Caustic soda does not dissolve grease — it turns it into soap. Alkali reacts with fats by saponification, producing water-soluble compounds that then drain away. At the same time, dissolving sodium hydroxide in water is exothermic and releases heat, and in a narrow pipe that heat has nowhere to go. A hot spot forms at the blockage, speeding the reaction up — and acting on everything else in the pipe as well.
What the heat does to the installation
PVC and PP soften at elevated temperatures. A single use generally leaves no trace, but repeated use, particularly at joints, deforms the material. A joint that has once pulled apart never returns to its original shape — it starts to leak, and in a wall or a floor that leak is discovered months later.
Rubber seals at joints and inspection points lose their elasticity. This is a common failure after repeated caustic soda treatment, and an expensive one, because it means opening up the installation.
Aluminium is attacked by alkali immediately. If any part of the installation is aluminium, caustic soda is out of the question.
Cast iron pipes tolerate alkali well enough in themselves. Their weakness is the rough inner wall, which deposits cling to far more readily than to smooth plastic, so the problem returns regardless of how it was cleared.
When it does not work
This is the scenario the label rarely mentions. If the soda does not break the blockage down, 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 acid drain cleaner and an alkali react violently, releasing heat and fumes.
- The plumber who opens the pipe is working in caustic solution, often with no warning that it is in there.
- The water that eventually gets through carries the alkali into the septic tank or the sewer system.
Septic tanks
A septic tank works by bacterial breakdown. A strong alkali destroys the bacterial culture, the system stops breaking waste down and fills up faster. The culture takes weeks to recover. During that period the odour from the vent is stronger, and the tank needs emptying earlier than planned.
| Caustic soda (NaOH) | Sodium bisulphate | |
|---|---|---|
| How it works | Saponification of fats, with exothermic dissolution | Acidic breakdown of deposits, no heating |
| Effect on PVC | Softens it with repeated use | None |
| Effect on seals | They lose elasticity | None |
| Septic systems | Destroys the bacterial culture | Compatible |
| Handling | Burns, splashing, fumes | Standard precautions |
| If it does not work | The alkali stays in the pipe | It rinses away with water |
What to use instead
For organic deposits — grease, soap, biofilm — a granulate based on sodium bisulphate breaks the deposit down with no exothermic reaction. GreenSwirl™ comes in a pack with two 233 g sachets: one sachet goes into the drain, is topped with a little warm water and left for 15 to 30 minutes, then flushed through with plenty of hot water.
For hair and solid objects, chemistry is not the answer in any form — you need a drain snake.
For a drain that runs freely but smells, the cause is not a deposit but a dried-out trap.