Choosing an alternate septic system for a homesite with thin soil over bedrock
David M. Gustafson, Extension Educator
James L. Anderson, Professor, Department of Soil, Water, and Climate
Sara F. Heger, Extension Educator, Department of Biosystems and Agricultural Engineering
Barbara W. Liukkonen, Extension Coordinator
On this page
- Problem overview
- Why did the homeowners choose a drip-distribution system?
- General design principles
- Instalation practices and costs
- Treatment effectiveness
Homeowners near Hastings knew their septic system wasn't in compliance with the current codes when they purchased the home (figure 1). They were very careful with water use and system maintenance, so they didn't have an obvious problem. Yet, they wanted to do the right thing and decided to upgrade their system to bring it into compliance. They weren't required to replace or improve their system, but wanted to minimize the potential risk to human health and the environment.
Bedrock at this site is only 12-18 inches below the surface, and the original seepage trenches were installed directly in the creviced limestone bedrock. The wastewater could enter fractures in the rock and increase the risk for contamination of groundwater and drinking water by entering their own and their neighbors' wells.
Working closely with Ravenna Township in Dakota County to ensure proper installation, inspection, and approval, the homeowners received permission to install a drip-distribution system (figure 2). As a general rule, your local government unit should always be contacted early in the process to get approval.
While drip-distribution systems have been used in wastewater applications across the United States, its use in northern states, such as Minnesota, is a relatively new phenomenon. Because the water lines in a drip-distribution system are placed at a shallow depth (where they provide water for lawns, for example), they may freeze during the winter. As a result, thermocouples (which measure soil temperature) are placed next to the lines to warn the homeowner when a freeze is imminent.
Figure-1. Map of the site near Hastings
Location- near Hastings.
Problem-system was installed in creviced limestone bedrock
Reason for upgrade-to protect groundwater
Site limitations-bedrock at 18 inches
Type of system-drip distribution
Installation cost-$9,800 including new septic tank, sand bed, drip tubing, and control unit
Installation time-two days
Unit size-500 linear feet of drip tubing
Capacity-600 gallons per day
Annual maintenance-tank pumping and cleaning; filter cleaning
Why did the homeowners choose a drip-distribution system?
The homeowners wanted to use much of their backyard for a garden and outdoor activities and preserve another large portion of their property to pasture horses. They were also concerned with saving several mature oak trees. These considerations, plus the location of their well and the shallow depth to bedrock, limited their options for a replacement system. A mound system would have been possible, but the somewhat sloping site would have made it a more expensive choice, and they would have lost most of their trees. As a result, they chose a drip-distribution system.
This is a subsurface drip-distribution system. It slowly emits wastewater with a system of electronics and valves to a couple of different feeds. We have some sand and soil in a very low-lying berm from which water is slowly emitted and treated-so it's much more friendly to the resources.
David Melbye, homeowner
Other factors influencing the homeowners' choice were the drip system profile was less intrusive in the landscape than a mound and the system could be installed around their oak trees. From a technical standpoint, a drip-distribution system provides longer contact times with the soil and places the effluent in contact with soil biota, which are most active in the upper soil horizons.
General design principles
A drip-distribution system uses septic tank effluent as a source of nutrients and moisture for plant growth. The effluent from the septic tank flows into a lift tank where it is pumped through a series of filters to remove the suspended solids. The filtered wastewater is pumped into a series of distribution tubes (see figure 2) in which drip emitters (figure 3) are placed at intervals of 12-24 inches. The drip emitters evenly distribute the waste-water over a large area and allow it to seep slowly into the soil where it is removed by plant uptake and evaporation from the surface of the soil. In addition, a drip-distribution system minimizes the risk of untreated water flowing through the soil.
A drip-distribution system works on the same basic principles as any other soil treatment system using filtration and bacterial decomposition of waste, but the method of application to the soil is different. The application system has three parts: the wastewater primary treatment (septic tank), the pump and filter in the lift tank, and the dispersal unit, which consists of a supply and return line (see figure 2).
The septic tank removes the larger solids and allows decomposition of some of the organic matter. The pump and lift station evenly distribute the wastewater through the soil. Pump selection and installation should follow standard design practices. For example, typical pressures in dispersal lines range up to 60 pounds per square inch and the pump must be sized to provide adequate flow volume and pressure for back-flushing the filters. The filters are a critical component of any drip-distribution system because they remove small suspended particles in the wastewater that could block the small exit holes in the drip emitters (figure 3).
The tubing that carries wastewater from the pump to the soil is the most complex part of this system. Thus, its design has improved significantly over the past 20 years. The tubing is connected to the feed line from the pump and, at the other end, is connected to the return line (figure 2). Every 12-24 inches, tiny orifices, or drip emitters, allow the wastewater to seep out into the soil. As the system operates, solids can build up in the drip tubing. The return line allows the system to be back-flushed automatically at regular intervals to remove these solids.
The size of the system is determined by the total household wastewater flow and soil acceptance rates. The area required for a drip irrigation system is somewhat larger than a conventional soil treatment system because the goal is to maximize the area loaded by a given volume of wastewater. For the site near Hastings, the chosen soil-sizing factor was 0.6 gallons per day/square foot, which requires 500 feet of drip tubing to treat 600 gallons of effluent per day.
Figure 2. Design of a typical drip-distribution system
Figure 3. Details of the tubing and drip emitters used in a drip-distribution system
Keys to trouble-free-operation of a drip-distribution system
Wastewater is effectively disposed of when added slowly to soil just below the surface over a wide area.
Spreading the wastewater evenly over the entire site is a significant benefit. To accomplish this, drip tubing and a pump are necessary parts of the system.
The small-diameter tubing and small emitters make the use of filters critical. Filters must be maintained for the system to operate effectively. The two commonly used filters are disc and spin. An example of a disc filter, which was chosen for the site near Hastings, is shown in figure 4.
Figure 4. Cross-section of a disc filter showing wastewater and rinse flows
The system must be properly laid out and installed to ensure that flushing occurs properly. This includes a return line that conveys the water used during the back-flushing cycle. Back-flushing will remove soil particles and slime accumulations.
When the pump shuts off, wastewater flows to the lowest point in the system and creates suction. Vacuum breakers release this suction and limit the amount of soil that is sucked into the drip emitters.
Installation practices and costs
The drip-distribution system took two days to install. During construction, care was taken not to compact the soil and destroy its ability to accept and treat waste-water. The total cost of construction was $9,800.
First, the land surface was roughened to ensure easy flow paths into the natural soil. Second, a two-foot layer of sand was distributed over the soil treatment area to create the required three feet of separation needed above the shallow creviced limestone bedrock. Third, drip line was placed on top of the sand and vacuum breakers installed at the high point to release suction when the pump shuts off. And last, the drip line was covered with six inches of topsoil and seeded with grass.
In experimental studies in northern Minnesota, drip-distribution systems have been shown to provide consistent, effective treatment of domestic wastewater. Regular monitoring, particularly of the temperature of the system, was also done to determine how well the system operated during cold weather. Although the monitored temperatures in the soil surrounding the drip tube have never gone below 32F, the system did have freezing problems in the first year of operation. The problem may have occurred due to compaction of the snow covering the system. During the second winter, no freezing problems occurred.
Drip-distribution systems have a septic tank and a lift station like other systems that use pressure distribution (for example, mound and at-grade systems). The septic tank should be cleaned periodically to remove accumulated sludge and scum. This is even more critical with a drip-distribution system because there are several filters that can become plugged, and the drip tubing itself has small perforations that are subject to plugging. For more information, see the Septic System Owner's Guide(a brief description, including how to order this manual, are shown at the right).
In addition to the septic tank, lift station, and pump, some key components need regular maintenance. The filters need to be cleaned manually or exchanged at least once a year. A series of pressure-release valves sense when the holes in the drip tubing are plugged with solids or are frozen shut. The system back-flushes the tubing on a regular basis, but may need to be back-flushed manually if this does not eliminate the problem. The tubing itself may have to be replaced.
The control panel monitors pressure changes across the system, as well as system temperature, pump performance, and daily flow. This allows the homeowner or hired professional to identify potential problems before they occur.
For more information about requirements and guidance concerning alternative systems in your county, contact your county planning and zoning department. In the seven-county Twin Cities metropolitan area, you may call the Metropolitan Council at 651-602-1005 for assistance in identifying the correct local office. In greater Minnesota, check with your county Extension office.
Septic system package available
A video was developed to accompany this and other septic system publications, or is included if you ordered the packaged Septic System Video and Guide. The video documents the construction of three alternative systems (including the one described here) and the reasons for selecting each type. It also includes interviews with homeowners and experts.
This video describes the basics of septic systems, including system features, safety, use, operation, maintenance, and troubleshooting. It also serves as a stand-alone resource, or may be purchased in a package along with written material titled Septic System Owner's Guide.
Septic System Owner's Guide. $4.00.
A fully illustrated guide for owners of septic systems. Includes information on safety, system features, use and operation, maintenance, and troubleshooting.
WW-07569 Revised 2000