Share Based Compensation (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
	May 18, 2021	Dec. 21, 2018	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Feb. 18, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common Stock, Par or Stated Value Per Share			$ 0.01	$ 0.01
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized		11,297,000
Share-based Payment Arrangement, Expensed and Capitalized, Amount			$ 17,703	$ 10,535
Proceeds from issuance of common stock			21,205	$ 10,091	$ 363
Intrinsic value, exercised			$ 60,370
Weighted average grant date fair value, granted (in dollars per share)			$ 9.00	$ 6.06	$ 3.87
Fair value of options vested			$ 4,434	$ 3,906	$ 4,064
Share-based compensation			15,524	$ 10,509	$ 19,903
Compensation cost not yet recognized, period for recognition			$ 20,177
Granted (shares)			612,000	823,000	1,114,000
Share-based Payment Arrangement, Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (shares)			11,083,000
Number of shares available for grant (shares)			2,089,000
Compensation cost not yet recognized, period for recognition			$ 8,872
Period for recognition for unrecognized compensation expense			2 years
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period			240,000	1,555,000
Stock-settled RSU's (shares)			1,209,000	750,000	2,002,000
Period for recognition for unrecognized compensation expense			2 years 2 months 12 days
Employee Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (shares)			5,100,000			5,000,000
Number of shares available for grant (shares)			4,340,000
Employee Stock Ownership Plan (ESOP), Plan [Domain]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common Stock, Par or Stated Value Per Share		$ 0.01
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized		5,000,000
Employee Stock Ownership Plan (ESOP), Compensation Expense			$ 887	$ 392	$ 400
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period			182,000	58,000	46,000
Special PSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (shares)	469,000
Special RSU
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value, granted (in dollars per share)			$ 16.92
Stock-settled RSU's (shares)			469,000
Period for recognition for unrecognized compensation expense			2 years 7 months 6 days
Granted (shares)	234,000
Evoqua Water Technologies Corp. Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards vesting percentage			25.00%
Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Issued During Period, Shares, New Issues			2,687,000	3,478,000	108,000
Shares withheld related to net share settlement (including tax withholdings) (shares)					884,000
Treasury Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Issued During Period, Shares, New Issues			(531,000)	531,000
Shares withheld related to net share settlement (including tax withholdings) (shares)					577,000