In 2005, faced with scum buildup and high peak flows, the City of Englewood, Ohio, needed to upgrade its wastewater treatment plant. The plant's two 90-foot-diameter, side-feed clarifiers were configured with influent pipe supported by the clarifier access bridge — a design used before skimming became a priority for wastewater plants. This arrangement actually prevented skimming of the basin's surface, resulting in periods of scum buildup.
The Englewood plant typically has flows in the range of 1.5 to 2.5 mgd, but has been experiencing peak flows averaging 10 mgd several times a year, stressing the plant's process. The solution was to upgrade the plant operation to a modified contact stabilization mode once the flows reached a predetermined rate. This mode reduces the solids loading to the clarifiers by diverting the high raw flow to the aeration basin's final compartment, thus preventing washout of solids from the aeration basin. The high hydraulic load required clarifiers to be as efficient as possible.
However, the 25-year-old drive unit and rotating mechanism had been replaced in 1997, due to wear and corrosion. Replacing the entire mechanism again did not sit well with city management.
Instead, the city's water and sewer department partnered with Siemens Water Technologies to develop a retrofit solution using peripheral-feed, peripheral-overflow clarifiers — a configuration that would increase flow capacity and allow surface and influent channel skimming. The retrofit can handle flows of up to 10 mgd, with a design flow of 2.5 mgd and an average flow of 1.5 mgd. At less than $350,000 per clarifier unit, the retrofit solved the plant's issues while saving construction costs and keeping clarifier down-time to a minimum.
One way to add skimming capability is to convert the clarifier to a siphon, center-feed unit. At the Englewood facility, this would require excavation outside the tank, cutting/trenching the basin floor, and installing an influent pipe in the center of the basin. The entire mechanism would have to be removed for the concrete and piping modifications to take place, and a new center pier would have to be added, incurring significant construction costs and disrupting the plant's operation.
A less labor-intensive, less invasive option is to retrofit the clarifier to operate as a peripheral-feed, peripheral-overflow design. With this plan, the influent is introduced through a hydraulically designed channel that extends around the entire basin's circumference.
The influent channel floor includes orifices to allow influent to enter the clarifier. In designing this channel, it is crucial to analyze the head loss required to drive the flow through the channel and orifices.
Many things must be considered to ensure that the flow travels the full length of the channel over the entire flow range, and enters the basin at a uniform rate over the entire periphery. Orifices must have specific diameters and spacing. In addition, properly sized orifice target baffles and specifically located skirt baffles of proper depth are necessary to optimize clarifier operation. The clarifier effluent is collected by an additional peripheral channel located next to the influent channel.
Project: Wastewater treatment plant secondary clarifier rehabilitation
A properly designed peripheral-feed, peripheral-overflow clarifier can provide 50% to 80% more hydraulic efficiency and can be designed with up to 50% less surface area than a conventional center-feed clarifier, resulting in a corresponding decrease in construction costs.
The clarifier makes full use of its tank volume by eliminating short-circuiting of the solids during higher flows. Also, locating the influent and effluent channels around the tank periphery allows the tank surface to be skimmed effectively. With this option, the existing mechanism can be used with add-on adapters, keeping costs to a minimum.
The combination of influent/effluent channels can be fabricated steel or poured-in-place concrete that is doweled to the basin wall. If the existing clarifier mechanism is still in good shape, as in the Englewood project, it can be easily modified to allow conventional main tank skimming. With peripheral feed, scum is concentrated in the influent channel, a relatively small area easily accessible from outside the basin; plus it allows numerous mechanical options for skimming the channel.
Some channel skimming options include a weir gate. The field-adjustable automated inverted weir gate is located at the end of the influent channel. Its operation is initiated by a land-mounted limit switch incorporated with a series of timers and counters that enable greater flexibility for removing scum, while keeping excess water in the removed scum to a minimum and reducing scum handling system costs. As it is lowered, the weir pulls the scum from the influent channel into a pit, significantly reducing losses related to a more conventional skimmer wiper/scum trough arrangement.
The timers allow the operator to schedule gate openings in relation to the approaching skimmer and the length of withdrawal time. Control counters enable the operator to dictate the frequency of gate cycles based on the number of clarifier rotations. This allows adjustment of the scum collection to match actual day-to-day requirements.
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 (firstname.lastname@example.org) is an application engineer with Siemens Water Technologies, based in Waukesha, Wis.
To see a video of Siemens' Rim-Flo clarifier, click here.