1. Biofilm-Friendly Surfaces

The porous, rough texture of concrete gives bacteria plenty of places to attach and thrive. Plastic’s smooth, slippery surface does not.  They are necessary because biofilms are the living engine of a septic system. Without them, the tank is just a container.

Why do those biofilm-friendly surfaces matter?

• Anchor for Bacteria – Wastewater bacteria do not just float around — they thrive when they can attach to a surface. A biofilm is basically a colony of bacteria sticking together in a slimy layer. Concrete’s rough, porous surface provides grip, while plastic’s smooth walls don’t.
• More Surface Area = More Treatment Power – The more surface bacteria can colonise, the more work gets done. A thick, well-established biofilm can break down solids, neutralise pathogens, and stabilise the whole system.
• Resilience and Stability – Free-floating bacteria can wash out of the tank during heavy use. But once bacteria are anchored in biofilms, they stay in place, survive longer, and keep working even when flow rates spike.
• Efficient Breakdown of Waste – Biofilms create micro-environments where different bacterial species cooperate — some break down solids, others remove nutrients, others neutralise toxins. This layered teamwork happens best when bacteria are attached, not drifting.
• Concrete’s Advantage – The pores and rough texture of concrete act like natural “real estate” for biofilms. Plastic’s smooth, inert surface offers far less space for colonisation, which can limit treatment efficiency.

Biofilm-friendly surfaces turn a septic tank into a living treatment plant. Concrete gives bacteria the foothold they need to thrive, while plastic makes it harder for those colonies to take hold and do their job.

2. Stable Temperatures

The steady internal environment in a concrete septic tank comes down to thermal mass — basically, concrete’s ability to absorb, store, and slowly release heat.

Why does stable temperature matter?

• High Heat Capacity – Concrete is dense and heavy. It can hold a large amount of heat energy without its temperature changing quickly. That means sudden outside changes (like a hot day or cold night) don’t instantly affect the liquid inside the tank.
• Slow Heat Transfer – Because of its thickness, concrete doesn’t transfer heat quickly. The soil outside might heat up or cool down fast, but that change takes a long time to reach the wastewater inside a concrete tank.
• Stable Internal Conditions – Bacteria in septic systems are sensitive to temperature swings — they work best in a steady, moderate environment. Concrete’s thermal mass smooths out external highs and lows, keeping the internal conditions more stable and bacteria more active.
• Plastic by Contrast – Plastic tanks are thin-walled and lightweight. They have little thermal mass, so they heat up and cool down quickly. That means the bacteria inside experience more stress from daily or seasonal temperature shifts

Concrete acts like an insulator and buffer, shielding bacteria from temperature swings, while plastic acts like a thin container that passes those swings straight through.

3. Balanced Chemistry

This is critical because the entire septic system depends on live bacteria to break down waste — and those microbes are very sensitive to their environment. If the chemistry drifts too far, the biology collapses.

Why does it matter?

• Bacteria Thrive in Neutral Conditions – The ideal pH for most septic bacteria is around 6.5–7.5. If the wastewater becomes too acidic or too alkaline, the microbes either slow down or die off. That means solids won’t decompose properly, and the tank will fill up faster.
• Prevents Odours and Toxic Byproducts – When the chemistry is unbalanced, the wrong types of bacteria take over (like sulphate-reducing bacteria). These can produce foul gases such as hydrogen sulphide, making the system smell and become more corrosive.
• Keeps the System Self-Sustaining – In a well-balanced environment, bacteria reproduce at the right pace to replace themselves and maintain treatment without constant additives. Unbalanced chemistry may force owners to rely on “starter” bacteria packets or chemicals — adding cost and hassle.
• Concrete’s Role – Concrete naturally helps buffer wastewater chemistry because it’s slightly alkaline. This neutralises excess acidity in the tank, keeping conditions stable for bacteria. Plastic, being chemically inert, doesn’t provide this benefit — if the wastewater chemistry drifts, bacteria are on their own.

Balanced chemistry is the difference between a septic system that quietly works for decades and one that fails early. Concrete tanks give bacteria an extra layer of protection by helping maintain that balance, while plastic offers no such support.

The result: a concrete tank sustains a stronger, more resilient bacterial ecosystem. That translates to more effective treatment and fewer problems over the long haul.