SCHEDULE I - Evoqua Water Technologies Corp. Condensed Statements of Changes in Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Condensed Financial Statements, Captions [Line Items]
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	$ (121,766)	$ (1,116)	$ (9,266)
Reconciliation of net income (loss) to cash flows provided by operating activities:
Foreign currency exchange (gains) losses on intercompany loans and other non-cash items	(8,202)	10,713	5,766
Changes in assets and liabilities
Accrued expenses and other liabilities	3,410	(9,159)	(22,851)
Prepaids and other current assets	2,088	9,447	(3,773)
Net cash provided by operating activities	158,358	125,196	81,017
Investing activities
Net cash provided by (used in) investing activities	11,992	(94,532)	(207,035)
Financing activities
Proceeds from issuance of common stock	18,927	363	137,605
Stock repurchases	0	0	(230)
Taxes paid related to net share settlements of share-based compensation awards	0	(1,270)	(8,807)
Net cash (used in) provided by financing activities	(89,449)	5,744	150,641
Change in cash and cash equivalents	83,120	27,516	23,111
Beginning of period		82,365	59,254
End of period			82,365
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(113,649)	9,523	(6,135)
Reconciliation of net income (loss) to cash flows provided by operating activities:
Net (loss) income of subsidiaries	(114,109)	9,293	(8,199)
Foreign currency exchange (gains) losses on intercompany loans and other non-cash items	(15)	0	0
Changes in assets and liabilities
Due from affiliates	(3,990)	0	0
Due to affiliates	(9,747)	1,343	8,812
Accrued expenses and other liabilities	160	0	(61)
Prepaids and other current assets	(24)	(161)
Net cash provided by operating activities	(14,076)	952	6,687
Investing activities
Contributed capital	0	0	(140,999)
Net cash provided by (used in) investing activities	0	0	(140,999)
Financing activities
Proceeds from issuance of common stock	18,927	363	137,605
Stock repurchases	0	0	(230)
Taxes paid related to net share settlements of share-based compensation awards	0		(8,807)
Net cash (used in) provided by financing activities	18,927	(907)	128,568
Change in cash and cash equivalents	4,851	45	(5,744)
Beginning of period	121	76	5,820
End of period	$ 4,972	$ 121	$ 76