Figure 1. Cross-section through a Micro-Mound
The Drip-Irrigation Micro-Mound is an approved on-lot wastewater distribution system that applies the treatment tank effluent to a sand layer resting on top of the natural soil. Most often the micro-mound is selected and installed on sites with slopes between 0 and 15% and that have soils with limiting zones at depths greater or equal to 16 inches when the limiting zone is rock, or 10 inches when the limiting zone is a seasonal high watertable (see Figure 1). The micro-mound is similar, in principle, to the long used elevated sand mound, but is lower and less intrusive on the landscape, with a maximum height of about 26 inches. The downside of the micro-mound is that the wastewater must be distributed to the micro-mound absorption area by a drip irrigation system, which is usually more costly than other distribution systems.
Overview of the Micro-Mound
The micro-mound is usually a four part wastewater disposal system, see Figure 2. The wastewater leaves the house via the building sewer and is deposited into a treatment tank. The treatment tank may be either a traditional septic tank (as shown in Figure 2) or an aerobic treatment tank (ATU). The treatment tank is sized and located in the normal way described in the regulations. The effluent discharged from the treatment tank then enters a dose/pump tank where it accumulates until it is pumped to the drip irrigation distribution system. The drip irrigation system is timed dose, which requires a larger dose tank (often in the range of 1,000 to 2,000 gallons) than is required for volume dosed units. Timed dosing simply means the period between doses is set and remains constant. For example, a micro-mound's drip irrigation system might be set to dose for 5 minutes every 6 hours designed with the capacity to drip irrigate all of the houses' wastewater every 24 hours. The drip irrigation system is controlled by central unit that monitors and controls when the pump runs, which drip zone receives wastewater, the final filtering of the wastewater before it enters the drip tubing, and the back flushing of the drip lines. The wastewater delivered to the driplines and emitters must be carefully filtered because drip emitters contain very small orifices that can easily be clogged with any suspended solids remaining in the wastewater. The flushing action is also designed to keep the drip tubes free of these suspended solids.
Each micro-mound contains a minimum of two zones. Wastewater is pumped alternately to one zone or the other. Figure 2 shows only one zone, so a complete micro-mound would have a second zone nearby. The pumped wastewater is transported to the mound through a series of pipes and finally through the 0.5-inch diameter Netafim drip tubing. The drip tubing includes small devices, called emitters, which reduce the pressure in the drip tubing to atmospheric so the water literally drips from the tubing into the sand surrounding the tubing. The tubing is laid in long straight lengths, usually not much longer than 100 feet, and spaced 6 to 9 inches apart. There is an emitter located every 2 feet along each drip tube.
The wastewater applied through the drip irrigation system percolates slowly downward through the 8 or 12 inches of sand below (see Figure 1) the drip tubes. The depth of sand under the drip tubes and above the natural soil surface is determined by the type of treatment tank used at the beginning of the micro-mound system. If a septic tank is used the depth of sand must be 12 inches. If an aerobic treatment tank is used, the depth of sand must be at least 8 inches. There must always be at least 2 inches of sand over the drip tubing.
After the applied wastewater passes through the sand layer where aerobic bacteria biodegrade any organic matter remaining in the wastewater, the water enters the natural soil below the mound and is dispersed to the uncontrolled environment.
Micro-mounds have become an attractive alternative on-lot wastewater system, especially on sites that have severe limiting zone limitations.
For additional assistance contact your local Sewage Enforcement Officer or County Extension Agent
Pennsylvania Association of Sewage Enforcement Officers (PASEO)
4902 Carlisle pike #268
Mechanicsburg, PA 17050
Pennsylvania Septage Management Association (PSMA)
Bethlehem, PA 18016