introduced separately at the inlet of the mixing apparatus it is equally possible to introduce a single fluid substance at the inlet which is to undergo mixing as it travels through the pipe elbows in a particular embodiment the invention is employed to mix a polymer with a liquid sludge the sludge has a solids concentration of 0 1 to 22 and is fed at a rate between 25 and 200 gpm a polymer solution of a flocculant having a dry solids concentration of 0 05 2 is fed at a rate between 1 and 15 volumetrically of the feed rate of the sludge the sludge and flocculant solutions pass through a vertical array of 12 pipe elbows consisting of four groups of elbows each with three elbows arranged in mutually perpendicular relation at the outlet of the elbows the sludge and flocculant are thoroughly and uniformly mixed each of the pipe elbows is composed of pvc and has a pipe diameter between two and three inches the pipe elbows are 90 u00b0 long sweep elbows and in each group are adhesively joined at their ends to form a smooth unbroken inner passageway without vanes paddles grooves etc each group of three elbows is connected to the next by means of flanges which are secured to the end elbows and are bolted together the flanges serve as a connect and release means for each of the groups of three elbows the mixture of sludge and flocculant is fed to an aging vessel in which the mixture has a detention time of the order of one.