SCHEDULE I - Evoqua Water Technologies Corp. (Details) - USD ($) $ in Thousands	12 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Debt Instrument [Line Items]
Long-term debt, gross	$ 754,943	$ 885,470
Net Cash Provided by (Used in) Operating Activities	178,705	177,026	$ 125,196
Net Cash Provided by (Used in) Financing Activities	130,344	108,117	(5,744)
Revision of Prior Period, Error Correction, Adjustment \| Understatement
Debt Instrument [Line Items]
Net Cash Provided by (Used in) Operating Activities	18,669
Revision of Prior Period, Error Correction, Adjustment \| Overstatement
Debt Instrument [Line Items]
Net Cash Provided by (Used in) Financing Activities	18,669
2014 Term Loan, due December 20, 2024
Debt Instrument [Line Items]
Long-term debt, gross	0	819,276
Parent Company
Debt Instrument [Line Items]
Net Cash Provided by (Used in) Operating Activities	(13,173)	(4,244)	952
Net Cash Provided by (Used in) Financing Activities	(19,882)	$ (9,095)	$ 907
Parent Company \| 2021 Credit Agreement
Debt Instrument [Line Items]
Long-term debt, gross	$ 511,105

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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