Question 2 of 5: Green Engineering, Mass Balance, LCA and the P2 Hierarchy

Question 2: Green Engineering, Mass Balance, LCA and the P2 Hierarchy (25 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Embedded Entropy and Complexity as an Investment

Complex, highly ordered materials embody large processing energy, so preserve them through reuse or remanufacture—e.g. remanufacturing a complex electronic module rather than shredding it to base metals—preventing the pollution of making it anew.

(b) Integration with Available Energy and Material Flows

Connect a process to existing energy/material streams—e.g. cogeneration using one fuel input for both electricity and useful heat, or industrial symbiosis routing one plant's waste heat or by-product as another's feedstock—turning waste into input and avoiding both disposal and virgin-resource use.

(c) Design for Commercial "Afterlife"

Design the product so it retains value after its first use—e.g. a modular, easily-refurbished photocopier or engine designed for remanufacture and resale—so it has a productive second life instead of becoming waste, preventing the resource use of making everything new.

(d) Aluminum Mass Balance

Current (100 − 91 = 9 t/yr pollution):

 100 t/yr Al ──►[ FACTORY ]──► 91 t/yr in products
                     └─────────► 9 t/yr pollution (stack PM, turnings, wastewater)

With pollution prevention (pollution halved to 4.5 t/yr, product held at 91, so input = 95.5 t/yr):

 95.5 t/yr Al ─►[ FACTORY (P2) ]──► 91 t/yr in products
                     └─────────────► 4.5 t/yr pollution

Preventing pollution lets the factory buy 4.5 t/yr less aluminum for the same output—cutting waste and material cost together.

(e) LCA: Plastic versus Metal Straws

(i) Functional unit: providing straws for serving drinks over a defined period (e.g. all straws a customer uses in a year)—so the many disposable plastic straws are compared fairly with the one reused metal straw. (ii) Four stages: raw-material extraction; manufacturing; distribution and use (for the metal straw, repeated washing); end-of-life. (iii) Stage of greatest impact: for the plastic straw, the recurring raw-material/manufacturing stage (a new straw every drink) plus end-of-life plastic waste/litter; for the metal straw, its energy-intensive one-time manufacturing (mining and forming metal) and the use-phase washing (water and energy) repeated over its life. The metal straw is greener only if reused enough times to amortize its high manufacturing burden—if discarded after few uses, it can be worse than the plastic it replaced.

(f) Ranking the Pollution-Prevention Measures

By desirability (most to least preferred), following the pollution-prevention hierarchy of reduce → reuse/recycle → treat → dispose:

1. Eliminating the need for the toxic material in the process (source reduction—best).
2. Substituting a less-toxic material (source reduction by substitution).
3. Capturing and recycling within the factory (in-process reuse/recycling).
4. Capturing and sending to an off-site recycling facility.
5. Collecting and destroying the material by incineration/chemical reaction (treatment).
6. Capturing, containing, and disposing of the material (disposal—least preferred).

The ranking prefers preventing the waste at source (eliminate, substitute) over managing it (recycle on-site before off-site), and treatment over mere disposal—the classic pollution-prevention/waste hierarchy.