---
title: "APC residue"
description: "APC residue (air-pollution-control residue, or APCr) is the fine fly-ash combined with reagent salts (calcium hydroxide, activated carbon, sodium bicarbonate) captured by the flue-gas-treatment train downstream of a WtE boiler. APC residue typically accounts for 2–5% of original waste mass — much less than incinerator bottom ash (IBA), but more hazardous because heavy metals (Hg, Pb, Cd) and dioxins concentrate here."
canonical_url: "https://sylio.co/glossary/apc-residue"
last_updated: "2026-06-28T02:29:41.152Z"
---

**APC residue** (air-pollution-control residue, or APCr) is the fine fly-ash combined with reagent salts (calcium hydroxide, activated carbon, sodium bicarbonate) captured by the flue-gas-treatment train downstream of a [WtE](/glossary/waste-to-energy) boiler. APC residue typically accounts for 2–5% of original waste mass — much less than [incinerator bottom ash (IBA)](/glossary/incinerator-bottom-ash), but more hazardous because heavy metals (Hg, Pb, Cd) and dioxins concentrate here.

## Classification and disposal

In the EU and UK, APC residue is classified as hazardous waste. Disposal routes include:

- Stabilisation and landfilling in specialised hazardous-waste facilities
- Underground storage (former salt mines in Germany)
- Treatment for partial reuse in construction materials
- Specialised commercial processing for metals recovery

## Sonic-horn relevance

APC residue is collected in [fly-ash hoppers](/glossary/fly-ash-hopper) below the boiler economiser, SCR, [baghouse](/glossary/baghouse) and any reagent-injection equipment. Hopper bridging is a frequent problem because APC residue is fine, sticky and partly hygroscopic. [Sonic horns](/glossary/sonic-horn) on APC-residue hoppers are routine specification on modern WtE plants.

## Related terms

- [Waste-to-energy](/glossary/waste-to-energy)
- [Incinerator bottom ash (IBA)](/glossary/incinerator-bottom-ash)
- [Baghouse](/glossary/baghouse)
- [Fly-ash hopper](/glossary/fly-ash-hopper)