6.6 Graywater Reuse, Rainwater Collection, Reclaimed Water & Subsurface Graywater

Graywater Reuse, Rainwater Collection, Reclaimed Water & Subsurface Graywater

Section 6.5 covered the rules common to all nonpotable systems. This section covers the three source-specific systems in IPC Sections 1302 through 1304, plus Chapter 14 subsurface graywater soil absorption systems. On the P3 exam, expect questions on allowed and prohibited sources, required valves, rainwater collection devices, pressure limits and absorption area calculations.

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On-Site Nonpotable Water Reuse Systems (IPC 1302)

Governing standard (1302.1): ASTM E2635 and Section 1302 govern on-site reuse systems, where the jurisdiction permits them.

Sources (1302.2 and 1302.2.1)

Wastewater containing urine or feces discharges to the sanitary drainage system per Chapter 7.

Collection, Treatment and Valves

• Traps (1302.3): Fixtures that discharge to a reuse system have traps per Section 1002.4.
• Collection pipe (1302.4): DWV piping materials per Section 702, installed per 704, jointed per 705 and sized per 710, like a sanitary drain.
• Filtration (1302.5): Filters have access, a pressure gauge or other service indicator, and shutoff valves immediately upstream and downstream.
• Retention limit (1302.6): Water containing untreated graywater may be held in collection reservoirs for not more than 24 hours.
• Flushing use (1302.6.1): Graywater used to flush water closets and urinals is disinfected and treated by a system complying with NSF 350.
• Bypass valve (1302.8.1): One three-way diverter valve listed to NSF 50 (or another approved device) is installed upstream of each tank or drain field. It diverts untreated water to the sanitary sewer for servicing and goes downstream of fixture traps and vent connections. Two shutoff valves may not serve as the bypass.
• Backwater valves (1302.8.2): One or more backwater valves (per Section 714) go on each overflow and tank drain pipe.
• Pressure (1302.10): Where pump pressure exceeds 80 psi static, a PRV reduces the distribution pressure to 80 psi or less.

Tank setbacks (Table 1302.7.1): 2 feet from the critical root zone of protected trees; 5 feet from lot lines, seepage pits, septic tanks and the water service; 10 feet from a public water main; and 50 feet from streams, lakes and water wells. Tank outlets are at least 4 inches above the tank bottom and do not skim the surface (1302.7.2).

     ON-SITE GRAYWATER REUSE SYSTEM — PLAN REVIEW FLOW (IPC 1302)

  Showers / tubs / lavs / washers ──► Trap + vent (DWV, Sections 702–710)
                                         │
                                         ▼
                         ┌── 3-way diverter valve, NSF 50 (1302.8.1) ──┐
                         │                                             │
                         ▼                                             ▼
               Filter (gauge + isolation valves)               Sanitary sewer
                         │                                     (bypass for service)
                         ▼
           Storage tank (≤24 h untreated, 1302.6) ── overflow/drain + backwater valve
                         │
                         ▼
           NSF 350 treatment for WC/urinal flushing (1302.6.1)
                         │
                         ▼
           Pump ──► PRV if >80 psi ──► PURPLE distribution (608.9)

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Nonpotable Rainwater Collection (IPC 1303)

• Collection surfaces (1303.2): Rainwater is collected only from above-ground impervious roofing of approved materials and, where approved, from vehicular parking or pedestrian walking surfaces.
• Debris excluders (1303.3): Downspouts and leaders connect to a debris excluder that removes leaves, sticks and needles before the tank.
• First-flush diverters (1303.4): These operate automatically, not with manual valves. Diverted water does not drain onto the roof, and the diverter has access for service.
• Gutter slope (1303.5.1): Gutters and downspouts slope at least 1/8 inch per foot with no pooling. Siphonic systems installed per the manufacturer are exempt.
• Gutter size (1303.5.2): Gutters are sized per Section 1106.6 and local rainfall rates.
• Roof washer drainage (1303.6): Water from roof washers and debris excluders may not go to the sanitary sewer. These devices self-drain between rain events and do not drain onto the roof.
• Tank setbacks (Table 1303.10.1): 2 feet from the critical root zone of protected trees, and 5 feet from lot lines, seepage pits and septic tanks.
• Backwater valves (1303.11.1): Required on each overflow and tank drain pipe.
• Fire protection use (1303.1.1): Storage and distribution for fire protection systems follow the International Fire Code.
• Gutter test (1303.15.1): At least 1 gallon of water is poured into the gutter end opposite the outlet. The gutter must not leak or hold standing water.
• Raw water quality (1303.15.9): ASTM E2727 is used to judge whether site conditions require treatment.

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Reclaimed Water Systems (IPC 1304)

Reclaimed water is treated wastewater delivered by a wastewater treatment facility.

• Pressure (1304.2): A PRV is required where supply pressure exceeds 80 psi static.
• Design (1304.3): The system follows accepted engineering practice. Distribution piping meets Section 605 for nonpotable water and is identified as purple per 608.9. The exception is outdoor irrigation piping downstream of the backflow preventer.

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Subsurface Graywater Soil Absorption Systems (IPC Chapter 14)

These systems send graywater from an on-site reuse system into the soil for subsurface irrigation. No disinfection (1401.5) or dye (1401.6) is required.

Graywater flow (Equation 14-1): $C = A \times B$, where:

• $A$ is the number of occupants. For homes, use 2 for the first bedroom plus 1 for each additional bedroom.
• $B$ is the residential flow: 25 gpd per occupant for showers, bathtubs and lavatories, plus 15 gpd per occupant for clothes washers or laundry trays.
• $C$ is the estimated graywater discharge in gallons per day.

Design loading rates (Table 1403.1.1):

Worked example: A three-bedroom house connects the showers, lavatories and clothes washer.

• $A = 2 + 1 + 1 = 4$ occupants
• $B = 25 + 15 = 40$ gpd per occupant
• $C = 4 \times 40 = 160$ gpd
• Percolation at 20 min/inch gives a factor of 0.8, so the area is $160 / 0.8 = 200\text{ sq ft}$.
• With 2-foot-wide trenches, that is 100 feet of trench, which is at the 100-foot limit for a single trench.

Installation rules:

• Percolation tests (1402.2.1): At least three in each system area.
• Absorption field setbacks (Table 1402.3): 2 feet from buildings and 100 feet from water wells.
• Seepage trenches (1403.1.2): 1 to 5 feet wide, spaced at least 2 feet apart, and no more than 100 feet in developed length each.
• Aggregate (1403.1.5): At least 6 inches of ½- to 2½-inch aggregate below the pipe and 2 inches over it, a cover, and 9 inches of soil backfill. Building paper may not be used.
• Distribution pipe (1403.2): At least 3 inches, with its top at least 8 inches below the original surface, sloped 2 to 4 inches per 100 feet. Materials are PE (ASTM F405) or PVC (ASTM D2729 or F1488).

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Plan Reviewer Verification Checklist: Source-Specific Systems

• Graywater Sources: Only allowed sources connect, and no kitchen, dishwasher, softener or RO reject water (1302.2).
• Bypass and Backwater Valves: An NSF 50 three-way diverter is installed downstream of traps and vents, and backwater valves are on the overflow and drain (1302.8).
• Treatment: Untreated storage is limited to 24 hours, with NSF 350 treatment for flushing (1302.6).
• Rainwater Devices: Debris excluder, automatic first-flush diverter, 1/8-inch-per-foot gutters, and no roof washer drainage to the sewer (1303).
• Pressure: A PRV is shown where pumped or reclaimed pressure exceeds 80 psi (1302.10, 1303.13 and 1304.2).
• Soil Absorption: Area equals C divided by the Table 1403.1.1 factor; trenches are 100 feet or shorter; distribution pipe is at least 3 inches (Chapter 14).