The purpose of a septic tank is to provide primary treatment of sewage that is suitable for final disposal in a sealed borehole or sand filter. This type of onsite sewage disposal system is a better alternative to cesspits in environmentally sensitive areas and in developments where there are high sewage flows.
Examples of situations where septic tanks, or a higher level of treatment, are likely to be required include:
- cave areas;
- disposal of wastewater less than 40’ away from inshore waters;
- disposal of wastewater less than 4’ above the mean high water (MHW) mark. Furthermore the cesspit shall be designed such that is has 6ft minimum depth below the invert of the lowest sewer pipe which with MHW being 1.3ft above Mean Sea Level suggests that the cesspit will require 12ft below grade to the water table to be acceptable. Otherwise a Septic Tank will be required;
- developments consisting of 10-50 bedrooms (1000-5000 Igpd) in environmentally sensitive areas;
- Any residential developments over 50 bedrooms.
Requirements
All development requiring a septic tank must be submitted to the Department of Planning. The Department of Health will assess the application at the building permit phase of the planning application.
A separate permit for a Water Right is required from the Environmental Authority if a waste disposal borehole is required for the tank. A copy of the Water Right application shall be submitted to the Department of Environment and Natural Rewith the building permit application.
Construction
All septic tanks shall be water tight and constructed of concrete block, fiberglass or polyethylene. Concrete block tanks must be constructed using Type 50 sulphate resistant cement and all inside surfaces shall be plastered (minimum ½”) and sponge floated smooth. They shall be designed and constructed to withstand all anticipated loads and meet Department of Planning structural requirements.
Design of Septic Tanks
Septic tanks shall have a minimum of 2 compartments. The inlet compartment shall not be less than two-thirds of the total capacity or less than 416 imperial gallons liquid capacity and a minimum of 3 feet wide and 5 feet long (inside measurements). The secondary compartment of the septic tank shall have a minimum capacity of 208 imperial gallons and a maximum capacity of one third of the total capacity. The minimum length shall be 2 feet 6 inches
The maximum liquid depth of a septic tank shall be 60 inches with a minimum depth of 30 inches. The preferred depth is 48 inches. The total depth shall be at least 8” greater than the liquid depth to allow for air space at the top.
The baffle wall between the first and second chambers shall be fitted with 3- 4” transfer pipes located 12” below the top of the liquid level measured from the top of the pipes
Rectangular tanks shall be constructed with the longest dimensions parallel to the direction of flow. Cylindrical tanks shall have a minimum inside diameter of 48 inches.
Inlets and Outlets
The outlet of the septic tank shall be elevated above the high water table so as not to be submerged at any time during tide, or rainfall related fluctuations. In the absence of a historical record of water table fluctuations at the site, the invert of the outlet shall be positioned at not less than 3 feet above ordnance datum.
The inlet and outlet on all tanks or tank compartments shall be provided with sanitary tees or baffles made of approved materials constructed to distribute flow and retain scum in the tanks or compartments. The tees or baffles shall extend a minimum of 6 inches above and 9 inches below the liquid level but shall not exceed one–third the liquid depth. A minimum of 2 inches of clear space shall be provided over the top of the baffles or tees. The bottom of the outlet shall be a minimum of 2 inches lower than the bottom of the inlet.
Manholes
Each compartment of a tank shall be provided with at least one manhole opening 24 inches square or diameter located over the inlet or outlet. Where the inlet compartment of a septic tank exceeds 12 feet in length, an additional manhole shall be provided over the dividing wall. Manholes shall finish flush with grade or as per manufacturer’s recommendations.
Manhole covers
Manholes shall be fitted with a water tight cover of concrete, steel, cast iron or other approved material.
Double Running Trap & Vent
A double running trap and vent shall not be installed on the drain line entering the septic tank.
Capacity of Septic Tanks
The minimum liquid capacity shall be 624 imperial gallons for all tanks except Klargester spherical septic tanks in which case the minimum liquid capacity shall be 616 imperial gallons. The liquid capacity of the septic tank shall be increased by 150 gallons for every additional bedroom above three bedrooms.
Other Buildings
The liquid capacities for septic tanks other than residential shall be calculated using values listed in Table 802.7.2 of the Standard Private Sewage Disposal Code 2007 published by the Southern Building Code Congress International and available at the Department of Health.
Installation
Septic tanks shall be located with a minimum horizontal distance between various elements as indicated in the table below. Prefabricated tanks subject to submersion in groundwater must be designed for such applications and anchored according to manufacturer’s requirements. A 3 inch thick compacted sand bedding foundation shall be provided for all septic tank installations other than those constructed with concrete block.
Minimum Horizontal Separation Distances for Septic Tanks
|
Element |
Distance (Feet) |
|
Building |
10 |
|
Potable Water Tank |
20 |
|
Foundation Wall |
10 |
|
Pond, Shoreline |
40 |
|
Boundary |
10 |
|
Well |
20 |
|
Swimming Pool |
20 |
Maintenance and Sludge Disposal
Septic tanks shall be cleaned whenever the combined thickness of settled sludge and floating scum occupy one-third of the tank’s liquid capacity. The minimum time interval between cleaning shall be 12 months. All septage shall be disposed at the Government Septage Facility, currently located at Tynes Bay, Palmetto Road..
Final Effluent Disposal Methods
The final disposal method shall be approved by the Department of Environment and Natural Resources. Boreholes and sand filters shall meet the following design criteria:
Boreholes
Boreholes shall be cased and sealed with an approved grout placed along the full depth of the annular space between the casing and the formation. The width of the annular space shall be a minimum of 2 inches. The sealed depth of the borehole shall be specified by the Environmental Authority (usually in the range of 40 feet to 110 ft below sea level) and there shall be at least 20 feet of open hole below the sealed portion. The borehole location shall be approved by the Environmental Authority.
Sand Filters
Sand filters shall be sized appropriately for anticipated flow rates. The minimum dimensions, for dwellings of up to 3 bedrooms, shall be 3’ x 4’ x 4’ deep. Discharge into the sand filter shall be via perforated PVC spreader pipes laid on the surface of the sand. Access shall be provided into the sand filter chamber for the purpose of cleaning and replacement of sand as necessary.