In wastewater applications, minimizing the dose is important to allow for separation in the tank


Sump, grinder and ejector pumps in the basement are used for two different types of pump applications: clean water or wastewater. Sump pumps deal with the footing drains and other clean water sources. These should never discharge into the soil treatment system and the discharge should even be directed away from the system to avoid additional water to the system. 

The second type of application is a sewage ejector or grinder, which is used to lift the wastewater in lower levels of the home into the system. In grinder pumps, a rotating blade shears or grinds sewage into smaller particles before pumping it. Since all sewage must pass through the grinding mechanism, a grinder pump may experience blockage as the grinding mechanism becomes dull or is clogged by foreign debris. These pumps are also used to lift sewage into the collection line or into the septic tank placed at a shallower bury depth.

In these applications, minimizing the dose is important to allow for separation in the tank. It is recommended that each dose into the tank be between 1 and 5 percent of the total septic tank to limit turbulence. Also, the pumped effluent should be delivered into the collection piping as far away from the septic tank as possible. This will allow for maximum energy reduction before entering the septic tank. When effluent is being delivered via a pump into the septic tank, the septic tank capacity is recommended to be increased by 50 percent of what would normally be required by state or local codes, with multiple compartments and an effluent screen and alarm. This will allow for better separation of the small solids that are being applied and will reduce the energy in the flow stream. Effluent pumps should not be used for this purpose, as they are only used with effluent that has undergone primary settling, such as in a septic tank.

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The other application where grinder pumps may be used is as part of a cluster system. In these applications, a grinder pump located outside the home pumps the effluent into a small-diameter pipe to a community cluster system. In 2004, the University of Minnesota collected effluent from seventeen cluster sites with various collections systems to compare the effluent. The systems had design flows between 6,000 and 45,000 gpd. Six of the sites have gravity collections systems, six have grinder pump collections systems, and one has a gravity collections system with a grinder pump lift station (and is classified as a grinder pump system). The septic tanks were measured for sludge and scum using a 1 1/4-inch-outer-diameter Sludge Judge. Wastewater from the septic tanks was tested for BOD and TSS two or three times per site from the effluent of the last septic tank. The hydraulic retention time (HRT) was calculated using the following equation:

HRT (days) = Tank Capacity (gallons) / Flow (gpd)    

The summary of the data is shown in Table 1.  

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Parameter Gravity Grinder
Median Design Flows HRT 1.4 days 1.5 days
Median Actual Flows HRT 2.4 days 5.5 days
Median BOD5 204 mg/L 268 mg/L
Median TSS 58 mg/L 64 mg/L
Table 1. Results of UMN Cluster Tank Study
 

When evaluating the sludge from grinder collections systems, it appeared courser and less digested than sludge from the gravity collections system, as shown in the picture.

This study does indicate that the effluent from grinder pump collection may not settle out as well in septic tanks. Current recommendations for septic tank sizing for a cluster system without additional pretreatment are: gravity collection — three days of design HDT; and grinder pump collection — four days.

About the author
Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association (MOWA) and the National Onsite Wastewater Recycling Association (NOWRA), and serves on the NSF International Committee on Wastewater Treatment Systems. Send her questions about septic system maintenance and operation by email to kim.peterson@colepublishing.com.

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