Bacteria are single-cell plants. Bacteria metabolize the organics in wastewaters with the production of new microbial cell mass.

The bacteria that can metabolize the maximum amount of the different organics, predominate. While most bacteria in wastewater treatment systems utilize organics for their metabolism, there is an important group of bacteria that utilize inorganic compounds for their metabolism. As a net result, the two groups of bacteria in wastewater, do not compete with each other for their nutrients and both grow in the same environment. The bacteria weigh approximately 10 – 12 g each.

Normal municipal wastewaters contain between 105 and 107 bacteria/ml. Bacteria use soluble food to reproduce by binary fission. They are about 0.5 to 1.0 micron in diameter.

Their shape falls in three categories:

Spherical (cocci),

Cylindrical (bacilli) and

Helical (spirilli); the spiral forms may be 15 microns long.

Metabolically, most bacteria in wastewater are heterotrophic. The autotrophic forms obtain energy by oxidation of inorganic substrates such as ammonia, iron and sulfur. There are a few autotrophic photosynthetic bacteria also. Depending on their organic metabolism reactions, the bacteria may be anaerobic or facultative.

Protozoans: Protozoa are single-cell animals that live on bacteria and small algae, helping to remove the dispersed bacteria and algae from the system.

They are much larger than the bacteria in wastewater. Four major groups have been identified:

Mastigophora, flagellated, usually parasites and some may cause disease, e.g. Giarida lamblia. These flagellated protozoa are not very efficient energy gatherers and cannot compete with the higher forms of protozoa e.g. Peranema, bodo, oikomonas, and monas.

Sarcodina, are characterised by amoeboid motion, some have flagella. Entamoeba histolytica causes dysentery.

Ciliata, largest and most varied group, are either free swimming with the help of cilia or stalked, attached to a solid body. The free-swimming ciliated protozoa are the most efficient protozoa and metabolize tremendous quantities of bacteria in wastewater e.g. Lionotus, paramecium, colpidium, euplotes, aspidiscus and stylonychia. When the energy level of the system decreases, the free-swimming ciliated protozoa give way to stalked ciliated protozoa, which are attached to flock particles and can metabolize bacteria in the nearby vicinity with a lower expenditure of energy than the free-swimming ciliated protozoa e.g. Vorticella, epistlis, opercularia, and carchesium. (Suctoria are a special group of stalked protozoa that eat free-swimming ciliated protozoa rather than bacteria).

Sporozoa, spore forming oblilgate parasites.

Species of protozoa known to have been transmitted by the ingestion of contaminated drinking water include:

Entamoeba histolytica (cause of amoebiasis)

Giardia spp.

Rarely, balantidium coli

These organisms can be due to human or animal fecal contamination.

Standard methods are not available to detect protozoa. When disease outbreaks occur and are associated with drinking water contamination by (pathogenic intestinal) protozoa, boiling of water may provide effective control. It leads to the inactivation of the above three.