Lagoon basics

What a wastewater lagoon is, how the cells divide the work, and what every term used across this site actually means.

Short answer

A wastewater lagoon is an earthen treatment basin where bacteria, algae, sunlight and wind break down sewage across one or more cells. Each cell has a job: BOD removal first, then nutrient removal, then settling and pathogen kill. About 4,657 discharging lagoon systems serve the United States, most in communities under 3,000 people.

Start here

Explainer

How a Wastewater Lagoon Works

A cell-by-cell explanation of how wastewater lagoons treat sewage using bacteria, algae, sunlight and wind, and what operators actually control.

Read the guide →

Glossary

21 terms, defined the way they are used in lagoon operation rather than in a textbook.

Aerated lagoon: A lagoon that adds mechanical oxygen and mixing rather than relying on wind and algae. It treats more load in less area and is far less weather dependent than a facultative cell.
Anaerobic: Without oxygen. Anaerobic conditions in a lagoon produce hydrogen sulfide, which is the rotten-egg odor, and slow the digestion of settled solids.
Areal loading: Organic load expressed per unit of surface area, in pounds of BOD5 per acre per day. It is the number that tells you whether a facultative cell is being asked to do more than its design.
Benthal feedback: The release of ammonia and phosphorus from an anaerobic sludge blanket back into the water column. It can keep effluent nutrients high even after influent loading improves.
BOD5: Five-day biochemical oxygen demand. The standard measure of the organic load bacteria must consume, and the input that drives how much oxygen a lagoon needs.
CBOD: Carbonaceous biochemical oxygen demand. BOD measured with nitrification suppressed, which matters because nitrification inside a test bottle can otherwise distort a result.
Cell: One basin in a lagoon system. Most systems run two or three in series, with the first removing BOD, the second handling nutrients, and the last settling solids and reducing pathogens.
Diffused aeration: Aeration using a shore-mounted blower feeding diffusers on the lagoon bottom. Fine bubbles transfer oxygen through the full water column and the rising plume mixes the cell bottom up.
Dissolved oxygen: Oxygen available in the water for aerobic bacteria. In a loaded facultative cell it can crash to zero in the hours before sunrise, which is why a midday reading can be misleading.
Facultative lagoon: A lagoon with an aerobic upper layer over an anaerobic bottom layer, oxygenated by wind and by algae during daylight. The most common small-community configuration.
Nitrification: The biological conversion of ammonia to nitrite and then nitrate. It requires oxygen, warmth and low BOD together, and it effectively shuts down in near-freezing water.
NPDES: The National Pollutant Discharge Elimination System, the federal permit program a discharging lagoon operates under. It sets the limits that define compliance.
Partial-mix aerated cell: An aerated cell mixed enough to distribute oxygen but not enough to keep all solids suspended. In these cells the horsepower needed for mixing often controls the design rather than oxygen transfer.
Retention time: How long wastewater stays in the system. Design retention is what the drawing says; effective retention is what you actually get after sludge accumulation and short-circuiting take their share.
Short-circuiting: Influent traveling from inlet to outlet without spending its design retention time in the cell. One of the two root causes behind most lagoon failures.
Sludge blanket: The layer of settled solids on the lagoon bottom. It steals live volume, releases nutrients through benthal feedback, and near the outlet it resuspends into effluent as unexplained TSS.
Sludge judging: Probing sludge depth on a grid across a cell to map the blanket. It is the measurement that should precede any removal spending decision.
SOTR: Standard oxygen transfer rate. The tested figure that lets you compare aerators on pounds of oxygen delivered per unit of energy rather than on horsepower.
Surface aerator: A floating unit that transfers oxygen by contacting water with air at the surface. Lower cost and relocatable, but it mixes mainly the upper water column.
TSS: Total suspended solids. A permit parameter and a diagnostic: sludge particles on an effluent TSS filter point at the blanket rather than at current biology.
Turnover: The mixing of a stratified water column, typically at ice-out in spring. It brings reduced compounds from the bottom to the surface at once and is commonly the year worst odor episode.