Daily STP Checklist: What Operators Must Check Every Day

An STP is a living machine. The billions of microbes doing the real work of cleaning your building's sewage need oxygen, food and stable conditions every single day — miss a day and they start to die, and a dead biology takes weeks and lakhs of rupees to rebuild. That is why the single most important maintenance activity in any building STP is not the annual overhaul or the quarterly service visit. It is the humble daily round the operator walks every morning.

This guide is that round, written out. It is the practical daily STP checklist a facility manager can hand to an operator, or use themselves, to catch the small problems — a tripped blower, a dropping DO, a choked screen — before they snowball into a plant that has stopped treating and a complaint from the pollution board.

Ninety percent of STP failures are not sudden. They are a small warning sign that nobody noticed for a week. The daily round exists to notice.

Before you start: what the daily round is for

The daily checklist is not paperwork for its own sake. Every check on it answers one plain question: is the biology alive and is the water leaving cleaner than it came in? Everything else — the readings, the smells, the logbook — is evidence for that one judgement. Do the round at roughly the same time each day, ideally in the morning after the overnight low flow, carry a notebook or the logbook itself, and actually write down numbers rather than trusting memory. Trends only show up when yesterday's number sits next to today's.

If you are new to how the individual units fit together, keep the how does an STP work guide open alongside this one — this checklist assumes you know an aeration tank from a clarifier.

The daily STP checklist, unit by unit

1. Dissolved oxygen (DO) in the aeration tank

This is the single most important number in the plant. The aerobic bacteria need dissolved oxygen to breathe and eat; too little and they suffocate, too much and you are burning electricity for nothing.

• Dip the DO meter into the aeration tank at mid-depth.
• Target band: 1.5–2.5 mg/L for a healthy activated-sludge or MBBR plant. If your target differs, follow the design value.
• Below 1 mg/L — the tank is oxygen-starved. Check the blower is running and the diffusers are not choked before anything else.
• Above 3–3.5 mg/L consistently — you are over-aerating; consider throttling the blower or reducing run hours to save power.

If your DO is chronically low or your air demand seems wrong, the aeration tank and air blowers and diffusers guides explain the underlying causes.

2. Blower and aeration — is it actually running?

• Confirm the duty blower is running and the standby is on auto/idle. Never run both continuously unless the design calls for it.
• Listen and feel: unusual noise, excessive vibration, or a hot casing all point to a bearing or belt problem.
• Look at the tank surface — you should see an even, rolling boil of fine bubbles across the whole tank. Dead patches mean choked or broken diffusers.
• Note the blower running hours and, if you have an ammeter, the current draw. A creeping current often signals a choked filter or failing bearing.

3. MLSS — colour, smell and settleability

You do not need a lab every day. Your eyes and nose are the fastest diagnostic tool on site. Scoop a sample of mixed liquor from the aeration tank into a clear jar or a settleability cone.

• Colour — a healthy tank is a rich chocolate / earthy brown. Black means it has gone septic (not enough oxygen); pale grey or thin means the biomass is too low or under-loaded.
• Smell — a healthy tank has a mild earthy, musty smell. A rotten-egg (H₂S) stink means septic conditions — check DO and blower immediately.
• Settleability — let the sample stand 30 minutes in a cone. Well-settling sludge drops to a clear boundary with clear water on top. Sludge that stays fluffy and refuses to settle (bulking) or floats back up is a warning worth logging.

For what the colour and settling are telling you biologically, see biological culture management and STP troubleshooting common problems.

4. Inlet flow and levels

• Check the raw sewage / equalisation tank level. A steadily rising level means the downstream plant is not keeping up — a pump or blower may be down.
• Note the inlet flow if you have a flow meter, and the treated-water flow at outlet. A big mismatch flags a problem.
• Confirm the equalisation tank pumps are cycling normally and transferring to the aeration tank at the design rate.

5. Screens and oil/grease trap

• Rake the bar screen clean of rags, plastics and solids. A choked screen backs up the inlet and can overflow the collection tank.
• Bag and remove the screenings; do not let them pile up in the plant room.
• Skim the oil and grease trap if floating scum has built up. Kitchen fats that get through will coat diffusers and foul the biology.

6. Pumps — the whole set

Walk every pump in the plant: raw sewage, transfer, sludge recirculation (RAS), filter feed and treated-water pumps.

• Confirm each duty pump runs and its standby is on auto.
• Feel for overheating and abnormal vibration; listen for cavitation (a gravelly rattle).
• Check for leaks at seals and flanges, and that valves are lined up correctly.
• Note anything tripping or short-cycling. Details in STP pumps and instrumentation and pump maintenance.

7. Clarifier / settling

• Look at the clarifier surface: it should be calm with clear water flowing over the weir.
• Rising sludge, floating clumps, or a scum blanket on top means either the sludge is old (return/waste it) or the tank has gone septic.
• Confirm the sludge return is working — no return means the biomass washes out of the plant.

8. Chlorine residual and disinfection

Before the treated water goes to the reuse tank, it must be disinfected.

• Check the chlorine dosing pump is running and the tank has solution.
• Test the residual chlorine at the outlet with a simple pool/DPD test kit. Aim for around 0.5 mg/L free residual in the treated water (follow your reuse requirement).
• No residual — dosing has failed or demand has spiked; treated water may be unsafe to reuse.
• Refill/mix the hypochlorite solution as needed. See the chlorination system guide, and size doses with the chlorine dose calculator.

9. Filters and treated water

• Note the pressure differential across the pressure sand and carbon filters. A rising differential means a backwash is due.
• Eyeball the treated water in the final tank — it should be clear, colourless and odourless. Cloudy or coloured water means something upstream is not finishing the job.

10. Logbook — write it all down

The round is only worth anything if it is recorded. Fill the daily logbook with every reading, every observation and every action taken. This is your evidence for the pollution board, your handover to the next shift, and your early-warning system when you compare today against last week.

A one-glance daily log format

Give the operator a simple sheet like this to fill each day:

The daily round in the wider maintenance rhythm

The daily checklist is the foundation, not the whole building. It sits inside a wider schedule — some tasks are simply too involved to do every day.

• Daily — this checklist: DO, blowers, MLSS, flow, screens, pumps, chlorine, logbook.
• Weekly — deeper cleaning, valve exercising, calibration checks: weekly STP maintenance.
• Monthly — sludge wasting review, lab testing, blower servicing: monthly STP maintenance.

Two things make the daily round pay off. First, consistency — the same person, same time, real numbers written down, so trends surface. Second, acting on what you see — a black tank or a zero chlorine residual is not a note for later, it is a job for now.

An STP that gets a proper daily round rarely fails a pollution-board inspection, rarely floods a basement, and rarely needs an emergency biology re-seed. One that does not, always does — just not on a day you were ready for it.

To go deeper on the machinery behind each check, browse the full Sewage Treatment Plants guide library, and if you suspect your plant is simply running its blowers too hard, the energy benchmark calculator will tell you whether your power bill is where it should be.