12.3 Base Case Water Use by Building Type & Matching Water Quality to Use
Key Takeaways
- EDGE estimates water use from the number of fixtures and from water usage loads — occupancy, usage rates and fixture flow rates — with base case patterns differing by building type.
- Recycled water and rainwater harvested on site are deducted from the improved case and rendered as water savings, exactly as on-site renewables are treated in the energy category.
- For greywater EDGE assumes all wastewater from kitchens and bathrooms is collected to meet toilet flushing demand, deducting only what is actually available if supply is insufficient.
- For blackwater EDGE assumes 80% of wastewater from flushing toilets is collected, treated and stored for future flushing or other outdoor uses.
- EDGE does not calculate water use for external activities such as car washing, and there is no international standard for building water calculation — the approach parallels the UK Water Efficiency Calculator for New Dwellings.
12.3 Base Case Water Use by Building Type & Matching Water Quality to Use
Exam Focus: Sub-topics 4.3.3 "Base case water use in each building type" and 4.3.4 "Water sources available, and the use of appropriate quality of water (mains-supplied, recovered, recycled, etc.)" are both named explicitly in the official outline and are commonly under-prepared.
How EDGE Builds Water Demand
The Methodology Report is candid that water is the simplest of the three categories to model and that no international standard governs it. The EDGE approach is described as similar to other calculators used around the world, such as the UK government's Water Efficiency Calculator for New Dwellings.
EDGE estimates annual water use through:
- the number of water fixtures — showers, taps, toilets and so on, and
- water usage loads — occupancy, usage rates, and the rate of water flow through the fixtures.
EDGE estimates fresh water use to determine overall consumption. Recycled water or rainwater harvested on site is then deducted from the improved case and rendered as water "savings" — structurally identical to how on-site renewable generation is handled in the energy category.
Why base case water differs by building type
The three inputs above are all typology-dependent:
| Building type | What drives base case water demand |
|---|---|
| Homes / Apartments | Residents present morning and evening; showers, kitchen taps, laundry, WC. Income category sets assumed occupancy |
| Hotels / Resorts | Guest showers and baths, laundry, kitchens, pools, landscape — very high per-occupant demand, scaled by star rating |
| Offices | Short occupancy hours, WC and hand basin dominated, low per-occupant demand, no showers in the base case |
| Retail | Large floor area, low occupant density, customer WC demand, some food service |
| Hospitals | 24-hour occupancy, clinical hand hygiene, laundry, sterilisation — the highest intensity of the common types |
| Education | Concentrated occupancy for part of the day, term-time only, WC dominated |
The practical consequence: a water measure worth 12 percentage points in a hotel may be worth 3 in a warehouse. Prioritise measures against the end uses that actually dominate that typology's base case. Fitting premium low-flow showerheads to an office that has no showers achieves nothing.
EDGE v3 has made these end uses more explicit — a Water End Uses field was added to the Area and Loads Breakdown section of the Design Tab, a Bath Tub field was introduced for Homes and Apartments, and the swimming pool type list was extended to cover indoor and outdoor heated pools.
The Recovery Assumptions EDGE Applies
These three published assumptions are precise and examinable.
Rainwater harvesting
EDGE calculates the maximum quantity collectable using rainfall data for the location and the roof area from the design inputs:
Total annual rainwater = Catchment area (m²) × Rainfall depth (mm) × Filter coefficient (assuming 20% losses) × Runoff coefficient
Note there are two reduction factors, not one. The filter coefficient assumes 20% losses through filtration and first-flush diversion, and the runoff coefficient separately accounts for the roof surface. Candidates who apply only the runoff coefficient overestimate yield substantially.
Greywater recycling
EDGE calculates the potential supply and reduces the demand for flushing toilets by that amount. EDGE assumes that all wastewater from kitchens and bathrooms is collected and stored to meet the demand for flushing toilets. If the quantity of wastewater is insufficient, then EDGE simply deducts the wastewater available from the total demand.
So greywater is credited against toilet flushing demand, and the credit is capped by whichever is smaller — the greywater available or the flushing demand. Oversizing a greywater plant beyond flushing demand yields no additional EDGE credit.
Blackwater treatment
EDGE calculates the potential supply and reduces the demand for flushing toilets by that amount. EDGE assumes that most of the wastewater (80%) from flushing toilets is collected, treated and stored to meet the demand for future flushing or other outdoor uses.
The 80% figure is the number to remember, and the distinction from greywater is the source: greywater comes from kitchens and bathrooms, blackwater from toilet flushing itself.
What EDGE Deliberately Excludes
EDGE does not calculate water use for external activities such as car washing. This matters when a client asks to claim a water-efficient car wash facility: it is good practice, and it is outside the EDGE water boundary. Setting that expectation early avoids an awkward conversation at Design Audit.
Matching Water Quality to Use
Sub-topic 4.3.4 asks about the appropriate quality of water for each use. The governing principle is that potable water is an expensively treated product and should be reserved for uses that require potability.
| Water source | Typical quality | Appropriate uses |
|---|---|---|
| Mains-supplied potable | Drinking quality | Drinking, cooking, hand basins, showers, food preparation |
| Harvested rainwater | Clean but not potable without treatment | Toilet flushing, irrigation, cooling tower make-up, general washdown |
| Recovered / treated greywater | Non-potable, treated | Toilet flushing, sub-surface irrigation |
| Treated blackwater | Non-potable, more heavily treated | Toilet flushing, restricted outdoor uses |
The engineering and regulatory constraints
- Dual plumbing is mandatory. Non-potable supply must run in a physically separate, clearly identified pipework system with no cross-connection to the potable network. Mis-labelled or cross-connected pipework is a public health failure, not a documentation defect.
- Local health regulation governs permissible uses. Some jurisdictions prohibit greywater reuse for surface irrigation of edible crops or for uses with human contact. Confirm the local rule before modelling the measure — this is exactly the country-specific requirement discussed in Section 11.3.
- Match treatment level to end use. Sub-surface drip irrigation tolerates a lower treatment standard than unrestricted surface irrigation.
The Expert's sequencing rule
Work in this order: first reduce demand at the fixture, then match quality to use by substituting non-potable sources for non-potable demands, and only then consider treatment-intensive recovery. Reducing demand is cheaper, has no ongoing operational cost, and cannot fail an audit because a treatment plant was not commissioned.
What proportion of wastewater from flushing toilets does EDGE assume is collected, treated and stored when blackwater treatment is claimed?
A team designs a greywater system with treated capacity well in excess of the building’s toilet flushing demand. What is the EDGE outcome?
Which two reduction factors does EDGE apply when calculating annual harvestable rainwater?
A client wants to claim a new water-efficient car washing facility in the project’s EDGE water savings. What should the Expert advise?