Organisms used in water purification

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A flowering water-purifying plant (Iris pseudacorus)

Biota are an essential component of most sewage treatment processes and many water purification systems. Most of the organisms involved are derived from the waste or water stream itself. However some processes, especially those involved in removing very low concentrations of contaminants, may use engineered eco-systems in created by the introduction of specific plants and sometimes animals. Some full scale sewage treatment plants also use engineered wetlands to provide treatment.



Parasites, bacteria and viruses may be injurious to the health of persons or livestock ingesting the water. These pathogens may have originated from sewage or from the excrement of domestic or wild animals or birds. Pathogens may be killed by ultraviolet sunlight unless that sunlight is blocked by plants or suspended solids.[1]

Suspended solids[edit]

Particles of organic material or mineral soil may be suspended in the water. Such materials may give the water a cloudy appearance. Organic materials may dissolve or decay causing the water to generate offensive odors.[2]


Compounds containing nitrogen or phosphorus may encourage growth of aquatic plants and animals causing increased concentrations of suspended organic material. Other dissolved minerals may be required nutrients at low concentrations or toxic at higher concentrations.[3]


Many dissolved or suspended metal salts exert harmful effects in the environment sometimes at very low concentrations. Some aquatic plants are able to remove very low metal concentrations, with the metals ending up bound to clay or other mineral particles.


Saprophytic bacteria convert dissolved organic impurities into living cell mass, carbon dioxide and water. These saprophytic bacteria may then be eaten by flagellates and ciliates which also consume suspended organic particles including viruses and pathogenic bacteria. Clarity of the water may begin to improve as the protozoa are subsequently consumed by rotifers and cladocera.[4] Purifying bacteria, protozoa, and rotifers must either be mixed throughout the water or have the water circulated past them to be effective. Sewage treatment plants mix these organisms as activated sludge or circulate water past organisms living on trickling filters or rotating biological contactors.[5]

Aquatic vegetation may provide similar surface habitat for purifying bacteria, protozoa, and rotifers in a pond or marsh setting; although water circulation is often less effective. Plants and algae have the additional advantage of removing nutrients from the water; but those nutrients will be returned to the water when the plants die unless the plants are removed from the water. Plants also provide shade, a refuge for fish, and oxygen for aerobic bacteria.[6] In addition, fish can limit pests such as mosquitoes. Fish and waterfowl feces return waste to the water, and their feeding habits may increase turbidity. Cyanobacteria have the disadvantageous ability to add nutrients from the air to the water being purified.

The choice of organism depends on the local climate different species and other factors. Indigenous species usually tend to be better adapted to the local environment.


A water-purifying plant (Iris pseudacorus) in growth after winter (leaves die at that time of year)

Different plant species are necessary in each of four depth-zones.[7]

  • 0–25 centimetres (0.0–9.8 in)
  • 40–60 centimetres (16–24 in)
  • 60–120 centimetres (24–47 in)
  • Greater than 120 centimeters (47 in)

Water purifying[edit]

Plants purify water by consuming excess nutrients and by de-acidifying it by removing carbon dioxide.




Different species required for each of 3 depth-zones. The fish need to be both herbivores and a native species in the area.

  • Surface
  • Middle
  • Bottom





Indigenous bacteria are preferred to ensure that good adaptation to local conditions. Bacteria can be grown by submerging straw (or other plant material) in water for several days. The bacteria automatically populate the material.



Extra considerations[edit]

For ecologic/self-purifying ponds, de-nutrified soil needs to be taken for the plants to prevent the possible growth of algae. Coconut fibre growing medium is best used to prevent soil from being spread around and sometimes to let the plants root in.

Example system[edit]

The following choices occupy all depth-zone niches and are mutually compatible in the same pond.


See also[edit]


  • Fair, Gordon Maskew, Geyer, John Charles & Okun, Daniel Alexander Water and Wastewater Engineering (Volume 2) John Wiley & Sons (1968)
  • Hammer, Mark J. Water and Waste-Water Technology John Wiley & Sons (1975) ISBN 0-471-34726-4
  • Metcalf & Eddy Wastewater Engineering McGraw-Hill (1972)


  1. ^ Fair, Geyer & Okun pp.31-3&31-4
  2. ^ Hammer pp.29&35
  3. ^ Metcalf & Eddy pp.256-258&492
  4. ^ Metcalf & Eddy pp.380&381
  5. ^ Fair, Geyer & Okun pp.32-12,32-31&34-2
  6. ^ Several types of plants required to form a water-purifying pond
  7. ^ Several-depth zones of plants need to be determined
  8. ^ a b c d e f Hammer pp.54&61
  9. ^ a b c d e f g h i j k l m Fair, Geyer & Okun p.34-8
  10. ^ Metcalf & Eddy p.381