What is the largest wastewater treatment plant in Iowa?

Walnut City Of STP in Walnut, treating an average of 0.99 MGD under NPDES permit IA0042986. Clarion, City Of STP in Clarion is second at 0.82 MGD.

How much wastewater do Iowa’s largest plants treat?

The 11 largest plants treat 2 MGD combined, 100% of the flow reported by all 678 publicly owned plants in Iowa.

How is plant size measured?

By reported average daily flow in million gallons per day (MGD) from EPA discharge monitoring reports, the standard measure of a plant’s size. Design capacity, the flow a plant is permitted to treat, is usually higher and is shown on each profile.

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The 11 largest wastewater treatment plants in Iowa

Name: Largest wastewater treatment plants in Iowa
Creator: Klarifi
License: https://klarifi.io/terms-of-service

Ranked by average flow · 2 MGD combined · 100% of the state's reported flow

Walnut City Of STP in Walnut is the largest publicly owned wastewater treatment plant in Iowa by reported average flow, treating about 0.99 million gallons per day. The 11 plants below treat 2 MGD between them, 100% of the flow reported by all 678 plants in the state, and serve about 9,413 people. Every plant links to its profile with permit, technology and compliance data.

All plants in Iowa →The 100 largest US plants →Iowa NPDES permits →

The 11 largest publicly owned wastewater treatment plants in Iowa by average flow (MGD), with population served, NPDES permit, expiry, treatment technology and three-year violation count.
#	Facility	City	Avg flow (MGD)	Population	NPDES permit	Expires	Technology	Violations (3y)
1	Walnut City Of STP	Walnut	0.99	747	IA0042986	Apr 2006	Aerated lagoons, Uncovered lagoons	40
2	Clarion, City Of STP	Clarion	0.82	2,979	IA0030945	Jan 2028	Aerated lagoons	13
3	Fredricksburg WWTP	Fredericksburg	0.17	973	IA0028991	Jan 2008	Aerated lagoons	0
4	Shellsburg WWTP	Shellsburg	0.13	1,100	IA0033332	May 2006	Clarifiers	0
5	Ossian City Of STP	Ossian	0.07	847	IA0042781	Sep 2006	Aerated lagoons, Uncovered lagoons	5
6	What Cheer WWTP	What Cheer	0.04	607	IA0032468	Apr 2006	Aerated lagoons, Uncovered lagoons	3
7	Lester City Of STP	Lester	0.04	296	IA0026620	Feb 2017	Uncovered lagoons	15
8	Birmingham City Of STP	Birmingham	0.04	373	IA0023426	Jul 2002	Aerated lagoons, Uncovered lagoons	16
9	Mitchell County WW Pretreatment Plant	Osage	0.03	841	MO0030821	Jun 2026	Carrousels / oxidation ditches, Biogas silos	111
10	Atalissa City Of STP	Atalissa	0.02	373	IA0070998	Apr 2006	—	4
11	Maharishi Vedic City - South	Fairfield	0.0079	277	IA0078417	Nov 2009	—	19

Largest plants in Iowa: questions

Sources and methodology

Flow, permit and violation figures come from the EPA's ICIS-NPDES discharge monitoring records. Population served comes from the Clean Watersheds Needs Survey 2022. Treatment structures are identified by Klarifi's review of aerial imagery, and capital improvement projects are extracted from published municipal budget documents. Violations are counted over the last three years of monitoring reports.

Data refreshed September 11, 2026.

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