Project Case Study: A balancing act

May 2005 » Feature Articles
Compared with the issues involved in process design, treatment plant hydraulics does not at first appear to be particularly difficult. The engineer needs to make certain that the weir in each successive process tank is lower than in the previous one, and that there is enough freeboard to prevent the tank from overflowing during high-flow conditions.

Dynamic models help engineers understand treatment plant hydraulics.

BY THOMAS WALSKI

Compared with the issues involved in process design, treatment plant hydraulics does not at first appear to be particularly difficult. The engineer needs to make certain that the weir in each successive process tank is lower than in the previous one, and that there is enough freeboard to prevent the tank from overflowing during high-flow conditions.

Engineers, however, don't just want to design a plant that doesn't flood the yard; they want to find the best elevations for tanks and weirs. The design process usually involves a good deal of trial and error-such as raising weirs, installing longer weirs or launders, and lowering tank walls-to balance hydraulic performance and the amount of cut and fill. The weir calculations can get tedious and difficult to visualize.

A software program, such as Bentley Systems' Civil Storm Dynamic (CSD), can help design engineers by performing these hydraulic calculations and providing a graphical user interface. CSD is a fully dynamic hydraulic tool that enables an engineer to solve for the flows and water levels in any complex system of conduits, channels, ponds, weirs, and control structures.

The program was designed primarily for use on land development projects, rather than treatment plants, but both applications have the same types of components-conduits, channels, and storage facilities.

CSD goes beyond solving steady-state hydraulic equations, and can route flows dynamically through a treatment plant using the full Saint-Venant equations for unsteady, one-dimensional flow. Because treatment plants include tanks, a change in flow at the headworks is not instantaneously realized throughout the plant.

Increased inflow to a tank manifests itself partly as an increase in water level and partly as an increase in outflow.

Additionally, when inflow to a tank changes, there is usually lag time before outflow equals inflow.

In a wastewater treatment plant, flow usually is measured at the headworks, but the unit process that is most sensitive to flow changes is secondary clarification.

The cycling of a pump at the headworks or in the collection system may have a marked effect on the flow at the plant influent flow meter, but, depending on the magnitude of flow changes and the plant hydraulics, it may not have a dramatic effect on flow through the subsequent treatment processes.

Dynamic flow routing can help engineers and operators understand the hydraulics of their plants and how those hydraulics respond to changes in flow.

Although the example cited here refers to wastewater treatment plants, much of the discussion is applicable directly to other trains of unit processes such as potable water treatment plants.

Water level profile Developing a hydraulic profile through a treatment plant is a key step in design and usually involves many “what-if ” analyses such as, “What if we lower this weir?” or “What if we increase the size of this tank?” The design engineer's two main tasks are performing the hydraulic calculations and viewing the results.

Using modeling software, the engineer can perform the calculations and present the results graphically. Figure 1 shows a plan-view schematic of a treatment plant with two trains containing primary clarifier, aeration basin, secondary clarifier, and chlorine contact tanks. Although this figure is a schematic, it is possible to draw the planview to scale.

Although the plan view is necessary to lay out the system, the profile view is the most important for hydraulic evaluation.

The profile for one of the trains from Figure 1 is shown in Figure 2. (For simplicity at this scale, Figure 2 does not contain any labeling.) With the profile view, the engineer can view quickly the impact of any design modifications.

Dynamic simulation A dynamic model can route flow through a treatment plant.

When the flow changes at the head of the plant because of a wetweather event, normal diurnal fluctuations, or cycling of an upstream pump, the flow does not instantaneously change throughout the entire plant. Rather, each process tank attenuates the change in flow, creating a time lag before the change appears in later processes, and making the change less pronounced as it proceeds downstream.

The specific response to the change in flow depends on the magnitude and rate of change, and the physical characteristics of the tanks and weirs.

Figure 3 shows the dynamic routing of flows at three points in a system when a pump in the headworks of the plant cycles on and off. Depending on the rate at which the pumps cycle, the increased flow only gradually affects the final pro-cesses.

Note how the flows in later processes respond to the sudden inflow changes occurring just before hours 1 and 1.5.

Depending on the rate at which pumps cycle, an increase in flow at the plant headworks may never be fully realized in the later processes. Figure 4 shows a situation where the pump cycles feeding the plant are generally so short that the flows in the final processes have not yet reached a new steady state before they change again.

In Figure 1, the tank elements were annotated with values for the tank water level. With a dynamic model, engineers can annotate any element with attributes such as level, depth, flow, and velocity; watch how these numerical values change over time; and analyze the effect of changing flow rates on plant hydraulics.

Thomas Walski is vice president, Engineering, for Haestad Methods' product line, Bentley Systems, in Waterbury, Conn. He can be contacted via e-mail at tom.walski@bentley.com.


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