Share-Based Compensation (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands					3 Months Ended		6 Months Ended
	Apr. 01, 2022	Oct. 01, 2021	Dec. 21, 2018	Oct. 01, 2018	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021	Sep. 30, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service period							3 years
Granted (shares)							2
Intrinsic value, exercises in period							$ 6,873
Fair value of options vested							5,317
Share-based Compensation expense					$ 6,068	$ 3,214	11,402	$ 6,290
Non-cash share-based compensation					5,386	3,214	10,589	6,233
Compensation cost not yet recognized, period for recognition					$ 27,863		27,863
Proceeds from issuance of common stock							$ 5,274	13,430
Employee Stock Ownership Plan (ESOP), Shares in ESOP			11,297
Common stock, par value (in dollars per share)					$ 0.01		$ 0.01		$ 0.01
Evoqua Water Technologies Corp. Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards vesting percentage					25.00%
Contractual term of options							10 years
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity incentive plan, number of shares authorized (shares)					11,083		11,083
Number of shares available for grant (in shares)					2,177		2,177
Compensation cost not yet recognized, period for recognition					$ 6,565		$ 6,565
Period for recognition for unrecognized compensation expense							1 year 10 months 24 days
Employee Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for grant (in shares)					3,950		3,950
Purchase price of stock, percentage				85.00%
Employee Stock Ownership Plan (ESOP), Shares in ESOP			5,000
Employee Stock Ownership Plan (ESOP), Compensation Expense					$ 198	$ 187	$ 419	$ 403
Shares issued under ESPP	48	60
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, par value (in dollars per share)			$ 0.01
Special RSU
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost not yet recognized, period for recognition					$ 11,063		$ 11,063
Period for recognition for unrecognized compensation expense							2 years 3 months 18 days
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Period for recognition for unrecognized compensation expense							2 years 2 months 12 days