Property, plant and equipment and construction in progress (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 126,438,769	$ 121,378,973
Less accumulated depreciation	67,557,951	71,695,202
Property, plant and equipment, net	58,880,818	49,683,771
Construction in progress	6,015,043	1,823,284
Land
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	3,435,361	3,435,361
Buildings [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	19,829,575	19,916,098
Plant and equipment
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	61,777,836	58,873,604
Distribution system [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	36,057,078	33,901,161
Office furniture, fixtures and equipment
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	3,635,184	3,413,702
Vehicles
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	1,431,719	1,444,182
Leasehold improvements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	244,221	237,027
Lab equipment
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 27,795	$ 157,838