6.5 Nonpotable Sources, Water Quality, and Leak Detection

Match Water Source to Use

After demand reduction, LEED's second lever is substituting nonpotable water for potable demand. Nonpotable water is any water not meeting drinking-water standards but suitable, after appropriate treatment, for uses like irrigation, toilet flushing, and cooling tower makeup. Knowing the recognized sources and what each is and is not safe for is the heart of this section.

Recognized Nonpotable Sources

Graywater is the term the exam tests most. It is wastewater from lavatories, showers, and clothes washers that has not contacted toilet waste or, in many codes, kitchen sink waste. Blackwater - toilet and often kitchen wastewater - is not graywater and needs more intensive treatment before reuse. Confusing the two is a classic distractor.

Water Quality Governs the Match

The defensible LEED answer always matches source quality to end use. An option that says "reuse any available nonpotable water for any purpose" is too broad and wrong. Higher-contact uses (toilet flushing inside an occupied space, cooling makeup) demand more treatment, filtration, and disinfection than subsurface drip irrigation. Local plumbing codes, cross-connection control (preventing nonpotable water from entering potable lines), and dual-piping (purple pipe) requirements all constrain what is allowed. Storage and treatment add cost, so demand reduction usually remains the cheaper first move.

Worked Scenario

A campus considers piping captured rainwater to building toilets. The team must confirm: adequate roof catchment and cistern storage for dry periods, filtration and disinfection to a quality safe for indoor flushing, code-permitted dual plumbing with backflow prevention, and clear marking of nonpotable lines. If any link fails, subsurface irrigation - a lower-quality-tolerant use - may be the appropriate destination instead. The source did not change; the match to use did.

Leak Detection and Operations

Nonpotable substitution and demand reduction can be undone by leaks. Leak detection finds water that is consumed without serving its function. It is an operations strategy supported by metering trends, submeter alarms, periodic inspection, and prompt repair. In LEED for Operations and Maintenance (O+M) rating systems, ongoing water tracking and leak response are part of maintaining certified performance. Detection by itself does not save water; it prompts the repair that does.

Cross-Connection Control and Dual Plumbing

Whenever nonpotable water enters a building, the dominant safety concern is preventing it from contaminating the potable supply. Cross-connection control uses backflow preventers and physical separation so nonpotable water can never siphon back into drinking-water lines. Nonpotable distribution typically runs in clearly marked purple pipe (the industry color for reclaimed water) kept entirely separate from potable piping. The exam may test that reclaimed and rainwater systems require dual plumbing and labeling; a single shared pipe network is never acceptable.

These code requirements, not LEED itself, often decide whether a substitution strategy is feasible.

Rainwater Sizing and Storage

A rainwater system is only as useful as its catchment and storage. The capturable volume depends on roof area, local rainfall, and a runoff coefficient; storage in a cistern must bridge dry periods between rain events. A common design failure is undersizing storage so the cistern empties during a drought just when irrigation demand peaks, forcing a fallback to potable water. LEED scenarios may describe a project that captures rainwater but lacks adequate storage - recognize that the gap is a sizing and reliability problem, not a source-quality problem.

Common Traps

• Graywater includes shower and laundry water; it excludes toilet (blackwater) flow.
• A nonpotable source is never "automatically appropriate" - quality, treatment, storage, and code must align.
• Substitution does not replace demand reduction; LEED still expects efficient fixtures and landscaping first.
• Leak detection is operations, not design - it preserves savings rather than creating new reduction.
• HVAC condensate and foundation dewatering are often-overlooked free nonpotable sources well suited to cooling makeup or irrigation.
• Municipal reclaimed water (purple pipe from the utility) may be the simplest substitution where the utility offers it, requiring no on-site treatment.

The defensible answer on any source question matches the water's quality to the demands of the end use, confirms code-compliant separation, and remembers that substitution sits second in the hierarchy behind demand reduction.

Putting the Sources in Order

When several nonpotable sources are available, prefer the one that needs the least treatment for the intended use, because treatment adds cost and energy. Municipal reclaimed water arrives pre-treated and is often the simplest choice where a purple-pipe network exists. HVAC condensate and foundation dewatering are essentially free byproducts well suited to irrigation or cooling makeup. Rainwater is widely available but needs filtration and storage. Graywater requires the most on-site treatment of the common sources and is constrained by code.

Whatever the source, the exam answer holds firm on one rule: match the water quality to the demand of the end use, and never assume a source is automatically appropriate.