Environmental Engineering: Fundamentals and Design - Chapter 11

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# Chapter 11 Wastewater Treatment

James R. Mihelcic, David W. Hand and Martin T. Auer

## Natural waste treatment systems

In small, rural communities, it may be too expensive to construct a mechanical municipal treatment plant. Here, wastewater lagoons or ponds provide an economical treatment option. The processes which occur in a lagoon are similar to those found in mechanical treatment plants.

Lagoons can have an aerobic upper layer (like the aeration tank in secondary treatment) and an anaerobic lower layer (like sludge digesters). In the upper, aerobic layer, bacteria break down organic matter to carbon dioxide and release nitrogen and phosphorus. Algae take up the nitrogen and phosphorus and release the oxygen required by bacteria. Additional oxygen is supplied from the air. Solids (algae and wastewater particles) that settle to the bottom, anaerobic layer are broken down by bacteria. Wastewater is held in lagoons for a minimum of six months before discharge to a receiving water such as a river. By that time, pathogens have died or been eaten by other organisms.

#### Credits

Lagoon photo:
Discharge into Lagoon photo:

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