Moving Effluent Is More of an Uphill Battle These Days

Today’s onsite system designs often require pumps carry water to a higher elevation for distribution. Follow these tips for proper pump selection and usage.

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We often get questions from homeowners about pump reliability in relation to the need for pumps somewhere in their system. Pumps these days are highly reliable if the correct pump is selected and installed. One pump is needed in most cases, but some systems require multiple pumps. We want to dedicate one column to explaining why we install pumps and some of the aspects for successfully using pumps. 

In what we sometimes like to call the “old days,” tanks were installed as deep as needed to receive sewage from the lowest level of the house. If the line was exiting from a basement this meant that the tank was installed at a depth of 10 feet or more. This made it very difficult to service the tank, much less repair or fix anything in the tank, such as replace baffles, etc. 

While we still occasionally see deep tanks, they are usually the result of older installations. We now recognize the tanks should be installed close to the ground surface, ideally with no more than a foot of cover. This makes it much easier to locate and access the tank for regular maintenance and any necessary repairs.  

The shallow location means there is often a sump and pump installed in the basement or lowest level of the house and the raw sewage is pumped up to the tank. A solids-handling pump is required. It is amazing how often we still see sewage effluent pumps or even clean-water sump pumps installed in these situations. They will not handle the solids and will frequently plug. 

Pressure and volume

One major concern with pumping raw sewage up into the septic tank is creating turbulence in the tank, interfering with settling of the solids in the tank. If the tank is not allowed to develop the three layers including the clear zone of liquid delivered to the next component, there is the potential for premature plugging of the effluent screen or solids delivered to the drainfield, causing plugging or excessive biomat development.

If a sewage ejector pump is used, the maximum dose when the pump runs should not exceed 5% of the total septic tank volume. For a 1,000-gallon tank, the amount delivered should not exceed 50 gallons per dose. We always recommend using a two-compartment tank or two tanks in series in these situations to ensure proper settling time is provided. The pump should deliver sewage at a rate of no less than 10 gpm so it will keep up with the discharge rate of appliances, such as washing machines and dishwashers. This is of less concern these days with the advent of low-water-use appliances.

Deep tanks usually meant the soil treatment trenches or bed were installed deep so there could be gravity flow out of tank. The soil treatment and dispersal area was installed below or out the zone in the soil where there would be maximum treatment and often would be in contact or near limiting soil layers such as perched water tables, dense soil layers or bedrock. To avoid these problems, maximum trench excavations are often limited by code to 4 to 5 feet. Installation of shallow trenches, as we recommend, often requires a pump tank and pump to deliver sewage effluent up to a distribution box or dropbox.

Watch flow rate

When effluent is pumped to a distribution or dropbox for gravity distribution in the trenches, effluent should not be delivered at a rate of more than 45 gallons per minute at the total dynamic head required for the elevation difference and friction in the supply line piping. This rate allows time for effluent to flow out the 4-inch diameter sewer pipe from the drop or distribution box to the trenches. 

A reader recently asked if since distribution or dropboxes can have multiple outlets, should the rate should be higher because effluent could flow out more than one outlet? The answer is no, the rate should be based on the scenario where all the flow will exit one pipe. In addition, the flow from the pump should be directed to a wall of the box without an outlet, or a box used with a baffle to dissipate the energy of the water as it enters the box. 

If it is necessary to pump to a pressure distribution system, low pressure dose trenches, sewage treatment mound or at-grade system, the pump requirements must meet the necessary flow rate to totally fill the distribution piping and deliver the desired dose amount at the required total dynamic head. 

Ideally the pump should only run four to five times per day to maximize pump life. This would require a minimum dose of approximately 25% of the estimated daily flow plus the amount of drainback from the supply pipe. This helps spread the flow out evenly during the day, while at the same time utilizing the whole soil treatment area. In some cases, it is desirable to add an additional tank to store effluent to handle peak flows and provide better flow equalization throughout the day. We will explore the topic of peak flows and flow equalization in a future column.

Pumps commonplace

With the advent of additional pretreatment devices (media filters, ATU’s constructed wetlands) over the last decade or two, an additional pump has become necessary to move effluent from one treatment component to another. These are designed and specified to meet the specific requirements of the pretreatment device.

Currently it is not unusual to have multiple pumps within the treatment train for systems. Recognizing the specific pump requirements for each situation has now become a necessary skill for any sewage treatment system installer or service provider. 


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