The sewage is defined as the waste water resulting from the various human activities, agriculture and industries and mainly contains organic and inorganic compounds, toxic substances, heavy metals and pathogenic organisms. The sewage is treated to get rid of these undesirable substances by subjecting the organic matter to biodegradation by microorganisms. The biodegradation involves the degradation of organic matter to smaller molecules (CO2, NH3, PO4 etc.) and requires constant supply of oxygen. The process of supplying oxygen is expensive, tedious, and requires a lot of expertise and manpower. These problems are overcome by growing microalgae in the ponds and tanks where sewage treatment is carried out. The algae release the O2 while carrying out the photosynthesis which ensures a continuous supply of oxygen for biodegradation.

The algae are also capable of adsorbing certain heavy toxic metals due to the negative charges on the algal cell surface which can take up the positively charged metals. The algal treatment of sewage also supports fish growth as algae is a good source of food for fishes. The algae used for sewage treatment are Chlorella, Euglene, Chlamydomnas, Scenedesmus, Ulothrix, Thribonima etc.