Sewage Treatment Plant (STP) — Working, Bacteria, BOD & Troubleshooting
How three chambers turn sewage into legal effluent on blown air, and what rots first when the blower stops.
Key Principles at a Glance 5 points
- Three chambers in series do three jobs: aeration digests organics on blown air, settling drops the sludge out, chlorine kills what settling cannot.
- Aerobic bacteria need free oxygen and make safe water plus CO2; starved of air the same tank goes anaerobic and brews methane, hydrogen sulphide and ammonia.
- Effluent must read under 50 mg/l BOD against raw sewage at 300–600, and under 250 faecal coliforms per 100 ml — the chlorine stage exists to hit the second number.
- A standby blower is fitted because an aeration chamber without air goes anaerobic within hours; chambers run 0.5–0.8 bar.
- Clear water is not disinfected water: high coliform with clear effluent means the chlorine tablets are spent.
1. Three Chambers in Series — Digest, Drop, Disinfect
Idea in one line: sewage cleans itself if bacteria get air — the plant is just three rooms that keep the air, the sludge and the chlorine in order.
The plant is three chambers in series: aeration (sludge + blown air, where bacteria digest organics), settling (sludge sinks, clarified water returns or moves on), and sterilisation (chlorine tablets kill what settling cannot). Air reaches the first chamber through a service-air line stepped down by a pressure-reducing valve and filter — chambers run 0.5–0.8 bar, and that pressure is the pulse of the plant.
How the rooms hand over: aeration holds the living sludge under constant air; settling lets that sludge sink while clarified water either returns to aeration or moves forward; sterilisation doses the clarified flow with chlorine tablets so bacteria counts — not just visible solids — fall inside limits.
2. Two Bacterial Worlds — and the Two Numbers That Certify Them
Idea in one line: the same sewage becomes clean effluent or poison gas depending on one input — free oxygen.
Aerobic — with free oxygen
Break organics into water, CO2, inert residue plus energy to grow more bacteria. Safe products, clean effluent — this is the design intent, and the blower exists to feed it.
Anaerobic — without free oxygen
Survive on chemically bound oxygen and rot organics into methane, hydrogen sulphide and ammonia — putrefaction. Corrosive, noxious effluent, and a maintenance atmosphere that can kill: ventilate before opening anything.
Two numbers certify the plant. Coliform count under 250 faecal per 100 ml — coliforms are the indicator organisms of sewage pollution, and each person sheds hundreds of billions daily carrying cholera, dysentery and typhoid risk. And BOD under 50 mg/l against raw sewage at 300–600 — BOD is the oxygen microorganisms need to stabilise the organics, measured on a litre sample incubated at 20 °C. The chlorine stage exists for the first number; the blower plus settling earn the second. Note the design intent behind the numbers: by discharge time, aerobic bacteria should go out dead, not alive.
Anaerobic pockets make STP internals a confined-space gas risk. Ventilate, test, and never open chambers alone — the standby blower keeps the process alive, not the person.
3. Troubleshooting — Fault, Cause, First Action
Idea in one line: read the effluent first — its colour and smell name the dead subsystem before any spanner turns.
The blower has a standby because an aeration chamber without air goes anaerobic within hours. If the duty blower dies, the standby starts at once — every hour without air pushes the plant toward putrefaction — and only then is the dead blower's cause (motor, belt, air path) chased.
| Fault | Why it happens | First action |
|---|---|---|
| Blower failed, standby running | Motor, belt or air-path failure; chambers fall off 0.5–0.8 bar | Start the standby blower at once — every hour without air pushes the plant anaerobic; then find the dead blower's cause |
| Black, foul effluent | Aeration starved — putrefaction products (H2S, CH4, NH3) instead of digestion | Restore air first, desludge excess, check chamber pressure back in range |
| Cloudy carryover from settling | Sludge blanket too high, water short-circuits to sterilisation | Desludge the settling chamber, verify return flow to aeration |
| High coliform despite clear water | Chlorine tablets spent — settling removes solids, not bacteria | Recharge chlorine tablets; clear water is not disinfected water |