Wastewater Treatment — Primary, Secondary & Tertiary

1. Treatment Levels Overview

1.1 Indian STP Typical Treatment Train

Influent → Bar screen → Grit chamber → Primary sedimentation → Aeration tank (Activated sludge) → Secondary clarifier → Chlorination → Discharge / Reuse

Sludge stream: Primary sludge + Return/waste activated sludge → Sludge thickener → Anaerobic digester → Sludge drying beds → Disposal/reuse

2. Preliminary Treatment — Screening and Grit Removal

2.1 Screening

Bar screens (coarse: 50–75 mm; medium: 20–50 mm; fine: 1.5–6 mm) remove floating solids — rags, paper, plastics, sticks — that would clog or damage pumps and downstream equipment. Mechanically cleaned screens are standard in modern STPs; manually cleaned screens used in small plants. Screenings are pressed and disposed to landfill or incinerated.

2.2 Grit Chamber (Detritor)

Grit chambers remove inorganic mineral particles (sand, gravel, cinders, eggshells) that would abrade pumps and settle in digesters, reducing their capacity. Flow is slowed to allow settling of particles > 0.2 mm diameter while allowing organic matter to remain in suspension.

3. Primary Treatment — Sedimentation

3.1 Primary Sedimentation Tank (PST)

The PST removes settleable and floating solids from screened and de-gritted sewage by gravity settling. Unlike water treatment clarifiers, PSTs handle sewage with higher organic content — the sludge collected is called primary sludge (raw sludge) and must be further treated.

3.2 Primary vs Secondary Sedimentation

4. Secondary Treatment — Biological Processes

Secondary treatment uses microorganisms to decompose dissolved and colloidal organic matter that passed through primary treatment. The microorganisms use the organic matter as food and convert it to CO₂, H₂O, and new cell mass, dramatically reducing the BOD.

4.1 Overview of Secondary Biological Processes

4.2 BOD Removal in Secondary Treatment

4.3 Typical Effluent Quality After Each Treatment Stage

5. Oxidation Pond

An oxidation pond (waste stabilisation pond, WSP) is a large, shallow basin where wastewater is treated by natural processes — sunlight, wind, algae, and bacteria — over a long hydraulic retention time (HRT). It is the simplest and lowest-cost secondary treatment option, but requires large land areas. Widely used in small towns and rural areas across India.

5.1 Types of Ponds in a WSP System

6. Imhoff Tank and Septic Tank

6.1 Imhoff Tank

An Imhoff tank is a two-compartment combined sedimentation-digestion unit used for small communities (up to 20,000 people). Sewage flows through the upper sedimentation chamber; settled solids drop through a slot into the lower digestion chamber without disturbance to the settling process. Gas from digestion escapes through gas vents (not through the settling compartment).

6.2 Septic Tank (IS 9601)

A septic tank is an on-site waterproof underground tank that receives raw sewage from a building and provides primary treatment by: (a) settling of heavy solids to the bottom (sludge), (b) flotation of grease and lighter materials to the top (scum), and (c) anaerobic digestion of sludge in the lower zone. The clarified effluent in the middle zone overflows to a soil disposal system (soak pit or leach field).

7. Tertiary Treatment

Tertiary treatment further purifies secondary effluent for sensitive discharge locations (bathing beaches, protected rivers, drinking water intakes) or for reuse (agricultural irrigation, industrial cooling, groundwater recharge).

7.1 Tertiary Treatment Processes

8. Sludge Treatment and Disposal

8.1 Sludge Sources

• Primary sludge: From primary sedimentation; 3–8% solids; high organic content; offensive odour; settles well
• Waste activated sludge (WAS): Excess biological sludge from secondary treatment; 0.5–1% solids; low organic content; more stable
• Digested sludge: After anaerobic digestion; 4–8% solids; reduced organic content; less odorous

8.2 Sludge Treatment Train

Primary sludge + WAS → Sludge thickener → Anaerobic digester → Sludge drying beds → Agricultural land application / Landfill

8.3 Anaerobic Sludge Digestion

8.4 Sludge Drying Beds

9. Effluent Standards — IS 2490

NMCG Ganga specific standards: For STPs discharging within 5 km of Ganga mainstream: BOD ≤ 10 mg/L, TSS ≤ 10 mg/L, FC ≤ 100 MPN/100 mL (more stringent); most existing Indian STPs cannot meet these without tertiary treatment upgrades.

10. Worked Examples (GATE CE Level)

Example 1 — Primary Sedimentation Tank Design (GATE CE 2022 type)

Problem: A sewage treatment plant receives 15 MLD of raw sewage (BOD = 250 mg/L, SS = 300 mg/L). Design a circular primary sedimentation tank with overflow rate = 28,000 L/m²/day and detention time = 2 hours. Find the tank diameter and side water depth. Also find the BOD and SS in the primary effluent (BOD removal = 50%, SS removal = 60%).

Answer: D = 26 m; H = 2.4 m; Primary effluent: BOD = 125 mg/L, SS = 120 mg/L

Example 2 — Overall BOD Removal Efficiency (GATE CE 2021 type)

Problem: A wastewater treatment plant has primary treatment removing 50% BOD and secondary treatment (activated sludge) removing 90% of remaining BOD. The raw sewage BOD₅ = 240 mg/L. Find (a) the effluent BOD₅ and (b) the overall treatment efficiency. Does the effluent meet the IS 2490 standard?

Answer: Effluent BOD = 12 mg/L; E_total = 95%; Meets IS 2490 (BOD ≤ 30 mg/L)

Example 3 — Oxidation Pond Design (GATE CE type)

Problem: Design an oxidation pond (facultative) for a population of 10,000 with sewage flow = 100 LPCD and BOD₅ = 200 mg/L. BOD surface loading = 200 kg/ha/day. Find (a) the pond area and (b) the HRT. Use Ce = Ci/(1 + k_rt) with k_r = 0.15 day⁻¹ to find effluent BOD.

Answer: Area = 1.0 ha; HRT = 10 days; Effluent BOD = 80 mg/L (adequate for land irrigation; needs further treatment for river discharge)

Example 4 — Sludge Volume and Digester Capacity (GATE CE type)

Problem: A STP serves 50,000 people. Primary sludge production = 35 g SS/person/day; sludge SS concentration = 5%. Find (a) the volume of primary sludge per day and (b) the required digester capacity for a 30-day digestion period. Density of sludge = 1000 kg/m³.

Answer: Daily primary sludge = 35 m³/day; Digester capacity = 2100 m³ (42 L/capita)

Example 5 — Septic Tank Volume (GATE CE type)

Problem: Design a septic tank for a building with 30 occupants. Assume sewage generation = 120 LPCD; retention time = 1.5 days. Determine the minimum volume of the tank.

Answer: Minimum septic tank volume ≈ 8.1 m³ (Method 1) or 6.5 m³ (IS 9601); adopt 8.1 m³

Example 6 — Sludge Drying Bed Area (GATE CE type)

Problem: A STP serving 80,000 people produces digested sludge at 0.15 L/capita/day. The sludge drying beds operate at an application rate of 150 L/m² per application and dry in 3 weeks. Calculate the total drying bed area required.

Answer: Required drying bed area = 3360 m² (2 sets of 1680 m² each)

11. Common Mistakes

Mistake 1 — Applying Secondary Treatment Efficiency to Raw Sewage BOD Instead of Primary Effluent BOD

Error: Computing secondary effluent BOD as raw sewage BOD × (1 – secondary efficiency) instead of primary effluent BOD × (1 – secondary efficiency).
Root Cause: Each treatment stage removes a fraction of its influent BOD, not the original raw sewage BOD. Secondary treatment receives primary effluent (already reduced by 50%), so its 90% removal applies to the already-reduced value.
Fix: Follow the sequential calculation: raw BOD → × (1 – E_primary) → primary effluent BOD → × (1 – E_secondary) → secondary effluent BOD. Overall efficiency formula: E_total = 1 – (1 – E₁)(1 – E₂) … confirms this approach.

Mistake 2 — Using Water Treatment Overflow Rates for Wastewater Primary Clarifiers

Error: Applying water treatment sedimentation overflow rate (12,000–24,000 L/m²/day) to a primary sewage sedimentation tank instead of the correct CPHEEO range (24,000–32,000 L/m²/day).
Root Cause: Both are sedimentation tanks, but wastewater primary clarifiers are designed at higher overflow rates than water treatment clarifiers because the sewage has higher suspended solids concentrations and the floc (primarily organic matter) is denser and settles faster than coagulated water floc. Higher overflow rates are acceptable.
Fix: Water treatment clarifier: 12,000–24,000 L/m²/day (with coagulation). Primary sewage clarifier: 24,000–32,000 L/m²/day. Secondary (final) clarifier for activated sludge: 16,000–24,000 L/m²/day (lighter biological floc).

Mistake 3 — Calculating VS Destruction in Digestion as Applied to Total Solids Instead of Volatile Solids

Error: Applying volatile solids (VS) reduction percentage (50–65%) to total solids (TS) instead of VS fraction.
Root Cause: Anaerobic digestion decomposes only the organic (volatile) fraction of sludge solids. Inorganic (fixed) solids remain unchanged. Volatile solids ≈ 70–80% of total solids for primary sludge.
Fix: VS_destroyed = VS_initial × VS reduction percentage. TS_remaining = TS_initial – VS_destroyed = FS_initial + VS_remaining. Never apply VS reduction % to TS directly.

Mistake 4 — Treating Oxidation Pond BOD Removal as the Same as Activated Sludge

Error: Claiming an oxidation pond achieves 90–95% BOD removal like activated sludge, leading to overoptimistic effluent quality predictions.
Root Cause: Oxidation ponds are slower processes. A simple facultative pond at 10 days HRT with k_r = 0.15 day⁻¹ achieves only 60% BOD removal (from Ce = Ci/(1 + k_rt) = Ci/2.5). Higher removals require longer HRT (30+ days) or multiple ponds in series.
Fix: Use the appropriate kinetic model for ponds (complete mix: Ce = Ci/(1 + kt); plug flow: Ce = Ci × e^–kt) with realistic k values for temperature and pond type, rather than assuming activated-sludge-level performance.

Mistake 5 — Designing the Digester for Fresh Sludge Volume Only, Ignoring Digested Sludge Storage

Error: Sizing the digester for digestion period × fresh sludge volume, without adding equal capacity for digested sludge accumulation between withdrawal cycles.
Root Cause: Sludge digestion tanks are not emptied continuously — digested sludge is withdrawn periodically (every few weeks). Between withdrawals, the digested sludge remains in the lower zone while fresh sludge is fed to the upper zone. The tank must accommodate both zones simultaneously.
Fix: Total digester volume = fresh sludge compartment + digested sludge storage compartment ≈ 2 × (daily sludge volume × digestion period). CPHEEO provides capacity as 70–100 L/capita for two-stage digestion.

12. Frequently Asked Questions

Q1. Why does India’s STP capacity significantly exceed its operational capacity, and what are the implications?

A 2021 CPCB assessment found that India has installed STP capacity of about 26,869 MLD against a sewage generation of ~72,368 MLD — only 37% of sewage generated can be treated even at full STP capacity. Worse, only about 60% of installed capacity actually operates (treatment capacity utilization). The gap exists for multiple reasons: many STPs are non-operational due to power supply failures, equipment breakdown, lack of O&M funds, or operator shortage; sewage collection networks (sewers) are incomplete, so STPs built in cities cannot receive sewage from unsewered slums; many STPs are under-loaded (sewers not yet connected) in early years while being over-loaded later. The implication is massive untreated sewage discharge to rivers — the primary cause of river pollution in India. The Namami Gange programme and AMRUT 2.0 together are investing over ₹40,000 crore in new and rehabilitated STPs specifically to address this gap, with performance-based contracts that pay operators only when effluent quality meets prescribed standards.

Q2. What is the UASB reactor and why is it preferred for Indian conditions?

The Upflow Anaerobic Sludge Blanket (UASB) reactor is a high-rate anaerobic treatment unit where sewage flows upward through a blanket of dense granular sludge. Methane-producing microorganisms in the sludge granules decompose organic matter rapidly, achieving 70–85% BOD removal without any external energy input for aeration. The reactor produces biogas (65% CH₄) that can be captured for cooking or electricity generation at the STP, offsetting operational costs. UASB reactors are particularly suitable for India because: (1) India’s warm climate (20–30°C year-round in most cities) sustains methanogenic activity without heating, unlike cold countries where UASB efficiency drops significantly in winter; (2) the energy-positive nature reduces the operational cost burden on cash-strapped municipalities; (3) the smaller footprint compared to oxidation ponds or trickling filters suits land-constrained urban sites; and (4) the relatively simple design requires less skilled operation than activated sludge processes. India leads the world in UASB capacity — Kanpur, Varanasi, Lucknow, Agra, and many other cities on the Ganga have large UASB-based STPs. The limitation is that UASB effluent (BOD ≈ 30–80 mg/L) requires post-treatment (a polishing pond or secondary treatment) before it can meet IS 2490 standards for river discharge.

Q3. How does the activated sludge process (covered in Civil_66) differ from the trickling filter?

Both are aerobic biological secondary treatment processes, but they differ fundamentally in how microorganisms are held. In the activated sludge process (ASP), microorganisms are suspended in the wastewater in the aeration tank — they are kept in suspension by mechanical aerators or diffused air, and their concentration is controlled by recycling settled sludge from the secondary clarifier back to the aeration tank. The ASP is a suspended growth process. In a trickling filter, microorganisms grow as a biofilm attached to the surface of a packing media (rock, plastic). Wastewater is sprayed over the media; it trickles downward while air flows upward, supplying oxygen to the biofilm. The trickling filter is an attached growth process. Key practical differences: ASP requires continuous energy input for aeration (major operating cost); trickling filters use passive aeration (lower energy). ASP handles variable loads better and achieves higher BOD removal (90–95%) in a smaller footprint; trickling filters are less expensive to operate, more resistant to toxic shock, but less consistent in performance (especially in cold weather). ASP produces a concentrated sludge that must be managed; trickling filters produce less sludge but include biological film sloughing that settles in the secondary clarifier.

Q4. When should a municipality choose an oxidation pond over an activated sludge process?

Oxidation ponds should be preferred when: (a) land is cheap and available in abundance — ponds require 10–100× more area than ASP for the same population; (b) the municipality has minimal operating budget and technical staff — ponds essentially run themselves with periodic monitoring; (c) the town is small (< 50,000 population) and located in a hot, sunny climate where natural treatment is effective year-round (most of peninsular and central India); (d) sludge management is a challenge — ponds generate little excess sludge compared to ASP, which produces large volumes of waste activated sludge requiring treatment and disposal. Activated sludge should be preferred when: land is expensive or unavailable (urban Indian cities); high effluent quality (< 10 mg/L BOD) is required; the plant is large (> 10 MLD) where operating efficiency justifies the capital cost; or advanced nutrient removal is needed (nitrification-denitrification is readily incorporated into ASP systems). In practice, many medium-sized Indian municipalities use UASB + facultative pond + maturation pond as a cost-effective combination — UASB provides high-rate primary-secondary treatment; ponds provide final polishing and pathogen removal using solar energy at low cost.