Annual report pursuant to Section 13 and 15(d)

Note 13 - Stockholders' Equity (Details Textual)

Note 13 - Stockholders' Equity (Details Textual)				1 Months Ended											12 Months Ended
Note 13 - Stockholders' Equity (Details Textual)	May 14, 2019 USD ($) $ / shares shares	Feb. 26, 2019 $ / shares shares	Jan. 22, 2019 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares	Aug. 31, 2020 USD ($) shares	May 31, 2020 USD ($) shares	Mar. 31, 2020 USD ($) shares	Sep. 30, 2019 USD ($) shares	Aug. 31, 2019 USD ($) shares	Jul. 31, 2019 USD ($) $ / shares shares	May 31, 2019 $ / shares shares	Mar. 31, 2019 $ / shares shares	Feb. 28, 2019 $ / shares shares	Jan. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2017 shares
Common Stock, Shares Authorized (in shares)				100,000,000											100,000,000	100,000,000
Common Stock, Par or Stated Value Per Share (in dollars per share) \| $ / shares				$ 0.001											$ 0.001	$ 0.001
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) \| $ / shares				1.90											$ 1.90
Stock Issued During Period, Value, Issued for Services \| $															$ 64,000	$ 4,927,000
Proceeds from Issuance of Common Stock \| $															$ 3,676,000	$ 29,380,000
Stock Issued During Period, Shares, Restricted Stock Award, Net of Forfeitures, Total (in shares)															691,820	533,655
Class of Warrant or Right, Weighted Average Exercise Price of Warrants or Rights (in dollars per share) \| $ / shares				$ 1.90											$ 1.90
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)															0	2,197,074
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total \| $				$ 3,300,000											$ 3,300,000
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year)															2 years 4 months 24 days
Share-based Payment Arrangement, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)										150,000
Share-based Payment Arrangement, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)										50,000
Share-based Payment Arrangement, Option [Member] \| Share-based Payment Arrangement, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year)										5 years
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Exercise Price (in dollars per share) \| $ / shares										$ 1.65
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)										3 years
Share-based Payment Arrangement, Option [Member] \| Share-based Payment Arrangement, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year)										5 years
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Exercise Price (in dollars per share) \| $ / shares										$ 1.87
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)										3 years
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)				3 years
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period, Number of Installments				6
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares)				1,714,252	367,500									263,660	8,056,280	1,271,222
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Fair Value \| $				$ 1,971,000	$ 380,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Outstanding \| $				14,400,000											$ 14,400,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Vested \| $															2,100,000
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Outstanding \| $				$ 2,500,000											$ 2,500,000
Officers and Directors Purchase Plan [Member]
Stock Issued During Period, Shares, New Issues (in shares)															5,453
Proceeds from Issuance of Common Stock \| $															$ 5,750
The 2014 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized (in shares)																	2,113,637
The 2014 Stock Incentive Plan [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year)																	10 years
The 2019 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized (in shares)				11,500,000											11,500,000
The 2019 Stock Incentive Plan [Member] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)						3 years
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period, Number of Installments						6
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Fair Value \| $						$ 670,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Issued in Period (in shares)						1,970,475
The 2019 Stock Incentive Plan [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year)															10 years
Board Members [Member]
Stock Issued During Period, Shares, Restricted Stock Award, Net of Forfeitures, Total (in shares)															356,492	202,905
Board Members [Member] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares)										60,560
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Fair Value \| $										$ 97,500
Prior Company Executive [Member]
Stock Issued During Period, Shares, New Issues (in shares)															347,892	161,362
Employee [Member]
Stock Issued During Period, Shares, New Issues (in shares)															1,776,680
Employee [Member] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)						3 years			12 months					3 years
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period, Number of Installments						3			3			3		6
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares)						17,500			134,419
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Fair Value \| $						$ 16,000			$ 231,000					$ 490,000
Employee [Member] \| Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)							3 years
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period, Number of Installments							3
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares)							830,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Fair Value \| $							$ 280,000
Consultant [Member]
Stock Issued During Period, Shares, Issued for Services (in shares)															67,522
Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)													1,260,000	232,461
Chief Executive Officer [Member] \| Share-based Payment Arrangement, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)													420,000
Chief Executive Officer [Member] \| Share-based Payment Arrangement, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)													420,000
Chief Executive Officer [Member] \| Share-based Payment Arrangement, Tranche Three [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)													420,000
Chief Executive Officer [Member] \| Share-based Payment Arrangement, Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year)														5 years
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Exercise Price (in dollars per share) \| $ / shares														$ 1.88
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)													3 years
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period, Number of Installments													3
Chief Executive Officer [Member] \| Share-based Payment Arrangement, Option [Member] \| Share-based Payment Arrangement, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year)													5 years
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Exercise Price (in dollars per share) \| $ / shares													$ 3.08
Chief Executive Officer [Member] \| Share-based Payment Arrangement, Option [Member] \| Share-based Payment Arrangement, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year)													5 years
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Exercise Price (in dollars per share) \| $ / shares													$ 3.68
Chief Executive Officer [Member] \| Share-based Payment Arrangement, Option [Member] \| Share-based Payment Arrangement, Tranche Three [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year)													5 years
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Exercise Price (in dollars per share) \| $ / shares												$ 4.18
Chief Executive Officer [Member] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares)								59,414
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Fair Value \| $								$ 109,000
Chief Financial Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)												250,000		56,698
Chief Financial Officer [Member] \| Share-based Payment Arrangement, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)												125,000
Chief Financial Officer [Member] \| Share-based Payment Arrangement, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)												62,500
Chief Financial Officer [Member] \| Share-based Payment Arrangement, Tranche Three [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)												62,500
Chief Financial Officer [Member] \| Share-based Payment Arrangement, Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year)														5 years
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Exercise Price (in dollars per share) \| $ / shares														$ 1.88
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)												3 years
Chief Financial Officer [Member] \| Share-based Payment Arrangement, Option [Member] \| Share-based Payment Arrangement, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year)												5 years
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Exercise Price (in dollars per share) \| $ / shares												$ 3.79
Chief Financial Officer [Member] \| Share-based Payment Arrangement, Option [Member] \| Share-based Payment Arrangement, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year)												5 years
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Exercise Price (in dollars per share) \| $ / shares												$ 4.39
Chief Financial Officer [Member] \| Share-based Payment Arrangement, Option [Member] \| Share-based Payment Arrangement, Tranche Three [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year)												5 years
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Exercise Price (in dollars per share) \| $ / shares												$ 4.89
Director [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)											172,915
Director [Member] \| Share-based Payment Arrangement, Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year)											5 years
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Exercise Price (in dollars per share) \| $ / shares											$ 2.48
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)											12 years
Company Employees [Member] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)							3 years
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period, Number of Installments							6
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares)							1,293,164
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Fair Value \| $							$ 440,000
January 2019 Public Offering [Member]
Stock Issued During Period, Shares, New Issues (in shares)			5,175,000
Shares Issued, Price Per Share (in dollars per share) \| $ / shares			$ 1.90
Proceeds from Issuance of Common Stock \| $			$ 9,800,000
Proceeds from Issuance of Common Stock, Net \| $			$ 9,100,000
May 2019 Public Offering [Member]
Stock Issued During Period, Shares, New Issues (in shares)	11,000,000
Shares Issued, Price Per Share (in dollars per share) \| $ / shares	$ 2.00
Proceeds from Issuance of Common Stock \| $	$ 22,000,000
Proceeds from Issuance of Common Stock, Net \| $	$ 20,300,000
ATM [Member]
Stock Issued During Period, Shares, New Issues (in shares)															3,217,426
Proceeds from Issuance of Common Stock \| $															$ 3,700,000
Veolia Warrants 1 [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)		2,000,000		2,000,000											2,000,000
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) \| $ / shares		$ 5.00		$ 5.00											$ 5.00
Warrants and Rights Outstanding, Term (Year)				10 years											10 years
Veolia Warrants 2 [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)		2,000,000		2,000,000											2,000,000
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) \| $ / shares		$ 7.00		$ 7.00											$ 7.00
Warrants and Rights Outstanding, Term (Year)				10 years											10 years
Veolia Warrants [Member]
Warrants and Rights Outstanding, Term (Year)		10 years
January 2019 Underwriter Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)				103,500										103,500	103,500
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) \| $ / shares				$ 1.90										$ 1.90	$ 1.90
Warrants and Rights Outstanding, Term (Year)				5 years											5 years
Class of Warrant or Right, Number of Securities Called by Warrants or Rights, Percent			2.00%
Veolia [Member]
Total Consideration for Service Agreement, Equity Shares (in shares)		2,350,000
Total Consideration for Service Agreement, Number of Installments		8
Stock Issued for Services, Quarterly Installments, Shares (in shares)		293,750
Stock Issued During Period, Shares, Issued for Services (in shares)																1,184,985
Stock Issued During Period, Value, Issued for Services \| $																$ 3,600,000
Clarios [Member]
Stock Issued During Period, Value, Issued for Services \| $																$ 1,300,000
Stock Issued During Period, Shares, New Issues (in shares)																807,436