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Property, plant and equipment and construction in progress (Details) - USD ($)
Dec. 31, 2016
Dec. 31, 2015
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Gross $ 123,861,456 $ 122,134,921
Less accumulated depreciation 70,777,351 68,391,751
Property, plant and equipment, net 53,084,105 53,743,170
Construction in progress 885,494 1,928,610
Land [Member]    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Gross 3,570,361 3,223,361
Building [Member]    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Gross 19,366,123 18,437,758
Distribution Systems [Member]    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Gross 33,482,674 31,726,766
Office furniture, fixtures and equipment [Member]    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Gross 3,405,633 3,210,819
Vehicles [Member]    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Gross 1,396,886 1,384,294
Plant And Equipment [Member]    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Gross 62,227,399 63,733,553
Leasehold Improvements [Member]    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Gross 261,147 260,519
Lab Equipment [Member]    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Gross $ 151,233 $ 157,851