An electrocoagulation reactor (ECR) apparatus and methods of using the ECR apparatus in the treatment of a fluid stream. The ECR apparatus includes a noncorrosive cylindrical cell housing a plurality of horizontally stacked electrode plates. The electrode plates are held in a pair of grooved, crescent shaped non-conductive inserts. The ECR apparatus further includes two end flanges each having integral flow diverters to facilitate a continuous single serpentine flow of the fluid in the cell. The ECR apparatus further includes a single flow inlet and single flow outlet.