysis (MFA) based on the law of conservation of mass. For a steady-state system boundary, the total material input equals total output plus internal accumulation ($\Delta S$): $$\sum_{i=1}^{m} \dot{M}_{\text{input}, i} = \sum_{j=1}^{n} \dot{M}_{\text{output}, j} + \frac{d S}{dt}$$ Where $\dot{M}_{\text{input}}$ represents primary raw material streams, $\dot{M}_{\text{output}}$ represents products, emi...