How do you convert raw sewage into clean water?

14 April 2020

Raw sewage from the typical household produces effluent whose strength is estimated by British Water to be 300 BOD. This is similar to 300 parts per million! BOD is the term used for Biological Oxygen Demand.

For many generations Septic Tanks were the standard way of treating sewage. Septic tanks work on the anaerobic principle, that is, the bacteria can live without oxygen. The bacteria live on the organic material in the effluent and produce unpleasant gases in the process. They are not called septic for nothing!

As sewage comes into the tank at 300 BOD treated effluent is released at the outlet pipe at the other end at 200 BOD. So we can see how inefficient the septic tank is. It retains, at best, only one third of what goes into it! The greyish dirty water then is sent to a soakaway where the anaerobic bacteria in the soil do their best to further digest the organic material.  Bacteria don’t live for long, they live, eat, breed and die. When they die they leave a black sludge.This sludge fills the pores of the soil so even in free draining sandy soils the ground under and around the soakaway gets saturated and the water in the soil then backs up into the septic tank and then into the drainage pipes around the house.

Eventually the soakaway is replaced if there is room in the garden.

Modern Sewage Treatment Plants work on the principle of aerobic digestion. That is, they need air or oxygen to operate. Sewage Treatment Plants come in different forms, from the filter bed (which introduces air by spraying water over stone media in the big filter beds operated by the big Water Companies such as Anglia water.) to Klargester’s Biodiscs (where discs dip slowly into and then out of the effluent giving the bacteria what they need. That is, food, water and air!). The smaller packaged plants blow air up through the effluent, much like in an aquarium. 

Now we have sewage final effluent as clean as 20 BOD or better. Sometimes down to 10 BOD. Compare that with roof water, say 5 BOD or tap water which of course is zero BOD. This water can safely discharged to the river system as fish can cope with it easily enough. This water is still carrying pathogens so is still not to be treated without care.

So this is how the two forms of treatment of sewage produce two very different standards of final effluent. Only the 20 BOD water can be discharged into rivers, the Septic Tank water has to be kept in the garden!

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