Constructed Wetlands

Constructed wetlands for wastewater treatment can be the ideal answer to your problem, combining excellent performance with minimal running costs. However, there are several varieties and many pitfalls.  If the wrong approach is taken, costly and messy problems can be created!  In 22 years of looking, there are probably more constructed wetlands in the UK not working properly than are!

So, it pays to have a bit of background knowledge about what works and what does not.  You’ve come to the right place…

Many wastewaters, such as sewage overflowing from a settlement tank, contain no oxygen. So, getting OXYGEN into the wastewater is one of the main aims of treatment. Powered package treatment plants achieve oxygenation using electricity. Drainage fields, following septic tanks, achieve oxygenation as the effluent passes through well aerated soil.  Waterlogging in drainage fields is prohibited. Natural wetlands are like waterlogged drainage fields. They contain very little oxygen. So, how do constructed wetlands achieve oxygenation? Well, they don’t! … except for vertical flow reed beds.

Reed beds
“Reed bed” is a term we in the UK tend to use interchangeably with “wetland”.  There are natural wetlands all over the UK, some of which are reed beds (and others, such as fens and bogs, which are not).  And most of our constructed wetlands used for wastewater treatment are planted with common reed (Phragmites australis), also known as Norfolk reed.  Many people even use “reed” to describe one of a number of other wetland plants, such as rushes.  So, please forgive my habituated use of the term “reed bed” to mean “constructed wetland”.

In the UK we use three main types of reed bed for wastewater treatment, all operating passively (without power for treatment) with treatment occurring as the water passes through the bed:

– Vertical flow: ie free-draining vertical down-flow
– Surface flow: also known as overland horizontal flow
– Horizontal flow: also known as subsurface horizontal flow

(Forced aeration reed beds also exist, which use power to introduce air from the base of waterlogged beds.  These are more like gravel-based package treatment plants with some redundant greenery and are a different type of technology altogether, which I am not dealing with here.)

Vertical flow wetlands are the only type that are not waterlogged. Only this type of reed bed can achieve sufficient oxygenation to treat sewage. This means they can treat relatively polluted water to a very high standard, even removing the ammonia.

Surface flow reed beds mimic natural wetlands (eg the Norfolk Broads) by water passing over the surface of soil from one end to the other. They do an excellent job of capturing fine solids and they are an excellent wildlife habitat. They are also simple to install and use virtually no gravel or sand. But they require the largest area of all reed beds and cannot be relied on to remove ammonia.

Horizontal flow reed beds are gravel filled and full to overflowing with wastewater. Designed so that the gravel captures pollutants in the wastewater, these beds nearly always block up after a while, mainly because there is no way for enough oxygen to get into the water. So, they usually need to be dug out and the gravel replaced and replanted. Very lightly loaded beds can remain unclogged for many years, but better to use a surface flow bed (no gravel) in those situations. So, despite being favoured by the mainstream UK water industry, hence becoming the most numerous type of reed bed in the UK, they are inherently unsustainable and best avoided (and Watercourse Systems never installs them). Most European countries rejected them years ago, or never took them up (eg France).