Property, plant and equipment and construction in progress (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 124,287,415	$ 123,861,456
Less accumulated depreciation	73,762,351	70,777,351
Property, plant and equipment, net	50,525,064	53,084,105
Construction in progress	1,823,284	885,494
Land
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	3,435,361	3,570,361
Buildings [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	20,135,753	19,366,123
Plant and equipment
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	61,437,751	62,227,399
Distribution system [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	33,986,305	33,482,674
Office furniture, fixtures and equipment
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	3,425,992	3,405,633
Vehicles
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	1,446,468	1,396,886
Leasehold improvements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	261,148	261,147
Lab equipment
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 158,637	$ 151,233