How to Choose an Off-Mains Wastewater System

Septic tanks allow the settlement, storage and partial decomposition of solids in wastewater.  The liquid effluent is then discharged from the tank and typically passed through an external drainage field.

Because of the way they are designed to work, septic tanks remove much of the solid matter, but in reality, only around 30 percent of the active pollutants.

Under H2 Building Regulations Drainage and Waste Disposal, septic tank discharges are classified as harmful to human health and the environment - hence why the effluent is passed through a drainage field, or mound, or a reed bed, for secondary treatment.

If ground conditions don’t allow efficient filtration (for example, if your house built on clay soil or in an area with a high water table) then the soluble waste effluent that the septic emits will collect near the surface – causing a foul-smelling, bog-like, health hazard. 

Since 2020, it has been an offence to discharge septic tank effluent into water courses and, if you think this is happening, you must take immediate steps to either a) connect your property to the mains sewer, or b) replace the tank with a sewage treatment plant or c) create a drainage field that meets the current British Standard BS6297 2007.

Sewage treatment plants are underground tanks containing bacteria that breakdown sewage before the effluent is discharged into a drainage field.  Treatment plans are so efficient that they typically treat more than 95% of the pollutants before any effluent is discharged.  

Packaged sewage treatment systems, with tertiary treatment such as packaged reed beds, are the current ‘gold standard’, and are more than 98% efficient.  These systems produce a clear effluent that is environmentally friendly and suitable for discharging even on sensitive sites.

A septic tank, by contrast, does not treat waste.  

The tank allows settlement of solids, creating a soluble waste which is then discharged and filtered through the ground across a large drainage field. 

The wastewater from a septic tank is not benign - it contains large amounts of harmful bacteria. Indeed, septic tank discharges are considered so polluting that some countries have passed legislation requiring them to be supported by a secondary treatment option, such as a reed bed, wherever they are installed.  

Septic tanks can offer a serviceable solution for some homes, but they need both the right ground conditions, plus the reed bed, to work effectively.

If the ground conditions don’t allow efficient filtration, then the soluble waste effluent from the tank won’t filter through the soil.  Instead, it will collect near the surface, and sooner or later this will become essentially a smelly bog….

Examples if unsuitable ground conditions include where the tank is installed on an impermeable substrate (such as clay soil) or where an area has a high water table and/or is at risk of flooding, because waterlogging will stop the soil from acting as a filter.  Under these conditions, the unfiltered waste will get washed either into ground water or into any surrounding surface water.  

Even if an area doesn’t routinely flood, the water table must be at least one metre below the septic tank outlet for the ground to do its job.

Determining whether a site is suitable for a septic tank and can support the required drainage field requires a professional drainage engineer who will carry out a percolation test.  

If you are in any doubt as to the suitability of your garden to filter wastewater, then it is sensible to take a prudent course and install a STP. 

It’s encouraging to see that off-mains drainage legislation in countries around the world has tightened up in recent years.  

However, the decision to relax legislation and requirements for reporting in some markets has caused concern among drainage management professionals.

Our takeout message is simple: off-mains drainage solutions should always be professionally designed and specified.  

Moral as well as legal responsibility for disposing of wastewater safely and considerately lies with the architect/specifier, developer, installer, and the property’s owner.  

Any attempt to ease the administrative burden on property owners should never be mistaken for permission to act irresponsibly and an owner’s personal liability remains unchanged.

Wastewater management is a complex, technical, highly regulated area, with legal restrictions on what you can and can’t do - so we recommend you get expert help.  Our Klargester team can offer you free advice every step of the way,

First, you will need a site inspection and a check on any existing solutions you might have on your property.

Second, you need advice on whether you can connect to the mains sewer perhaps with help from a pump station (you must, if you can)

Third, if the decision is that you need to manage your wastewater using a sewage treatment system, then further investigation must be carried out to find out the right solution for you to establish if:

a)   Your property is in a Groundwater Source Protection Zone

b)   The height of the water table around your property in winter (established by Trial Site Assessment Hole)

c)   Your ground’s suitability to act as a drainage field; this check is known as a percolation test 

All this will determine which off-mains system is right for your property.  If the ground is poorly draining (e.g. clay), or at risk of flooding, or if the winter water table is less than one metre below the septic tank outlet, then you will need to install a sewage treatment plant (see above) rather than a septic tank.

Klargester is a market-leading brand in drainage, including septic tanks, sewage treatment plants, and pump stations.  We can arrange a free consultation and site inspection, including running performance tests on any existing solutions on your property to check this will meet your needs. 

If required, we can offer you the chance to select from our range of high-quality, reliable wastewater treatment plants, all of are fully compliant with all industry standards. 

If cost is one of your main concerns, then rest assured that the cost of our Clearwater BioAir Domestic Sewage Treatment Plant including installation and drainage field, compares very favourably with a standard septic tank with its much larger drainage area.

You currently don’t need a permit to discharge domestic sewage if you meet the general binding rules for discharges to a surface water or the general binding rules for discharges to ground.  But, even if you don’t need a permit your sewage treatment system must meet the relevant British Standard that was in place at the time it was installed.

As we’ve indicated, neither septic tanks nor sewage treatment plants should be considered a fit-and-forget solution. 

Both systems need to be maintained and regularly emptied by a specialist service team.  Our wastewater team offer an exceptional and cost-effective aftersales service to ensure you enjoy years of trouble-free, effective operation.

Go to General Binding Rules for small domestic wastewater treatment plants, which is overseen by the Environment Agency.