If the lot does not pass the perc test, some municipalities may allow you to build an alternative engineered system. Where the problem is soil that is too dense (or in some cases, too permeable), too shallow (over bedrock or hardpan), or the water table is too high, a “mound” system is often the first choice as it is works much the same as a standard system, only with the leach field raised above the natural grade. Alternative systems may also be required in waterfront properties to help protect water sources.
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 maintenance 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 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 raises 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, they 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 professionally by a service company. However, some 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 pretreatment of the effluent before it reaches the leach field, a mini version of a sewage-treatment plan. 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.
- 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 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, effluent is dosed onto the top of the filter and collected at the bottom where it is pumped to the drain field. Some sand filters recirculate the effluent back to the tank several times before distributing it to the drain field. While most systems include a pump and controls, gravity distribution is possible in some cases.
- 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.
- Aerobic treatment system: Instead of a leach field, these systems use an aerobic process to treat the effluent in a large sand and gravel filter bed. Pumps circulate the effluent through perforated pipes in the filter bed, while blowers run constantly to inject air into the treatment tank. Aerobic systems can produce higher quality wastewater than a traditional system and may include a disinfectant component before the purified wastewater is discharged. These systems are often housed above grade in box built from pressure-treated timbers and are a growing choice for waterfront property and other environmentally sensitive areas. Technically these are not septic systems, but mini sewage-treatment plants.
- Drip distribution/irrigation: This uses a pump to distribute effluent through a filtering device to an array of drip tubes over a large area. 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.
- 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 spent time planting, trimming, and weeding the wetlands area.