What Is the Difference Between a Septic Tank and a Sewage Treatment Plant?

It's not glamorous but having an effective way to remove and manage wastewater is crucial for maintaining a safe, hygienic property. Among the different systems used for this purpose, three are the most common: central sewage systems, septic tanks, and sewage treatment plants.

In the UK, most properties are connected to a central sewage system – a vast network of underground pipes that transport large amounts of wastewater to a treatment facility. In remote or rural areas, however, many are instead linked to a decentralised septic tank or sewage treatment plant.

If your property can't connect to a central sewage system, you might be wondering whether a septic tank or a sewage treatment plant is the best fit for your needs. Despite sharing a common goal, the two have some major differences, and getting a good grasp on them will help you pick the right choice for your home or business.

At Greenstar, we've been working with septic tanks and sewage treatment plants for more than three decades. Read our guide below to discover the differences between the two systems, including how they work, what they cost to install, which is more eco-friendly, and more.

What are septic tanks and sewage treatment plants?

To begin, let's take a look at what the two systems actually are.

Septic tanks are buried, watertight containers made from concrete, fibreglass, or plastic. The tank separates solid and liquid waste, with the solid waste dissolving or sinking to the bottom and the liquid waste getting discharged into a leach field for additional treatment.

Sewage treatment plants go a step further – they don't just separate waste but also treat it for contaminants. This means that the water leaving a sewage treatment plant is much cleaner than what you'd get from a septic tank and can be legally released directly into a local watercourse.

How do septic tanks and sewage treatment plants work?

Septic tanks and sewage treatment plants follow a similar process, but they differ in how extensively they treat wastewater. First, let's break down how septic tanks work:

1. When wastewater from toilets, sinks, and household drains enters a septic tank, solid materials settle at the bottom, forming sludge. Lighter particles like grease and oils float to the top, creating a scum layer. The remaining liquid, called effluent, occupies the middle layer.
2. As the septic tank fills, the natural bacteria in the sewage start breaking down the organic matter in the sludge and scum layers. This microbial digestion converts solids into liquid and gas.
3. The partially treated effluent leaves the septic tank and flows into a drain field or leach field. There, it undergoes further treatment as it percolates through the soil, acting as a natural filter to remove remaining impurities and harmful microorganisms.

Sewage treatment plants follow the same steps as septic tanks up to the point where the solids and liquids are separated. Here's what happens after they diverge:

1. Once the liquid is separated, it moves into a second chamber, leaving the solid waste behind. An air pump then injects oxygen into the system. This encourages the growth of aerobic bacteria, which gets to work breaking down the remaining sewage, turning it into a cleaner effluent within the tank.
2. The liquid, now infused with bacteria, then flows into a final chamber. The bacteria settles at the bottom to be recycled back into the first chamber. The clean effluent is now ready for discharge.
3. This discharge can either go into a soakaway or, thanks to the additional cleaning steps, be released directly into a ditch, stream, or another watercourse (subject to approval from the local environmental authority).

In both septic tanks and sewage treatment plants, the sludge separated during the treatment process must be regularly collected and disposed of in an environmentally responsible manner.

Are sewage treatment plants or septic tanks more environmentally friendly?

Septic tanks typically have a greater impact on the environment compared to sewage treatment plants. This is because the discharge from septic tanks may contain untreated wastewater, which can potentially contaminate groundwater and surface water bodies. On the other hand, sewage treatment plants employ more thorough treatment processes, preventing the release of untreated sewage into waterways. The treated effluent is usually cleaner than natural water bodies, reducing the risk of waterborne diseases and protecting aquatic ecosystems.

Do septic tanks or sewage treatment plants cost more?

Sewage treatment plants generally come with higher costs than septic tanks. There are a few reasons for this:

• Sewage treatment plants have more complex designs than septic tanks, so their construction and installation tend to cost more. This is especially true when there's no existing soakaway or watercourse to discharge the effluent to.
• Sewage treatment plants require a continuous supply of electricity to operate the air pump. This ongoing need for electricity is an additional cost that septic tanks don't have.
• While septic tanks might need to be emptied more frequently, sewage treatment plants are more susceptible to wear and tear due to their additional components. This means that sewage treatment plants demand more regular attention and service visits compared to septic tanks.

Should you install a septic tank or sewage treatment plant on your property?

Deciding between a septic tank and a sewage treatment plant comes down to your budget and wastewater treatment needs. Typically, septic tanks work well for single households or small development sites with enough space for a soakaway system. On the flip side, sewage treatment plants are a better fit for homes with multiple occupants, larger development sites, or commercial properties.

Trust Greenstar for first-rate septic tank and sewage treatment plant installations

For over 30 years, Greenstar has been the South's go-to choice for septic tank and sewage treatment plant installations, repairs, and emptying. Our fully accredited team have the expertise to design and install the ideal treatment system for your property, guaranteeing a smooth and stress-free experience throughout.

If you're not sure whether a septic tank or sewage treatment plant is better for your property, no worries — we're happy to share our expert recommendations to help you choose the perfect system. This way, you can be confident that your property will be properly serviced for years to come.

Ready to get started? Get a quote online or call us on 01202 355 344 today! We're here to answer any questions you may have and can walk you through our installation process in more detail. We're looking forward to hearing from you!