Abstract: A sidestream pipeline, adapted for attachment to a mainstream pipeline, is provided with end closures to permit the introduction of mechanical cleaning devices or chemical compositions to the sidestream pipeline to remove biofilm and treat corrosion-causing constituents; also provided are a plurality of removable coupons closely fitting within holes in the sidestream pipeline wall, and duplicating the material and configuration of the interior surface of the mainstream pipeline. The coupons can be individually withdrawn from the lowermost surface position during cleaning operations to avoid contact with the mechanical cleaning devices and can be isolated from teh chemical treatment. The coupons are removable for laboratory study and analysis.