This is an academic problem that I need confirmation for. It’s a LEED-NC 2009 project.
• 242 FTEs. No visitors, etc. 50:50 gender ratio = 121 of each.
• All urinals are waterless.
• All water closet (WC) flush valves are dual flush (1.6-gpf | 1.1-gpf).
• 11 of the 40 total WCs are plumbed with reclaimed industrial rinse water. 11/40 = 72.5% potable & 27.5% (or 0.275 integer) non-potable. Of these 11 reclaimed WCs, 7 are in a women’s bathroom, the other 4 in a men’s.
When we populated the WE Pre-1 Form in LEED Online we used 351 Daily Uses (72.5%) for the Baseline (1.6-gpf) and Installed (1.35-gpf) rates for potable water WCs. On a second row we entered 133 Daily Uses (27.5%) for the reclaimed-source WCs. We entered 242 Daily Uses for urinals (1.0-gpf and 0.0-gpf). This yielded a 53.78% reduction for flush fixtures.
However, the WEp1 Form does not state how many gallons/year of reclaimed water are generated and this value is needed to populate the WEc2 form. We figured there are 484 WC flushing events/day as follows: (6 X 121 FTE) – (2 X 121FTE urinals flushing) = 484 WC flushes/day. Our weighted daily flush rate = [Sum of (1.35-gpf X 29) + (0.0-gpf X 11)] = 0.99-gpf. Then, 484 flushes/day X 0.99-gpf X 0.275 X 250 days/year = 32,942.25-gal./year reclaimed water. This is the volume we entered in the WEc2 LEED Online form for On-Site treated Wastewater, yielding a 66.35% reduction. So far, so good.
However, out of curiosity I did a separate calculation using the same fixtures, daily uses and rates, assuming that 35% (7/20 WCs) of all female WC flushes are reclaimed and male WC flushes 20% reclaimed (4/20 WCs). For the Design case flush rates this yielded a total reduction of 49,913 gal/year of reclaimed water = 73% reduction. If I used the Baseline flush rates this yielded 60,500 gal/year reduction (77%).
We earn the credit with room to spare. But, inquiring minds want to know: which reduction calculation should I use to match a GBCI-accepted methodology?