Siemens' Rim-Flo circular clarifier uses influent and effluent channels located around the wastewater tank periphery to effectively skim the entire tank surface and the influent raceway. Images: Siemens
The Englewood Wastewater Treatment Plant in Ohio uses automated weir gates to efficiently remove scum from influent channels.

Retrofitting logistics

Once the Englewood Water and Sewer Department decided that a retrofit was the best option, plant management enlisted Siemens to evaluate its current wastewater flows — taking into account the available upstream/downstream head loss and influent/effluent piping locations — and recommend a design. The design, based on the company's Rim-Flo clarifier, included a field-welded steel peripheral influent/effluent trough system that compensates for existing concrete basin tolerances and variations.

Influent is introduced into a channel surrounding the periphery of the tank. The channel has a varied cross section that helps maintain a constant velocity in the channel to prevent settling out of solids. The confined influent provides a uniform distribution around the periphery. Orifices in the feed channel floor are sized and spaced by computer, to provide a controlled head loss that ensures equalized flow distribution into the tank around the periphery. Orifice spacing also helps prevent solids from being deposited on the channel floor.

Since hydraulic loadings can vary so widely, it's important to determine the exact relationship of the channel widths and cross section to the orifice size and locations, tank size, and overflow rates. With the clarifier, these calculations are determined by a computer program based on a data bank of field experience.

As the controlled flow enters the tank through the orifices, it is deflected by a target baffle on the underside of the feed channel. This baffle, and the orifice length, eliminates the “jetting” action into the basin. Instead, the flow is diffused rapidly and completely in the large area between the tank wall and the influent skirt baffle. The skirt baffle defines a clear liquid zone, and its cross-sectional area is such that the inlet velocities are controlled at up to 5 feet/minute at maximum flow.

Flow enters the clarifier uniformly and at low velocities, below the skirt baffle and above the normal sludge blanket. The flow moves outward, toward the center, where it meets flow from the other side. The flow then moves up and back toward the peripheral effluent channel in a gentle circular motion. The entire tank volume is used, eliminating eddies that cause short-circuiting, while solids uniformly drop out of suspension.

A scum baffle attached to the effluent launder prevents collected surface scum in the main tank from entering the effluent channel. A hinged wiper assembly and spring-loaded closure plate ensure constant contact between the scum blade, scum baffle, and beach as the blade travels up the beach and deposits into the main tank scum trough.

A hinged blade assembly, mounted on an extension arm, provides influent channel skimming. The arm is supported by the main tank skimmer support truss, which directs scum to the weir gate for removal. This design prevents scum from bridging in the areas of a wider influent channel width. The gate is lowered as the skimmer approaches; the type and volume of the floatable material determines the operating time cycle.

Efficient and affordable

A retrofit ideally makes use of existing basins and equipment to save the plant construction and installation costs, while allowing an easy way to upgrade aging equipment for higher flow rates, better performance, and perhaps higher energy efficiency. The clarifier retrofit made sense for Englewood's water treatment plant, since all the components replaced in 1997 — including the rotating mechanism, drive unit, and access bridge — could be reused with the use of adaptors and extensions.

Equipment modifications were designed to keep installation costs to a minimum. They included:

  • Raising the bridge to provide clearance for the influent channel skimmer support
  • Adding extensions to the center pier and center cage
  • Adding spacers for the bridge connection to the tank
  • Extending the existing counterweight support truss arm and evaluating it for the new skimmer loads
  • Furnishing an additional truss panel for simple field-welding.

    The retrofit of the two clarifiers was staged over a period of two years, to spread out the cost and allow the city to meet its budget. The first clarifier was updated in 2006 and the second in 2008. Both units were upgraded during summer months when the wastewater treatment flow was low.

    — Jeffrey Schneider ( is an application engineer with Siemens Water Technologies, based in Waukesha, Wis.


    To see a video of Siemens' Rim-Flo clarifier, click here.