How Off Mains Drainage can contribute to River Eutrophication

One of the biggest causes of off-mains drainage pollution is septic tanks that have been placed in the wrong location. 

Septic tanks can be reasonably effective drainage solutions, but only when they are in the right conditions ie. on substrate where there is adequate drainage, and the water table is more than one metre below ground.  The reason is because septic tanks don’t significantly treat sewage and need a large drainage field and a reedbed through which their effluent can be filtered before it reaches groundwater.  The septic tank itself can remove significant levels of solids (passively, through gravity), but only around 30% of the active pollutants.  

Where septic tanks have drainage fields that cannot filter their effluent, these pollutants – such as nitrogen, phosphorous and ammonia can find their way into water courses via groundwater.  If your property is located on clay, for example – which is the underlying substrate for around half of the UK – then the effluent will not be able to drain freely.

Factor in poor maintenance and a septic’s efficiency drops still further.

The latest concern is that housing developments could add to the eutrophication of water courses already significantly contaminated by other pollutants such as agricultural run-off.  To try and reduce the risk, Natural England has told 74 local planning authorities with protected sites, that development in ‘catchments’ around some waterways cannot proceed if the new homes would increase levels of nutrients.

For their plans to be approved, developers must make sure their schemes ‘nutrient neutral’. This means that any nitrates entering the water system as a result of a new development must be offset by the removal of an equivalent amount of nitrates.

This marks the latest tightening of regulations designed to help protect inland water courses.  Since 2020, it has also been illegal for septic tanks to discharge directly into surface water.  Owners of septic tanks that release effluent directly into a ditch, stream, canal, river, surface water, or drain, must either connect to the mains sewer, or replace the tank with a sewage treatment plant, or create a drainage field that meets the current British Standard BS6297 2007.

Sewage Treatment Plants (STPs) offer a significantly superior alternative to septic tanks. Sewage treatment plants provide an environment where aerobic bacteria are cultivated to break down sewage.  The bacteria survive by using biological matter in the sewage as food.  Sewage treatment plants are highly efficient, treating around 90% of the pollutants. They produce a clear overflow that is environmentally friendly and suitable for discharging even on sensitive sites.

It's worth noting that effluent from a sewage treatment plant is allowed to discharge into a watercourse if it achieves a sufficiently high quality, but this is regulated and may require registration with the Environment Agency/Environmental Protection Agency. (Applications for discharges can be made online, but always seek advice first).

A Klargester BioDisc offers exceptional, market-leading sewage treatment performance, using bacteria to breakdown waste.  It is also odour-free and silent offering peace of mind to homeowners.

(*Source: Home Builders Federation)

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.