Low-pressure distribution (LPD) systems are an emerging alternative to the conventional septic tank systems that are commonly used at locations outside of municipal sewer service areas. ARRO engineers have done extensive work with LPD systems in Maryland, which has been progressive in the use of LPD systems, and offer their experienced insights into the advantages and challenges of working with these non-conventional systems.
LPD systems, developed as part of pressure dosing and sand mound system research conducted at the University of Wisconsin in the 1970s, have been in use in some parts of the country since the early 1980s. LPD systems are commonly used to overcome site conditions that can present obstacles to traditional septic systems, such as steep slopes, shallow depth to bedrock, high seasonal water tables, and low soil percolation rates. In some states, for example, LPD systems are permitted with the bottom of the system only 1 foot above bedrock, allowing installation in challenging site conditions. Additionally, when combined with “innovative” treatment technologies, these systems can significantly reduce nitrogen loading in watersheds where nitrogen pollution is of concern.
Wastewater system regulation
Prior to discussing the components of LPD system designs, it is important to understand that individual state codes typically give the authority to regulate wastewater disposal systems to state regulatory agencies. So, certain regulations, details, and specifications cited below may not apply in all areas. Typically, representatives of each state’s Department of the Environment (or Health), or their designees, are assigned to review onsite wastewater treatment within the state. These individuals oversee system design and installation, as well as conduct case studies and keep appraised of new technological trends in onsite wastewater treatment systems.
When a new system is to be installed or a failed septic system needs to be replaced, the authorized individual assigned to the region selects or recommends an appropriate system for use at a site based on site conditions and the results of soils testing. This selection process is done in concert with a local sanitary/health official — in many cases either a representative of the local health department, a municipality, or municipal authority, whichever agency issues septic system permits for a given area. Design and installation of the system will typically be supervised and reviewed by both of these parties throughout the entire process. If it is determined that a LPD system will be a good fit for the particular site, local health departments or sanitary officials often maintain a list of professional consultants with experience in LPD and innovative system design.
State laws typically give the appropriate regulatory agency the authority to allow the use of “non-conventional” or “innovative” systems on a case-by-case basis, which commonly use LPD as the method of effluent distribution. Depending on the state, a number of treatment practices may be approved by the state regulatory agency; a list of approved practices is often available through local health departments or sanitary officials or online at the state agency’s website.
Some states encourage the use of innovative systems for their potentially greater nitrogen removal capabilities relative to conventional septic systems. In Maryland, for instance, new homes in areas near the Chesapeake Bay now require the use of onsite treatment systems other than septic tanks. In other states, such as West Virginia, innovative systems are only permitted on larger sites or sites with failed conventional systems. And in some states, the use of innovative and LPD systems is still being evaluated by state environmental/health agencies.
The specific treatment units used in innovative/LPD systems are typically proprietary technologies that are reviewed and approved by state agencies. Some treatment units can resemble small-scale wastewater treatment plants, utilizing aerobic batch treatment and denitrification through a series of chambers, or trickling filter/fixed film biological treatment processes. Other practices use organic biofilter media or other types of filter media for treatment.
In some states, this process is handled by independent testing firms employed by the state that maintain testing facilities where prototype treatment practices can be run over time and their performance analyzed. In some cases, technologies not yet added to the approved list for a state agency may be tested at specific sites on a case-by-case basis, and if a treatment practice shows acceptable performance over time, it may be added to the approved list.
LPD system design
In an LPD system, wastewater flow is first collected and processed by the treatment unit. A pump discharges the effluent from a dosing chamber following the treatment unit through a force main to a discharge manifold in the disposal area. The manifold connects to a series of perforated lateral pipes, typically wrapped in geotextile filter fabric (see Figure 1).
The perforations, commonly referred to as orifices, are evenly spaced along the length of the manifold. Lateral sizing and orifice sizing and spacing depends on the jurisdiction, although orifices are almost always either 1/4 inch or 5/16 inch in diameter, with 5/16-inch orifices sometimes preferred because of less potential of clogging by gravel fill. Some areas require calculated orifice spacing for even distribution (discussed later in this article) within a range such as 3 to 10 feet; others specify a uniform orifice spacing, typically 6 feet.
The effluent, under 2 to 3 feet of pressure “head” (about 1 pound per square inch), is discharged through the orifices in the laterals, with the pressurized condition ensuring a uniform dose throughout the disposal area. LPD systems are designed to provide uniform dosing of absorption areas, allowing the entire absorption area to be used in treating effluent, much like an elevated sand mound.
Manifold configurations are typically either end manifold, where laterals extend in only one direction away from the manifold, or center manifold, where laterals extend in both directions (see Figure 2). The manifold configuration depends on site conditions and the location of the dosing chamber relative to the manifold. Typically, a center manifold configuration is recommended for trenches that are 75 feet or longer to minimize the effect of lateral friction on system operation.
Design guidelines always specify that laterals run perpendicular to the slope of the site (on contour), to ensure uniform dosing and simplify design calculations, and on sloping sites, to be as long as possible along the contour to provide the most disposal area possible over the minimum elevation differential. Generally, design guidelines call for turn-ups/cleanouts at the end of some or all laterals for inspection and cleaning, and call for perforations at the crown of the pipe or otherwise elevated to facilitate lateral drainage after dosing and allow for trapped air to escape.
Laterals for a standard trench LPD system are usually buried 15 to 18 inches below grade, near the top of a gravel bed of 6 inches or greater thickness and underlain by several feet of sand fill for filtration purposes. Some jurisdictions allow the use of additional gravel thickness for filtration, beyond the minimum required, to reduce the total trench length required. Some states also allow a deep trench system configuration, which utilizes deep — approximately 8 feet — trenches designed to allow the walls of the trenches to provide absorption as well as the bottom, reducing the necessary disposal area significantly. In a deep-trench LPD system, the lateral pipes are located in a gravel layer at the bottom of the trench rather than near the top, and sand fill material is provided above the gravel.
Sand mound systems and other at-grade/above-grade configurations, such as capping fill, can be utilized with low-pressure dosing as well. These systems may be necessary when challenging site conditions — such as a very shallow water table or very low soil permeability — are even more pronounced than those typically associated with LPD systems. Sand mound systems rely on mounds of sand fill material above grade to provide additional filtration beyond that provided by site soils. The laterals are run through gravel absorption trenches within the mounds, and discharged effluent passes through a layer of sand before being absorbed into the soil.
State design requirements for LPD systems typically set the minimum dose volume per dosing cycle based on the total volume of the pipe network. This is done to ensure that the manifold and laterals achieve and maintain a full/pressurized condition for a sufficient length of time to provide uniform dosing to the absorption area. The required volume varies from state to state, but is typically five to seven times the total volume of the laterals being dosed plus some multiplier of the volume of the manifold and the volume of any force main piping from the treatment unit to the manifold, or some minimum dose (typically 100 gallons), whichever is greater.
LPD systems are often designed to operate on dosing/resting cycles, which maintain aerobic conditions in the soil. Evidence suggests that aerobic soils provide better treatment and system hydraulic performance. Large-scale systems, such as may be found at larger commercial sites, places of worship, or public buildings, sometimes use multiple dosing areas, each with its own manifold and laterals. At these sites, each dosing area is sufficient in size and design characteristics to meet the required effluent loading on its own. The additional zones are provided for redundancy and safety reasons and to help maintain aerobic conditions by alternating dosing of zones. Dosing of these zones is alternated on a routine basis via valves and control systems in a manner deemed appropriate by the regulatory agency; on some sites, zones are rested for months at a time and then alternated.
Additionally, some states require that effluent distribution be even across the disposal area, requiring differing orifice size, spacing, and quantity between laterals to ensure relatively balanced discharge across the system. Even dosing is typically considered to be a condition where no two laterals/trenches differ in discharge per unit length by more than 10 percent to 25 percent, depending on the jurisdiction.
As noted previously, some states currently have less stringent design requirements or simply specify the number and spacing of lateral orifices. When the drainage field is small with a uniform elevation throughout, the calculations can be fairly simple, but when a drainage field must be installed with laterals at varying elevations, as is often the case in residential designs, the calculations become significantly more complex. The calculations also become an iterative process when dealing with larger manifolds, long laterals, and high flows where lateral and manifold friction can become significant.
A potential issue to consider when designing laterals in states with these design requirements is that when laterals are at differing elevations because of slope across the disposal area, the difference in elevation causes a difference in pressure head between laterals, which directly influences the discharge rate. In states with these requirements, proper design of orifice size, spacing, and quantity for the laterals is especially important and requires the services of a qualified design professional.
Because of the unique aspects of each site, it is strongly recommended that property owners considering an LPD system, or in need of engineering consulting services related to an LPD system, seek a firm with sound experience in LPD system design and with a profound understanding of the regulatory approval and permitting processes applicable to the project site.
For additional information about onsite wastewater treatment, the U.S. Environmental Protection Agency’s Onsite Wastewater Treatment Systems Manual (available online at www.epa.gov) is a good place to start, as well as applicable state and local regulatory agency websites.
Robert F. Barrick, P.E., is a project engineer in ARRO’s Hagerstown, Md., office. He can be contacted at firstname.lastname@example.org. Lancaster, Pa.-based ARRO provides professional engineering and environmental consulting services to municipal, industrial, and commercial enterprises throughout the mid-Atlantic region.