SCHEDULE I - Evoqua Water Technologies Corp. Condensed Consolidated Balance Sheets (Details) - USD ($) $ / shares in Units, $ in Thousands	Sep. 30, 2020	Oct. 01, 2019	Sep. 30, 2019	Sep. 30, 2018
Condensed Financial Statements, Captions [Line Items]
Current assets	$ 695,712	$ 637,220
Cash and cash equivalents	193,001		$ 109,881	$ 82,365
Prepaid and other current assets	18,715	21,867
Total assets	1,844,458	1,782,660	1,737,848
Total liabilities	1,362,395	1,418,175
Common stock, par value $0.01: authorized 1,000,000 shares; issued 119,486 shares, outstanding 117,291 at September 30, 2020; issued 116,008, outstanding 114,344 shares at September 30, 2019	1,189		1,154
Treasury stock: 2,195 shares at September 30, 2020 and 1,664 shares at September 30, 2019	(2,837)		(2,837)
Additional paid‑in capital	564,928		$ 552,422
Retained deficit	(62,664)	(176,313)
Total Evoqua Water Technologies Corp. equity	480,144	361,422
Total liabilities and shareholders’ equity	$ 1,844,458	$ 1,782,660
Common stock, par value (in USD per share)	$ 0.01		$ 0.01
Common stock authorized (in shares)	1,000,000,000		1,000,000,000
Common stock issued (in shares)	119,486,000		116,008,000
Common stock outstanding (in shares)	117,291,000		114,344,000
Treasury stock (in shares)	2,195,000		1,664,000
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Current assets	$ 10,871		$ 335
Due from Affiliates	5,821		0
Cash and cash equivalents	4,972		121
Prepaid and other current assets	78		214
Investment in affiliate	489,745		385,175
Total assets	500,616		385,510
Due to affiliates	0		9,747
Total liabilities	0		9,747
Common stock, par value $0.01: authorized 1,000,000 shares; issued 119,486 shares, outstanding 117,291 at September 30, 2020; issued 116,008, outstanding 114,344 shares at September 30, 2019	1,189		1,154
Treasury stock: 2,195 shares at September 30, 2020 and 1,664 shares at September 30, 2019	(2,837)		(2,837)
Additional paid‑in capital	564,928		552,422
Retained deficit	(62,664)		(174,976)
Total Evoqua Water Technologies Corp. equity	500,616		375,763
Total liabilities and shareholders’ equity	$ 500,616		$ 385,510