Organisms used in water purification

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.

Impurities

Pathogens

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

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]

Nutrients

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]

Metals

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.

Organisms

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.

Plants

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

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

• Nymphea alba; for temperate climates, depth 60–120 cm
• Phragmites australis, for temperate climates but is invasive in many areas.
• Sparganium erectum, for temperate climates, depth 60–120 cm.
• Iris pseudacorus, for temperate climates, depth 0–20 cm but is also invasive.
• Schoenoplectus lacustris, for temperate climates.
• Carex acutiformis, for temperate climates.

Oxygen-supplying

• Stratiotes aloides, for temperate climates, depth 40–60 cm, .
• Hydrocharis morsus-ranae, temperate climates, depth 40–60 cm. Extremely invasive and is listed on the Washington State Noxious Weeds list.
• Acorus calamus, for temperate climates

Shade/refuge-supplying

• Hydrocharis morsus-ranae, temperate climates, depth 40–60 cm,.
• Nuphar lutea, temperate climates, depth 60–120 cm, develops floating leaf.

Fish

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

Surface

• Leuciscus leuciscus, for temperate climates.
• Leuciscus idus, for temperate climates.
• Scardinius erythrophthalmus, for temperate climates.

Middle

• Rutilus rutilus, for temperate climates.

Bottom

• Tinca tinca, for temperate climates.

Bacteria

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.

• Alcaligenes^[8]
• Bacillus^[8]
• Beggiatoa^[9]
• cyanobacteria
• Flavobacterium^[8]
• Pseudomonas^[8]
• purple sulfur bacteria^[9]
• Sphaerotilus^[9]

Protozoa

• Amoeba^[9]
• Arcella^[9]
• Blepharisma^[8]
• Didinium^[9]
• Euglena^[9]
• Hypotrich^[8]
• Paramecium^[9]
• Suctoria^[10]
• Stylonychia^[9]
• Vorticella^[9]

Insects

• Chironomidae bloodworm larva^[9]
• Podura aquatica water springtail^[9]
• Psychodidae drain fly or filter fly larva^[9]

Extra considerations

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 to sometimes to let the plants root in.

Example system

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

Plants

• 0–20 cm—Iris pseudacorus, Sparganium erectum
• 40–60 cm—Stratiotes aloides, Hydrocharis morsus-ranae
• 60–120 cm—Nymphea alba, Nuphar lutea
• Deep—Myriophyllum spicatum

Fish

• Surface—Leuciscus leuciscus, Scardinius erythrophthalmus, ...
• Middle—Rutilus rutilus
• Bottom—Tinca tinca

See also

• Aquatic plant
• Water purification
• Treatment pond

Sources

• 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)

Notes