SCHEDULE I - Evoqua Water Technologies Corp. Condensed Consolidated Balance Sheets (Details) - USD ($) $ / shares in Units, $ in Thousands	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Condensed Financial Statements, Captions [Line Items]
Current assets	$ 678,458	$ 695,712
Cash and cash equivalents	146,244	193,001	$ 109,881
Prepaid and other current assets	21,871	18,715
Total assets	1,868,891	1,844,458
Total liabilities	1,286,672	1,362,395
Common Stock, Value, Issued	1,223	1,189
Treasury Stock, Value	(2,837)	(2,837)
Additional paid‑in capital	582,052	564,928
Retained deficit	(11,182)	(62,664)
Total Evoqua Water Technologies Corp. equity	580,671	480,144
Total liabilities and shareholders’ equity	$ 1,868,891	$ 1,844,458
Common Stock, Par or Stated Value Per Share	$ 0.01	$ 0.01
Common Stock, Shares Authorized	1,000,000,000	1,000,000,000
Common Stock, Shares, Issued	122,173,000	119,486,000
Common Stock, Shares, Outstanding	120,509,000	117,291,000
Treasury Stock, Shares	1,664,000	2,195,000
Parent Company
Condensed Financial Statements, Captions [Line Items]
Current assets	$ 51,777	$ 10,871
Due from Affiliates	39,982	5,821
Cash and cash equivalents	11,681	4,972
Prepaid and other current assets	114	78
Investment in affiliate	517,479	489,745
Total assets	569,256	500,616
Due to affiliates	0	0
Total liabilities	0	0
Common Stock, Value, Issued	1,223	1,189
Treasury Stock, Value	(2,837)	(2,837)
Additional paid‑in capital	582,052	564,928
Retained deficit	(11,182)	(62,664)
Total Evoqua Water Technologies Corp. equity	569,256	500,616
Total liabilities and shareholders’ equity	$ 569,256	$ 500,616