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What are the main types of Sewage Treatment Plant, how do they work and what are the benefits?

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What are the main types of Sewage Treatment Plant

 If you live in a house that is not connected to mains drainage or are looking to build a property where access to mains drains is not available, then you will need to install a small domestic sewage system. Each type of modern system utilises a tank where a form of treatment occurs. The two types of tank in widespread use are a Septic Tank and a Sewage Treatment Plant.

A Septic Tank only offers primary treatment, which is very rudimentary.  This leads to the output quality being poor and as such this type of system cannot be used without an additional form of secondary treatment.

A Sewage Treatment Plant is the catch all for a modern system that incorporates an acceptable level of secondary treatment. This allows the system to be used for more methods of discharge than a Septic Tank. This guide deals with the different types of secondary treatment that different tanks utilise and what the benefits of each are.

Activated Sludge Process (ASP)

This type of tank has two key elements. A treatment area and a settlement area. Stage 1: the waste from a property enters the tanks treatment area. Within this zone an air diffuser keeps the bacteria alive by providing them with oxygen. The bacteria that live inside the treatment area help to break down the solids/sludge inside the system.

Once the level of waste exceeds a limit in the treatment zone the wastewater flows over into the settlement and return area. The waste settles with the heavy solids falling to the bottom of the tank while the most active element rises to the top. The active element is filled with “good” bacteria, elements of this are passed back to the treatment area to reinforce the level of bacteria. The element just below the surface is allowed to escape from the tank.


The length of time the waste spends in the treatment area is dependent on the inflow of wastewater. This leads to a large variability in effluent quality. We would not advise using this type of tank to discharge to a sensitive area or if you have any form of effluent quality limit.

The system is simple, with the air compressor being the only mechanical part. The air compressor has to run constantly leading to higher electrical costs and  than say a batch cleaner.

The tanks are normally tall, this increases installation costs and difficulty to the extent that an installation may not be possible.

Fixed bed reactor/ Basic Aeration Unit/ Submerged Aerated Filter

A basic aeration system combines both media and aeration to treat waste coming from a property. This is the most popular and common type of Sewage treatment Plant. Unlike most plants these systems have three chambers; a primary settlement chamber, a secondary treatment chamber and a final settlement chamber.

Firstly, waste flows into the primary settlement chamber, where the solids sink to the bottom of the chamber. A T-Piece holds back the solids allowing the residual wastewater to flow into the secondary chamber. The secondary chamber is where the magic happens. The chamber is filled with media balls and pumped full of oxygen via an air diffuser at the bottom. Media balls are spherical plastic shapes that create a huge surface area for “good” bacteria to live and break down the pollutants left in the wastewater.

The treated water then flows into the final settlement tank where any suspended solids sink to the bottom of the chamber and the cleaned effluent is discharged out of the tank. Finally, any solids or sludge at the bottom of the third chamber is transferred back to the primary chamber via a sludge return. This seeds the primary chamber and allows treatment to occur here as well.


The length of time the waste spends in the treatment area is dependent on the inflow of wastewater. This leads to variability in effluent quality. However, as there is a primary chamber the effluent quality is generally better and more consistent than an ASP system.

The system is simple, with the air compressor being the only mechanical part. This leads to low maintenance costs. The air compressor has to run constantly leading to higher electrical costs than say a batch cleaner.

Sequence Batch Reactor (SBR)

SBR sewage treatment plants have the best and most consistent effluent quality level of the different types of Sewage Treatment Plant.  They work in a similar way to the ASP but the key difference is the time the waste water spends in the treatment zones is controlled. This allows for the optimal level of treatment.  A small computer and a system of valves and airlifts control the flow.


The effluent quality is the best on the market, the compressor does not run consistently which reduces maintenance, noise impact and electrical costs. The main downside is the system is more complicated than a basic aerator and relies on the control panel to function which is costly to replace should it go wrong.

Non-electric filter or worm tanks

These are not practical and effluent quality is unlikely to consistently meet the standards necessary. Moreover, the outlet depth is normally such that a pump is required, which is of course electrical. Please check the tank has the appropriate British standards and is listed on the British Water approved list of Sewage Treatment Plants in line with Environment Agency recommendations.


Install a Septic Tank or a decent Sewage Treatment Plant, avoid at all costs.

Rotating Disc System / Rotating Biological Contractor (RBC)/ Biodisc

These systems use a large water wheel that rotates vertically, half in the wastewater and half out. This ensures aerobic contact and the aeration of the wastewater moving it between different chambers. 

These systems are pushed by architects and builders who have no idea what they are doing. Please avoid. 


Maintenance costs are large, initial tank cost is high. You have expensive electrical and mechanical parts operating in damp conditions. The tanks themselves have a large footprint with a huge ugly lid. Avoid.

Our Core Services: The Sewage Treatment expert Installers

On the 1st January 2020, all Septic Tanks that discharge to a Stream, Watercourse or Drainage Ditch will become illegal and will require replacing with a Sewage Treatment Plant. Please see here for more details.

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Expert wastewater treatment engineers who specialise in Septic Tank and Soakaway replacements as well as Sewage Treatment Installations. We are based in the New Forest, Hampshire but operate across the South Coast as well as Wiltshire, Somerset, Devon, Sussex, Surrey, Oxfordshire, and Berkshire.

The Home of Septic tank and private drainage system solutions. 

Based in the New Forest, Hampshire on the border with Dorset and Wiltshire we operate an expert team of off-mains drainage specialists. This encompasses Septic Tank, Soakaway and drainage system problems, installations, blockages, and servicing. In addition, any issues relating to cesspits and sewage treatment plants.

Homeseptic offers high quality, professional advice, repairs and installation for all aspects of effluent cleansing and disposal across the south of England. Whether you’re looking for septic tank installation, sewerage treatment plants or systems, piping or soakaway solutions, Homeseptic employs only the most competent engineers in the drainage and treatment industry and knows exactly which solutions suit each property.

We only install and offer quality systems we believe in, from suppliers we know and trust.

New Forest Philosophy

As we have said Homeseptic is based in the New Forest. A great location in between the cities of Southampton, Salisbury and Bournemouth. We are a group of good friends who grew up together in the area.  Individually we all have grown tired of the way other Drainage firms are run. As a result, this is why we set up Homeseptic. If you want to know more please check out our About Page

We want to actually put the customer first and turn up to do a great job that we can be proud of. Unfortunately, this is not the case in the market, with most companies pressuring their workers to sell unnecessary works on a visit. Due to this, we offer upfront quotations with any variations discussed before we start work.

We are honest hard working professionals, which we hope you will agree with. As a result, it will certainly not turn up in flash sales vehicles and give you a load of chat and then do a terrible job. Our team will come with the job in mind and the aim of giving you the best value we can in order to facilitate a quality installation. 

We cover all of Hampshire, West Sussex, Dorset, Somerset, and Wiltshire. Please get in touch and see what you think.

What are the main types of Sewage Treatment Plant, how do they work and what are the benefits?
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What are the main types of Sewage Treatment Plant, how do they work and what are the benefits?
A guide to the main types of sewage treatment plant
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