Caustic soda clears a blockage by getting hot. Dissolving sodium hydroxide in water is exothermic — it releases heat. In a narrow pipe that heat has nowhere to go, so a hot spot forms at the blockage and melts the grease. The difficulty is that the same heat reaches everything else in the pipe.
What the heat does to the installation
PVC and PP soften at elevated temperature. A single use usually leaves no trace. Repeated use, especially at joints, deforms the material. A joint that has once pulled apart does not return to shape — it starts to weep, and a leak inside a wall or floor is discovered months later.
Rubber seals at joints and access covers lose elasticity. This is the most common failure after repeated caustic treatment, and the most expensive, because it means opening the installation.
Aluminium is attacked by alkali immediately. If any part of the installation is aluminium, caustic soda is not an option at all.
Cast iron tolerates alkali but not what deposits afterwards on the roughened inner wall. Blockages become more frequent after treatment, not less.
When it does not work
This is the scenario the label rarely mentions. If the soda does not clear the blockage, it stays in the pipe as a strong alkali and the drain is still full. Three things follow:
- The next product poured in mixes with the alkali. An acidic drain cleaner and caustic soda react violently, releasing heat and vapour.
- The plumber who opens the pipe is working in caustic, often without being told it is there.
- Whatever water eventually gets through carries the alkali into the septic tank or the network.
Septic systems
A septic tank works by bacterial digestion. Strong alkali destroys the culture, the system stops breaking waste down and fills faster. The culture re-establishes over weeks. In the meantime the vent smells stronger and the tank needs emptying earlier.
| Caustic soda (NaOH) | Sodium bisulphate | |
|---|---|---|
| Mode of action | Exothermic reaction, melting | Chemical breakdown, no heating |
| Effect on PVC | Softens with repeated use | None |
| Effect on seals | Loss of elasticity | None |
| Septic systems | Destroys the bacterial culture | Compatible |
| Handling | Burns, splashing, vapour | Standard precautions |
| If it fails | Alkali stays in the pipe | Flushes away with water |
What to use instead
For organic deposit — grease, soap, biofilm — a sodium bisulphate granulate breaks material down without an exothermic reaction. GreenSwirl™ comes as two 233 g sachets; one sachet is dosed into the drain, flushed with lukewarm water and left to work.
For hair and solid objects, no chemistry of any kind is the answer — use a drain snake.
For a drain that runs freely but smells, the cause is not deposit but a dry trap.