Greywater (GW) is the largest single source of recyclable water in any Indian commercial or hospitality project — typically 40-60 % of total wastewater volume. Properly treated, it can offset 30-50 % of non-potable demand (flush, landscape, AC make-up). This article walks through the design framework, treatment train selection, and IGBC/GRIHA point capture.
What counts as greywater
Greywater = wastewater from fixtures with low organic + low pathogen load:
- Bath / shower drains
- Wash basin drains
- Laundry drains (with soap restrictions)
Excluded (this is blackwater, requires separate treatment):
- Toilet drains (urine + faeces)
- Kitchen sinks (high oil/grease/organic)
- Floor drains in food preparation areas
Hospitality + healthcare projects typically separate GW + BW at the fixture; office + retail separate at the manhole.
Treatment train tiers
| Tier | Treatment | Use category | IGBC/GRIHA credit |
|---|---|---|---|
| 1 | Sedimentation + sand filter + chlorine | Landscape only | Partial |
| 2 | + Multimedia filtration + UV | Landscape + flush | Full |
| 3 | + Activated sludge + UF | Cooling tower make-up | Bonus |
| 4 | + RO + UV + ozone | Cleaning + non-contact non-potable | Bonus |
Indian commercial standard practice (2022+) is Tier 2 — meets IGBC + GRIHA requirements without expensive RO.
Sizing the GW system
Step 1: estimate GW generation
- Hotel guestroom: 70-100 L/p/day GW (out of 200-250 total)
- Office: 5-10 L/p/day GW (basin + occasional shower)
- Mall / retail: 4-6 L/p/day GW
- Hospital ward: 80-120 L/p/day GW (bathing dominant)
Step 2: estimate non-potable demand
- Flush: 25-40 L/p/day (commercial); 30-50 (hospitality)
- Landscape: 5-10 L/m²/day depending on plant + climate
- Cooling tower make-up: based on chiller plant cycles
Step 3: match supply + demand
- If GW > demand: route excess to recharge or sewer
- If GW < demand: supplement with RWH or municipal
Worked example: 200-key Mumbai hotel
Inputs:
- 320 guests × 90 L/p/day GW = 28,800 L/day GW generated
- Flush demand: 320 × 35 = 11,200 L/day
- Landscape (1000 m² × 6 L/m²/day): 6,000 L/day
- Cooling tower make-up: 4,500 L/day
- Total non-potable demand: 21,700 L/day
Match: 28,800 GW / 21,700 demand = 132 % offset. The system delivers 100 % non-potable replacement with 7,100 L/day excess (route to recharge).
Tier 2 treatment train sizing
For 28,800 L/day GW (1,200 L/h):
- Equalisation tank: 3-hour HRT = 3,600 L
- Sedimentation tank: 1-hour HRT = 1,200 L
- Multimedia filter: 1.5 m³/h capacity, 600 mm diameter pressure vessel
- UV reactor: 1.5 m³/h, 4-lamp medium-pressure
- Treated water tank: 1-day buffer = 28,800 L
- Booster pump set: 4.5 m³/h @ 30 m head (flush + landscape distribution)
Pipe and tank colour-coding
CPCB Greywater Manual + NBC 2016 Pt 9 require visible identification:
- Greywater pipes: lavender (RAL 4001) with 100 mm “GW” stencil at 3 m intervals
- Treated water pipes: pale blue with “TW” stencil
- Potable: white with “POTABLE” stencil
- Sewer: black
Tank labelling must include the warning “NON-POTABLE — DO NOT DRINK” in 75 mm letters, English + local language.
IGBC + GRIHA credit capture
IGBC v3 WE-3 (Water Efficiency — Wastewater Treatment + Reuse):
- 1 point: 50 % treated wastewater reused
- 2 points: 75 % reused
- 3 points: 100 % reused (zero liquid discharge intent)
GRIHA Criterion 27 (Wastewater Treatment + Reuse):
- 1-3 points based on % of generated wastewater treated + reused on-site
Documentation needed:
- Greywater + blackwater segregation drawing
- Treatment train flow diagram + sizing calc
- Reuse-application plan (flush + landscape + cooling tower)
- Annual water-balance calc
Operational + maintenance
GW systems fail at operations, not at design. Common failures:
- UV lamp ageing (replace at 8000 hr, ~12 months)
- Multimedia filter media exhaustion (backwash daily, full media replacement at 3 yr)
- Microbiological re-contamination in treated tank (oxidant residual + tank cleaning)
- Plumbing cross-connections during retrofits (annual hydrostatic test mandatory)
Operator training + AMC contract are non-negotiable for hospitality + healthcare.
From the Field — Engineer’s Notebook
A 280-key Bengaluru hotel commissioned a Tier 2 GW system in 2019. By 2022, the operator was buying tanker water for landscape because the GW plant produced “smelly, brown” water. Investigation: the multimedia filter had not been backwashed for 14 months, the UV lamp was 24 months past replacement, and the chlorine residual in the treated tank was zero. The plant was physically functional but operationally abandoned. Lesson: design AMC + monitoring into the operations contract before handover. The operator’s brand SOP should mandate weekly water-quality checks + monthly equipment inspection.
5 common mistakes
1. Not separating GW + BW at fixture. Combined waste = no GW system possible. Coordinate with architect at SD stage.
2. Treatment train under-sized for peak flow. GW arrives in 2-3 hour peaks (morning shower window). Equalisation tank must absorb peak.
3. No colour-coded piping. CPCB + NBC mandatory; failure = compliance audit fail + retrofit cost.
4. No oxidant residual maintenance in treated tank. Bacteria regrow within 24 hours of chlorine depletion.
5. No O&M budget at handover. GW systems fail at year 2-3 from neglect. Operator must own the AMC + monitoring.
Designer’s checklist
- [ ] GW + BW segregated at fixture (not at manhole)
- [ ] GW generation estimated by occupancy + use
- [ ] Non-potable demand matched (offset target 75-100 %)
- [ ] Treatment train tier selected against reuse application
- [ ] Equalisation + sedimentation + filtration + disinfection sized for peak flow
- [ ] Treated water buffer tank sized for 1-day demand
- [ ] Booster pump sized for distribution head + flow
- [ ] Pipe + tank colour-coded per CPCB + NBC
- [ ] IGBC WE-3 / GRIHA Criterion 27 documentation prepared
- [ ] AMC + O&M plan written into operations handover
- [ ] Annual hydrostatic + cross-connection test scheduled
Pairs with: RWH Yield Calculator, Rainwater Harvesting per CGWA