Mixing of the Metro Wastewater Treatment Plant effluent with the Mississippi River below St. Paul

As a result of this study the following description of the Metro Wastewater Treatment Plant (WWTP) effluent zone in the Mississippi River below St. Paul can be given: • The shape and the size of the mixing zone changes with season. Only summer conditions were analyzed specifically. • The longest mixing zone can be expected in fall or spring when the mixing is controlled by transverse spreading under neutrally buoyant conditions. Such conditions may also exist at very high flows during other seasons. o Between June and August the Metro WWTP effluent plunges below the water surface before reaching the end of the diverging outlet channel. The process also occurs in May and ends sometime in September. The effluent water therefore enters the Mississippi River as a submerged flow (underflow). Water temperatures and higher dissolved solids content in the effluent relative to the oncoming Mississippi River water are the cause of the plunging phenomenon and the underflow. • The river becomes vertically stratified below the outlet due to the sinking of the effluent into the deepest part of the river main channel. • The spread of the effluent underflow across the river bottom during June, July and August appears essentially completed within 0.3 miles or less from the outlet, the actual distance depending on river flow rate. An analysis of the initial transverse spreading process (nearfield) has not been .made.· • The most appropriate model for the summer Metro WWTP effluent mixing zone is therefore a model which considers the plunging, transverse underflow and vertical mixing in the river.