Summary of Significant Accounting Policies (Impact of ASC Topic 842 Adoption) (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Oct. 01, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Prepaid and other current assets	$ 18,715	$ 21,867
Current assets	695,712	637,220
Property, plant, and equipment, net	364,461	336,396	$ 333,584
Operating Lease, Right-of-Use Asset	45,965	42,073
Assets	1,844,458	1,782,660	1,737,848
Accrued expenses and other liabilities	143,389	115,435	101,839
Liabilities, Current	349,555	335,817
Obligations under operating leases, excluding current installments	37,796	29,308
Other non‑current liabilities	98,456	81,906
Liabilities, Noncurrent	1,012,840	1,082,358
Liabilities	1,362,395	1,418,175
Retained deficit	(62,664)	(176,313)
Total Evoqua Water Technologies Corp. equity	480,144	361,422
Total accumulated other comprehensive loss	482,063	364,485	365,822	$ 362,016	$ 216,575
Liabilities and Equity	$ 1,844,458	1,782,660
Accounting Standards Update [Extensible List]	us:gaap_AccountingStandardsUpdate201602Member
Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Prepaid and other current assets			21,940
Current assets			637,293
Property, plant, and equipment, net			333,584
Operating Lease, Right-of-Use Asset			0
Assets			1,737,848
Accrued expenses and other liabilities			101,839
Liabilities, Current			322,221
Obligations under operating leases, excluding current installments			0
Other non‑current liabilities			78,661
Liabilities, Noncurrent			1,049,805
Liabilities			1,372,026
Retained deficit			(174,976)
Total Evoqua Water Technologies Corp. equity			362,759
Total accumulated other comprehensive loss			365,822
Liabilities and Equity			$ 1,737,848
Revision of Prior Period, Accounting Standards Update, Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Prepaid and other current assets		(73)
Current assets		(73)
Property, plant, and equipment, net		2,812
Operating Lease, Right-of-Use Asset		42,073
Assets		44,812
Accrued expenses and other liabilities		13,596
Liabilities, Current		13,596
Obligations under operating leases, excluding current installments		29,308
Other non‑current liabilities		3,245
Liabilities, Noncurrent		32,553
Liabilities		46,149
Retained deficit		(1,337)
Total Evoqua Water Technologies Corp. equity		(1,337)
Total accumulated other comprehensive loss		(1,337)
Liabilities and Equity		$ 44,812