Septic Tanks
Septic Tank vs Sewage Treatment Plant
The practical differences between a septic tank and a sewage treatment plant, and how to work out which suits your site.
In short: a septic tank settles solids and relies on a drainage field to finish treating the liquid; a sewage treatment plant actively treats the wastewater itself, producing effluent clean enough to usually discharge straight to a ditch or stream. The right choice for you depends mainly on whether your site can accommodate a drainage field.
Side-by-side comparison
| Septic Tank | Sewage Treatment Plant | |
|---|---|---|
| How it treats waste | Settles solids only | Actively treats waste (aeration) |
| Needs a drainage field | Yes, in almost all cases | Not usually |
| Can discharge to a ditch/stream | Generally no, without a permit | Generally yes, under the GBR |
| Electricity required | No | Yes (for the aerator) |
| Typical upfront cost | Lower | Higher |
| Ongoing running cost | None | Modest (electricity + servicing) |
| Best suited to | Sites with space for a drainage field | Constrained sites, or discharge to a watercourse |
How to decide
- Do you have room for a drainage field, and does the ground drain well? If yes, a septic tank is usually the simpler, lower-cost route. See our percolation test guide.
- Is your only realistic discharge point a ditch or stream? A treatment plant is usually the compliant choice. See septic tank discharging to a ditch.
- Is the site very constrained (small plot, high water table)? A treatment plant’s smaller footprint often makes it the only workable option.
A specialist site assessment is the reliable way to settle this — ground conditions vary enough that generic advice can only take you so far.
Can I convert a septic tank to a treatment plant later? + −
Yes, this is a common upgrade path — particularly where a drainage field has failed and a treatment plant is a more practical replacement than a new field.
Is a treatment plant always better than a septic tank? + −
Not necessarily — where a drainage field works well, a septic tank is simpler, cheaper to run, and has fewer mechanical parts to maintain. The right choice is site-specific.
Do both need emptying? + −
Yes, both need periodic desludging, though treatment plants often need it somewhat less frequently than an equivalent septic tank due to the more active treatment process.