With Storm Intensity and Frequency, Hydraulic Engineering is Increasingly Necessary to Enable Wastewater Conveyance Systems to cope with Unpredictable Change.
Resilience of a system, be it natural or engineered, is described as its inherent (or built-in) ability to accommodate acute and often drastic fluctuations of its operating parameters and return to normal levels of service without sustaining permanent damage or change. Adaptability refers to the system’s ability to gradually adapt to changes in what is considered “normal”. Hydraulic engineering is increasingly required to enable wastewater conveyance systems to cope with both planned and unpredictable change.
The Challenge in Brief
From the perspective of resilience and adaptability, both new and old pumped wastewater conveyance systems face several key challenges:
Inflow and Infiltration (I&I) Are Increasing
The gravity sewer system feeding a lift station1 deteriorates with age, resulting in groundwater inflow and rainfall-driven infiltration along the pipes/joints, at manholes and into wet wells, contributing to the flow requirements of the station.
The Design Storm is Changing
While wastewater systems are designed to accommodate a certain level of I&I, this value changes drastically with increasing storm intensity – aggravated as a result of climate change. For example, a 100-year design storm for a wastewater system designed in 1970 occurs at a far greater frequency today. Also, what is actually the new 100-year storm may convey a far greater peak flow than estimated in 1970.
Pump Station Turn-Up Ratios are Increasing
The turn-up ratio is a hydraulic term describing the variation pumps must handle, defined as the ratio of wet weather flow (WWF) to dry weather flow (DWF) or base flow. Pump selection and operation in an environment with a large range in flow is challenging, made worse by increased I&I and peak storm frequency. This often requires a range of duty pumps, variable-speed drives or even additional, dedicated storm pumps, increasing costs for space below ground, machinery, and electrical systems.
Power Outages are Increasingly Frequent
Power outages result from electrical system failures at the grid, local, or even facility level. As extreme weather events become more frequent, the likelihood of power failure, pump shutdown, and surge (hydraulic transient) events increases.
Urban Areas are Intensifying
The load on conveyance systems may be increased significantly by urban intensification. Infill studies and capacity allocations typically consider the increased DWF but not the potential for future I&I.
Halls Mill Force Main Network, Mobile, Alabama
The Halls Mill wastewater system consists of five (5) lift stations with design flows ranging from 1,000 gallons per minute (gpm) for its smallest station to 6,000 gpm for its largest station. Individual force mains2 ranging from 12 inches to 36 inches in diameter connect these 5 lift stations to a large common discharge force main that is 48 inches in diameter and over 9 miles in length. This force main discharges to the William Wastewater Treatment Plant in Mobile Bay via a vertical overflow.
The system presents the following key challenges:
- Frequent power failures at each of the lift stations, causing large pressure surges in the force main that have resulted in joint movement, air valve float collapse, and pipe breaks;
- Lack of pump start/stop coordination between different lift stations, resulting in rapid changes in flows and pressures; and
- Capacity issues during heavy rainfall periods, potentially resulting in surcharge in upstream sewers and flooding of the lift station and its immediate environs. An off-line WWF storage tank (by others) is used to accept a large volume during storms that flow-back to the wet well afterwards.
WSP (now part of WSP | Parsons Brinckerhoff) collaborated with the owner and other consultants to identify the causes of hydraulic transients and to devise a system of closed surge tanks (e.g., hydrodynamic or pressure vessels) to absorb rapid changes in pressure resulting from normal or emergency pump shut-downs or re-starts. This enables the system to withstand any combination of operational inputs, making it more resilient. It also limits all transient pressures, decreasing fatigue in the system and the resulting wire break frequency in the reinforced concrete pipe on the main line. Together with improved air handling to maintain capacity, plus the existing and proposed WWF off-line storage tanks, the system’s adaptability has been maximized.
Unnamed Force Main Network, Sewer District X Outside Charlotte, North Carolina
This system contains four (4) lift stations with design flows ranging from 140 gpm for its smallest station to 875 gpm for its largest station. Individual force mains ranging from 4 inches to 8 inches in diameter connect these 4 lift stations to a common discharge force main that is 12 inches in diameter and over 9 miles in length.
The system presents the following key challenges, some of which are influenced by climate change:
- Need to reverse the direction of flow, as a result of the planned construction of a new wastewater treatment plant (WWTP) at the system’s opposite (southern) end. The system currently discharges to a plant to its north. The construction of a new plant highlights the system-wide growth and capacity constraints that designers are often asked to conform to;
- Flooding of the system’s largest lift station and its immediate environs, with a very high wet weather flow/dry weather flow multiple anticipated in the future, thereby requiring specialized pump selection and careful station design; and,
- Due to budget limitations, there was a need to prioritize/phase construction with an emphasis on targeting the most effective strategies and reusing existing system components wherever possible. Potential reuse of the pumps required careful assessment of “efficiency versus cost of new purchase” to justify the decision to relocate them (to pump in a different direction).
Hydraulic modeling revealed the following additional challenges at this location:
- Pumping backwards results in significant portions of the force main being pumped downhill. This required a detailed hydraulic transient analysis to identify areas susceptible to sewage column separation, air pocket formation, and potential relocation and resizing of air release valves;
- Accommodating the high turn-up ratio at pumping station 5 creates high system heads when the storm pump is on. These target heads cannot be met by the smaller lift stations; and
- The high turn-up ratio presented the challenge of pump selection for a large range of heads and flows at lifting station 5.
WSP | Parsons Brinckerhoff recommended the following key design features to increase the long-term resilience and adaptability of the system:
- Employ a staged pump system comprised of: two (2) active duty, high head, medium volume pumps; one (1) identical standby pump to accommodate peak dry weather flows; one (1) active duty, medium head, high volume peak wet weather pump; and one (1) identical pump on standby.
- Place variable frequency drives (VFDs) at lift station 5 in order to maintain conveyance even at a low level (critical flood mitigation measure).
- Provide a sensor-actuated (SCADA driven) system to “back off” (turn off) smaller lift stations at a wet well level set-point in order to allow the medium-head storm pump to rapidly drain the wet well at pumping station 5, significantly lowering the risk of flooding there.
- Re-route the smaller lift stations to the wet wells of larger ones to reduce target heads;
- Employ soft starters and ramped shutdowns to minimize wear-and-tear from hydraulic transients. For the same reasons, re-size sewage combination air valves (SCAVs) to avoid excessive water column separation and the upsurge pressures that could result upon air expulsion (pump restart).
- Place a large detention-tank tiered wet well at pumping station 5, to accommodate the high turn up ratio during major storms.
- Install a SCADA-activated pinch valve (pressure sustaining valve) at the discharge point to the new WWTP, in order to prevent the force main’s downhill segments from draining/ emptying when pumps are off and system heads are low. This will reduce corrosion due to air overlying stagnant areas when the line stops for any length of time.
Lessons Learned and Conclusion
Using a detailed computer model of steady-state and transient hydraulics, it is possible to refurbish a system with multiple lift stations and force mains without requiring complex SCADA controls and interlocks to coordinate pump starts; while also improving system resilience and adaptability.
The high turn-up ratios in sewer flow are a result of both aging infrastructure and climate change, and they are here to stay. Designers should therefore prepare to employ staged pump systems (i.e., a combination of SCADA-driven low-flow to high-flow pumps) to accommodate the wide range of flows efficiently. Using a high number (3 or 4) of identical pumps often results in diminishing returns of flow for every additional pump engaged, due to steep system curves for an economical force main, such that “more pumps are less” effective in handling peak storm-driven flows. This is a key aspect of resiliency and adaptability of the pumped sewage system.
Flow should be consolidated where possible. Smaller lift stations struggle to perform when discharging to systems with large, continuously operating lift stations. Gravity flow or station-to-station pumping should be used where possible to discharge to larger wet wells and pumps that handle the total flow. Connecting directly to large common discharge headers may present pump selection issues and operational constraints; although pressure vessels can lessen the impact of un-coordinated or un-planned pump starts or stops (or power failures).
Hydraulic modeling is an invaluable tool in minimizing long-term costs and it should be employed in the earliest possible stages of design. Minor system features can be identified by hydraulic modeling that can make large budgetary differences, for example, a pressure sustaining valve or right-sized air valve. For large or complex projects, consider both 3D flows in sumps and hydraulic transients for operations.
Retrofitting and modifying wastewater systems is a daunting and costly task. The design of new systems and lift stations should employ long-term resiliency thinking.
1Lift stations contain the pumps, valves and electrical equipment necessary to move sewage from low to high elevation, making them an essential component of many municipal sewer systems.
2Force main is a principal conduit through which water is pumped as distinguished from one through which it flows by gravity.