If you’re researching decentralized greywater systems for apartments or designing an independent unit that works, this guide is for you. It explains what decentralized greywater tanks are, why they make sense in Indian apartments, how to size and place them, simple treatment options, plumbing and safety rules, maintenance, realistic costs, and local vendors. Written in plain English, it focuses on practical steps you can act on immediately.
What is a Decentralized Greywater System?
A decentralized greywater system treats and stores wastewater from showers, washbasins, and laundry (not toilets) close to where it is produced—typically at the building block, wing, or apartment level—and reuses it for non-potable purposes such as toilet flushing, gardening, and washing cars.
Why it matters in India:
Water stress is increasing, apartment complexes use large volumes for flushing and landscaping, and decentralized systems can significantly reduce fresh-water demand while lowering municipal sewer loads. Several pilot projects in India report freshwater savings of 30–40% when greywater is reused efficiently.
Decentralized vs. Centralized
Centralized: One large STP for the whole society. Suitable for very large complexes with space and skilled operations. Higher capital cost and longer installation time.
Decentralized: Tanks or units per block/wing or even apartment. Lower piping complexity, modular scale-up, easier retrofits, independent operation, and faster payback.
For most mid-rise apartment societies in India, decentralized systems are ideal: they minimize disruption, allow phased rollout, and give local control.
Key Components of a Decentralized Greywater Unit
A typical apartment-level unit includes:
- Source separation & plumbing: Greywater lines separated from toilets.
- Primary settling / grit trap: Removes hair, lint, and sand.
- Biological treatment: Small biofilters, aerobic media, or compact MBBR/FF units.
- Disinfection: Chlorine dosing or UV treatment to control pathogens.
- Storage buffer tank: Holds treated greywater with overflow to sewer and locked supply outlets.
- Pump & controls: Float switches and alarms for supply to toilets or irrigation.
- First-flush & bypass: Diverts very dirty loads to sewer.
A compact system combining settling, biofiltration, and disinfection is usually sufficient for non-potable reuse if maintained properly.
Sizing & Placement Rules of Thumb
- Estimate greywater generation: Each person produces ~50–80 litres/day. Multiply by occupants for daily volume.
- Decide reuse fraction: 50–70% of greywater can be reused for flushing and gardening.
- Buffer/storage: Provide 0.5–1 day of treated water storage. Small blocks: 1,000–2,000 L; larger wings: 5,000–10,000 L.
- Treatment sizing: Design biofilter surface/media and retention time for expected load.
- Siting tips: Basements, service rooms, or block-level plant rooms with easy maintenance access. Avoid placing units inside living flats unless ventilation and safety are assured.
Plumbing & Safety Considerations
- Strict source separation: Greywater must not mix with toilet water. Label or colour-code pipes.
- Backflow prevention: Use non-return valves; treated water must not enter potable lines.
- Signage & locking: Restrict access to tanks; label outlets “For toilet flushing / garden use only.”
- Overflow routing: Connect overflow to sewer to prevent backups.
- Microbial checks: Test for E. coli, residual chlorine, or UV performance regularly.
- Municipal approvals: Seek guidance or file a technical note with local authorities if reusing water at scale.
Treatment Options
- Sediment filter → Bio-filter → Chlorine/UV: Compact, reliable, low-power, and widely used.
- Constructed vertical flow bio-reactor: Gravel/media beds with intermittent dosing; low maintenance for moderate flows.
- Membrane + UV: UF/MF and UV for near-clear water, suitable for laundry or cooling reuse.
Choose systems that your society can maintain. Simpler biological filters with a disinfection step are often most sustainable.
Operation & Maintenance
Maintenance drives long-term success. Typical schedule:
- Daily/Weekly: Check levels, pumps, float switches; skim grease/lint.
- Monthly: Backwash media, clean filters, check chlorine/UV.
- Quarterly: Microbial sampling (E. coli), odor check, water quality tests.
- Annually: Replace gaskets, UV lamps, service pumps, inspect tanks.
Most failures result from neglected lint/grit traps. Use accessible pre-filters and schedule cleaning.
Costs (India, 2025 Estimates)
- Small apartment unit (0.5–2 m³/day): ₹50,000–₹2,00,000 installed.
- Block/wing unit (2–10 m³/day): ₹2–7 lakh installed, including tank and pumps.
- O&M: ₹3,000–₹15,000/year, scaling with size.
Obtain multiple quotes and consider lifecycle costs. Slightly higher upfront cost for durable components usually saves on maintenance.
Local Vendors & Pilots
- JalSevak Solutions: On-site scalable greywater recycling for residential buildings.
- Nijhuis Saur Industries: Decentralized greywater solutions for larger deployments.
- Era Hydrobiotech / Ecoagulation: Packaged domestic and commercial units with installation support.
- Academic/NGO pilots: Pilot studies document gravity-driven, low-cost designs suitable for apartments.
Always ask for references, test reports, and clear O&M agreements.
Mini Case Example
A mid-size apartment wing installed a 5 m³/day biofilter + UV unit supplying 40 flats. Results: ~35% reduction in freshwater for flushing and gardening, with payback in 4–6 years. Keys to success: effective pre-filters, daily caretaker checks, and quarterly microbial reports.
Rollout Plan for Societies
- Survey: Measure current water use and greywater volumes.
- Pilot: Install a 2–5 m³/day unit in one block or wing.
- Monitor 3–6 months: Track water savings, maintenance time, and resident feedback.
- Refine: Adjust pre-filtration and controls.
- Scale: Roll out to other wings or adopt a hybrid block + apartment-level approach.
Pilot programs reduce upfront risk and build resident confidence.
Final Checklist Before Signing a Contract
- Confirm separation of grey and blackwater lines on drawings.
- Vendor provides O&M schedule and AMC quote.
- Review sample test reports from reference sites.
- Clear overflow routing and backflow prevention in place.
- Resident communication plan and labelled outlets.
- Check permits or clearances with municipal/health authorities.