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India recycles less than 10% of its 72,000 million litres of daily sewage, while urban water demand escalates 5–7% annually. As borewells deplete and municipal supplies falter, the question of safety of STP treated water for direct consumption gains urgency.
Modern STPs remove 95–99% of contaminants, yet potable recycled water remains regulated due to trace risks. SKF Elixer’s Vulcan STP with AABR and advanced polishing delivers sewage water purification to near-drinking standards in 5–500 KLD plants, enabling safe non-potable reuse and paving pathways to future potability.
This blog dissects the STP process, post-treatment quality, legal barriers, practical reuse, and emerging technologies. Whether you oversee an apartment in Mumbai or a hotel in Goa, learn the facts behind treated sewage safety and sustainable applications.
Understanding the STP Treatment Process
Preliminary Screening and Grit Removal
Bar screens with 6–10 mm spacing capture large debris like plastics, sanitary items, and vegetable matter, removing 2–5 kilograms of total suspended solids (TSS) per kilolitre of influent—equivalent to 200–500 kilograms daily in a 100 KLD plant.
Aerated grit chambers with air flow at 0.3–0.6 m³/min/m² length settle sand and inorganic grit (specific gravity >2.65) at velocities of 0.2–0.3 m/s, preventing abrasion on pumps and pipes that would otherwise cost ₹20,000–₹50,000 in annual repairs.
This stage reduces TSS by 20–30% and protects downstream biology from physical damage, ensuring consistent hydraulic flow without blockages that could spike effluent TSS above 100 mg/L.
Primary Sedimentation
Clarifiers with 2–4 hour hydraulic retention time (HRT) and surface loading rates of 1–2 m³/m²/hr allow gravity settling of settleable solids, achieving 50–70% TSS removal and 30–40% BOD reduction. Flocculants like alum (10–20 mg/L) enhance coagulation, forming flocs that settle at 0.5–1 m/hour.
Primary sludge at 1–2% solids is scraped to hoppers, yielding 50–100 kilograms per day for a 100 KLD plant—dewatered to 20–25% solids via centrifuges for disposal at ₹10–15 per kilogram. This step lowers organic load on biology by 60–80 kilograms BOD daily, preventing overload and effluent BOD exceeding 100 mg/L.
Biological Treatment – AABR Core
Fixed synthetic media with 200–300 square metres surface area per cubic metre provides attachment for microbial biofilms in stainless steel tanks. Continuous flow with fine bubble diffusers (oxygen transfer efficiency 30–40%) at 0.4–0.6 kWh/KL maintains dissolved oxygen 2–4 mg/L. Biofilms degrade 95%+ BOD (200–400 mg/L influent to below 10 mg/L) through aerobic respiration and 80–90% ammonia conversion via nitrification-denitrification in anoxic microzones.
Hydraulic retention 12–18 hours ensures 6-log pathogen reduction (>99.9999%) via predation and starvation. Vulcan’s SS construction resists corrosion from pH swings 6–9, delivering odour-free operation with H2S <1 ppm.
Secondary Clarification
Tube or lamella settlers with loading 2–3 m³/m²/hr separate biomass at settling velocities 0.5–1 m/hour, returning activated sludge (RAS) at 50–100% of influent flow to maintain mixed liquor suspended solids (MLSS) 3,000–4,000 mg/L.
Waste activated sludge (WAS) at 0.3–0.5 kilograms per kilogram BOD removed is 30–50 kilograms daily for 100 KLD—preventing carryover that would raise effluent TSS above 20 mg/L.
Tertiary Polishing
Dual media filters (sand 0.5–1 mm + anthracite 1–2 mm) at filtration rates 10–15 m/hr reduce TSS to below 5 mg/L and turbidity below 1 NTU. Granular activated carbon (GAC) columns with empty bed contact time (EBCT) 10–15 minutes adsorb residual organics and odour compounds to below 1 mg/L.
UV disinfection at 40 mJ/cm² or chlorination with 0.5–1 mg/L residual chlorine achieves total coliform below 10 MPN/100 mL—ensuring 90,000–95,000 litres reusable daily from 100 KLD inflow.
Water Quality Parameters After Treatment
Physical Parameters
Turbidity drops from 200–500 NTU raw to below 1 NTU via filtration, matching BIS drinking water limit of 1 NTU and preventing pathogen shielding. Total dissolved solids (TDS) remain 500–1,000 mg/L as minerals pass through biology—acceptable for non-potable but requiring RO for drinking (BIS <500 mg/L preferred). Colour reduces to below 5 Pt-Co units from 100–500 raw via carbon adsorption, eliminating visual staining in reuse applications.
Chemical Parameters
BOD falls below 5–10 mg/L (98%+ removal) vs. BIS drinking <2 mg/L—sufficient for irrigation but marginal for potability. COD reduces to below 20–50 mg/L (95%+ removal) from 400–800 mg/L raw. Ammonia drops below 1–5 mg/L (90%+ nitrification); nitrate stays below 10 mg/L via denitrification. Trace metals (if present) adsorb on carbon to below 0.01 mg/L.
Microbiological Parameters
Total coliform below 10–100 MPN/100 mL (6–7 log reduction) vs. BIS 0 MPN/100 mL for drinking. E. coli absent in 100 mL samples via UV/chlorine—safe for non-contact but regrowth risk in storage.
Emerging Contaminants
Pharmaceuticals reduced 80–90% to 0.001–0.01 µg/L via carbon/UV; pesticides below 0.1 µg/L. Long-term exposure data limited at ng/L levels.
Limitations and Legal Restrictions for Potable Use
Trace Organic Micropollutants
APIs like diclofenac persist at 1–10 ng/L; endocrine disruption potential at chronic intake—conventional STP removes 70–90%, requiring AOP for >99.9%.
Pathogen Regrowth Potential
Chlorine residual (0.5 mg/L) decays in 24 hours; biofilms in distribution pipes regrow coliform if TDS >500 mg/L or temperature >25°C.
Psychological and Cultural Barriers
Public aversion to “sewage-origin” water despite treatment surpassing many river sources.
Legal Framework
CPCB prohibits direct potable reuse; BIS 10500 mandates 0 coliform/100 mL—STP effluent <100 fails. FSSAI bans recycled water in food processing.
Infrastructure Risks
Dual plumbing cross-connections contaminate supply—₹5–10 lakh detection/retrofit per incident.
Safe Reuse Applications for Treated Water
Toilet Flushing
40% residential demand (50–60 litres/person/day); 90,000 litres/day from 100 KLD saves ₹1.5–2 lakh/year at ₹60/KL.
Gardening and Landscaping
BOD <30 mg/L safe; 90,000 litres irrigates 1,800 sqm (50 litres/sqm)—no sodium accumulation.
Cooling Towers
TDS <1,500 mg/L; 20–30% make-up recycling saves ₹2–3 lakh/month in 500 KL/day.
Vehicle Washing and Construction
TSS <50 mg/L prevents spotting; 500–1,000 litres/car cycle.
Groundwater Recharge
BOD <10 mg/L avoids aquifer clogging; 90,000 litres/day recharges 1–2 borewells.
Future Technologies for Achieving Drinkable Recycled Water
Advanced Oxidation Processes (AOP)
Ozone (5–10 mg/L) + UV (40 mJ/cm²) + H2O2 (5 mg/L) degrades APIs >99.9% to <1 ng/L—₹5–8/KL add-on.
Reverse Osmosis (RO)
0.0001 micron membranes reject TDS 95–99%, salts, viruses—energy 2–3 kWh/KL.
Forward Osmosis + Biomimetic Membranes
Draw solution recovers water at 0.5–1 kWh/KL; aquaporin membranes mimic cell transport—70–80% recovery.
Multi-Barrier Systems
STP + RO + UV + remineralisation (Ca 20–40 mg/L, Mg 10–20 mg/L)—taste-matched to BIS.
Real-Time Sensors + AI
Online TOC (<0.5 mg/L), NDMA (<10 ng/L) sensors with AI predictive modelling—alerts at thresholds.
Conclusion
STP treated water is safe for flushing, irrigation, and industrial reuse—removing 99%+ contaminants—but not yet approved for drinking due to trace risks and regulations. SKF Elixer’s Vulcan AABR achieves <10 mg/L BOD and <100 MPN coliform, enabling 90% non-potable recycling. Future AOP + RO will bridge to potability.
Adopt proven sewage water purification for sustainability today. Contact SKF Elixer.
FAQs
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1. What does STP treatment remove from sewage?
95%+ BOD (<10 mg/L from 200–400 mg/L), 99% TSS (<5 mg/L), 6-log pathogens (<100 MPN/100 mL), 80–90% ammonia—via screening, AABR biology, filtration, UV in Vulcan STP.
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2. Why can't STP water be drunk directly?
Trace APIs (1–10 ng/L), potential regrowth, TDS 500–1,000 mg/L, and zero coliform rule (BIS 10500)—legal ban despite 99% purity; cross-connection risk ₹5–10 lakh retrofit.
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3. How safe is treated water for toilet flushing?
BOD <10 mg/L, TSS <5 mg/L, coliform <100 MPN/100 mL—90,000 litres/day from 100 KLD Vulcan saves ₹1.5–2 lakh/year at ₹60/KL, no health risk.
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4. What future tech makes recycled water drinkable?
AOP (ozone-UV) degrades APIs >99.9%; RO rejects TDS 95–99%; mineralisation adds Ca/Mg—₹15–20/KL total.
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5. How does Vulcan STP enable groundwater recharge?
BOD <10 mg/L, nitrate <10 mg/L prevents soil clogging; 90,000 litres/day infiltration recharges 1–2 borewells annually—sustainable in water-stressed societies.
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