Contains the full lesson along with a supporting toolkit, including teachers’ notes.
The term ‘biosolids’ refers to the products of sludge treatments that are pasteurisedto a specified standard so that they are suitable for use as fertilisers. Sludge, in this context, generally refers to sewage sludge although the term can also be applied to other semi-solid material of biological origin such as agricultural slurry, silage liquor, dairy waste andeffluent from the paper industry.
Why is it important to treat waste water?
The release of nutrient-rich material into rivers, lakes and coastal waters leads to eutrophication – a rapid growth of microorganisms (e.g. filamentous algae) in surface water that limits the light available to plants in deeper water. The resulting decrease in dissolved oxygen has a detrimental effect on fish and other aquatic animals. The metabolic by-products of suchalgal blooms are often toxic. The untreated sludge is also unsightly and smelly and can harbour pathogens.
It can take years for some ecosystems to recover from seriouseutrophication and contaminated water courses may become unusable as sources of drinking water.
About 50% of lakes in Europe and North America are eutrophic. This can sometimes happen naturally if rivers leach large amounts of nutrients from rock strata into relatively small bodies of water. However, the effect is more usually due to human mismanagement or carelessness.
The treatment process is designed to:
It is desirable to remove the solid material from the sludge before it begins to putrefy, i.e. before large scale sieves. The resulting more concentratedsludge is digested anaerobically in closed tanks. If this is done at a higher temperature (up to 55˚C) the digestion is faster. This process greatly reduces the volume of solid material and releases nutrients into solution.
The digested sludge can be removed by means of screens and then pasteurised and dried for use in agriculture or as landfill.