After discussing how clean water gets to our taps in Part 1 – From source to tap – we now get onto the slightly less glamorous side. Although the processes involved in wastewater treatment are not a million miles away from the steps detailed in Part 1, there are some key differences – especially when it comes to dealing with ‘sludge’.


Firstly, screening is used to filter out non-processable, non-biodegradable waste – essentially rubbish – which is then dried and sent to landfill.


Primary waste treatment is used to remove larger materials. The water is held in tanks to allow for the material within the tank to settle. Heavy matter will sink to the bottom and form a sludge, while lighter or floating materials rise to the top and form a scum.  The water is then drained away to be further treated and the left-over material is extracted for further processing.


The secondary stage is mainly focused on breaking down the biological components of the water via biological processes. This is carried out in three ways:


Biofiltration uses filters to catch and remove any sediment in the water, with some filters also acting as biological filters. Other filters, such as sand filters, are also used at this stage.



Aeration involves pumping air into the water to allow aerobic (air consuming) bacteria to develop, grow and carry out a biodegrading process in order to clean the water of smaller organic particles. Essentially, the air fuels the bacteria to consume organic waste.

Oxidisation pools

The water is then held in shallow lagoons and exposed to sunlight to allow algae to form. The algae produce oxygen, further fuelling aerobic bacteria, allowing them to effectively consume and degrade any remaining waste. This waste then sinks to the bed of the lagoon before the water is pumped away.


Tertiary treatment is the final stage before the water is released back into the environment. Tertiary treatment often involves passing the water through reed beds - aquatic plant-based systems which allow bacteria, fungi and algae to digest the sewage and clean the water - to ensure the water is organically clean before redistribution into lakes and rivers.

If the treatment site is located near sensitive streams, such as fish habitats, there may be additional stages of treatment to remove nitrates, ensuring that there is no risk of rapid algae growth which could suffocate the surrounding ecosystem.


Not forgetting about that sludge that sank to the bottom - yes it also gets treated. This waste material can be recycled and used to create energy pellets for burning, or to create fertiliser for farms. The methane produced by the sludge is harnessed and used to power the process of drying and producing these by-products.

Some 166,500 people are employed in all these processes to ensure that the highest standards are preserved at every step. Maintaining these processes, as well as the vast infrastructure of pipes and sewers, requires a massive amount of labour, hardware and yes – money. But for that cost, you can be sure that the quality of the water you use every day is amongst the highest in the world.


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