he beauty of a typical standard system comprised of a septic tank and a drainfield is that it uses no electricity or mechanical devices. Aside from periodic pumping of accumulated septic tank solids, the system operates by natural processes. Gravity that deceptively elegant and often overlooked principle provides all the power needed for water and wastes to flow through the system. Treatment and disposal of the wastewater is accomplished by natural physical, chemical, and biological processes in the soil of the absorption system.
From The Septic System Owner's Manual
Given adequate site and soil conditions, septic systems with gravity-flow drainfields can provide successful sewage treatment and disposal for decades some say practically indefinitely when properly constructed and maintained.
Just what is an “advanced system”? What we call here “advanced systems” are those used where the conventional gravity-fed system will not provide adequate treatment, typically due to insufficient land, high groundwater, proximity to rivers, bays, lakes, etc., poor soil, or shallow soil over bedrock. An advanced system is, in the broadest sense, one which incorporates some modification of, or addition to, the standard gravity-powered tank and drainfield setup. A wide variety of advanced systems have been developed in response to the needs of different site conditions. Since these systems are used for sites with limited soil or other problems, the margin of safety (otherwise afforded by optimal soil and site conditions) is limited, and any failure is likely to be difficult (and expensive) to correct.
Types of Advanced Systems
In the original version of this book, we described five advanced systems:
- Dosed-flow systems
- Mound systems
- Sand filters
- Gravel filters
The idea was to tell homeowners how each works, for two reasons:
- So that if you were involved with construction of a new system you would understand some basic principles and be better informed when talking to regulators and engineers
- Or in case you already had an advanced system, you would understand how to maintain it properly
In this revised edition we have amended the description of the above five systems, added a number of new (to us) advanced systems, and given references for more complete information.
“Dosed-flow” refers to controlling the flow of effluent to the drainfield, as opposed to the continuous gravity flow of conventional systems. In addition to evenly distributing the water in the drainfield, timed doses allow the system to rest and recover between loadings, and avoid peak hydraulic loads on the drainfield. This helps maintain a stable biomat and prevents the oxygen in the vadose zone (area where air is available to soil) from being depleted. New systems of tanks, level sensors, controllers and pumps have made the systems more reliable than they were in the past.
Dosing Without Pressure
Here effluent is distributed to the drainfield in periodic, large-volume doses. Dosing is achieved by a pump (see p. 88) or, if the drainfield is located far enough downhill from the tank, with an energy-free dosing siphon. Effluent accumulates in the dosing tank (pump chamber) up to a predetermined volume, at which point the pump or siphon is activated, and the accumulated effluent goes to the drainfield in a single dose. The pump or siphon then shuts off until enough effluent has again accumulated in the dosing tank. There are also dosing systems with timers that dose periodically. There is also the “flood-dosed” system that is designed so that the pump fills all the pipes at once, thus utilizing the entire field. Some tanks are designed with the pump chamber in the same tank instead of using two separate tanks.
Dosing with Pressure
A pressure-dosed system, also called a pressure distribution system, provides for periodic dosing of large volumes of effluent. All of the pipes within the drainfield are filled during each dosing cycle, and a uniform volume of effluent is distributed out of each hole in the network; this allows the soil to drain between applications. Drainage brings air into the drainfield, and reduces excessive biomat growth and soil clogging. This even distribution of effluent over the entire drainfield length provides more contact with soil organisms and, therefore, enhanced treatment. This uniform distribution allows the soil to remain well aerated and prevents soil clogging from excessive loading.
Update note: In our opinion, mounds are, in most cases, no longer an appropriate technology. Many of the devices listed in this chapter provide superior treatment to mounds, at lower cost, and with much less environmental destruction. Yet, so many mounds have been built that we offer the following description for homeowners who have to maintain them.
Mounds are basically raised drainfields. They were developed in the 1940s for sites with impermeable soils, high groundwater, or other limiting conditions. They were used extensively until the last decade, when a combination of mound failures, high costs, and newer, more efficient technology, has superceded them (in areas where regulators are up-to-date). If you are in an area where mounds are still required, look through the alternatives below, and initiate discussions with your health officials.
The mound consists of a sand bed over lightly tilled native soil, a gravel distribution bed on top of the sand bed, and a cover of topsoil over the entire mound. Septic tank effluent is distributed evenly over the sand bed through a pressurized network of small-diameter perforated pipes installed in the gravel bed. (Many of the early mounds that were not pressure-dosed failed.) Controlled dosing in the gravel bed aids in uniform distribution of the effluent into the sand. Final treatment and disposal occur in the native topsoil beneath the built-up mound.
Problems with Mounds
- They are huge (about 40 feet wide by 80 feet long for a 3-bedroom home with 450-gallon-per-day flow) and are an intrusion in the landscape.
- Routine monitoring and maintenance of the pump system is needed to ensure continued proper operation. When the power goes out, they stop working.
- They are expensive (pump, pump chamber, pressure distribution network, clean graded sand, engineering fees, heavy equipment used in construction).
- When the soil is saturated from rainfall, they tend to leak around the toe.
- They are ecologically destructive. Large amounts of sand and gravel are mined from riverbeds and transported to the site. The land upon which they are constructed cannot be used for anything else.
- Careful design and construction is required. Mound system failures have occurred due to compaction of topsoil, improper configuration or orientation on sloping sites, undersized sand bed, a poorly designed or constructed pipe network, and sand material that is too fine or too coarse.
For further information on mound systems, review articles in Small Flows Quarterly and Pipeline, available on line and by mail from National Small Flows Clearinghouse at:
There are three types of sand filters: the intermittent sand filter, the bottomless sand filter, and the recirculating sand filter.
Intermittent Sand Filter
This consists of a watertight basin with a bed of sand 2436 inches deep with a distribution pipe network on top, and an underdrain below. The distribution network and underdrain system are laid out in the layers of gravel above and below the sand bed. A medium-grade, clean sand is used for the sand bed, and pea gravel is used for the distribution pipe and underdrain layers. (See p. 92.) The intermittent sand filter produces a high-quality effluent for disposal into difficult soil. It essentially serves as a substitute for treatment that would otherwise be achieved by good-quality drainfield soil.
The unit is usually constructed below ground, with a geotextile filter fabric over the top of the gravel distribution bed, and soil backfill up to native grade. However, some filters are built open to the ground surface with a removable wood cover or are constructed completely above ground in an enclosed structure similar to a raised garden bed.
How an Intermittent Sand Filter Works
Septic tank effluent is periodically (intermittently) applied to the sand filter through the pipes at the sand bed surface. The treated, filtered effluent is collected by the underdrain system below the sand bed, and then typically flows to the drainfield directly or via a second dosing tank for final disposal.
Size of Sand Filters and Drainfields
A typical 2- to 3-bedroom house (300-gallon-per-day flow) would use a sand filter unit about 19 feet by 19 feet and 4 feet deep. Since a sand filter produces better quality, clearer effluent than a septic tank, higher drainfield application rates may be allowed in some cases and a correspondingly smaller drainfield is possible. For further info, see:
Bottomless Sand Filter
This is a sand filter that provides wastewater treatment not provided by excessively permeable (Type 1A) soil. This technology is being used for homes on sites where soil and site conditions preclude the use of conventional or shallow drainfields. There is a containment vessel consisting of four sides, but open at the bottom. Typical would be 12 inches of gravel with the distribution pipe, and under that 24 inches of sand or crushed glass; on top of the gravel is 612 inches of earth backfill. Pretreated effluent is applied via the pipes in the middle of the gravel layer. It then trickles down through the sand media in a time-dosed mode, and from there into the underlying soil.
See “NOPD II Helps Rhode Island Improve Coastal Pond,” Small Flows Quarterly, Spring 2000 (Vol. 1, Issue 2), which describes bottomless sand filters and other methods of remediation in critical environmental areas; go to p. 10 of:
Recirculating Sand Filter
A significant advance in recent years has been the development of recirculating filters with both sand and synthetic media. They are much smaller in footprint (for example, 4 x 8 feet) than a typical sand filter, and suitable for use in environmentally sensitive areas (sites on the coast, rivers, etc.). These devices recirculate effluent and provide a level of treatment superior to many centralized treatment plants at a fraction of the cost, especially when there is a necessity to remove nutrients like nitrogen.
The recirculating sand filter typically consists of three parts: a septic tank, a recirculating unit, and a drainfield, which can be a bottomless sand filter, shallow drainfield, or a standard drainfield). Effluent is recirculated between the recirculating tank and the sand filter, or drainfield, where it is applied in even doses. Note: Less land is required for a recirculating sand filter one-fifth of the land area of an intermittent sand filter (see p. 92).
For info on recirculating sand filters, see:
Sand-lined trenches are often used in excessively permeable (Type 1A) soils. Unlike an intermittent sand filter, they are not contained in a watertight vessel. Typical would be 12 inches of gravel with the pipe, and under that 24 inches of sand or crushed glass; on top of the gravel is 612 inches of earth backfill. Proper function requires that effluent to the sand filter be distributed over the media in controlled, uniform doses. To achieve accurate dosing, these systems require a timer-controlled pump, pump chambers, electrical components and distribution network, with a minimum of four doses per day spread evenly over a 24-hour period. The effluent is absorbed into the native soil at the bottom of the sand-lined trenches, which accomplishes disposal and further treatment.
For a good paper titled “Sand-Lined Trench Systems,” which includes drawings of trenches and bottomless sand filters, by the Washington State Department of Health, see:
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