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Sunday, February 18, 2018

Septic System Design

The first step in the septic system design process is to identify locations on the property that have suitable site conditions to support an onsite septic system. For more information on septic systems in general, please see our Description of a Typical Septic System page. Consideration of potential sites must take into account the following regulatory constraints:

  • The drainfield must be at least 50 feet away from any downhill slope greater than 25%; however, the MOA can waive this requirement, on a case-by-case basis.
  • Percolation test: If the percolation test results yield an absorption rate of less than 60 minutes per inch, a conventional system can be installed. Percolation rates between 60 and 120 minutes per inch necessitate using an Advanced Wastewater Treatment System (AWWTS). If it takes greater than 120 minutes for the water level in the percolation hole to drop 1 inch, the soil is unsuitable for an onsite septic system.
  • There must be adequate room to install 2 drainfields (primary and reserve) on the lot and not encroach upon the protective radii of the neighboring wells and proposed well for the lot. The protective radii are summarized as follows:
    • Private wells (single-family residence and duplexes) have a 100 foot protective radius.
    • Class "C" wells have a 150 foot protective radius.
    • Class "A" or "B" wells have a 200 foot protective radius.
  • The septic system must be at least 100 feet away from any surface water. The MOA can waive this on a case-by-case basis. If the lot is covered with snow it may be difficult to assess whether there are surface waters present. Clearly, there are some areas where surface waters are not a concern, and there are others where it is always questionable. Typically, it is not possible for an engineer to make absolute statements regarding surface waters during winter conditions, unless he/she has done work in the immediate vicinity during summer months. The only other options are to talk to the neighbors owning adjacent developed lots, or to rely on statements from the sellers.
  • If the groundwater supply (aquifer) has a high concentration of nitrates (a by-product of decomposing organic material), the MOA may prohibit the installation of a conventional onsite septic system and require the installation of a more expensive AWWTS system that is designed to reduce nitrates.  There are two nitrogen reducing systems approved for use in the Municipality of Anchorage, the  AerocellTM system, manufactured by Quanics Inc, and the AdvantexTM system manufactured by Orenco Systems Inc.  The MOA classifies these as "Catetory III Nitrogen Reducing Systems"

Prior to making the initial site visit and identifying a potential septic system site, records research is done on the subject and adjacent properties to identify the locations of pertinent wells, septic systems, structures, surface waters, wetlands, easements, and topographical features within the vicinity of the subject property. This information is usually compiled from records on file at the MOA, ADEC, and/or GEG historical records.  In some cases it may be necessary to retain a registered Land Surveyor to survey the location of specific features, like streams, wetlands, existing wells, and other features.  This information is compiled and used to create a preliminary site plan. In short, there is a considerable amount of preparatory work before the initial site visit.

With the site plan in hand, the engineer can make some preliminary determinations as to what areas on the lot may be potential septic sites. If the property lines are not clearly identified in these areas, they should be flagged by a registered land surveyor prior to the initial site visit.

During the site visit, the engineer will make a visual assessment of the potential septic sites previously identified and select the site/s which are best suited. Typically, the following objectives are considered:

  • If possible, the septic system should be downhill from the existing or proposed house so as to insure that there is gravity flow from the house to the septic tank and drain field. There are cases where this cannot be achieved and the installation of a lift station is necessary.
  • Ideally, the septic system should not be under an existing or proposed driveway/parking area.
  • The septic site should be as accessible as possible for purposes of construction and maintenance.
  • The septic tank must be in a location where it can be reached by a pump truck.
  • The septic system should not be in a drainage swale or depression that may seasonally carry or have standing water in it.
  • The drainfield must have adequate separation to any existing septic systems, structures, and other features specified with MOA code.

With the septic site(s) selected, the field engineer then narrows the site selection down to the most suitable areas and establishes the location for the test holes. If the property is undeveloped, and this is the first septic system to be installed on it, the MOA will also require that the engineer establish the location for a reserve (future) drainfield site. In short, there will need to have the room to install two septic systems; however, only one will have to be installed. If the property is currently developed and the existing septic system is being upgraded (because it has failed) then identification of a reserve drain field site is not required, unless the number of bedrooms in the house has increased since the original septic system was installed. If so, then a reserve drainfield site will need to be identified. Both the upgrade and reserve drainfield will need to be adequately sized to serve the current or proposed number of bedrooms in the residence.

Once the test holes are excavated, the soils are classified and percolation tests are performed. Per MOA requirements the groundwater levels are monitored in the test hole for a period of at least 7 days. The information and data gathered in this investigation are used to establish what type of septic system can be installed and how large the drainfield will be. The size of the septic tank is not dependent upon the site conditions, but rather the number of bedrooms in the residence.

Typically, the most cost effective system to install is a conventional septic system; however, the soil and site conditions may be such that installation of a conventional septic system is not feasible. For marginal sites, the utilization of an AWWTS may be necessary.  For some properties the site conditions may be so poor that the only viable alternative is to install a holding tank.

Utilizing the information gathered to this point, the engineer will prepare the design drawings and specifications. This package is then submitted to the applicable regulatory agency for plan review and issuance of a permit. For single-family residences and duplexes inside the MOA (Girdwood to the Knik River), the permit is issued by the MOA, Development Services Department. For multi-family dwellings the permit (called a certificate to construct) is issued by the State of Alaska, ADEC. Both regulators charge a fee for review of the design package and issuance of the permit.

The cost for the contractor to install a septic system can vary widely, depending up the type of system, the size of the system, and the unique site conditions. The property owner should obtain bids from several reputable contractors that specialize in the installation of septic systems.  During the installation of the system, the engineer must perform mandatory inspections at various stages of the construction. The final step of the process is for the engineer to prepare record drawings of the new system and submit the drawings to the applicable regulatory agency for review and final approval the system.

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