Self-Cleaning Trench-Type Wet Well Design

Transcription

1 Self-Cleaning Trench-Type Wet Well Design *Trooper Smith, P.E., Freese and Nichols, Inc. *10814 Jollyville Rd. Building 4, Suite 100 Austin, TX, PH: (512) ABSTRACT Wastewater treatment rarely yields awe-inspiring structures. If asked, the general public s most favorable comment about a wastewater treatment plant is that they have never noticed it. And yet, for the North Texas residents in cities served by the Central Regional Wastewater System of the Trinity River Authority, the design of Pump Station 13B with the unwieldy description of a self-cleaning trench-type wet well with vertical turbine solids handling pumps represents an inspired, and inspiring, application of innovative technology to familiar conflicts in publicly-operated services. This project addressed limited budget, limited space and increased service demand by exploring the value of a vertical, self-cleaning pumping configuration where a more conventional, horizontal pump design had been recommended for the 50-million-gallon per day pump station. The design team, led by Freese and Nichols, consulted quality control services from national expert Robert L. Sanks, Ph.D., P.E., who has authored and edited standard texts on pump-station design. The resulting design fit the new pump station and its three vertical pumps into an 18 ft.-wide trench. Because the design no longer required removal or relocation of nearby roadways, structures and pipelines, construction costs were reduced to $8.5 million, saving the Trinity River Authority $2.9 million, well more than the pumping design engineering fee. In addition to conserving existing structures, land and funds for the Authority, which is funded through fees to municipalities it serves in Dallas/Fort Worth, the design s self-cleaning features provide long-term savings by reducing debt service and staff time required for regular maintenance. The pump station design was the first self-cleaning trench-type wet well application in Texas and the largest return activated sludge application of the vertical pump configuration in the United States. KEYWORDS Trench-type wet wells, ANSI/HI 9.8, self-cleaning, pump intake, sequent depth, modeling. INTRODUCTION Trinity River Authority was faced with an impending increase in service demand and needed to upgrade their Central Regional Wastewater System (CRWS) Treatment Plant with a new 50 MGD RAS pump station. This was complicated by the all-to-familiar restrictions of a plant now land-locked by surrounding treatment plant development, as seen in in the photo of the bare pump station site in Figure 1. Several alternatives were evaluated for the design and construction of TRA s Pump Station 13B (PS-13B) and a sustainable solution was chosen that conserved land, reduced disposal of materials and improved efficiency in wastewater treatment at this plant.

2 Figure 1: Pump Station Site Pre-Construction The solution was the design of an innovative 50 MGD self-cleaning trench-type wet well return activated sludge (RAS) pump station. The idea introduced an existing technique which had not been constructed previously for major plants in Texas. The trench-type, wet well design occupies less than one-quarter the space of more traditional designs. The narrow design of the trench also reduced the construction costs of the wet well. The self-cleaning feature adds another dimension to this project s innovation and value. It reduces both the cost of staff time and downtime in plant operation for cleaning. To the team s knowledge and coordination with pump manufacturers, the self-cleaning trench-type well design had not been employed previously within the state. By exploring this concept and applying it to the TRA site, the team resolved the conflicts inherent in many plant expansions. This project introduced Texas wastewater plant designers to a concept that can reduce construction costs for constricted sites and will continue to yield savings through its self-cleaning features. Using the vertical turbine solids handling (VTSH) pumps further widened engineering practice, since vertical turbine pumps are used more frequently in water treatment applications. METHODOLOGY There were several obstacles to overcome during the design phase. The biggest problem was the confined space that was available for the construction. The complexity of the area lay in fitting a 50-MGD RAS pump station into a site shared by the following: 84-inch primary clarifier effluent pipe 60-inch final clarifier effluent pipe Caustic soda storage facility 2- to 12-inch utility pipes, including non-potable water, drain pipes, etc. Electrical duct banks 12-foot-wide concrete roadway The intricacy of the site of PS-13B can be seen in Figure 2.

3 Figure 2: PS-13B Site The team on the project performed a site analysis and examined several pump station arrangements to determine their overall footprints; the data collected for each option can be seen in Table 1. The drypit/wet-pit horizontal non-clog pump station was familiar to the plant staff but had a large footprint and involved relocation of pavement, a 60-inch FCE line, a 4-inch PCE line, and a caustic soda building. The wet pit yielded a smaller footprint, called for no existing facility relocations and had lower probable construction costs. The design team then investigated a deeper trench-type well design, which required only an 18-foot internal width for the structure and reduced probable construction costs to $8.5 million. While VTSH pumps are used primarily for raw wastewater applications, the team researched their use for a RAS application and found that the trench-type wet well would be a suitable environment for the pumps. The 25 MGD VTSH pumps were oriented in a vertical configuration. Table 1: Pump Station Options Pump Station Type Length/Width Depth Preliminary design budgeted estimate Horizontal Non-Clog Centrifugal Pump Station (Wet Pit/Dry Pit) Vertical Non-Clog Centrifugal Pump Station (Wet Pit/Dry Capital Cost, 3 Pumping Units Opinion of Probable Construction Cost $11.4 million 67 /65 26 $915,000 $10.9 million 59 /54 35 $945,000 $11.4 million VTSH Pump Station (Wet Pit) 49 /43 29 $1,718,000 $9.5 million VTSH Trench-Type Wet well 57 /18 38 $1,718,000 $8.3 million; actual bid & award $8.5 million

4 Another obstacle the design team faced was the low experience level of everyone involved. The plant staff was unfamiliar with the VTSH pumps and even the design team lacked experience in designing of a trench-type wet well. To fill in these gaps they completed two tasks. One, they visited large 36-inch VTSH pumps in Phoenix, Arizona and also completed reference calls to a majority of the Fairbanks Morse largecapacity VTSH installations. Two, they hired Robert Sanks as their quality control (QC) reviewer and also used computer models from Montana State University. Robert L. Sanks, Ph.D., P.E., co-edited Pumping Station Design, 3rd Edition (and previous editions), and co-authored the American National Standard for Pump Intake Design, which includes the self-cleaning, trench-type wet well design. Trench-Type Wet Well Trench-type wet wells were invented by D.H. Caldwell in They are suitable for design flows greater than 3 MGD. The pump intakes are confined in a deep narrow trench which is substantially lower than the upstream inlet pipe, as seen in Figure 3, and it is suitable for different pump types and arrangements. The advantages to a trench-type wet well include the hydraulic environment for pump intakes and importantly for this project was the minimum footprint size, a small floor area to accumulate sludge or grit, and the ease and quickness of cleaning. For TRA s pump station, these outweighed the disadvantages of minimal storage capacity, increased depth, and clogging if the pumps are not used. Figure 3: Trench-Type Wet Well Vertical Turbine Solids Handling Pumps VTSH pumps combine the proven technologies of vertical turbine type pumps with that of a non-clog pump design. Most of the pump uses standard vertical turbine pump technology such as column pipe, shafting, bearings and discharge head. This design differs from standard vertical turbine pumps by using an overhung impeller with no bearing in the suction bell (tail bearing), a non-clog impeller, a non-clog diffuser (bowl) with two or three vanes, and a splitter in the column to prevent the accumulation of material at the intersection of the enclosing tube with the discharge head elbow. The line shaft and bowl bearing are flushed with clean water from an external source. The pump is operated with the impeller and shaft submerged in the wet well and the motor is mounted above the wet well. The motor carries the hydraulic thrust of the pump and the motor thrust bearing must be sized accordingly.

5 These pumps were specifically designed to handle large amounts of solids and long stringy materials, while maintaining a balanced hydraulic flow with low radial loading on the pump shaft and bearings. Typical applications include raw sewage, return activated sludge, mixed liquor, primary effluent, secondary effluent, raw water and industrial waste. RESULTS The Design Adding the self-cleaning system enhanced the project greatly. During normal operation, RAS flows travel across the wet well water surface at a velocity of 1 foot per second (FPS) or less. The wet well s pumps pull the slower-moving flows downward from the surface to the pumps suction intake for return to the aeration basin. For cleaning, the RAS velocity is increased significantly. An upstream isolation gate reduces influent flow and an Ogee ramp provides a hydraulic jump that continues to increase RAS velocity, as massed sludge and scum move across the wet well to the pump farthest from the influent side. With the water level lowered by the reduced influent flow and with the far pump running at full speed, scouring velocity is achieved and the pump forcefully ejects the solids. This process is pictured in Figures 4 and 5. Figure 4: Illustrative Section of Pump Station 13B - Cleaning Cycle - Draining

6 Figure 5: Illustrative Section of Pump Station 13B - Cleaning Cycle - Solids Removal Developing the Design The team used two modeling programs, UnifCrit 2.2 and Trench 2.0 (both available from Montana State University), for calculating parameters used in developing the design. UnifCrit 2.2 calculates the uniform and critical flow depths for circular channels. Trench 2.0 calculates the water surface profile and associated hydraulic parameters for a specific trench-type wet well. Output from Trench 2.0 can be seen in Figure 6. Figure 6: Output from Trench 2.0

7 The team, with specific input from Robert Sanks, designed the Ogee ramp and flow splitter to lower frictional losses and conserve energy, maximizing the hydraulic jump for more mixing of settled solids in the incoming RAS flows. The Ogee ramp in the trench-type wet well can be seen during construction in Figure 7. Figure 7: Ogee Ramp Construction The flow splitter extends from the wet well entrance to beyond the second of three pumps to minimize flow vortices developed during the cleaning cycle. This design provided high velocity to the influent RAS, thereby conserving energy and creating a powerful hydraulic jump. Specifications called for 316 stainless steel for the flow splitter. The design team also designed a hydrocone and vane for the third pump in the trench to eliminate surface vortices usually carried to the last pump. Two anti-rotational baffle plates one attached to the wall and the other attached to the pump counter circulation of RAS behind the last pump. CONCLUSIONS Social, Economic and Sustainable Design Consideration Effective wastewater treatment is basic to sustainability. Considering the increased service demand faced by TRA and the restrictions on the site, this project team examined an overlooked option and delivered a sustainable solution that conserved land, reduced disposal of materials and improved efficiency in wastewater treatment at this plant. The final pump station is pictured in Figures 8.

8 Figure 8: TRA's Pump Station 13B Completed The self-cleaning feature delivers long-term economy. Inspection of the TRA pump station takes approximately 20 minutes, and cleaning of the wet well is controlled by an automated system at a centralized location. Inspection and cleaning without the self-cleaning system would require as much as a full day, and would involve dewatering, for which the staff would enter the wet well and insert a temporary pump to remove RAS below the pump suction. In this project, not relocating pipelines and structures enabled TRA to avoid a host of environmental concerns regarding disposal of materials and the fabrication of new materials. This pump station treads lightly on the site where it resides. Client Satisfaction The design of this pump station exceeded the client s needs. The design team fully delivered on all the expected needs, but also met all their wants. The pump station fit into a constricted site without removal or relocation of existing structures and large-diameter pipelines. Self-cleaning and improved maintenance capabilities were incorporated. There will be minimum accumulation of sludge and grit inside the wet well due to the narrow dimensions of the trench. Then there are the savings. The savings to TRA exceeded the engineering fee for this project, exemplifying the value of good engineering design. The final design specifications and costs can be seen in Table 2.

9 Table 2: Final Design Specifications REFERENCES Calhoon, Joel. Computer Models (UnifCrit2.2 and Trench 2.0). Montana State University. Jones, G. M., R. L. Sanks, G. Tchobanoglous, B. E. Bosserman II (2005) Pumping Station Design, 3 rd Edition, Butterworth- Heinemann, Woburn, Massachusetts. Sanks, Robert L. American National Standards Institute (1998) American National Standard for Pump Intake Design ANSI/HI 9.8; Hydraulic Institute: Parsippany, N.J. Sanks, Robert L. Professor Emeritus, Montana State University, Bozeman, MT, USA