What Is a Septic Tank and Why Do They Fail?
Septic tanks have been used for over a century to treat household waste in homes not connected to the mains sewer. When designed and maintained properly, they can last for decades. But thousands of homeowners across Surrey, Hampshire and Berkshire struggle with septic tank problems, including tanks that smell, overflow, or need emptying every few months.
The truth is: most of these “bad tanks” aren’t bad tanks at all. They’re tanks set up to fail.
🟡 What Is a Septic Tank?
A septic tank is an underground chamber that separates waste into three layers:
- Sludge (solids) sinks to the bottom
- Scum (fats, oils and grease) floats to the top
- Effluent (liquid) flows out through a pipe into a drainage field, where the soil treats it naturally
A septic tank is not a soakaway pit. Soakaways are only for rainwater. Septic tanks must discharge into properly designed drainage fields (land drains).
🚫 Why Do Septic Tanks Fail?
1. Roof water and surface water
The number one killer of septic tanks is rainwater wrongly connected to the foul system.
- Builders or handymen often “make life easy” by running a downpipe into a foul drain.
- Customers don’t realise what’s happening until it’s too late.
- The inflow of water dilutes the effluent, stirs up the sludge, and pushes thick sewage into the drainage field.
- Once the land drains clog, there is no fix.
👉 Every homeowner should have a note ready for anyone working on gutters or drains:
“Do not connect roof water or surface water to my septic tank.”
2. Fats, oils and grease
Even when customers swear they “don’t put fat down the sink,” the evidence often shows up in the inspection chamber: a thick floating layer of grease.
- Fat doesn’t break down in the tank
- It blocks pipes and reduces settlement
- It coats the drainage field and shortens its life
3. Wrong size or poor design
A tank that’s too small can’t separate waste properly. Solids get pushed straight out into the drainage field.
In the wrong soil (clay, waterlogged ground), septic tanks don’t work at all — a treatment plant is the only option.
🟢 How Long Should a Septic Tank Last?
With the right design and no roof water, no fats, no shortcuts:
- A septic tank can work efficiently for 20 years or more
- With minimal emptying (just as needed, not constantly)
- With no damage to the environment
The difference between a tank that lasts two years and one that lasts two decades comes down to design, care and knowledge.
✅ Need Help With Your Septic Tank?
At ASL Limited, we’ve been helping homeowners across Surrey, Hampshire and Berkshire for over 30 years. Our engineers know how to design, survey and maintain septic systems that last.
Call us today for a site survey or advice: 08000488701
