5.2 Rainwater Harvesting & Greywater Recycling Systems

5.2 Rainwater Harvesting & Greywater Recycling Systems

Exam Focus: Mastering rainwater yield calculations, catchment runoff coefficients, greywater vs. blackwater source differentiation, dual-pipe hydraulic design, and storage tank sizing logic is vital for the EDGE Expert Exam.

While installing low-flow indoor fixtures reduces overall water demand, achieving advanced water performance—such as 40% to 50%+ operational water savings—frequently requires substitution of potable municipal water with alternative non-potable water sources. Rainwater Harvesting (RWH) and Greywater Recycling Systems (GWRS) are two primary strategies evaluated within the EDGE software to meet non-potable end-use demands such as toilet flushing, urinal flushing, cooling tower makeup, and outdoor irrigation.

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Rainwater Harvesting Principles & Catchment Mechanics

Rainwater harvesting involves collecting, filtering, storing, and distributing precipitation runoff from building roofs and impervious hardscape surfaces.

Roof Surface Runoff Coefficients ($C$)

The volume of rainwater collected depends heavily on the surface material of the catchment area. Different roofing materials absorb or retain varying amounts of rainfall.

First-Flush Diversion & Filtration

Initial rainfall washes accumulated airborne dust, bird droppings, foliage debris, and heavy metals off the catchment surface. To protect storage tanks and filtration systems, a first-flush diverter automatically isolates and discards the first 1.0 to 2.0 mm of rainfall before directing clean water to the main storage vessel. Typical mechanical filter efficiency ($\eta$) ranges between 0.85 and 0.90 (85% to 90%).

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Rainwater Harvesting Calculation Equations

1. Annual Harvestable Rainwater Yield Equation

$\text{Annual Yield } (Y \text{ in Liters}) = P \times A \times C \times \eta$

$\text{Annual Yield } (Y \text{ in } m^3) = \frac{P \times A \times C \times \eta}{1,000}$

Where:

• $P$ = Annual precipitation (mm/year)
• $A$ = Roof catchment footprint area ($m^2$)
• $C$ = Runoff coefficient (dimensionless, 0.0 to 1.0)
• $\eta$ = First-flush and filter efficiency factor (dimensionless, typically 0.85)

2. Rainwater Tank Sizing Rules of Thumb

In EDGE modeling and sustainable plumbing design, storage tanks are sized using two standard engineering methods:

1. Percent Yield Method: Size storage tank capacity to hold 5% of total annual harvestable yield ($V_{tank} = 0.05 \times Y_{annual}$).
2. Dry-Season Security Method: Size storage capacity to supply 14 to 21 days of peak dry-season non-potable demand ($V_{tank} = \text{Daily Non-Potable Demand} \times 14 \text{ to } 21 \text{ days}$).

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Greywater Recycling Technologies & Source Segregation

Greywater refers to untreated wastewater generated from domestic personal hygiene and laundry activities. It contains significantly lower organic loads and pathogen concentrations than blackwater.

Source Stream Categorization

Greywater Treatment Process Stages

Modern greywater recycling plants utilize multi-stage treatment to produce clear, odor-free non-potable water compliant with international reuse standards (e.g., BOD $< 10 \text{ mg/L}$, TSS $< 10 \text{ mg/L}$, Turbidity $< 2 \text{ NTU}$, E. coli $< 10 \text{ CFU}/100 \text{ mL}$):

1. Pre-Screening & Equalization: Coarse lint strainers and sedimentation tanks remove hair, lint, and gross particulates while balancing peak morning flow surges.
2. Biological Oxidation (MBR / MBBR): Membrane Bioreactors (MBR) or Moving Bed Biofilm Reactors (MBBR) utilize beneficial aerobic bacteria to digest dissolved organic compounds.
3. Ultrafiltration & Disinfection: Microfiltration membranes (0.04 micron pore size) remove suspended solids and bacteria, followed by Ultraviolet (UV) light disinfection and residual chlorination to prevent bio-fouling in storage tanks.

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Integrated Non-Potable Water Balance Math in EDGE

In the EDGE App, non-potable recycled water cannot exceed total non-potable building demand. The water balance equation enforced by EDGE is:

$\text{Usable Recycled Volume} = \min \left( \text{Supply}_{Rain + Greywater}, \quad \text{Demand}_{Flushing + Irrigation + Cooling} \right)$

$\text{Net Potable Water Savings (\%)} = \left( \frac{\text{Baseline Potable Demand} - (\text{Proposed Potable Demand} - \text{Usable Recycled Volume})}{\text{Baseline Potable Demand}} \right) \times 100$

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Dual-Pipe Plumbing Architecture & Regulatory Compliance

To safely distribute recycled greywater or harvested rainwater throughout a building, plumbing codes mandate a dedicated dual-pipe plumbing system.

Critical Engineering Standards

• Color-Coded Piping (Purple Pipe): Non-potable distribution pipes must be color-coded purple (Pantone 512C or RAL 4001) or continuously stenciled with warning labels: "CAUTION: NON-POTABLE WATER - DO NOT DRINK".
• Physical Air-Gap Backflow Protection: Municipal backup potable water connections to rainwater or greywater day tanks MUST feature an approved physical air gap (minimum twice the diameter of the supply pipe, never less than 25 mm) above the overflow rim to eliminate cross-contamination risk.
• Dual Riser Isolation: Potable water lines and non-potable purple pipe risers must run in separate chases or maintain minimum physical clearances.

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Evidence Documentation & On-Site Audit Verification

During the EDGE audit process, the Auditor verifies greywater and rainwater interventions using technical submittals:

1. Hydraulic Calculation Sheet: Signed engineer calculations showing rainfall yield, greywater supply volumes, non-potable flush/irrigation demand, and storage tank sizing.
2. Plumbing Schematic & Riser Diagrams: Full MEP drawings highlighting dual-piping distribution networks, treatment plant layout, pump specs, and backflow air gap details.
3. Equipment Cut Sheets: Technical spec sheets for greywater treatment plants (MBR/MBBR capacities in $m^3/day$), UV disinfection dosage ($mJ/cm^2$), and rainwater filtration equipment.
4. On-Site Inspection: Physical verification of purple piping, treatment plant installation, tank capacity nameplates, and operational testing of automated backup supply valves.