Electrostatic precipitator vs scrubber vs catalytic converter: what exactly does each remove, and why does a converter need unleaded petrol?
Zoya Khan12th Sample
3 Oct 2026 · 1 min read
I've been reading the air pollution section slowly because I find the three devices easy to blur.
What I have so far:
- Electrostatic precipitator (thermal power plants): removes particulate matter. NCERT says it can remove over 99% of particulates, with electrode wires giving particles a charge and collecting plates attracting them.
- Scrubber: removes gases like sulphur dioxide, by passing exhaust through water or lime spray.
- Catalytic converter: fitted in automobiles with the expensive metals.
My gaps. First, I don't really get how the charging works in the ESP: where does the charge come from and why do the particles move to the plates? Second, what are the actual metals and reactions inside the catalytic converter, and why must vehicles that have them use unleaded petrol? Third, NCERT says particles of 2.5 micrometres or less are the most harmful. Why that size specifically and not larger particles?
Also if someone can give me a clean way to pair each device with what it removes and where it is used, my notes would finally stop looking like a spiderweb. Thank you in advance 🌿
7 answers & replies
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[+][–]Nisha Verma12thSample· 17d ago
ESP mechanism: it has electrode wires kept at several thousand volts. These produce a corona, which releases electrons. Electrons attach to dust particles, giving them a net negative charge. The collecting plates are earthed, so they attract the charged dust particles and the dust sticks to the plates. The air goes out clean, the dust is shaken off later. Over 99% of particulate matter is removed this way.
We get the PM2.5 stuff every winter in Delhi so the topic is, sadly, personal for me 😅
[+][–]Diya Menon11thSample· 16d ago
ahh I think the ESP also removes SO2 since it's a power-plant device and power plants release SO2, so it cleans all pollutants from smoke? I don't think it only does dust?
[+][–]Kavya NaikDropperSample· 16d ago
Diya, no. ESP only deals with particulates because it works on electric charge on solid/dust particles. Gases like SO2 pass through it. For SO2 you need a scrubber, where exhaust is passed through a spray of water or lime and the gas gets dissolved or neutralised. Same power plant, two different jobs.
[+][–]Siddharth RaoDropperSample· 16d ago
Catalytic converter: contains platinum, palladium and rhodium as catalysts, coated on a honeycomb-like surface. As the exhaust passes through it:
- unburnt hydrocarbons become CO2 and water
- carbon monoxide and nitric oxide become CO2 and N2 (nitrogen gas)
Lead (from leaded petrol) coats the catalyst and makes it inactive, which is why a car with a converter must run on unleaded petrol. If you used leaded petrol the converter would be "poisoned" and stop working. That's basically a one-liner for NEET.
[+][–]Pooja Iyer12thSample· 16d ago
Size question!! PM2.5 means particles of diameter 2.5 micrometres or less. These are so tiny they can be inhaled deep into the lungs. NCERT says they cause breathing and respiratory symptoms, irritation, inflammation and damage to lungs, and even premature deaths. Bigger particles tend to get trapped in the nose and upper tract, so the real danger is the tiny ones that go all the way down. Fun fact I use: human hair is way thicker than 2.5 micrometres, so you can't even see these.
[+][–]Rahul DubeyDropperSample· 16d ago
Pairing for revision (one line each):
- ESP: particulate matter, thermal power plants
- Scrubber: gases such as SO2, industries/power plants
- Catalytic converter: CO, NO, unburnt hydrocarbons, automobile exhaust
- Related: Delhi's public transport shifted to CNG, which burns more efficiently and leaves less unburnt; Bharat Stage norms (like Euro norms) set limits on vehicle emissions.
Ask yourself the device, the pollutant, and the place. If you have those three in a row, the options usually answer themselves.
[+][–]Zoya Khan12thSample· 16d agoAuthor
this is exactly the spiderweb remover I wanted, thank you all. Writing the three-in-a-row method on a sticky note.
