Wastewater engineering

In short

Wastewater engineering deals with the collection, characterisation, treatment and safe disposal or reuse of sewage and industrial effluent, so that discharged water does not damage rivers, land or public health. It covers quantity estimation, laboratory characterisation (BOD, COD, solids, nitrogen), unit operations and processes (screening, grit removal, sedimentation, biological treatment, disinfection), sludge handling, and effluent standards set under the Water (Prevention and Control of Pollution) Act, 1974 and CPCB/MoEFCC notifications. It is one of the highest-weightage sections of GATE Civil (Environmental Engineering) and of university Environmental Engineering-II papers. Design and operation jobs sit with state PHEDs, Jal Nigams, Jal Boards, NMCG/AMRUT and Namami Gange STP projects, and with consultants and EPC firms such as L&T, VA Tech Wabag, Tata Projects, SUEZ India, Thermax and NEERI-linked research teams.

Explained

Quantity, characteristics and codes

• Sewage = spent water supply plus added solids; sanitary (domestic) sewage, industrial effluent, storm water; combined vs separate systems. Dry weather flow (DWF) is the sewage flow without rain.
• Quantity: take 70-80% of the water supply as sewage; add infiltration/inflow. Typical Indian design water supply 135 lpcd (CPHEEO/BIS for towns with sewerage) → sewage ≈ 100-110 lpcd. Peak factor (CPHEEO sewerage manual): 3 up to 20,000 persons, 2.5 for 20,000-50,000, 2.25 for 50,000-7.5 lakh, 2 above that.
• Physical characteristics: total solids TS = suspended (SS) + dissolved (DS); each splits into volatile (organic) and fixed (mineral). Fresh sewage is grey, musty; stale sewage is black and smells of H₂S. Typical Indian raw sewage: SS 200-400 mg/L, BOD₅ 200-300 mg/L, COD 400-600 mg/L.
• Chemical: pH 6.5-8; organic nitrogen and ammonia (TKN 30-50 mg/L); phosphorus 5-10 mg/L; chlorides, sulphates; oil and grease.
• Biological: bacteria, viruses, protozoa, helminth eggs; E. coli / faecal coliform as indicator organisms (MPN per 100 mL).
• BOD: oxygen used by microbes to oxidise biodegradable organics; standard test 5 days at 20 °C (BOD₅), incubated dark, dilution with seeded water. COD: chemical oxidation by K₂Cr₂O₇ in acid with Ag₂SO₄ catalyst — measures total oxidisable matter, done in ~3 h. ThOD is stoichiometric. Generally COD > BOD_ultimate > BOD₅; BOD₅/COD ≈ 0.4-0.6 for domestic sewage (easily biodegradable), < 0.3 suggests industrial/refractory load.
• TOC measures carbon directly; relative stability and oxygen consumed are older indices.
• Codes and law: CPHEEO *Manual on Sewerage and Sewage Treatment* (2013 revision) is the practice manual; IS 4764 (sewage effluent discharge), IS 1622/3025 (sampling and test methods); Water Act 1974 and Environment (Protection) Act 1986 with Rules; EP Rules Schedule-I/VI effluent standards; CPCB consent to establish/operate; Prohibition of Employment as Manual Scavengers Act, 2013 governs sewer entry. 2017 MoEFCC STP standards (BOD 10-20, TSS 20-50 mg/L, faecal coliform 100-1000 MPN/100 mL depending on class/metro) have been litigated and amended — verify the current notification before quoting numbers.

Self-purification and effluent disposal

• Disposal routes: dilution in a stream, land disposal (sewage farming, broad irrigation), oxidation ponds/lagoons, sea outfall, reuse for irrigation, construction and industrial cooling.
• Self-purification of a stream: dilution, sedimentation, oxidation by microbes, reaeration, sunlight, and dying off of pathogens. Zones downstream: degradation → active decomposition → recovery → clear water.
• Deoxygenation (BOD exertion) competes with reaeration from the atmosphere; the result is the DO sag (Streeter-Phelps) curve with a critical point of minimum DO.
• Reoxygenation capacity rises with turbulence, shallow depth and a large existing deficit (reaeration rate = k_R × D); cold water holds more DO at saturation, though k_R itself rises with temperature.
• Design DO is usually kept ≥ 4 mg/L for fish life; dissolved oxygen saturation at 20 °C ≈ 9.17 mg/L, at 25 °C ≈ 8.25 mg/L.
• Eutrophication from N and P causes algal blooms, night-time DO crashes and fish kills — hence nutrient removal in sensitive catchments.

Preliminary and primary treatment

1. Screens: coarse (bar, 50-100 mm), medium (20-50 mm), fine (< 20 mm / mesh). Velocity through bars kept ~0.8-1.0 m/s to avoid grit deposition and screen blinding. Mechanically raked in modern STPs.
2. Grit chamber: removes inorganic sand/gravel > 0.2 mm. Horizontal-flow type designed for velocity 0.2-0.3 m/s (0.3 m/s typical), detention 45-60 s; velocity held constant by a proportional (Sutro) weir or parshall flume. Aerated and vortex (detritor) grit units are common now.
3. Skimming tank / oil and grease trap: compressed air floats grease; mainly for industrial and combined flows.
4. Primary sedimentation tank (clarifier): detention 1.5-2.5 h; surface overflow rate 30-40 m³/m²·day; weir loading 100-300 m³/m·day; removes 50-70% SS and 25-40% BOD. Rectangular or circular with scraper mechanism; sludge drawn from hopper.
5. Chemically enhanced primary treatment (CEPT) with alum/FeCl₃/polymer raises SS removal above 80% — used where land is scarce.

Secondary (biological) treatment

• Principle: microorganisms convert soluble and colloidal organics into new cell mass and CO₂, and the biomass is then separated by settling. Attached growth (fixed film) vs suspended growth.
• Trickling filter (attached growth): sewage sprayed by rotary distributor over 1.8-2.5 m stone media (or plastic media in high-rate towers); biofilm oxidises organics; underdrains collect effluent; sloughing sheds excess film.
  • Low-rate (standard): hydraulic loading 1-4 m³/m²·day, organic loading 80-320 g BOD/m³·day, no recirculation, BOD removal 80-90%, nitrified effluent.
  • High-rate: hydraulic loading 10-40 m³/m²·day, organic loading 500-1000 g BOD/m³·day, recirculation ratio 0.5-3, removal 65-85%.
  • Problems: ponding (clogged media), psychoda (filter) flies, odour, cold-weather drop in efficiency.
• Activated sludge process (ASP, suspended growth): aeration tank + secondary clarifier + return activated sludge (RAS); excess biomass wasted as WAS.
  • Key parameters: MLSS 1500-4000 mg/L (MLVSS ≈ 0.7-0.8 MLSS); F/M ratio 0.2-0.5 kg BOD/kg MLVSS·day (conventional); SRT (θc) 5-15 d; HRT 4-8 h; return ratio 0.25-1.0; aeration 0.5-1.5 kg O₂ per kg BOD removed.
  • Modifications: conventional plug flow, complete mix, tapered aeration, step aeration, extended aeration (F/M 0.05-0.15, SRT 20-30 d, no primary settling), contact stabilisation, high-rate ASP, oxidation ditch (Carrousel/Pasveer loop with brush/disc aerators), SBR (fill-react-settle-decant-idle in one tank; very common in new Indian STPs), MBBR and MBR (membrane separation, footprint-saving, used in metro and in-situ projects).
  • Sludge settling indices: SVI = (settled volume mL/L × 1000)/MLSS mg/L; good 50-150 mL/g. High SVI = bulking sludge (filamentous growth, low DO, low F/M, nutrient deficiency); other troubles: rising sludge (denitrification in clarifier), pin floc, foaming (*Nocardia*).
  • Aeration: diffused (fine-bubble membrane discs, blowers) or mechanical (surface/turbine aerators); DO maintained 1.5-2 mg/L in the tank.
• Secondary clarifier: SOR 16-24 m³/m²·day, detention 2-2.5 h, solids loading 3-6 kg/m²·h; its job is both clarification and thickening of RAS.
• Anaerobic treatment — UASB: upflow sludge blanket with granular biomass, HRT 8-10 h, upflow velocity 0.5-1.0 m/h, organic loading 1-10 kg COD/m³·day; produces biogas (~0.3-0.35 m³ CH₄ per kg COD removed), very little sludge, no aeration power — but BOD removal only 60-75%, so a polishing pond (1 day) follows. Many Ganga-basin STPs (Kanpur, Mirzapur) use UASB + FPU.
• Waste stabilisation ponds: anaerobic → facultative → maturation series. Facultative pond: depth 1-1.5 m, detention 15-30 d, surface BOD loading 150-350 kg/ha·day (lower in cold climates); algae supply oxygen by day. Cheap, land-hungry, excellent pathogen (coliform) removal. Aerated lagoon is a pond with mechanical aerators, detention 2-6 d.
• Nutrient removal: nitrification (autotrophs, needs SRT > 8-10 d, 4.57 g O₂/g N, alkalinity consumed) followed by anoxic denitrification (needs carbon; recovers oxygen and alkalinity); phosphorus by biological P-removal (anaerobic/aerobic cycling) or chemical precipitation with alum/ferric/lime.
• Disinfection: chlorination (contact 30 min, residual 0.5 mg/L), UV (preferred where chlorinated by-products and reuse matter), ozone. Required where effluent is reused or discharged to bathing-quality waters.

Sludge treatment and disposal

• Sequence: thickening → stabilisation (digestion) → conditioning → dewatering → disposal/reuse.
• Thickening: gravity thickener (primary sludge 4-8% solids), dissolved-air flotation or gravity-belt/centrifugal thickening for WAS.
• Anaerobic digestion: hydrolysis → acidogenesis → acetogenesis → methanogenesis; mesophilic 35 °C with 15-30 d SRT (high-rate heated, mixed digesters ~15 d); biogas 60-70% CH₄, ~0.9-1.1 m³ per kg VS destroyed, used in gas engines for in-house power. Needs pH 6.8-7.2 and low volatile acid/alkalinity ratio; upset by shock loads, heavy metals and sulphide.
• Aerobic digestion suits small plants and extended-aeration sludge; simpler but power-hungry and gives no gas.
• Dewatering: sludge drying beds (common and cheapest in India), centrifuge, belt filter press, filter press; digested sludge is conditioned with polymer. Cake 18-30% solids.
• Disposal/reuse: co-composting, soil conditioner/manure (subject to heavy-metal limits), sanitary landfill, incineration (rare). Septage/FSSM under the National FSSM Policy 2017 brings septic-tank sludge to co-treatment at STPs.
• Sludge volume ∝ 1/solids content: halving solids doubles volume — the governing idea in all sludge calculations.

On-site and decentralised systems

• Septic tank: 2-compartment, detention 24-48 h (for ≤ 20 users), sludge accumulation allowance per IS 2470 Part 1; desludged every 2-3 years; effluent needs soak pit, dispersion trench or filter (IS 2470 Part 2) — never direct discharge.
• Imhoff tank: settling chamber above a digestion chamber, separated by slots, so gas does not disturb settling.
• Twin-pit pour-flush latrine (Swachh Bharat Mission standard for rural India), bio-digester / biotoilets (Indian Railways), DEWATS (settler + anaerobic baffled reactor + planted gravel filter) for campuses and colonies.
• Reuse: treated sewage for gardens, flushing, construction, power-plant cooling (Nagpur-Koradi and Chennai models) and industry — mandated in several state policies and in AMRUT reuse targets.

Key points

• Sewage quantity - usually 70-80% of water supplied, plus infiltration; peak factor 2-3.
• BOD₅ - oxygen demand over 5 days at 20 °C; the standard measure of biodegradable organic load.
• COD vs BOD - COD is chemical, fast and almost always higher; ratio BOD₅/COD < 0.3 indicates poorly biodegradable waste.
• Grit chamber velocity - held at 0.2-0.3 m/s so organics stay in suspension while sand settles.
• Primary clarifier - SOR 30-40 m³/m²·day, detention ~2 h, removes 50-70% SS and 25-40% BOD.
• Overflow rate - governs settling, not detention time; particles with settling velocity ≥ SOR are fully removed (ideal tank).
• F/M ratio - kg BOD applied per kg MLVSS per day; sets sludge age, oxygen demand and effluent quality.
• SRT (sludge age) - total MLVSS in system divided by MLVSS wasted per day; > 8-10 d needed for nitrification.
• MLSS/RAS - aeration-tank solids are maintained by returning settled sludge; return ratio from solids balance.
• SVI - settled sludge volume per gram of solids; 50-150 mL/g healthy, > 200 means bulking.
• Trickling filter - fixed-film, low-rate 80-90% BOD removal; high-rate uses recirculation; prone to ponding and flies.
• Extended aeration/oxidation ditch/SBR - low-F/M, long-SRT variants giving stable, well-nitrified effluent in small footprints.
• UASB - anaerobic, no aeration power, yields biogas, needs post-treatment pond; widely used in Ganga-basin STPs.
• Stabilisation ponds - cheapest and best for pathogen removal, but need large land and long detention.
• Anaerobic digestion - mesophilic 35 °C, 15-30 d, biogas 60-70% methane, stabilises and reduces sludge.
• Self-purification - deoxygenation versus reaeration gives the DO sag curve; critical DO is the design check.
• Standards and law - Water Act 1974, EP Act 1986, CPCB consent; CPHEEO Manual 2013 for design; STP effluent limits revised by MoEFCC notifications, so cite the current one.
• Sludge volume - inversely proportional to solids content; thickening before dewatering saves cost.

Formulas and examples

Example 1 — BOD test and ultimate BOD. A 5 mL sample is diluted to 300 mL. Initial DO = 8.6 mg/L, DO after 5 days at 20 °C = 3.2 mg/L. k_D = 0.1 /day (base 10).

• Dilution factor = 300/5 = 60.
• BOD₅ = (8.6 − 3.2) × 60 = 5.4 × 60 = 324 mg/L.
• 1 − 10^−0.1×5 = 1 − 10^−0.5 = 1 − 0.3162 = 0.6838.
• L₀ = 324/0.6838 ≈ 474 mg/L ultimate BOD.
• BOD₃ = 474 × (1 − 10^−0.3) = 474 × (1 − 0.5012) = 474 × 0.4988 ≈ 236 mg/L.

Example 2 — DO sag in a river. Sewage 0.5 m³/s with BOD_ult 180 mg/L and DO 0 mg/L joins a river of 5.0 m³/s with BOD_ult 2 mg/L and DO 8.2 mg/L. Both at 20 °C (DO_sat = 9.17 mg/L). k_D = 0.1/day, k_R = 0.4/day (base 10).

• Total Q = 5.5 m³/s.
• L₀ = (0.5×180 + 5.0×2)/5.5 = (90 + 10)/5.5 = 100/5.5 = 18.18 mg/L.
• DO_mix = (0.5×0 + 5.0×8.2)/5.5 = 41/5.5 = 7.45 mg/L → D₀ = 9.17 − 7.45 = 1.72 mg/L.
• t_c = [1/(0.4 − 0.1)] log₁₀{(0.4/0.1)[1 − 1.72(0.3)/(0.1×18.18)]}

= (1/0.3) log₁₀{4 × [1 − 0.516/1.818]} = 3.333 log₁₀{4 × 0.7162} = 3.333 log₁₀(2.865) = 3.333 × 0.4572 = 1.52 days.

• D_c = (0.1/0.4) × 18.18 × 10^−0.1×1.52 = 0.25 × 18.18 × 10^−0.152 = 4.545 × 0.7047 = 3.20 mg/L.
• Minimum DO = 9.17 − 3.20 = 5.97 mg/L — above the 4 mg/L limit, so the discharge is acceptable at this dilution.

Example 3 — Primary clarifier and grit chamber. Town of 1,00,000 persons, water supply 135 lpcd, 80% reaching the sewers.

• Q = 1,00,000 × 135 × 0.8 /10⁶ = 10.8 MLD = 10,800 m³/day = 0.125 m³/s.
• Primary clarifier at SOR 35 m³/m²·day → A = 10,800/35 = 308.6 m²; circular tank d = √(4×308.6/π) = √392.9 ≈ 19.8 m (say 20 m).
• At 2.0 h detention, V = 10,800 × 2/24 = 900 m³ → depth = 900/308.6 = 2.92 m (plus 0.5 m freeboard, 0.3 m sludge zone).
• Grit chamber for peak flow (factor 2.25) = 24,300 m³/day = 0.28125 m³/s at v = 0.25 m/s → cross-section A = 0.28125/0.25 = 1.125 m²; for detention 60 s, length L = v × t = 0.25 × 60 = 15 m; with depth 1.0 m, width ≈ 1.13 m.

Example 4 — Activated sludge design. Q = 10 MLD, influent BOD₅ S₀ = 250 mg/L after primary settling 150 mg/L, required effluent S = 20 mg/L, MLVSS X = 2500 mg/L, Y = 0.6 kg/kg, k_d = 0.06 /day, θc = 10 days.

• V = [10,000 × 0.6 × 10 × (150 − 20)] / [2500 × (1 + 0.06×10)] m³ (using Q in m³/day × mg/L consistently)

= (10,000 × 0.6 × 10 × 130)/(2500 × 1.6) = 78,00,000/4000 = 1950 m³.

• HRT = 1950/10,000 × 24 = 4.68 h — within the 4-8 h range.
• F/M = (10,000 × 150)/(1950 × 2500) = 15,00,000/48,75,000 = 0.308 kg BOD/kg MLVSS·day — conventional range.
• Sludge wasted P_x = 0.6 × 10,000 × (150 − 20)/10⁶ × 1000/(1 + 0.6) kg/day = (0.6 × 10 × 130)/1.6 = 487.5 kg/day.
• Return ratio, with X_r = 10,000 mg/L: R = 2500/(10,000 − 2500) = 0.33.
• Oxygen ≈ Q(S₀ − S)/0.68 − 1.42 P_x = (10 × 130)/0.68 − 1.42 × 487.5 = 1912 − 692 = 1220 kg O₂/day (carbonaceous only).
• Secondary clarifier at SOR 20 m³/m²·day → A = 10,000/20 = 500 m² → d ≈ 25.2 m.

Example 5 — Sludge volume after thickening. Sludge solids 2000 kg/day at 1.0% solids, specific gravity 1.02; thickened to 4.0% solids.

• V₁ = 2000/(1000 × 1.02 × 0.01) = 2000/10.2 = 196 m³/day.
• V₂ = 2000/(1000 × 1.02 × 0.04) = 2000/40.8 = 49 m³/day.
• Volume reduced to one-fourth — digester size and pumping cost fall in the same proportion.

Where it is used

• Municipal STP design and operation: Delhi Jal Board, BWSSB, CMWSSB, MCGM and state Jal Nigams run plants from a few MLD to 500+ MLD; roles include design engineer, O&M engineer and process/lab chemist.
• Mission programmes: Namami Gange/NMCG hybrid-annuity STPs, AMRUT 2.0 sewerage and reuse projects, Swachh Bharat FSSM and septage treatment plants.
• EPC and technology firms: L&T Water, VA Tech Wabag, SUEZ India, Thermax, Ion Exchange, Triveni, Toshiba Water — bid, design and commission SBR/MBR/UASB plants.
• Industrial ETPs and CETPs: textile (Tirupur zero-liquid-discharge), tannery (Kanpur, Ranipet), distillery spent wash, pharma, paper and sugar; CETPs in GIDC, MIDC and SIPCOT estates.
• Regulators and consultants: CPCB and state pollution control boards (consent, monitoring, online effluent monitoring systems), EIA consultants, NEERI and IIT research groups.
• Reuse and resource recovery: treated sewage to thermal power plants and industry (Nagpur-Koradi, Chennai tertiary plants), biogas-to-power at large STPs, co-composted sludge as soil conditioner.

Interview questions

• Q: Why is BOD measured at 5 days and 20 °C? A: Five days captures most carbonaceous demand before nitrification interferes, and 20 °C is a reproducible average stream temperature; the condition is a standardised convention, not a natural limit.
• Q: Why is COD always greater than BOD? A: Chemical oxidation attacks both biodegradable and non-biodegradable organics, while BOD only measures what microbes can oxidise in the test period.
• Q: What governs removal in a sedimentation tank — detention time or surface area? A: Surface overflow rate (Q/A); in an ideal tank any particle with settling velocity ≥ Q/A is removed regardless of tank depth.
• Q: Why is grit-chamber velocity kept near 0.3 m/s? A: It is high enough to keep lighter organic matter in suspension and low enough to let inorganic grit settle, so the grit removed is clean and does not putrefy.
• Q: What is sludge age and why does nitrification need a long one? A: Sludge age (SRT) is the average time biomass stays in the system; nitrifying autotrophs grow slowly, so they wash out unless SRT exceeds about 8-10 days at Indian temperatures.
• Q: What causes sludge bulking and how is it fixed? A: Filamentous growth from low DO, low F/M, nutrient deficiency or septic feed; corrected by raising DO, adjusting F/M and RAS, chlorinating the return sludge or adding selectors.
• Q: Why does a UASB plant need a polishing unit? A: Anaerobic treatment removes only 60-75% BOD and little pathogen load, so a 1-day polishing pond or aerated unit is added to meet discharge standards.
• Q: Compare trickling filter and activated sludge. A: The trickling filter is a fixed-film process with lower power, simpler operation and lower efficiency; ASP is suspended growth with higher efficiency and better effluent but higher aeration power and more skilled operation.
• Q: Why is an SBR preferred in new Indian STPs? A: It combines aeration and settling in one tank, needs no separate clarifier or RAS pumping, handles variable flow, and gives nitrified low-BOD effluent in a small footprint.
• Q: What is the DO sag curve and what does the critical point tell you? A: It is the DO profile downstream of a discharge where deoxygenation outruns reaeration; the critical point gives the lowest DO and its distance, which decides whether aquatic life standards are met.
• Q: Why is anaerobic digestion preferred over aerobic for large STP sludge? A: It needs no aeration power, destroys 40-50% of volatile solids, reduces pathogens and odour, and produces methane that can run the plant's own gas engines.
• Q: An STP's effluent BOD suddenly rises while MLSS falls. What would you check? A: Sludge loss over the clarifier weir (bulking or hydraulic overload), excess sludge wasting, loss of DO from blower failure, or a toxic industrial shock load killing the biomass.

Common mistakes

• Using base-10 and base-e BOD rate constants interchangeably — k = 2.303 k_D; mixing them makes every BOD answer wrong.
• Reporting BOD₅ as ultimate BOD, or forgetting the dilution factor in the bottle test.
• Designing a clarifier on detention time alone and ignoring the surface overflow rate (or vice versa).
• Unit slips in aeration-tank formulas — mg/L with m³ and MLD; always reduce to kg/day before dividing.
• Using MLSS where MLVSS is required in F/M and SRT (MLVSS ≈ 0.7-0.8 × MLSS).
• Getting the oxygen-balance signs wrong: nitrogenous demand (4.57 g O₂/g N) is added, the biomass term (1.42 × P_x) is subtracted.
• Quoting old effluent standards (BOD 30 mg/L) when the applicable notification or state consent condition is stricter, and ignoring the local SPCB consent limit.

Related topics

Sewer network design · Water treatment plant design · Water supply engineering · Solid waste and air pollution · Environmental impact assessment · Open channel flow.