IS 2470 Part 1 : 1985Code of Practice for Septic Tanks - Part 1: Design Criteria and Construction

IS 2470 (Part 1) is the code of practice for septic tanks — Design Criteria and Construction. It governs the dimensioning, materials, structural design, and construction of on-site sewage treatment by septic tank for individual buildings, small clusters, and rural / peri-urban settlements not served by piped sewerage.

Use IS 2470 Part 1 when designing on-site sanitation for: - Individual residences without piped sewer connection - Bungalows, farmhouses, holiday homes in rural areas - Small institutions (schools, primary health centres, anganwadi) up to ~300 users - Industrial/commercial sites in unserved zones - Initial / interim sanitation while piped sewer is being laid

IS 2470 Part 2 covers secondary treatment (soakaway, dispersion trench, filter beds) — the downstream of the septic tank. Both Parts must be used together for a complete on-site system.

Don't use IS 2470 alone for: - Settlements > 300 persons — design as a centralised treatment plant per CPHEEO Manual on Sewerage and Sewage Treatment - Industrial wastewater with chemicals/heavy metals — chemical treatment is needed beforehand - Hospital wastewater — pathogen load demands additional disinfection - Areas with high water table (< 1.5 m below ground) without designed lining

Sewage from toilets, washing, and kitchens flows by gravity into a watertight underground tank, where it is held for 24-48 hours. Inside the tank:

1. Sedimentation — heavy solids settle to the bottom as sludge (60-70 % of removal) 2. Floatation — fats, oils, grease (FOG) rise to the top as scum (~10 %) 3. Anaerobic digestion — microbes break down organic matter (CH₄, CO₂, water as by-products) 4. Effluent (clarified middle layer) overflows from the tank to a soak pit, dispersion trench, or sand filter for further treatment

The septic tank is a physical separator + partial digester, not a full treatment system. The effluent still has high BOD (~250 mg/L), pathogens, and nutrients — secondary treatment (Part 2) is essential before discharge.

A typical 4-person household septic tank is desludged once every 3-5 years. Frequency depends on actual use, food waste in drains, and detergents (high detergent use slows microbial activity).

Tank sizing (IS 2470 Part 1 Table 1, simplified):

Volume = (Q_inflow × T_retention) + Sludge accumulation

• Q_inflow = water consumption per user × number of users (typically 90-120 L/p/d for residential)
• T_retention = 24-48 hours (governs primary settling)
• Sludge accumulation = 30-50 L per user per year × desludge interval (years)

Liquid capacity for residential (households):

| Number of users | Liquid capacity (m³) | Approx tank dimensions (L × W × Liquid depth, m) | |---|---|---| | Up to 5 | 2.0 | 1.5 × 0.75 × 1.5 | | Up to 10 | 2.5 | 1.8 × 0.9 × 1.5 | | Up to 15 | 3.5 | 2.0 × 1.0 × 1.7 | | Up to 20 | 4.5 | 2.3 × 1.1 × 1.8 | | Up to 30 | 6.5 | 2.7 × 1.3 × 1.8 | | Up to 50 | 10.0 | 3.5 × 1.6 × 1.8 |

Add ≥ 0.3 m freeboard above liquid level for scum and gas accumulation.

Tank proportions (Clause 6.2): - Length-to-width ratio: 2:1 to 4:1 (longer = better settling) - Liquid depth: 1.0 m minimum, 1.8 m maximum - Two-compartment tank recommended (first compartment 2/3 volume, second 1/3) — improves sludge retention

Inlet and outlet (Clause 6.3): - Inlet pipe: enters via T-fitting, with bottom of T at least 200 mm below liquid surface - Outlet pipe: T-fitting, bottom of T at 0.4 × liquid depth from top - Inlet always 30-50 mm higher than outlet (gravity flow, no surge back)

Materials: - Walls: 230 mm brick masonry in 1:5 cement-sand mortar, or 150 mm RCC M20 with 12 mm cover both faces; inside plastered 12 mm thick 1:3 cement-sand smooth finish - Floor: 150 mm RCC M20 over 75 mm PCC bedding - Cover slab: RCC M20 with 12 mm bars @ 150 c/c both ways; weight 250 kg minimum to deter unauthorised opening; manholes 600 × 600 mm with cast-iron / pre-cast cover - Vent pipe: GI / PVC, 100 mm diameter, extending ≥ 600 mm above eaves of nearest building, with mosquito proof mesh

• IS 2470 Part 2:1985 — disposal of septic tank effluent (soakage pit, dispersion trench, filter bed).
• IS 1742:1983 — code of practice for building drainage.
• IS 1172:1993 — code of basic requirements for water supply, drainage, sanitation in buildings.
• IS 4985:2021 — uPVC pipes (often used for inlet/outlet pipework).
• IS 13592:2013 — uPVC pipes for soil and waste discharge (DWV — sewer connections to tank).
• IS 458:2003 — RCC pipes (large-diameter sewer, leach pit lining).
• IS 651:1992 — salt-glazed stoneware pipes (legacy alternative).
• IS 456:2000 — RCC design (for tank walls, floor, cover slab).
• IS 875 Parts 1-2 — dead and live loads on cover slab.
• IS 6403:1981 — bearing capacity of soil under tank.
• CPHEEO Manual on Sewerage and Sewage Treatment — central authority for sanitation design in India.
• Swachh Bharat Mission (Gramin) Twin Pit Toilet design — alternative for rural households at lower cost.
• NBC 2016 Part 9 — Plumbing Services (the broader National Building Code).

1. Tank too small. Most failures stem from undersizing — design for actual occupancy, not nominal house size. Rule: minimum 2 m³ regardless of users, even for a single-occupant cottage. Underdesigned tanks fill rapidly with sludge, retention drops below 24 hours, effluent quality crashes. 2. Single compartment instead of two. Single-compartment tanks let sludge swept into the soakaway over time, clogging it. Two-compartment design is mandatory for any tank > 3 m³. 3. Inlet without T-fitting. Direct horizontal inlet creates a turbulent jet that disturbs the scum layer and breaks anaerobic stratification. Always use T-fitting with the bottom 200 mm submerged. 4. Vent pipe too short or absent. Anaerobic digestion produces methane; without venting, gas builds up, scum hardens, and tank operation fails. Vent must reach above eaves and have mosquito mesh. 5. Tank built without water-tightness test. Hydrostatic test (filled to liquid level for 7 days, drop ≤ 25 mm/day) is mandatory before commissioning. Many tanks leak from day 1, contaminating soil. 6. Tank built within 7.5 m of a drinking water source. IS 2470 specifies minimum separation distances from wells, water mains, and habitation: - 7.5 m from any drinking-water well or borewell - 1.8 m from foundation of any building - 5.5 m from boundary of property 7. Soakaway designed without percolation test. IS 2470 Part 2 requires soil percolation rate measurement before sizing the soakaway. Pour-test in a 300 mm dia × 300 mm deep test pit; record minutes per inch. 8. No desludging plan. Owner often unaware that the tank needs desludging every 3-5 years. Provide a maintenance schedule + emergency contact. 9. High water table without lined / floating-resistant design. If groundwater rises above tank bottom in monsoon, the empty tank can float (uplift > weight). Anchor with weighted floor or use design that accounts for buoyancy. 10. Direct discharge to drain or stream. Septic tank effluent is NOT compliant with surface water discharge norms. Always provide secondary treatment (soakaway, filter bed, constructed wetland) before any release.

Standard design cascade for a building/cluster without piped sewer:

1. Population estimation — number of users at peak occupancy (don't underdesign for guest rooms, expansion). 2. Water demand (CPHEEO) — typical 90-135 L/p/d residential; 60 % of supplied water becomes wastewater (the discount is for evaporation and external water use). 3. Septic tank sizing (this code, IS 2470 Part 1) — Q × T + sludge accumulation. 4. Soakaway / filter bed sizing (IS 2470 Part 2) — based on soil percolation rate; sufficient to dispose of full daily inflow. 5. Site layout — septic tank away from water sources, foundations, boundary; secondary treatment downstream of tank, ideally lower elevation. 6. Structural design — RCC tank walls, floor, cover slab per IS 456:2000; buoyancy check if water table high. 7. Inlet / outlet plumbing — uPVC per IS 13592 or salt-glazed stoneware; T-fittings; vent pipe. 8. Construction — water-tightness test, percolation test verification, plaster smooth finish for sludge cleaning. 9. Commissioning — fill with water + a starter culture (cow dung slurry or commercial bacteria pack) to establish anaerobic flora. 10. Operation manual — desludging cadence, what NOT to flush (oils, plastics, harsh chemicals — they kill the microbial culture), maintenance contact.

For large institutional/commercial use (> 100 users), consider packaged sewage treatment plants (SBR, MBBR, MBR) per CPHEEO instead — they produce treatable effluent that meets discharge norms and have smaller footprint per person.