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Amount of direct costs of the business combination including legal, accounting, and other costs incurred to consummate the business acquisition.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

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Amount of liabilities incurred by the acquirer as part of consideration transferred in a business combination.

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For contingent consideration arrangements recognized in connection with a business combination, this element represents an estimate of the high-end of the potential range (undiscounted) of the consideration which may be paid.

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The number of countries in which the entity operates as of balance sheet date.

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The net result for the period of deducting operating expenses from operating revenues.

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The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

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The cash inflow associated with the amount received from the sale of a portion of the company's business, for example a segment, division, branch or other business, during the period.

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Cash inflow representing an adjustment to the purchase price of a previous acquisition.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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Acquisitions and Divestitures (Narrative) (Details) £ in Thousands, $ in Thousands, $ in Thousands								12 Months Ended
	May 25, 2019 GBP (£)	May 25, 2019 USD ($)	Aug. 31, 2018 CAD ($)	Aug. 31, 2018 USD ($)	Jul. 26, 2018 USD ($)	Mar. 09, 2018 USD ($)	Jan. 31, 2018 USD ($)	Sep. 30, 2019 USD ($) country	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Aug. 31, 2018 USD ($)
Business Acquisition [Line Items]
Revenue								$ 1,444,441	$ 1,339,541	$ 1,247,424
Earnings before interest, taxes, depreciation and amortization (EBITDA)								59,672	66,846	69,204
Proceeds from sale of business								$ 0	430	$ 0
Number of countries in which entity operates \| country								10
ATG UV [Member]
Business Acquisition [Line Items]
Payments to acquire businesses, gross	£ 5,500	$ 6,931
Transaction costs								$ 193
IsH20Top Group
Business Acquisition [Line Items]
Business combination, consideration transferred			$ 3,651	$ 2,804
Payments to acquire businesses, gross			$ 3,171	$ 2,435
Earn Out Payment, Period			1 year	1 year
Proceeds from Previous Acquisition			$ 226	$ 175
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High			$ 480								$ 369
IsH20Top Group \| General and administrative expense
Business Acquisition [Line Items]
Transaction costs											$ 188
ProAct
Business Acquisition [Line Items]
Payments to acquire businesses, gross					$ 133,772
Revenue									8,042
Earnings before interest, taxes, depreciation and amortization (EBITDA)									$ 590
ProAct \| General and administrative expense
Business Acquisition [Line Items]
Transaction costs					$ 1,067
Pacific Ozone
Business Acquisition [Line Items]
Business combination, consideration transferred						$ 2,000
Payments to acquire businesses, gross						6,557
Consideration transferred, liabilities incurred						$ 8,557
Earn Out Payment, Period						3 years
Proceeds from Previous Acquisition						$ 934
Pacific Ozone \| General and administrative expense
Business Acquisition [Line Items]
Transaction costs						$ 191
Pure Water
Business Acquisition [Line Items]
Business combination, consideration transferred							$ 993
Payments to acquire businesses, gross							3,706
Consideration transferred, liabilities incurred							4,699
Proceeds from Previous Acquisition							461
Pure Water \| General and administrative expense
Business Acquisition [Line Items]
Transaction costs							$ 132
Memcor [Member]
Business Acquisition [Line Items]
Transaction costs								$ 2,795