If the lot does not pass the perc test, some municipalities may allow you to build an alternative engineered system. Alternative systems may also be required on waterfront properties and other environmentally sensitive areas to help protect water sources.
Where the problem is soil that is too dense, too shallow over bedrock or hardpan, or the water table is too high, a “mound” system is often the first choice as it works much the same as a standard system, only with the leach field raised above the natural grade.
Alternative systems cost more because they have more moving parts and electrical components (pumps, monitors, alarms) and greater complexity. They require greater monitoring and maintenance to prevent problems. They also require expert design and installation. If either the designer or the installer is unfamiliar with the technology being used, it may not perform as intended.
There is no one-size-fits-all solution. Design of a system is specific to the soil type, site conditions, and usage levels. Make sure your designer and installer both have proven track records with the specific system you are using.
Some states and municipalities only accept system types certified in their jurisdiction and may require that the owner maintain a service contract with an approved vendor. Proper maintenance is critical for success with alternative systems.
The mound uses a series of small distribution pipes set in a layer of gravel on top of a layer of sand, typically one to two feet deep. The tops and sides are covered with topsoil (see illustration). A mound system has one extra component, a dosing chamber (also called a pump chamber), which collects wastewater coming out of the septic tank.
Using a pump and a float system, the dosing chamber pumps effluent at a controlled rate for uniform distribution into the leach field. Most have an alarm system that alerts the owner, or maintenance company, if the pump fails or the water rises too high in the tank. Observation tubes are also recommended which allow easy inspection of the tank without digging up the access port. Also monitoring wells are often installed to monitor the conditions in and around the leach field.
The biggest costs are the additional equipment as well as the earthwork and extra materials needed to build the mound. Depending on the system design and the local cost of sand and gravel, a mound system can add $10,000 or more to the price of a conventional system – often costing over $20,000 in some areas.
Also, mound systems require more frequent pumping and more monitoring and maintenance because of the added complexity. Annual maintenance costs can be as high as $500.
OTHER ALTERNATIVE SEPTIC SYSTEMS
There are a wide range of alternative septic systems on the market and new ones arriving all the time. Some are geared more to community systems serving several homes and are usually monitored and maintained professionally by a service company.
Some alternative systems are well-suited to individual homes, although the cost, complexity, and maintenance of these systems need to be carefully considered. Most use electric pumps or siphons as well as filters, all of which need more monitoring and maintenance than a traditional system.
Some larger community systems use pre-treatment of the effluent before it reaches the leach field, a mini version of a sewage-treatment plant. But most single-home systems rely on bringing in natural or manufactured materials that require less surface area for leaching than the poorly draining native soils would require. There are many variations and combinations of systems and components used including:
- Pressurized dosing: This uses a holding tank and pump to force the effluent though the distribution piping more evenly and in controlled doses, improving the performance of the leach field. It can be used to rehabilitate a leach field or in combination with other systems such as a mound system, sand filter, plastic leach fields, or drip irrigation.
- Plastic chamber leach field: This is a standard septic system with an alternative leach field, which may be downsized in some jurisdictions, making it well suited to small building sites. The half-pipe plastic chambers create the void for effluent flow, so no gravel is needed. One example, the Infiltrator System, has been in use for over 20 years, and according to the company can support traffic loads with only 12 inches of compacted cover. The additional cost of the plastic components is partly offset by the savings on gravel and smaller size of the drain field.
- Sand filter: This is a large sand-filled box, 2-4 ft. deep, with a watertight liner of concrete or PVC. The sand is used to pre-treat effluent, by filtration and aerobic bacteria, before disposal to the leach field. The boxes are usually partially or fully in the ground, but can also be above-grade when required. In the most common configuration, a pump and controls are used to evenly dose the effluent on top of the filter, although gravity distribution is possible in some cases. The treated effluent is collected at the bottom where it is pumped or gravity-fed to the drain field.Some sand filters recirculate the effluent back to the tank several times before distributing it to the drain field.Most sand filters are used for pre-treatment, but they may also be designed as the primary treatment. This is called a “bottomless sand filter” as the effluent passes directly into the soil beneath the filter (see photo above). Sand filters must be well-engineered and constructed, and properly maintained, to avoid frequent clogging of the sand. Read more on Sand Filters.
- Aerobic treatment system: These systems use an aerobic process to treat the effluent, typically using an multi-chamber concrete tank placed underground. The most complex systems contain four chambers to collect, aerate, purify, and pump the effluent. The first chamber acts like a smaller version of a traditional septic tank. In the second “treatment” tank, an air pump is used to constantly inject air into the effluent. The air supports the growth of aerobic bacteria which process sewage more efficiently than the anaerobic bacteria in a typical septic system. In some systems, a third and fourth chamber are used to further clarification and pump out the purified water. So-called “fixed-film” systems also use a synthetic media filter to enhance the bacterial process.When properly maintained, aerobic systems can produce higher quality wastewater than a traditional system and may include an added disinfectant before the purified wastewater is discharged. Discharge may be to a reduced-size drain field or sprayed over a large area in rural settings. Technically these are mini sewage-treatment plants rather than septic systems, which rely mainly on anaerobic treatment. They are often referred to as ATU’s (aerobic treatment units) or BATs (best available technology units). These systems are expensive to install and maintain, so are mainly used where high-quality treatment is needed in a limited area or with poor soils. They are becoming more common on waterfront properties. Read more on Aerobic Treatment Systems.
- Drip distribution/irrigation: This uses a pump to distribute effluent through a filtering device to an array of shallow drip tubes spread over a large area. A pretreatment unit is usually required to deliver relatively clean water to the system. The water may used to water a lawn or non-edible plants, which remove the nitrogen from the wastewater. This type of system can be used with shallow soils, clays, and on steep slopes. Because the tubes are near the surface, freezing can be a problem in cold climates. As with other alternative systems, expect high installation costs, along with extra monitoring and maintenance.
- Constructed wetlands. For the ecologically minded who want to play an active role in recycling their wastewater, these can work in almost any type of soil. The system uses a man-made shallow pond, which is lined and filled with rock, tire chippings, or other media. The media provides a habitat for special plants that treat wastewater and provide a pleasant environment. Wastewater from the septic tank is distributed by a perforated pipe across the media bed, where plant roots, bacteria, and other microorganisms break down the pollutants. A second pipe at the back of the wetland collects the treated water. The homeowners must plan to spend time planting, trimming, and weeding the wetlands area.
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