Assessment, Permitting and Response Revenues—Assessment, Permitting and Response revenues are generated from multidisciplinary environmental consulting services. The majority of the contracts are fixed-price or time and material based.

Measurement and Analysis Revenues—Measurement and Analysis revenues are generated from emissions sampling, testing and reporting services, leak detection services, ambient air monitoring services and laboratory testing services. The majority of the contracts are fixed-price or time-and-materials based.

Remediation and Reuse Revenues—Remediation and Reuse revenues are generated from operating and maintenance (“O&M”) services (on biogas and waste water treatment facilities), as well as remediation, monitoring and environmental compliance services. Services on the majority of O&M contracts are provided under long-term fixed-fee contracts. Remediation, monitoring and environmental compliance contracts are predominantly fixed-fee and time-and-materials based.

Disaggregation of Revenue—The Company disaggregates revenue by its operating segments. The Company believes disaggregating revenue into these categories achieves the disclosure objectives to depict how the nature, amount, and uncertainty of revenue and cash flows are affected by economic factors. Disaggregated revenue disclosures are provided in Note 21, Segment Information.

Contract Balances—The Company presents contract balances for unbilled receivables (contract assets), as well as customer advances, deposits and deferred revenue (contract liabilities) within contract assets and accounts payable and accrued expenses, respectively, on the consolidated statements of financial position. Amounts are generally billed at periodic intervals (e.g., weekly, bi-weekly or monthly) as work progresses in accordance with agreed-upon contractual terms. The Company utilizes the practical expedient to not adjust the promised amount of consideration for the effects of a significant financing component as the period between when the Company transfers services to a customer and when the customer pays for those services is one year or less. Amounts recorded as unbilled receivables are generally for services the Company is not entitled to bill based on the passage of time. Under certain contracts, billing occurs subsequent to revenue recognition, resulting in contract assets. The Company sometimes receives advances or deposits from customers before revenue is recognized, resulting in contract liabilities.

The following table presents the Company’s contract balances as of December 31:

	2021		2020
Contract assets	$	40,139	$	38,576
Contract liabilities		27,907		6,114

Contract assets acquired through business acquisitions amounted to $0.5 million and $6.5 million as of December 31, 2021 and 2020, respectively. Contract liabilities acquired through business acquisitions amounted to $0.5 million and zero as of December 31, 2021 and 2020, respectively.

Revenue recognized during the year ended December 31, 2021, included in the contract liability balance at the beginning of the year was $4.1 million. The revenue recognized from the contract liabilities consisted of the Company satisfying performance obligations during the normal course of business.

The amount of revenue recognized from changes in the transaction price associated with performance obligations satisfied in prior periods during the year ended December 31, 2021 was not material.

Remaining Unsatisfied Performance Obligations—Remaining unsatisfied performance obligations represent the total dollar value of work to be performed on contracts awarded and in progress. The amount of remaining unsatisfied performance obligations increases with new contracts or additions to existing contracts and decreases as revenue is recognized on existing contracts. Contracts are included in the amount of remaining unsatisfied performance obligations when an enforceable agreement has been reached. As of December 31, 2021, and 2020, the estimated revenue expected to be recognized in the future related to performance obligations that are unsatisfied was approximately $108.7 million and $24.4 million, respectively. As of December 31, 2021, the Company expected to recognize approximately $87.5 million of this amount as revenue within the next year and $21.2 million the year after.

Accounts Receivable, Net—Accounts receivable, net as of December 31, consisted of the following:

	2021			2020
Accounts receivable, invoiced	$	101,709		$	57,228
Accounts receivable, other		1,385			1,139
Allowance for doubtful accounts		(4,581	)		(4,265	)
Accounts receivable—net	$	98,513		$	54,102

The Company extends non-interest-bearing trade credit to its customers in the ordinary course of business. Accounts receivable are shown on the face of the consolidated statements of financial position, net of an allowance for doubtful accounts. The allowance for doubtful accounts is established at the origination of an account in accordance with ASU 2016-13, Financial Instruments—Credit Losses (Topic 326). ASC 326 requires the Company to estimate the lifetime expected credit losses on such instruments and to record an allowance to offset the receivables. In determining the allowance for doubtful accounts, the Company analyzes the aging of accounts receivable, historical bad debts, customer creditworthiness and current economic trends. During the first quarter of 2020, there was a global outbreak of a new strain of coronavirus, COVID-19. The COVID-19 pandemic has added uncertainty to the collectability of certain receivables, particularly in industries hard hit by the pandemic. As a result, the Company recorded $6.3 million of bad debt reserve during the first quarter of 2020. The bad debt adjustment included a $5.5 million reserve for one customer in the Company’s Remediation and Reuse segment in which management concluded to discontinue select service lines as of June 30, 2020 (Note 21).

The Company has one customer who accounted for 10.0% of revenue for the year ended December 31, 2021. The Company did not have any customers that exceeded 10.0% of revenue during the years ended December 31, 2020 and 2019. The Company has one customer who accounted for 23.1% and 10.2% of our gross accounts receivable as of December 31, 2021 and 2020, respectively. The Company performs ongoing credit evaluations and based on past collection experience, the Company believes that the receivable balances from these largest customers do not represent a significant credit risk.

	Beginning Balance		Bad Debt Expense		Charged to Allowance			Other⁽¹⁾		Ending Balance
Year ended December 31, 2021	$	4,265	$	1,135	$	(1,548	)	$	729	$	4,581
Year ended December 31, 2020		1,327		4,532		(2,633	)		1,039		4,265
Year ended December 31, 2019		453		1,246		(546	)		175	$	1,327