GLOBAL WASTEWATER CHALLENGES - PT. 3 Global Wastewater Challenges Place wastewater treatment station, which means only pre- Pressure on Aging Infrastructure screened sewage has to be pumped? Pumps & Systems January 2016 Part 3 of 6 Shifting trends in water use and a changing sewage composition cause complex problems for the world’s sewer systems. Figures 1 (left) and 2 (right). Two general designs of By Horst Sturm (KSB) a submersible pumping station (Images and graphics courtesy of KSB) Third of Six Parts The content and mix of the pumped fluid matters greatly. Often, “sewage” is the only information When a system is not working as expected, users and application engineers receive when asked to specify engineers must see the complete picture, especially in pumps. But providing a detailed definition of the wastewater transport systems. While the increasing particular fluid (i.e. unscreened raw sewage, pre- use of wipes is a challenge, as discussed in Parts 1 screened sewage, stormwater, sludge) is vital for and 2 of this series (Pumps & Systems, November selecting the hydraulic/impeller. Especially in raw and December 2015), different systems will handle sewage, increased wipe use compounds the challenge this problem differently, and different fluid content for pump selection, system engineering and operation. will influence performance. Wastewater is not all the same. Depending on the operation or the system’s This article will exclude pre-screened wastewater and design, the sensitivity can vary for the particular kind focus on the transport of raw sewage to treatment of wastewater. plants. Several international initiatives attempt to clarify and Pump Station Designs define different wastewater. Engineering and planning The first design aspect to consider is the pump sump of wastewater facilities requires knowing what kind and the intake’s design. Even if the sewer needs a of wastewater or sewage must be pumped for trouble- particular velocity to ensure that all solids can pass, free performance and energy efficiency. the flow should not be too fast and too hard to create vortex. The inlet can carry a lot of air if it jumps Types of Stations in from a higher head. Engineers must consider There are three general types of wastewater pumping these factors. For combined pumping stations that stations, depending on pump types used: conventional have to pump stormwater and wastewater, a good dry pit pumping stations, submersible pumping compromise between minimum velocity to transport stations and submersible pumping stations in dry pit all the content and maximum velocity to avoid vortex installation. The type that works best depends heavily and entrained air has to be found. If the station only on the wastewater pumped. Is it a transport station pumps raw sewage, the main risk is a slow flow can for raw sewage only? Is it a station for stormwater cause solids to settle. These stations often run only (including surface water)? Is it a combined station minutes per day and mostly with low flow. for sewage and stormwater? Or is it inside in a CONQUER THE CLOG 1 GLOBAL WASTEWATER CHALLENGES - PT. 3 Figure 1 is an example of a stormwater station that has other requirements as a raw sewage station. Even if the stormwater station can handle different solid loads, the operating velocity will be mostly high and it will be flushed during operation. The flow will be much higher and more powerful depending on the particular storm event. Most of these stations will hold back larger material of the heavy loads by a screen before it comes to the pumping stations. The high flow will flush solids and wipes, but high flows can create vortex in the sump and cause unbalanced inflow into the pump’s suction. These stations also face higher risk of air drawn into the sump and pump suction. For this reason, the inflow to the sump should not be too high or too close to the pumps. Also, having the inflow below the max water level is a good idea, but engineers must ensure that no Figure 3. Example of a large combined stormwater backflow can occur. and sewage station If the intake level is higher than 1 to 1.5 meters (3.3 to For a combination of stormwater and sewage 5 feet), a dissipation chamber or “balcony” in front of pumping station, engineers must consider aspects the intake to the sump is recommended. This feature of both (see Figure 3 and Image 1). Even if a can contribute to a smaller floor footprint. 1 pumping station is flushed in a storm event, the sewage’s settling solids can pose problems. Another A dissipation balcony breaks the flow’s power to disadvantage arises if the same pump size handles dry- prevent unnecessary vortex and helps to remove air weather flows and high flows during storms. before it enters the pumps. The fillet to the opposite wall should have a slope of approximately 10 degrees. Figure 2 shows a raw sewage pumping station sump. The footprint of the bottom level is much smaller, and the sump, wall and fillets are designed to guide the fluid with the solids and wipes to the suction area. Any area of dead zone would pose a risk of materials settling. The fillets to the wall should have a slope of 45 degrees or up to 60 degrees for heavy cases. A sump designed for raw sewage pumping stations with a larger footprint can cause problems. A growing volume of settling substances may eventually cover the pumps because they do not run often (sometimes just 15 minutes per day), and the raw sewage is thick and Image 1. Example of a combined station dries quickly. If more volume for storage is needed, it is better to use it from the vertical space. For these cases, a jockey pump is often selected. Even if this pump is available, it may seem better to use 2 CONQUER THE CLOG GLOBAL WASTEWATER CHALLENGES - PT. 3 variable frequency drives (VFDs) for speed reduction level down as much as possible. An additional small of the larger pumps for dry-weather flow, but this emptying pump may be helpful. A small mixer in causes trouble because of low velocity. the sump can help to create turbulences and mix the floating sludge with the other pumped medium before Even if a pump can handle the mixture’s fluidity pumping. portion, the pump’s tip speed and the pipe’s velocity may not be sufficient to properly transport solids Inflow to the Sump and rags. The pump should operate in the Hydraulic Institute’s recommended Preferred Operating Range. When possible, the inflow should be below the water level. Often, backflow must be prevented or other Combined stations must be able to pump very high geodetic conditions prevent the inflow from being that flows during certain events as well as low flows that deep. If the inflow is above the water level, the water can carry a lot of solids and wipes during dry weather. jumps into the sump and air can be entrained. Submersible Pumping Stations For high flows, the available energy in the flow can cause turbulences on any obstruction or the opposite Sump volume is calculated according to the necessary wall. A baffle wall or a dissipation chamber can storage volume of fluid, which is related to the prevent these effects. They can remove air before it maximum switching frequency. However, dead zones reaches the pumps, reduce the flow’s energy and share and zones that can cause vortex or swirls should be the total flow between pumps in parallel operation. avoided. It is recommended to have fillets with a slope A dissipation chamber or “balcony” may be needed of approximately 45 degrees, but 60 degrees may be in cases with an inflow level more than 3.2 feet above better for heavy loads. The circular sump is typically the water level.1 preferred, but rectangular sumps are possible. Normally, inflow velocity into the sump cannot Between submersible pumps, splitters are be defined in a tight limit, but a proven value is an recommended. They are relatively inexpensive and average of 3.3 feet per second. minimize the influence of the flow from the pumps sitting next to each other. Also below the suction of Pipe Work the pump, a splitter can improve the hydraulic flow into the pump and prevent unbalanced load. Because wastewater can change in content and flow over time and different stations handle different Proper design and shape of the sump on the pump’s wastewater without detailed definition, optimum backside helps to establish good flow and hydraulic velocity for wastewater cannot be mathematically conditions in the sump to improve the transport of defined. solids. For heavy cases and large sumps, a small mixer may be used. Before the pumps start, the mixer can However, global wastewater transport experience bring everything into suspension and help handle the provides recommended values. Some tolerances are load in a more homogenized condition. Compared possible depending on load. Especially in the case of with a valve on a discharge pipe or on a pump with VFD operation, engineers and end users must consider the same purpose, a mixer is more flexible and minimum velocities. Many breakdowns could be powerful to flush the sump. A valve is pointed in one avoided with this consideration. direction only and can clog. For trouble-free operation, dimensioning and For stations that have floating sludge on top of the positioning of pipes and valves play an important role sump level, it is important not to have a constant (see Image 2 [page 76] and Figures 4 and 5).