Sewage Tanks

New and Replacement Tanks

All new and replacement treatment tanks shall be Aerobic Treatment Units (ATUs) compliant with the National Sanitation Foundation (NSF), Standard 40, Class 1, unless otherwise approved. Tanks shall be sized at a minimum treatment capacity of the greater of either 500 gallons per day or 120 gallons per bedroom per day.

Trash and Pump Tanks

Trash and pump tanks shall be a minimum of 1000 gallons.

Holding Tanks

Holding tanks are utilized on a temporary basis until public sewer connection can be made, or on rare occasions, where the installation of a system on an existing lot improved by an existing structure cannot be accomplished. All holding tanks must meet the following requirements:

  • Approval – The use of holding tanks is approved on a case-by-case basis.
  • Material – Tank materials shall be watertight and of a similar material used for other sewage tanks.
  • Location
    • Tanks must be located within 10 feet of an all-weather access road.
    • Holding tanks shall meet the setback requirements for tanks and not within 20 feet of a building.
    • Accidental spillage shall not create a nuisance.
  • Sizing 
    • One- and Two-Family Homes– Systems serving these structures shall be sized as follows:
      Number of Bedrooms
      Tank Capacity (in gallons)
    • Other Residential Structures – The minimum holding tank size is based on the greater of 1,000 gallons or 400 gallons per bedroom.
    • Non-Residential Structures – The minimum holding tank size is based on at least 5 times the estimated or measured flow rate serving the tank but not less than 2,000 gallons.
  • Number of Tanks– Not more than 4 holding tanks shall be installed in series.
  • Tank Outlets – Where sewage tanks are used as holding tanks, the outlet shall be sealed.
  • Flotation – The holding tank shall be protected from flotation via tank weight, earth anchors, or shallow burial depth.
  • Manhole – Manholes must meet the following requirements:
    • Size – Manholes shall be a minimum 24-inch square or 24-inch inside diameter.
    • Extension – The manhole shall extend through the cover to within 12 inches of finished grade but not closer than 6 inches from grade.
    • Cover and Grade – The manhole cover must be covered with a minimum of 6 inches of soil. Finished grade shall slope away from the manhole.
    • Locking Device – The manhole cover shall be locked to prevent unauthorized entry.
    • Service Port – The cleanout port in the manhole cover shall be a minimum of 8 inches in diameter, extend to a minimum of 4 inches above finished grade, and have an effective locking plug or brass cleanout plug.
  • Vent – Each tank shall be serviced by a minimum 2-inch diameter vent extending a minimum of 12 inches above grade and terminating with a return bend fitting or approved vent cap.
  • Warning Device – Alarms shall meet the following requirements.
    • Level – A high-water alarm shall be installed to activate a minimum of 1 foot below the inlet pipe. Commercial installations shall alarm when the capacity is at 75%.
    • Type – Alarms shall be audible or visual.
    • Equipment – Alarm equipment shall be located outside the holding tank.
  • Graywater Systems – Tanks used with graywater systems shall receive soil waste only (i.e. toilet) and may be a minimum of 1,000 gallons in capacity.
  • Pumping and Maintenance Schedule – A pumping and maintenance contract shall be submitted to St. Charles County prior to the installation of the tank. Adherence to the schedule is a condition of approval.
  • Maintenance Affidavit – A maintenance affidavit shall be submitted to the County annually by an OWTS Installer licensed with St. Charles County.