Greywater Reuse Estimator — Possible Water & Cost Savings

How This Greywater Estimator Works

• Recoverable volume: Shower + bathroom tap (per person) + laundry (per household), adjusted for capture efficiency and treatment losses.
• Storage smoothing: A simple tank model caps storage and bleeds a % daily to mimic hygiene flushes.
• Reuse allocation: Volume is allocated to toilets and/or irrigation per your priority or 50/50 split.
• Savings: Mains volume saved, bill savings using your tariffs, optional sewer credit, and optional CO₂ savings for water supply/WWTP.
• Economics: Simple payback = CAPEX ÷ (annual bill savings − O&M).

Limitations

• Not a design tool. Real systems need code compliance, backflow protection, and treatment sized to peak flows.
• Kitchen effluent and blackwater are excluded; many regions disallow kitchen sinks for greywater reuse.

Greywater Basics: How Reuse Saves Water, Money, and Energy

Greywater is lightly used water from showers, baths, bathroom sinks, and laundry. Unlike blackwater (toilets) and often kitchen wastewater (grease and food solids), greywater can sometimes be reused for non-potable purposes such as toilet flushing or landscape irrigation. The Greywater Reuse Estimator helps you understand your recoverable volume, how storage affects availability, and what that means for mains water savings, bill reductions, and optional CO₂ benefits from lower water supply and wastewater treatment.

Where the Water Comes From

• Showers & baths: Often the largest and most regular source. Flow rate × time × people gives a reliable baseline.
• Bathroom sinks: Small but steady contributions from handwashing and brushing teeth.
• Laundry: Highly variable by machine efficiency and load size. Many systems only capture a fraction of cycles.

Kitchen sinks and dishwashers are commonly excluded by code due to fats, oils, and food residues.

From Source to Use: The Journey

1. Diversion & capture: A portion of greywater is diverted to the system. We model this with a capture efficiency to reflect plumbing losses and downtime.
2. Treatment & quality losses: Filtration, settlement, or disinfection may require periodic backwash or cartridge flushing. The estimator applies a percentage loss to represent these processes.
3. Storage: Tanks increase reuse by buffering day-to-day variations, but hygiene matters. The model includes a simple capacity limit and an optional daily “bleed” to mimic refresh cycles and residence-time limits.
4. Allocation to demands: Greywater is assigned to toilet flushing and/or irrigation based on your priority or a split rule. Toilet reuse typically returns to sewer; irrigation usually infiltrates or evaporates.

Benefits You Can See

• Mains water saved: Lower incoming water volumes reduce consumption charges.
• Sewer credits (where applicable): If irrigated water bypasses the sewer, some utilities reduce wastewater fees.
• Cost savings: Water + sewer tariffs determine the annual bill reduction. Subtract estimated O&M to see net impact.
• Simple payback: CAPEX ÷ (annual savings − O&M) gives a quick, high-level payback estimate.
• CO₂ footprint: Fewer cubic meters through supply and treatment can lower indirect emissions.

Good Practices & Practical Tips

• Keep greywater lines separate from potable lines and follow cross-connection control requirements.
• Match tank size to peak shower periods rather than just averages; more storage isn’t always better if turnover slows.
• Use low-sodium, low-boron detergents if irrigating plants; check plant tolerance and soil salinity over time.
• Design for maintenance: easy filter access, schedules for cleaning, and clear indicators for users.
• Expect seasonal swings: irrigation demand rises in warm months; bathroom sources are fairly steady year-round.

Important Limitations

This estimator is an awareness tool and not a design or permitting guide. Real systems must meet local plumbing and health codes, include backflow protection, and be sized for peak flows and real fixture performance. Kitchen wastewater and toilets are excluded. CO₂ factors and tariffs vary by region; adjust inputs to mirror your utility bills and local data.

Glossary

• Capture efficiency: Percentage of greywater successfully diverted into the system.
• Treatment loss: Water used for filter backwash or media flushing, not available for reuse.
• Residence time: How long water remains in storage; shorter is generally better for hygiene.
• Sewer credit: A billing reduction when reused water does not return to the wastewater system.

Tip: Save a shareable link after tuning assumptions (e.g., different tank sizes, capture efficiencies, or tariffs) to compare scenarios with stakeholders.