Question 4 of 5: Streamlined Comparative LCA – Municipal Tap Water vs. Disposable Bottled Water

Question 4: Streamlined Comparative LCA – Municipal Tap Water vs. Disposable Bottled Water (24 marks)

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Topic (iii) is selected: municipal tap water vs. single-use disposable bottled water, in an urban Canadian setting.

Topic description, system boundary and functional unit

Both systems deliver the identical service — potable drinking water to an urban consumer — so a direct comparison is well posed. The comparison is drawn cradle-to-grave for both systems, and the functional unit is chosen as "delivering 1000 litres of drinking water to an urban Canadian consumer," not "one bottle" or "one litre from the tap," since the two systems have radically different packaging, treatment, and distribution infrastructure per litre delivered and a naive per-container comparison would obscure the scale mismatch entirely. The targeted outcome is a relative comparison of embodied energy/GHG (production, treatment, and transport), water consumption (net, beyond the water itself), and end-of-life packaging fate, since these are the categories in which the two systems differ most sharply.

System inventory, impact factors and engineering judgment

For the municipal tap-water system, the dominant inputs are water treatment (coagulation/filtration/disinfection at a centralized municipal plant, a comparatively modest per-litre energy draw for most Canadian surface-water sources) and distribution pumping energy through the existing municipal pipe network; there is no packaging manufacture, no bottle-filling line, and no long-haul transportation burden, since the water is delivered essentially to the point of use through infrastructure that is already in place and shared across the entire municipal population. The principal output/waste stream is minimal: pipe-network losses (Canadian municipal systems typically lose some fraction of treated water to leakage, an inventory item worth flagging even though it is outside the functional unit's direct control) and the periodic chemical/energy burden of the treatment plant itself.

For the bottled-water system, the inventory is dominated by stages the tap-water system simply does not have: PET bottle manufacture (petrochemical feedstock extraction, polymerization, and blow-moulding, each carrying its own embodied energy and GHG burden per bottle, on top of whatever source-water treatment the bottler applies — minimal for some spring waters, but often more energy-intensive than municipal treatment where reverse osmosis or ozonation is used for purified water); packaging (shrink-wrap, cardboard trays/cases, secondary plastic); and transportation, frequently over far longer distances than municipal distribution (inter-provincial or cross-border trucking is common for major bottled-water brands sold in Canadian urban centres), adding a diesel-combustion GHG and criteria-pollutant burden with no equivalent in the tap-water system. Refrigeration at retail (a use-phase energy input largely absent from the tap-water comparison, since tap water is typically consumed without dedicated retail cold-chain energy) is a further burden specific to the bottled system. End-of-life is the bottled system's clearest weak point: even with Canada's relatively strong PET recycling infrastructure relative to other plastics, actual bottle-to-bottle recycling rates remain well below 100%, and a material fraction is landfilled or, in the worst case, becomes environmental litter with associated persistence and marine/wildlife-ecotoxicity impacts far outside anything the tap-water system generates.

Impact-category assignment: global warming potential (dominated by PET production and transportation energy for the bottled system; dominated by treatment/pumping electricity, itself grid-mix-dependent, for the tap system), fossil resource depletion (petrochemical feedstock for PET vs. negligible for tap infrastructure that is already sunk-cost/shared), solid-waste/persistence (bottle manufacture and imperfect recycling vs. essentially none for tap water), and water consumption in the narrower sense of process/cooling water beyond the delivered litre itself (bottling and PET production both consume process water in addition to the water eventually sold). Engineering judgment required: the comparison is sensitive to the assumed local electricity grid mix for both systems' treatment/pumping/manufacturing energy (a low-carbon grid, common in much of Canada, narrows the GHG gap for both, but does not close the packaging and transportation gap that is specific to the bottled system), the assumed transportation distance (a locally-bottled regional brand narrows the gap far more than a nationally-distributed one), and the assumed end-of-life recycling rate actually achieved in the specific municipality being studied rather than a national average.

Impartial assessment

Published comparative LCAs of bottled vs. tap water consistently find that bottled water's life-cycle burden per litre delivered is substantially higher than municipal tap water's across nearly every impact category studied — typically by one to three orders of magnitude for embodied energy and GHG emissions — driven primarily by PET production and transportation rather than by the water treatment step itself, which is often broadly comparable between the two systems. This is a case where the engineering conclusion is directionally robust rather than genuinely close: unlike the plastic-bag vs. cotton-tote comparison of Question 1(a), where realistic assumptions can plausibly reverse the ranking, no combination of defensible Canadian grid-mix, transportation-distance, or recycling-rate assumptions is likely to make bottled water's life-cycle burden fall below tap water's for the identical functional unit. The legitimate remaining engineering judgment is therefore not "which system wins" but rather quantifying how much higher the bottled-water burden is under a given municipality's specific circumstances (local grid mix, typical bottled-brand sourcing distance, and actual achieved recycling rate), and identifying the specific use cases (e.g. genuine water-quality or emergency-supply concerns) where bottled water's higher life-cycle burden might nonetheless be judged an acceptable trade-off against a non-environmental requirement the tap system cannot meet.