Science & Technology in Action

3rd Edition


Higher Education Authority

Returning the nutrients to the land, in a form that is not easily leached out, restores the fertility of the soil in an environmentally sustainable way.
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Lesson excerpt

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.

Treatment processes
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:

  • remove solid material and convert it into useful products (e.g. fertilisers) that are safe to use, are easily handled and have no unpleasant side-effects.
  • remove or greatly reduce the soluble nutrients from the water, especially phosphorus and nitrogen compounds, before it is returned to the natural environment.
  • be environmentally sustainable.

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.

True or False?

  1. Pasteurisation is a process of heating materials in order to destroy microorganisms. true
  2. Using biosolids as agricultural fertiliser is ecologically unsustainable. false
  3. Eutrophication helps the environment by providing nutrients such as nitrates and phosphates for plants. false
  4. In anaerobic digestion organic matter is broken down into simpler substances by air or oxygen. false
  5. Nitrosifying bacteria use the oxidation of ammonia as their energy source. true
  6. Denitrifying bacteria convert nitrites to nitrates. false
  7. Nitrates and phosphates are essential for plant growth. true
  8. Soluble phosphates are salts of phosphoric acid. true
  9. Chemical and biological methods are used to reduce the concentration of phosphates in treated water before it is returned to the environment. true
  10. Thermophilic bacteria are removed from biosolids by pasteurisation. false
  11. Pathogens are either bacteria or viruses. false
  12. Returning the nutrients to the land, in a form that is not easily leached out, restores the fertility of the soil in an environmentally sustainable way. true