Density currents form in all activated sludge clarifiers. The dense influent cascades downward and become a current moving in a horizontal plane just above the sludge blanket, attracting lighter solids and short-circuiting the main clarification volume of the tank. The density currents carry the solids to the tank wall and deposit them into the effluent channel, increasing effluent suspended solids and reducing the clarifier’s hydraulic capacity. An EPA consultant, working at the Stamford, CT WWTP in the early 1980’s, conceived of an inclined baffle mounted to the clarifier wall that would intercept these currents and redirect them toward the center of the tank. Several years later, NEFCO implemented that concept by designing and installing the first full-scale, fiberglass Density Current Baffle at the Stamford plant.

The original NEFCO Stamford Baffle was inclined at 45° and had a relatively small horizontal projection factor. That baffle reduced the clarifier’s total suspended solids (TSS) by roughly 40%. The design quickly became the industry standard. NEFCO’s baffle included two proprietary features: an integrally molded support bracket that simplified installation, and gas vents molded into the mounting flange at the top of the baffle. The 45° baffle continues to be produced by others.

In clarifiers with outboard launders, the baffle mounts to the clarifier wall with the bottom of the baffle positioned mid-way between the top of the sludge blanket and the bottom of the weir. In clarifiers with inboard launders, the baffle will perform most effectively mounted to the tank wall below the launder, so long as there is sufficient vertical clearance to mount the baffle with the bottom of the baffle two feet or more above the top of the blanket. If there is insufficient vertical clearance, the baffle should be mounted to the lower inboard corner of the launder trough. The width of the launder should be included in calculating the horizontal projection of the baffle, but the projection should never be less than 24”.