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Every day, an average Indian household of four generates 300–400 litres of greywater (wastewater from bathrooms, kitchens, and laundry) that looks relatively clean, smells mild, and is therefore quietly poured into storm drains, open plots, or backyard soak pits. Multiply that by 150 million urban households and the volume becomes staggering: roughly 50,000 million litres of soiled, detergent-rich, nutrient-heavy water released untreated every single day. As several studies on sewage treatment challenges in urban India show, this daily greywater load is now overwhelming drains and local ecosystems.
Most people believe greywater is harmless compared to blackwater from toilets. Reality tells a different story. In 2025, untreated greywater has emerged as one of the biggest contributors to urban lake foaming, perennial drain clogging, mosquito breeding, soil salinization, and groundwater contamination—issues that underline why cities urgently need advanced wastewater treatment solutions to manage both greywater and blackwater effectively.
What Exactly Is Greywater and Why It Is Almost Always Mismanaged
Greywater constitutes 65–75 % of total domestic wastewater. It contains soap, shampoo, oil, food particles, detergents (high in sodium, phosphates, and sulphates), hair, lint, and significant amounts of organic matter. Typical characteristics in Indian cities:
- BOD: 150–300 mg/L
- COD: 400–800 mg/L
- Total suspended solids: 100–250 mg/L
- Phosphates: 10–40 mg/L
- High sodium absorption ratio (SAR >15)
Because it does not smell as bad as toilet waste, most apartments, independent houses, and even large townships simply route greywater into stormwater drains or shallow soak pits. Municipal corporations rarely penalise this practice, and building bylaws often remain silent on separate greywater treatment.
How Untreated Greywater Damages Soil, Lakes, and Urban Drainage
- Soil degradation and plant death
Continuous irrigation with raw greywater raises soil pH and sodium levels within months. In Bengaluru’s Whitefield and Electronic City layouts, apartment societies that have been using untreated greywater on lawns for 5–7 years now report dead patches, stunted trees, and hard, impermeable soil that no longer absorbs rainwater. - Clogging of stormwater drains
Oil, grease, hair, and food particles from kitchen sinks accumulate inside drains, reducing carrying capacity by 40–60 %. During light rain itself, roads turn into rivers because drains are choked with greywater sludge. - Lake and water-body eutrophication
Phosphates and nitrates in greywater act as super-fertilizer for algae. Bellandur Lake (Bengaluru), Powai Lake (Mumbai), and numerous urban lakes in Hyderabad regularly foam and catch fire because apartment complexes upstream discharge hundreds of KLD of untreated greywater directly or indirectly. - Mosquito breeding and disease vector proliferation
Stagnant greywater mixed with organic waste creates perfect conditions for Aedes and Culex mosquitoes. Several dengue and chikungunya outbreaks in Delhi-NCR and Kerala colonies have been traced to open greywater channels.
The Algae Bloom Connection Nobody Talks About
When phosphates from a single 500-flat society (approximately 80–100 KLD greywater) reach a lake, they trigger explosive growth of blue-green algae. One gram of phosphorus can produce up to 100 grams of algal biomass. The algae die, sink, and decompose, consuming all dissolved oxygen and releasing toxins. Fish die, the lake turns anaerobic, and methane bubbles make the surface foam — a sight now common in at least 40 major Indian cities.
Practical Greywater Treatment Solutions That Actually Work
Thankfully, greywater is far easier and cheaper to treat than combined sewage:
- For independent houses and small apartments (<50 flats)
- Simple three-chamber grease trap + planted gravel filter (reed-bed system) costing ₹40,000–₹80,000 can treat 2–5 KLD and produce water perfect for gardening.
- Soil biotechnology (SBT) units popular in Maharashtra and Goa occupy just 4–6 m².
- For medium societies (100–500 flats)
- Compact physico-chemical systems: screening → oil & grease trap → coagulation-flocculation → settling → sand + carbon filtration. Capital cost ₹6–10 lakh for 20–50 KLD.
- For large apartments, malls, and institutions
- Dedicated greywater treatment plants using AABR or MBBR technology followed by ultrafiltration. A 100 KLD system costs ₹18–28 lakh and produces water with BOD <10 mg/L suitable for flushing, cooling towers, and car washing.
Why Small-Scale STPs or Dedicated Greywater Plants Are Now Essential
Many forward-looking builders and resident associations have already separated greywater lines at construction stage. The benefits are immediate and measurable:
- A 400-flat society in Bengaluru’s Sarjapur Road treats 120 KLD greywater and reuses every drop for flushing and gardening — zero freshwater used for non-potable needs, saving ₹48–55 lakh annually in tanker bills.
- A five-star hotel in Chennai installed a 150 KLD greywater system in 2022 and reduced municipal water consumption by 64 %.
- A 300-bed hospital in Pune uses treated greywater for all cooling towers and saves ₹22 lakh per year while staying well within pollution-board norms.
These plants occupy only 30–40 % of the footprint of a conventional combined STP because blackwater volume is much lower and the organic load is easier to handle.
The Bigger Picture
Untreated greywater is the low-hanging fruit we have ignored for too long. While we debate multi-thousand-crore centralized sewerage projects, millions of litres of detergent-laden water continue to choke our cities from within. Treating greywater at source — whether through a ₹50,000 reed-bed system for a bungalow or a ₹25-lakh modular plant for a high-rise — is the quickest, most affordable way to stop lake foaming, save stormwater drains, protect urban greenery, and reduce freshwater demand by 50–70 %. As highlighted in many energy-efficient wastewater treatment solutions, decentralised treatment fixes the problem where it starts instead of waiting for city-scale upgrades.
Every apartment, school, office, and hotel that lets greywater flow untreated is actively harming the city it calls home. Request price quotation on STP plant.
Frequently Asked Questions (FAQs)
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1. Is greywater really more harmful than toilet water to lakes?
Yes, in terms of nutrient pollution. Blackwater has higher BOD but lower phosphates; greywater’s high detergent content makes it the primary trigger for algal blooms.
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2. Can we mix greywater and blackwater again after treating them separately?
No need. Keeping them separate reduces treatment cost and complexity, and allows targeted reuse (greywater for gardening/flushing, blackwater after full STP for non-contact uses).
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3. How much does a 50 KLD greywater treatment plant cost?
₹12–25 lakh (2025 prices) for a fully automatic system with tertiary polishing, including civil work and commissioning.
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4. Is it mandatory to treat greywater separately?
Not yet nationwide, but Tamil Nadu, Karnataka, Rajasthan, and Maharashtra already mandate greywater management in new large projects, and NGT is pushing similar rules in Delhi-NCR and Punjab.
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5. How much water can an apartment save by recycling greywater?
A typical 200-flat society can reuse 80–120 KLD, meeting 100 % of flushing + gardening demand and cutting freshwater consumption by 50–65 %.
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