Greywater is the wastewater from baths, lavatories, kitchen sinks, and washing machines — distinguished from blackwater (toilet, urinal). It accounts for 50-80% of a building’s wastewater volume. With proper treatment, it can be reused for toilet flushing, irrigation, cooling tower make-up, and floor washing. Indian green-building credits — IGBC v3 WC, LEED v4.1 WE, GRIHA — all reward greywater recycling.
This guide covers source segregation, treatment options, design loads, and compliance with Indian codes.
What greywater is, and what to do with it
Source water types:
| Source | Designation | Characteristics |
|---|---|---|
| Bath, shower, lavatory | Light grey | Low BOD/COD, mild organics |
| Kitchen sink | Heavy grey | High BOD, oils, food particles |
| Washing machine | Medium grey | Soap, detergent, lint |
| Toilet, urinal | Black | High pathogen load |
For most applications, light greywater + medium greywater = treated greywater for reuse. Kitchen waste (heavy grey) is sometimes excluded due to high oil/grease content; some systems include it with grease trap pre-treatment.
Reuse applications (in descending priority):
1. Toilet flushing (highest reuse percentage; aligned with bath/lavatory greywater)
2. Cooling tower make-up (with chlorination)
3. Subsurface irrigation
4. Floor washing
5. Landscape irrigation (above-ground)
Step 1: Source segregation at design stage
The biggest design decision is how greywater is segregated from blackwater. Three approaches:
Approach A: Plumb separately during construction
Two parallel drainage systems: one for greywater (bath, lavatory, kitchen, washing), one for blackwater (toilet, urinal). Requires planning at architectural stage; cannot be retrofitted easily.
Approach B: Combined drainage with treatment plant separation
All wastewater goes to one drainage system, which feeds a sewage treatment plant (STP). The STP separates greywater stream during processing.
Approach C: Hybrid — light greywater segregated, others combined
Bath and lavatory greywater segregated for reuse; kitchen + washing machine + blackwater go to STP.
For new construction, Approach A is preferred (lower treatment cost). For retrofit, Approach B is the only option (no second drainage line).
Step 2: Treatment requirements
NBC 2016 Pt 9 and IS 14687 set minimum treatment standards for reuse water.
For toilet flushing reuse:
| Parameter | Maximum |
|---|---|
| BOD₅ | 10 mg/L |
| Suspended solids | 10 mg/L |
| Total coliforms | <200 CFU/100mL |
| Faecal coliforms | <14 CFU/100mL |
| Free chlorine residual | 0.5-1.0 mg/L |
| Turbidity | <5 NTU |
For cooling tower make-up:
| Parameter | Maximum |
|---|---|
| BOD₅ | 5 mg/L |
| Suspended solids | 5 mg/L |
| Free chlorine | 1-2 mg/L |
| pH | 6.5-8.0 |
| Total dissolved solids | <500 mg/L |
For irrigation:
| Parameter | Maximum |
|---|---|
| BOD₅ | 30 mg/L |
| Suspended solids | 30 mg/L |
| Total coliforms | <500 CFU/100mL |
| Free chlorine | 0.2-0.5 mg/L |
| pH | 6.0-8.5 |
These targets drive the treatment chain selection.
Step 3: Treatment plant selection
For commercial buildings up to 5,000 occupants, three common treatment chains:
Chain 1: Conventional biological + chlorination
Screening → equalization tank → biological treatment (extended aeration or trickling filter) → secondary clarifier → sand filter → activated carbon → UV/chlorination → reuse storage tank.
Pros: well-proven, low capex.
Cons: large footprint, higher BOD/COD output, requires more space than membrane systems.
Capex: ~₹2-4 lakh per kL/day of capacity.
Chain 2: Membrane Bio-Reactor (MBR)
Screening → equalization → MBR (combined biological + ultrafiltration membrane) → UV/chlorination → reuse storage tank.
Pros: small footprint (50-70% smaller than conventional), high quality output, BOD < 5 mg/L typical.
Cons: higher capex, membrane fouling requires periodic cleaning.
Capex: ~₹4-7 lakh per kL/day of capacity.
Chain 3: Sequencing Batch Reactor (SBR)
Screening → equalization → SBR (single tank for fill, react, settle, draw, idle) → sand filter → carbon → UV/chlorination → reuse tank.
Pros: very compact for small flows, automated batch processing, no clarifier needed.
Cons: not continuous flow; requires tankage for stormwater equalization.
Capex: ~₹3-5 lakh per kL/day of capacity.
For a 1,000-occupant office building (~ 10 kL/day greywater), MBR is typically chosen for its small footprint despite higher capex.
Step 4: Sizing the treatment plant
Greywater generation per occupant:
| Building type | Greywater (L/person/day) |
|---|---|
| Office (no shower) | 25-35 |
| Office (with shower) | 50-70 |
| Hotel guestroom | 100-150 |
| Residential | 80-120 |
| Commercial / retail | 15-25 |
| Hospital | 80-120 |
For 1,000-occupant office: 30 L/person × 1,000 = 30,000 L/day = 30 kL/day greywater. Treatment plant sized for 30 kL/day plus 25% peaking factor = 38 kL/day capacity.
Step 5: Storage tank sizing
Treated greywater storage tank typically holds 1-2 days demand:
For office building, treated greywater used for toilet flushing only — typical demand 30-40 L/person/day for flushing (actually slightly higher than greywater generated by bath/lavatory alone).
Demand vs supply check: 1,000 occupants × 35 L flush/day = 35 kL/day. Greywater generated 30 kL/day. Greywater supply is 86% of flush demand. Mix-up tank with municipal water makes up the 14% gap. This is a real-world scenario — most buildings end up greywater-deficit for flush demand.
Storage tank: 1.5 day buffer × 35 kL = ~55 kL capacity, roughly 2 m × 5 m × 5.5 m underground tank.
Worked example: 1,000-person office building
Greywater sources:
- Lavatory and bath: 1,000 × 25 L/day = 25,000 L/day
- Kitchen: 1,000 × 5 L/day = 5,000 L/day
- Total: 30,000 L/day = 30 kL/day
Treatment: MBR sized for 38 kL/day (25% peaking)
Reuse: toilet flushing, 35 L/person/day = 35 kL/day demand
Supply-demand:
- Daily greywater treated = 30 kL/day (after losses)
- Toilet flush demand = 35 kL/day
- Make-up from municipal water = 5 kL/day (16%)
Treated water storage tank: 50 kL underground, with chlorinator at outlet, sand filter polish, dual-pump set (one duty, one standby).
Cooling tower make-up
For projects where greywater exceeds toilet flush demand, cooling tower make-up is the typical secondary use. Cooling tower consumes 1-2% of total chilled water flow as evaporation — 5-10 kL/day for a 200 TR plant.
Quality must be more stringent (lower TDS, controlled chemistry). Typically requires additional polishing — reverse osmosis or ion exchange after MBR.
Five common greywater system mistakes
1. Mixing greywater with blackwater early in design. Once mixed, separation is impossible without retrofit.
2. Sizing for “average” demand, not peaking. 25-30% peaking factor required for reliable system.
3. No storage tank. Treatment plant runs 24/7 at variable rate; no buffer = poor reliability.
4. Inadequate chlorination for reuse. WHO requires free chlorine 0.5+ mg/L for any reuse application.
5. Cooling tower make-up without TDS control. Untreated TDS rise → scaling, biofouling, capital damage.
Quick checklist
- [ ] Source segregation strategy (separate plumbing or combined)
- [ ] Greywater generation rate calculated per occupancy
- [ ] Treatment plant sized for design flow + 25% peaking
- [ ] Treatment chain matched to reuse application’s quality target
- [ ] Storage tank ≥ 1.5 day capacity
- [ ] Chlorination + UV for pathogen kill
- [ ] Make-up water connection from municipal supply
- [ ] Compliance with NBC 2016 + IS 14687 + IGBC/LEED targets
References: NBC 2016 Pt 9 (Plumbing); IS 14687 Indian Code for Greywater Treatment and Reuse; IGBC Green New Buildings v3 WC; LEED v4.1 BD+C WE Indoor Water Use; CPHEEO Manual on Sewerage and Sewage Treatment.
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