Giving a Lift

Pump tanks and related components must be designed and installed with an eye toward convenience for the maintenance and service provider
Giving a Lift

For those riding the Treatment Train for the first time, welcome aboard the systematic process of evaluating the components of an onsite wastewater treatment system. In our series so far, we have discussed the wastewater source, the septic tank, and the gravity soil treatment area (drainfield).

Now let’s look at a pump tank and its role in a pump-to-gravity system. There are many types of pump tanks, including lift stations, process tanks, recirculation tanks, surge/equalization tanks, internal pump basins, and dosing tanks. Dosing tanks may be either pump- or siphon-operated. Here we’ll look at the basic dosing pump tank typically used in a pump-to-gravity system.

 

Considering options

There are a number of pump tank options, depending on the designer. In some cases, small-diameter (24- to 30-inch) vertical pump basins are specified. These work well for demand-dosed systems but lack one consideration: power outages and storage volume. A thoughtful design factor is at least 24-hour storage to prevent overflows.

When selecting a pump tank, recognize the needs of the service provider to establish and verify gallons per inch for the sake of drawdown and dose volume. Square concrete and vertical cylindrical tanks are easy to calculate, but round buoy or horizontal cylindrical tanks make it difficult to determine gallons per inch unless the manufacturer provides a calibration chart.

Why is this so important? One of my favorite designers has driven into my head, “It’s all about the dose!”

A typical pump tank consists of the tank itself, a pump, the discharge assembly, liquid-level controls, and a control panel. No matter the tank material (concrete, plastic, fiberglass), the tank must be fitted with access risers.

Also important is a watertightness test to ensure a good seal at the access risers and all other tank plumbing connections. Leaks at any connection may allow inflow during the wet months, and as we all know, hydraulic overloading is the number one cause of failure of the soil treatment area.

A properly designed system includes an effluent screen in the septic tank. If no screen is installed, or if the design employs a combo (two-compartment) tank, a pump screen should be installed to prevent fouling of the pump or transfer of unwanted solids to the soil treatment area.

 

Choosing a pump

Two types of pumps are typically specified: high-head multistage or low-head centrifugal. Multistage pumps are often located in a flow inducer that supports the pump and helps control the liquid level in relation to the pump intake. Centrifugal pumps typically have short legs on the bottom but should always be positioned on a 3- to 4-inch block/pedestal to make sure no accumulated solids are drawn into the pump.

The discharge assembly should have a few key elements to ensure proper operation and serviceability. A check valve is typically located just above the pump to prevent backflow from the supply line and reduce pump run time. Where freezing is an issue, drainback is desired, and a check valve might not be specified.

When a check valve is installed, it is important to have a small bleeder hole between the pump and the valve to prevent air lock and pump cavitation. Next in the discharge assembly is a quick disconnect to allow for pump removal and servicing. The most common quick disconnect is a union, but a camlock connector can be used.

Rubber couplers with band clamps or compression couplings are not recommended, as they will often self-disconnect under normal operating pressure. An isolation valve should be located after the quick disconnect to prevent backflow when servicing the pump or check valve.

And finally, a pump pull rope attached to the top of the pump is highly recommended, especially for centrifugal pumps, as they are heavy, and the discharge plumbing is offset and may break if used to lift the pump. A 1/2-inch or larger polypropylene rope is best, as it is strong and corrosion resistant. It also floats if it gets lost in the tank (and is less likely get sucked into the pump intake).

There are a couple of other considerations where the discharge line exits through the pump tank. As all serviceable points should be accessible from the ground surface, the discharge line must be properly fitted and aligned so as not to stress the plumbing and cause premature failure. It is also important to silicone-seal the grommet where the discharge line exits the tank riser to prevent possible inflow. (This is a good practice for all tank risers through fittings.)

 

Keeping control

Next let’s look at the control system. There are many types of liquid-level control devices, from ultrasonic to pressure transducers to control floats. All perform the same task, sending a signal or connecting a circuit to activate a timer or mechanical device, such as a pump, valve or blower.

For simplicity, let’s talk about a two-float system with a pump on/off float and a high-level-alarm float. Here we may see one of two options for on/off floats. A wide-angle mechanical float relies on a ball inside the float housing that shifts back and forth as the float moves up and down, connecting a circuit and turning the pump on and off.

Vertical mechanical floats have an adjustable float that travels up and down on a shaft and activates a micro-switch. This is the most common high-level-alarm float.

Whatever the type of float, good corrosion-resistant fasteners must be used. We often see hardware-store-grade stainless steel band clamps used as float fasteners. Guess what: The worm drives on these clamps are not stainless and typically corrode away. Only good marine-grade band clamps are truly all stainless. Non-corrosive plastic float fasteners also work well.

In well-designed systems, the floats are attached to a removable float tree that, with enough cord length, allows removal of the floats for testing and service. Whether the float cords are wired into an internal or external splice box or run through conduits to the control panel, the passages must be sealed to prevent unwanted gases from entering the control panel, where corrosion can cause irreparable damage. Use a sealant such as electrician’s putty that can be easily removed for float replacement or repairs.

 

Types of panels

Control panels also vary widely, from telemetry Internet-based to simple on/off functions (discussed here). Most panels have a few basic features that are valuable for long-term system performance monitoring. An HOA (Hand/Off/Auto) switch assists in the routine maintenance of the system and allows testing of the pumping system.

One useful test for tracking pump wear is an amperage/voltage test. With baseline startup information on volts at rest, volts while pumping and current draw (amps) you can catch a pump that is beginning to struggle before it fails, so you can replace it during regular service, not in an emergency.

Most control panels can be fitted with an event counter and hour meter. These devices track how many times the pump starts and stops and record the cumulative run time, enabling you to calculate average daily water use and flow in gallons per minute. This helps verify if the system is being used according to design, and if perhaps the pump and supply system needs recalibration.

Last but not least is the method of pressure dosing to the soil treatment area. Various types of distribution boxes can be used in a pump-to-gravity system. Another method is to use a pressure manifold with valves or orifices to distribute the effluent equally. These and other distribution devices require routine inspection, verification of even distribution, and service and rebalancing.

 

In conclusion

So, let’s recap the critical issues around the pump tank that can help or hinder the efforts of a service provider.

1. Has the tank assembly been certified as watertight?

2. Are all components accessible from the ground surface?

3. Is there a pump screen at or upstream of the pump?

4. If the pump is a centrifugal, is there a pump block/pedestal?

5. Is there a check valve if appropriate? And is there a bleeder hole below it?

6. Is there an appropriate quick-disconnect device?

7. Is there an isolation valve?

8. Is the discharge plumbing properly installed and sealed?

9. Are the floats easily serviced and adequately secured?

10. Are all the float cords properly sealed to prevent gas transfer and corrosion?

11. Does the control panel allow proper testing and system performance verification?

To learn more about O&M on pump tanks and the variables only touched on here, check www.nawt.org for a training session near you. Next in O&M Matters, we’ll look at media filters as final treatment and dispersal components.

 

About the author

Kit Rosefield is an adjunct instructor at Columbia Community College and a trainer for NAWT and the California Onsite Wastewater Association. His company, Onsite Wastewater Management in Mi Wuk Village, Calif., has a consumer education service at www.septicguy.com. Reach him at 209/770-6760 or kit@septicguy.com.



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