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WASTEWATER – Complete Wet Well-Dry Pit Pump Station Systems

Basic information on the design of this type of pump station system is provided here.

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(Romtec Utilities encourages you to use our Pump Station Design Services. Please contact us to discuss your projects)

INTRODUCTION

Wet well-dry pit pump stations preceded submersible pump stations by many decades. Until a sealed centrifugal pump was designed, the pumps had to be located where they would not get wet. Thus, the principal advantage of a wet well-dry pit (wet-dry) wastewater lift station is that the pumps are not in the sewage.

There are two distinct vessels: a wet well into which the wastewater flows, and a dry underground vault (dry pit) in which the pumps, valves and other equipment are located. Suction lines near the bottom of the wet well are connected through the walls of both the wet well and dry pit to the pumps.

Access to the underground dry pit, which is designed for entry by operation and maintenance staff, is by way of a hatch in the top, equipped with a ladder or stairs. The dry pit is typically equipped with lighting and must have adequate ventilation for worker safety.

The disadvantages of wet-dry pump stations are that they tend to be more expensive than comparable submersible pump stations, and they also take up more space.

Since the pumps are not submerged, they can be non-sealed, air cooled pumps. It should be noted that submersible pumps may also be used in a dry pit, especially in sites that may be subject to occasional flooding.

In designing a wastewater wet-dry lift station, the type, size and number of pumps is determined by:

  1. Characteristics of the sewage being pumped
  2. Size of the wet well and manholes, storage vessels and sewer lines upstream from the pump station
  3. Peak flow to the pump station
  4. Total dynamic head (TDH) against which the pumps must work

Characteristics of the Sewage Being Pumped

The constituency of wastewater and the other things in it can be quite varied, depending on the source. For example, the wastewater from a residential subdivision likely contains water, solid and liquid human waste, lint and soap from clothes washing, food waste and oils from kitchens, paper and other fiber and the occasional child’s plastic toy. By contrast, the wastewater from a prison contains all that plus rags, trash, contraband and other items that can clog pumps and build up inside of pipes, manholes and wet wells.

Therefore, wastewater pump stations must be designed for the sort of sewage flowing to them. If a sewer collection system is equipped with tanks for separating out the solids before they reach the pump station, then the wet well, pumps and discharge pipes are designed for pumping mostly liquid. On the other hand, if raw sewage and everything else that can be flushed down the toilets of a college dormitory are flowing to the pump station, then macerator pumps or separate grinders are probably necessary. Romtec Utilities designs wastewater pump stations for the sort of stuff that has to be pumped, no matter what the source. 

Macerator or Chopper Pumps

If the wastewater contains high concentrations of fiber, rags and trash, it may be necessary to install a macerator (also referred to as a grinder) to pulverize solids in the gravity influent line upstream of the wet well or even in the wet well itself, at the point where the influent enters the wet well. A macerator uses electric power and may require additional space on the pump station site, so it is very important to determine the need for this additional equipment early in the sewage pump station design process. Romtec Utilities designs wastewater pump stations with macerators sized for the unique demands of the particular site and the sewage being handled.

Upstream Storage

In the event a wastewater pump station has a catastrophic failure (pumps clog, no power, no back-up pumping, etc.) the volume of storage upstream from the pump station in manholes and other storage vessels, even sewer lines, comes into play in determining the amount of time before an overflow will occur. This information, which is provided by the project’s site engineer, may be factored into the wastewater pump station design.  

Wet Well

Romtec Utilities wet-dry pump stations for wastewater use a cylindrical wet well of precast concrete or reinforced fiberglass. The wet well receives influent from a gravity sewer line. The diameter and depth of the wet well are largely determined by the peak flow to the pump station, the depth of the influent line and the size and number of pumps.

Precast concrete wet wells are delivered to the project site in sections. The base, one or more barrels and the top are fitted with sealing gaskets and are lifted with a crane and stacked in alignment below ground to create the complete wet well. The suction pipes that connect through the wet well to the dry pit are usually preinstalled. Fiberglass wet wells can be delivered to the site as a single piece with all internal parts fully-assembled.

To prevent damage to concrete wet wells from sewer gasses an interior lining or coating can be specified. Exterior surfaces of the concrete can also be coated to prevent moisture incursion and protect the concrete. These materials are installed or applied by Romtec Utilities at the factory, and they are fitted with weld strips and additional sealing in the field.

Romtec Utilities offers wet wells from 4 feet to 12 feet in diameter (ID) and up to 40 feet in depth. The wet well in every Romtec Utilities wastewater pump station is sized for the specific site requirements, which can be quite varied and include:

  1. Depths of influent and suction lines
  2. Current and future peak flows and average flows to the pump station
  3. Storage available in wet well and upstream manholes (for calculating time to overflow)
  4. Pump and suction pipe sizes (physical size and pump power rating)
  5. Average number of pump starts per time hour for optimum pump efficiency
  6. Relationship between suction pipe area of influence and wet well diameter

The interior walls of the wet well base are angled to help direct solids to the suction pipes and keep them from accumulating within the wet well base. This feature helps prevent the build up of a solid layer of fat, oil and grease (FOG). Romtec Utilities wet well bases can be lined with fiberglass or coated with epoxy sealer to provide a slick, non-porous surface that discourages collection of FOG.

The exact location, angle and diameter of influent and discharge lines are predetermined and can be prefabricated in a complete package pump station. Coring of concrete or fabrication of fiberglass is done at the factory to eliminate the need for field work and ensure precise fit of the influent and discharge lines to the wet well. Flexible seals in all wet well penetrations are also factory installed.  

Depending on the wet well type, other equipment such as influent deflector panels or drop tubes, pump accessories and liquid level sensors can be factory-installed or field installed. The wet well top includes the access hatch with fall protection and attachments for liquid level sensors and other items

Dry Pit

The dry pit is made of precast concrete or reinforced fiberglass. These assemblies are delivered on trucks and installed in the excavated site with a crane.

The entry and exit points for suction lines and discharge lines are pre-cored or fabricated and fitted with flexible seals. In many instances the pumps, piping, valves and other equipment can be factory assembled and pressure tested, eliminating costly construction work in the field.

The depth, length and width of the dry pit are largely determined by the depth of the wet well, size of the pumps, diameter of the discharge lines and any requirements for special valves, flow metering, bypass pumping port or other equipment located in the dry pit. Access to all equipment within the dry pit is required, so access hatches, ladders, platforms, lighting and ventilation equipment are also included. Romtec Utilities uses all applicable safety requirements governing confined space entry in the design of the dry pit.

Pumps

Romtec Utilities offers pumps of virtually every type from the leading manufacturers. The specific type and size of pumps in a package wet-dry pump station for are determined by many criteria, including: best performance and value, customer preference for a particular brand or type of pump, overall value and other factors.

Several types of pumps are commonly used in wet-dry wastewater pump stations:

  1. Solids-handling, non-clog pumps that pass 3-inch diameter solids
  2. Grinder pumps with impellers that grind solids
  3. Chopper pumps that macerate difficult solids such as rags

It should be noted that submersible pumps are frequently used in dry pit applications, especially in sites subject to occasional flooding. These pumps are designed to operate while submerged, so flooding of the site will not damage them.

The pump electrical cables and sensor wires are connected to the pump station control panel through a junction box or pump disconnect panel. Romtec Utilities designs these connections to meet all applicable electrical codes.  

Some wet-dry pump stations have pumps of two different sizes. The smaller pump handles small demand, and the larger pump(s) operates when the flow into the wet well or demand for water from the pump station is greater.

Piping

The type, diameter and length of the pump discharge and force main piping are predetermined in a package pump station. Romtec Utilities pump stations generally include all piping through the last valve connected to the pump station end of the force main. Common pipe materials are stainless steel, ductile and PVC.

Liquid Level Sensors

One or more liquid level sensors are suspended in the wet well to report the fluid level. This data is used by the pump station controller to turn the pumps on or off and, in the event of a problem, to turn on alarms and transmit alarm data. The common types of liquid level sensors are: micro switch floats, multi-electrode probes, pressure transducers and ultrasonic sensors.   

The type of sensors used is determined by customer preference or by the controller chosen for the pump station. Many controllers can operate with any type of level sensor, while other controllers are exclusive to one type of sensor. Romtec Utilities designs with all types of liquid level sensors and controllers.   

Valves, Pressure Sensor, Bypass Pumping Port, Pigging Port

Valves and other equipment in the pump discharge lines are typically located within the dry pit.

A single non-contact pressure sensor is located in the force main, although some pump systems require individual pressure sensors for each discharge line.

Often specified for wastewater lift stations are equipment such as:

  1. A bypass pumping port for connecting an emergency pump, in the event of a major failure within the pump station
  2. A pigging port for sending a “pig” into the force main for cleaning and/or removing an obstruction.
  3. Chemical injection or other equipment for reducing odor in the wet well and force main

Flow Calculating & Metering

Some wastewater pump stations must measure the volume of fluid being pumped over a period of time. An easy and inexpensive way to accomplish this is by calculating the flow by multiplying the volume of water pumped with each pump start by the number of pump starts.

Flow metering may be necessary for proving the discharge volume over a period of time. An electromagnetic flow meter inserted in the force main can be located in the dry pit or in a separate underground vault after the dry pit. To allow maintenance of the flow meter without disabling the pump station, a bypass line with piping and valves to pump around the flow meter is included in the design. A flow readout device, usually located at the pump station control panel, receives electronic flow data and reports it to a display panel. Transmission of flow data to a remote computer is also available.

Control Panel & Electrical Power

Typically, the electrical controls of the pump station are located in a control panel within a weatherproof enclosure. Various devices control the pumps, receive operational and fault data from sensors within the pumps and the wet well, report elapsed operational time for each pump and report operational status and alarm conditions both locally and to remote sites through telephone circuits, radio transmissions and other means.

The control panel in a Romtec Utilities pump station can be placed in a variety of locations, generally wherever the customer wants and needs it to be. As an important part of the design process, Romtec Utilities produces a scale drawing showing the equipment layout on the site. No two pump stations have the same equipment layout.

The source and type of electrical power to the pump station site is an important design factor. Romtec Utilities can design the pump station for whatever type of single-phase or three-phase power is available to the site. Generally, the control panel is designed to operate just the pump station. Other electrical demands at the site, such as lights and building heat are not considered in the pump station design, unless specifically required by the customer.

SCADA

Pump stations can be equipped with SCADA (Supervisory Control and Data Acquisition) devices to both report operational data to remote sites, such as an online computer terminal at the wastewater authority office, and to receive operational instructions and requests for operational data from the same computer or other computers or mobile data devices. This two-way communication capability provides many benefits to pump station operators. Romtec Utilities designs sewage pumping systems to work with all kinds of communications hardware and software.   

Electrical Generator

A standby electrical generator that operates the pump station during power outages may be required. The generator is usually sized to operate just the pump station; however a larger generator can be specified for operation of the pump station plus other equipment such as space heaters and exterior lights. A generator using natural gas, propane or diesel fuel can be permanently located at the pump station site and wired to the control panel through an automatic transfer switch that senses the loss and return of grid power and switches the generator on or off accordingly.

The generator can be housed in a sound-attenuating enclosure. The generator can also by installed within a building equipped with a ventilation system.  Alternatively, a portable generator can be brought to the site and plugged into the control panel to temporarily supply electricity until grid power returns.  

Other Optional Equipment

All design parameters to accommodate any additional optional equipment are predetermined in the design of the packaged pump station, so the optional equipment is integral to the complete lift station design. Romtec Utilities specializes in designing pump stations that accommodate various specialized equipment required by the customer, the sewer authority or regulations.   

Shelter or Building

A structure to shade the control panel enclosure and provide shelter from weather for operation and maintenance personnel may be required. It may also be necessary to house the pump station electrical control panel, generator and other equipment within a building designed for equipment security and shelter. Romtec Utilities supplies a wide range of shelters and buildings that can be custom fitted to the specific requirements of any wastewater pumping system.