The design and operation of residential wastewater systems can be complicated and confusing for many. That is why it is important for professional contractors to know what goes into properly selecting and installing a residential wastewater pump, as there are many factors that go into choosing the right pump. As a homeowner, price is often top of mind, and the most efficient pump may be the most expensive option.
Ultimately, the right selection is a mix of what pump the user can afford and if it falls on the correct intersection on the pump’s curve, so it will run the longest, be the most efficient and do the best job for the application. Below are key factors
to properly size a wastewater pump to ensure the longevity of a residential wastewater system.
6 Steps for Sizing a Residential Wastewater System
Selecting a wastewater system involves determining the appropriate pump capacity and specifications to meet the wastewater system requirements. Here are six key steps to sizing a residential wastewater pump:
1. Comply With State & Local Codes
The first and most important step to sizing a wastewater system is to check state and local codes for requirements and comply with regulations to ensure the legality of the system. Regardless of advice on installation from engineers or professionals, state and local codes take precedence. To access valuable resources related to wastewater codes, engage with industry organizations like the Submersible Wastewater Pump Association (SWPA) and the Sump and Sewage Pump Manufacturers Association (SSPMA). These organizations offer educational opportunities, training and a wealth of information on wastewater products and compliance with codes.
2. Determine Size of Solids
After ensuring the legality of the wastewater system is in accordance with local codes, decide the size of solids based on the four types of pumps: sump, effluent, sewage or grinder. The first question to ask a user is “what are we pumping?” The pump that is chosen will depend on the application. The size of the solids for the specific application will dictate what type of pump is needed. Keep in mind the size of the pipe will also affect the total dynamic head (TDH). Make sure when picking a pipe size, the system will be able to maintain a scouring velocity of 2 feet per second.
3. Decide Required Capacity
Deciding the appropriate capacity, flow or discharge required by the installation is an essential step for ensuring efficiency. One simple tool is the bathroom count method, where the number of bathrooms in a residential property is used to estimate the required pumping capacity. Each bathroom corresponds to a specific flow rate, such as 30 gallons per minute (gpm) for two bathrooms or 40 gpm for three bathrooms. However, it is crucial to ensure the bathrooms have standard fixtures to obtain accurate results. In cases where the fixtures deviate from the norm, an alternative approach is the fixture count method. This method involves tallying the total number of fixtures, such as toilets, showers and kitchen sinks, and using a predetermined count-per-fixture value to calculate the total count.
4. Calculate Total Dynamic Head
After selecting the required pumping capacity, the next step in the process is to calculate the TDH. TDH comprises three components: vertical lift, friction loss and any pressure requirements specific to the job. Vertical lift represents the distance from the pump to the highest point in the system.
Friction loss is caused by the resistance created as liquid flows through the pipe. It is necessary to account for
the lengths of straight pipe, along with fittings such as elbows and check valves. Utilize the appropriate friction loss chart
for the specific pipe material such as plastic or steel. A few rules of thumb to consider include:
- Friction loss increases as pipeline increases. The more piping there is, the more friction loss will need to be overcome.
- Friction loss increases as the flow rate increases. The more water that is being pumped through the system, the more friction loss will occur.
- Friction loss decreases as the pipe size increases. The bigger the pipe, the less friction loss will occur.
To calculate TDH, use the following equation:
Vertical lift + friction lift + pressure requirement = TDH in feet
Most systems will not need to account for the pressure requirement.
5. Size the Receiver Basin
The next step is to size the receiver basin if one is required. The receiver basin must be large enough to accommodate
the pump and allow room for switches.
The basin should have sufficient
drawdown, ensuring a minimum one-minute runtime for pumps with a rating of 2 horsepower or lower.
The basin selected should also have enough runtime to evacuate the liquid within the system during each cycle, avoiding the accumulation of residual liquid. As a general rule of thumb, the receiver basin should have a capacity that is three to four times the pump capacity. For a simplex system with one pump, the diameter of the basin should be at least 24 inches, while for a duplex system with two pumps, a diameter of 36 inches is recommended.
6. Choose Electrical Service
First, decide the motor type based on the available phase. Wastewater pumps can operate on single- or three-phase systems. Single-phase systems typically range from 115 to 240 volts (V), while three-phase systems commonly use 200, 230, 460 or 575 V. For three-phase systems, it is important to ensure the panel or pump includes heaters for overload protection and the pump aligns with the available service. For example, if the available service is single-phase, 230 V, then a single-phase, 230 V pump should be selected.
In choosing the panel, a checklist can be followed to make the appropriate choice:
- Determine the number of pumps required.
- Identify if it is a simplex or a duplex system.
- Consider the phase and voltage requirements.
- Take note of the amp draw or horsepower of the pump.
- Decide whether the pump will be located indoors or outdoors.
Proper pump selection is critical in minimizing life cycle costs and reducing energy consumption. By matching the equipment more precisely to actual system demands, pumps will use less power and require less maintenance, reducing costs and extending equipment life.
Maintaining Pump Longevity
Pump longevity can be influenced by other factors beyond improper sizing. One such factor is the modern wastewater stream, which has been inundated with flushable wipes and other unconventional solids, such as baby wipes, paper towels and feminine hygiene products. Unlike toilet paper, these materials do not break up easily or quickly and can clog sewage treatment equipment and home septic systems. These items can wrap around or break the impeller, shortening the pump’s lifespan and backing up the system.
Current pump technologies commonly used to address modern wastewater challenges and avoid system damage include grinder pumps and vortex impellers.
Grinder pumps differ from sewage pumps in that the pump incorporates a grinder, or cutting system, as well as an impeller to effectively process wastewater in the modern sewage system. Grinder pumps should be used in high head applications.
Vortex pumps have a recessed impeller design that creates a tornado-like vortex, pushing the waste out with little to no contact when passing through the impeller. This design is less prone to clogging, especially with disposable waste items like flushable wipes. However, they may provide less head (TDH) due to the larger gap between the impeller and the volute.
Self-cleaning impeller pumps are able to efficiently transport fluids with a lower risk of clogging, making the pump a practical solution for residential applications.
Integrating Technology to Enhance Efficiency
The need to leverage technology to develop new and smarter pump technologies to stay ahead of the ever-changing residential wastewater environment has never been more evident.
In addition to newer non-clog technology, smart monitoring technologies are entering the residential wastewater market. Equipped with integrated intelligence, these wastewater pumping systems sense the operating conditions
of their environment and provide feedback to help minimize downtime and enhance efficiency. Monitoring equipment that detects common threats against wastewater pumps like overheating and seal leaks or failure provide peace of mind through real-time insights into the status of the wastewater pump system, sending alerts when maintenance is required.
With the challenges of modern wastewater pumping growing more complex, residential wastewater pump technology must keep pace. Pump maintenance and replacement can be expensive, which increases the cost of investment, so residential wastewater pumps with non-clog technology that minimize maintenance and decrease downtime are fast becoming the
preferred choice.